wb 2 suggested ans

Chemistry A Modern ViewExperiment Workbook 2　Suggested answers

Contents

PART V ACIDS AND ALKALIS

Chapter 15 Acids15.1 To investigate properties of dilute acids 3

15.2 To study the role of water in exhibiting characteristic properties of acid 6

15.3 To investigate the corrosive nature of concentrated acids 10

Chapter 16 Alkalis16.1 To investigate the action of dilute alkalis on ammonium compounds 11

16.2 To investigate the action of dilute alkalis on aqueous metal ions to form metal

hydroxide precipitates 12

16.3 Action of concentrated sodium hydroxide solution on meat (T) 14

Chapter 17 Indicators and pH17.1 To find pH values of some common substances 15

Chapter 18 Strength of acids and alkalis (Extension)18.1 To compare the relative strength of acids and of alkalis 17

Chapter 19 Neutralization and salts19.1 To investigate the temperature change associated with a neutralization

reaction 22

19.2 To prepare sodium sulphate crystals from an acid-alkali titration 23

Chapter 21 Simple volumetric work (Extension)

©Aristo Educational Press Ltd. 2003 - 1 -

21.1 To prepare standard ethanedioic acid solutions 24

21.2 To find the molarity of a given hydrochloric acid 25

21.3 To determine the concentration of ethanoic acid in commercial vinegar 26

Chapter 22 Rate of reaction (Extension)22.1 To investigate the effect of concentration on the rate of reaction 27

22.2 To investigate the effect of surface area on rate of reaction 28

22.3 Effect of temperature on the rate of reaction 33

PART VI CHEMICAL CELLS AND ELECTROLYSIS

Chapter 23 Chemical cells in daily life23.1 To compare the service life of a zinc-carbon cell with an alkaline manganese

cell of the same size (T) 38

Chapter 24 Simple chemical cells24.1 To make simple chemical cells and construct part of the Electrochemical

Series 39

Chapter 26 Cell reactions (Extension)26.1 To construct a simple chemical cell using inert electrodes 41

26.2 To examine and compare the internal structures of a well-used and a new zinc-

carbon cell 42

Chapter 27 Electrolysis (Extension)

27.1 Electrolysis of dilute sulphuric acid (T) 43

27.2 Effect of concentration on preferential discharge of ions 44

27.3 Effect of electrodes on products of electrolysis 47

27.4 Electroplating 48


PART VII PRODUCTS FROM IMPORTANT PROCESSES

Chapter 28 Chlorine and hypochlorite28.1 To investigate the oxidizing property of chlorine water 51

28.2 To make chlorine bleach 52

28.3 Action of chlorine bleach on coloured substances 56

28.4 Action of acid on chlorine bleach 57

Chapter 29 Sulphuric acid and sulphur dioxide29.1 To investigate properties of concentrated sulphuric acid (S/T) 58

29.2 To dilute concentrated sulphuric acid (S/T ) 60

29.3 To prepare sulphur dioxide and test for its properties 61

29.4 To bleach coloured papers and flower petals with sulphur dioxide (S/T ) 65

MICROSCALE EXPERIMENT

M1 Electrolysis using a microscale Hoffman apparatus 66


Experiment 15.1 To investigate properties of dilute acids

Test Observations

2 M hydrochloric acid 2 M ethanoic acid

1. ■ Test with a blue litmus paper. turns red turns red

2. ■ (a) Clean a 2 cm length of magnesium

ribbon with sandpaper. Add the

cleaned ribbon to the acid. Quickly

place the test tube upright in a test

tube rack. Immediately cover the

mouth of the tube with an inverted

rubber stopper (Figure 15.1).

Leave the tube to stand, until the

magnesium has dissolved

completely.

rapid

effervescence of a

colourless gas; the

magnesium dissolves

quickly to give a

colourless solution

effervescence of a

colourless gas; the

magnesium dissolves to

give a colourless

solution

■ (b) Remove the stopper and quickly

put a burning splint into the mouth

of the tube (Figure 15.2).

a ‘pop’ sound is heard a ‘pop’ sound is heard

(c) Name the gas evolved. hydrogen hydrogen

3. ■ (a) Repeat Step 2(a), using 2 calcium

granules instead of a magnesium

ribbon. (Caution! Handle calcium

with forceps, never with bare

fingers.)

rapid

effervescence of a

colourless gas; the

calcium granules

dissolve quickly to give

a colourless solution

effervescence of a

colourless gas; the

calcium granules

dissolve to give a

colourless solution

■ (b) Test any gas evolved with a

burning splint.

a ‘pop’ sound is heard a ‘pop’ sound is heard

(c) Name the gas evolved. hydrogen hydrogen

(d) Write a general word equation for

the reactions involved in Steps

2(a) and 3(a).

acid + metal → salt + hydrogen


Test Observations

2 M hydrochloric acid 2 M ethanoic acid

4. ■ (a) Add 1/4 spatula measure of

copper(II) oxide to the acid. Warm

the mixture gently (Figure 15.3).

(Caution! Wear safety spectacles.)

the black copper(II)

oxide solid dissolves to

give a blue solution

the black copper(II)

oxide solid dissolves to

give a blue solution

(b) Write a general word equation for

the reactions involved here.

acid + metal oxide → salt + water

5. ■ (a) Add 1/4 spatula measure of

calcium hydroxide to the acid.

Warm the mixture gently.

(Caution! Wear safety spectacles.)

the calcium hydroxide

dissolves to give a

colourless solution

the calcium hydroxide

dissolves to give a

colourless solution

(b) Write a general word equation for


acid + metal hydroxide → salt + water

6. ■ (a) Add 3 spatula measures of

anhydrous sodium carbonate to the

acid. Pass any gas evolved through

limewater (Figure 15.4).

rapid

effervescence of a

colourless gas; the

carbonate dissolves

quickly to form a

colourless solution;

limewater turns milky

effervescence of a

colourless gas; the

carbonate dissolves to

form a colourless

solution; limewater

turns milky

(b) Name the gas evolved. carbon dioxide carbon dioxide

(c) Write a general word equation for


acid + carbonate → salt + carbon dioxide + water


7. ■ (a) Repeat Step 6(a), using sodium

hydrogencarbonate instead of

sodium carbonate. Pass any gas

evolved through limewater.

rapid

effervescence of a

colourless gas; the

hydrogencarbonate

dissolves quickly to

form a colourless

solution; limewater

turns milky

effervescence of a

colourless gas; the

hydrogencarbonate

dissolves to form a

colourless solution;

limewater turns milky

(b) Name the gas evolved. carbon dioxide carbon dioxide

(c) Write a general word equation for


acid + hydrogencarbonate

→ salt + carbon dioxide + water

8. Hydrogen gas.

Hydrogen ion, H+(aq).

Yes.

9. c. No.

d. No.

Hydrochloric acid reacts with the carbonate to give a salt, carbon dioxide and water. The

H+(aq) ions are removed as water. Thus the characteristic acidic properties are lost.

e. Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)

10. a. red


salt; hydrogen

salt; water

salt; carbon dioxide; water

b. hydrogen ions H+(aq)


Experiment 15.2 sample laboratory report

Title: To study the role of water in exhibiting characteristic properties of acid

PurposeTo study the role of water in exhibiting characteristic properties of acid.

Apparatus and chemicals used• Watch glass (dry) • Magnesium ribbon (2 cm length)

• Dry test tubes (150 × 18 mm) in rack • Citric acid crystals (dry), 2 g

• Stopper that fits 24 mm diameter test tube • Deionized water

• Matches/lighter • Wooden splint

• Blue litmus paper (dry)

Chemical reactions involvedMg(s) + 2H+(aq) Mg2+(aq) + H2(g)

2H2(g) + O2(g) 2H2O(l) ('pop' sound test)

Procedure(A) To compare the action of solid citric acid and its aqueous solution on dry blue litmus

paper

1. (a) Half a spatula measure of dry solid citric acid was added to a dry watch glass.

(b) A dry blue litmus paper was dipped into the solid acid (Figure 1a)

(c) Any colour change of the dry blue litmus paper was recorded.

2. (a) 1 cm3 of water was added to the solid citric acid.

(b) A dry blue litmus paper was dipped into the aqueous solution (Figure 1b).

(c) Any colour change of the dry blue litmus paper was recorded.


(B) To compare the action of solid citric acid and its aqueous solution on magnesium

3. (a) A piece of magnesium ribbon was put into a dry test tube (Figure 2a).

(b) 3 spatula measures of solid citric acid were added to the test tube (Figure 2b).

(c) Any observation occurred was recorded

4. (a) Water was added to the test tube (from Step 3) to a depth of 3 cm.

(b) The tube was shaken to dissolve the citric acid crystals. It was quickly placed upright in

a test tube rack. The mouth of the tube was immediately covered with an inverted

rubber stopper. The tube was allowed to stand for 5 minutes (Figure 3a).

(c) Any observation occurred was recorded.

(d) After 5 minutes, the stopper was removed and a burning splint was quickly put into the

mouth of the tube (Figure 3b).

(e) Any observation of the experiment was recorded.


Observation1. (Reference to Step 1): No observable change.

2. (Reference to Step 2): The blue litmus paper changed to red.

3. (Reference to Step 3): No observable change.

4. (Reference to Step 4): The citric acid gradually dissolved to form a colourless solution. At the

same time, colourless gas bubbles were evolved from the surface of the magnesium ribbon.

The magnesium ribbon gradually became smaller. The bottom of the test tube became warm.

The gas gave out a 'pop' sound with a burning splint, which was identified as hydrogen.

Interpretation1. No observable change because there is no hydrogen ions in the absence of water. Thus, solid

citric acid does not show acidic property.

2. After the addition of water, hydrogen ions that can turn the blue litmus paper red, are formed.

3. No observable change because there is no hydrogen ions in the absence of water. Thus, solid

citric acid does not show any acidic property.

4. Citric acid ionizes in water to give H+(aq) ions. It reacts with magnesium to give hydrogen

gas.

Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)

It shows acidic property.

Discussion1. It is essential that the apparatus and chemicals used must be dry at the start, so that the effect

of adding water can be compared correctly.

2. It is not a good practice to cover the test tube with student 's thumb because there may be

some acid on the rim of the test tube. If a right-sized stopper is used to cover the test tube, do

not leave it unattended when there is effervescence in the test tube. Pressure will build up

inside the tube and the stopper may shoot out. This will accidentally hurt students' eyes. The

stopper used should have a bigger size (suitable for test tube with 24 mm diameter).

3. Unreacted magnesium should never be disposed of into the sink. It should be collected by the

teacher and the laboratory technician will take care of it.

4. Magnesium metal surface should be cleaned with sand paper to remove any oxide layer

together with the grease. It should be noted that holding the metal by hand would make the

metal greasy.

ConclusionWithout water, an acid does not show the usual acidic properties. Water must be present for an acid


to give H+(aq) ions, which are responsible for the typical acidic properties.

Answers to questions for further thought1. It changes blue litmus paper to red colour.

It conducts electricity.

It reacts with metal oxides and hydroxides to give salts and water.

It reacts with carbonates and hydrogencarbonates to give salts, carbon dioxide and water.

2. (a) When it is dissolved in water, the solid acid ionizes to form H+(aq) ions, which react

with sodium hydrogencarbonate. There is effervescence, carbon dioxide gas being

given off.

(b) H+(aq) + HCO3−(aq) H2O(l) + CO2(g)

(c) It should be stored in a dry cool place.

3. In the presence of water, acidic gas will dissolve and ionize to form H+(aq) ions. Thus, it can

change the blue litmus paper to red colour.

4. (a) No colour change. It is because pure ethanoic acid liquid contains no water.

(b) From blue to red. In the presence of water, ethanoic acid ionizes to form H+(aq) ions.

Thus, it can show acidic properties.


Experiment 15.3 To investigate the corrosive nature of

concentrated acids

1. d. A ‘pop’ sound is heard.

Hydrogen.

2. Yes.

Concentrated hydrochloric acid.

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

3. c. No signs of reaction no matter the acid is cold or hot.

Yes. A dilute aqueous solution of a typical acid does not react with copper and those

metals below copper in the metal reactivity series.

4. Brown fumes are evolved quickly to fill the tube.

1; 4; 1; 2; 2

No. An aqueous solution of a typical acid reacts with those metals, which are above copper in

the reactivity series. In such reactions, the gas liberated is hydrogen.

5. a. zinc; the same; faster; acidic; acidity; corrosive; acidity

b. no; oxidizes; very corrosive; oxidizing


Experiment 16.1 To investigate the action of dilute alkalis on

ammonium compounds

4. A characteristic pungent choking smell of ammonia.

Ammonia.

5. b. It turns from red to blue.

(NH4)2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2NH3(g) + 2H2O(l)

6. Ammonia; ammonium sulphate; sodium hydroxide; ammonia; pungent, choking; red; blue


Experiment 16.2 To investigate the action of dilute alkalis on

aqueous metal ions to form metal hydroxide

precipitates

Test Observations

0.5 M NaOH(aq) 1 M NH3 (aq)

1. Add a piece of red litmus paper to

2 cm3 of an alkali solution in a test tube

(Figure 16.4).

turns blue turns blue

2. (a) Using a teat pipette, add 10 drops of

sodium hydroxide solution to 6 cm3

of water in a test tube (Figure 16.5).

Rub a little of the diluted solution

between your fingers.

(Caution! (1) Wash your hand

immediately afterwards with plenty

of water. (2) Omit this step if there

is a wound in your fingers. This is

to avoid possible irritation.)

(b) Rub a little of the 1 M ammonia

solution (undiluted) between your

fingers.

it has a slippery (soapy)

feel

it has a slippery (soapy)

feel


6.

Metal ions Addition of a little

NaOH(aq)

Addition of excess

NaOH(aq)

Addition of a little

NH3(aq)

Colour of precipitate

formed (if any)

Does the precipitate

dissolve in excess

NaOH(aq)?

Colour of precipitate

formed (if any)

Pb2+(aq) white yes white

Cu2+(aq) blue no blue

Fe2+(aq) dirty green no dirty green

Fe3+(aq) reddish brown no reddish brown

K+(aq) (no precipitate) (not applicable) (no precipitate)

Zn2+(aq) white yes white

7. Yes.

Hydroxide ion, OH-(aq).

NaOH(s) + H2O(l) → Na+(aq) + OH−(aq)

NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)

8. a. Pb2+(aq), Cu2+(aq), Fe2+(aq), Fe3 +(aq), Zn2+(aq)

b. Pb2+(aq) + 2OH−(aq) → Pb(OH)2(s);

Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s);

Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s);

Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s);

Zn2+(aq) + 2OH−(aq) → Zn(OH)2(s)

9. Pb2+(aq), Zn2+(aq)

10. a. red; blue


soapy

metal hydroxides

Pb(OH)2(s), Cu(OH)2(s), Fe(OH)2(s), Fe(OH)3(s), Zn(OH)2(s)

b. hydroxide ion OH−(aq)


Experiment 16.3 Action of concentrated sodium hydroxide

solution on meat (T)

1. Pale pink or slightly yellow (answer varies).

Yes.

5. The chicken foot becomes smaller. It turns reddish yellow and somewhat ‘translucent’. The

solution becomes pale red.

The chicken foot becomes white. There is no other apparent change.

6. The chicken foot crumbles (breaks up) when stirred. A mixture of skin, meat and white oily

mass floats. Small pieces of bones sink to the bottom.

7. There is no apparent change.

8. Concentrated sodium hydroxide solution is very corrosive on meat.

9. He should wash the affected part with plenty of water (for at least a few minutes), then report

to the teacher.

10. corrosive; eat; chicken feet


Experiment 17.1 To find pH values of some common substances

4. (1) 4

(2) 10

(3) 2

(4) 3

(5) 9

(6) 10

(7) 7

(8) answer varies

(9) 1

(10) 14

(11) 12

(12) 7

5. (a) distilled water, sodium chloride solution

(b) soft drink (7-up), vinegar, lemon juice, 1 M hydrochloric acid

(c) soap solution, Philips Milk of Magnesia/ window cleaner, limewater, 1 M sodium

hydroxide

12. pH scale

Indicator

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Methyl orange red orange yellow

Litmus red purple blue

Phenolphthalein Colourless very pale

pink

red

13. Red / yellow.

Not sure.

Acidic / not sure.

14. Red / blue.

Not sure.

Acidic / alkaline.


15. Colourless / red.

Not sure.

Not sure / alkaline.

16. No, we could only get a rough idea of the pH, acidity or alkalinity of a solution.

17. By using universal indicator or pH meter.



Title: To compare the relative strength of acids and of alkalis

PurposeTo compare the relative strength of

(A) acids and

(B) alkalis.

Apparatus and chemicals usedApproach 1: To compare the relative strength of acids and of alkalis by measuring

electrical conductivities

(A) For comparing the relative strength of acids

• Beaker (100 cm3) • Hydrochloric acid (0.1 M)

• Power supply (0 − 24 V) / 6 V d.c. supply • Ethanoic acid (0.1 M)

• Electrode rod holder

• 2 carbon rods

• 3 connecting wires, with crocodile clips at both ends

• Digital multimeter (or milliammeter)

• Measuring cylinder (100 cm3)

(B) For comparing the relative strength of alkalis

• Same as in (A) • Sodium hydroxide solution (0.1 M)

• Ammonia solution (0.1 M)

Approach 2: To compare the relative strength of acids and of alkalis by measuring pH

values

(A) For comparing the relative strength of acids

• 2 test tubes (150 × 18 mm) in rack • Hydrochloric acid (0.1 M)

• Glass rod • Ethanoic acid (0.1 M)

• Measuring cylinder (100 cm3)

• pH paper (pH range 1 − 14) with colour chart

(B) For comparing the relative strength of alkalis

• Same as in (A) • Sodium hydroxide solution (0.1 M)

• Ammonia solution (0.1 M)


Chemical reactions involvedHCl(aq) H+(aq) 1 Cl−(aq)

CH3COOH(aq) CH3COO−(aq) + H+(aq)

NaOH(s) + water Na+(aq) + OH−(aq)

NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)

Approach 1: To compare the relative strength of acids and of alkalis by measuring


Procedure(A) To compare the relative strength of acids by measuring electrical conductivities.

1. 80 cm3 of 0.1 M hydrochloric acid was put into a small beaker (100 cm3).

2. The apparatus was set-up as shown in Figure 1.

1 2

3. The range selector of the multimeter was adjusted to a suitable range for measuring d.c.

current. The reading was noted immediately.

4. The electrode rod holder was taken out of the beaker. The carbon rods were washed with

running tap water.

5. The beaker was emptied and washed well with water.

6. Steps 1 to 5 were repeated. 80 cm3 of 0.1 M ethanoic acid was used instead.

(B) To compare the relative strength of alkalis by measuring electrical conductivities.

7. Steps 1 to 5 were repeated. 80 cm3 of 0.1 M sodium hydroxide solution was used instead.

8. Steps 1 to 5 were repeated. 80 cm3 of 0.1 M ammonia solution was used instead.



values

(A) To compare the relative strength of acids by measuring pH values.

9. (a) A glass rod was dipped into a little 0.1 M hydrochloric acid in a test tube to get a drop

of the solution (Figure 2a).

(b) The drop of acid was applied to a piece of pH paper (Figure 2b).

10. The colour of the pH paper was matched with the colour chart. The pH value was then

recorded.

11. Steps 9 to 10 were repeated using 0.1 M ethanoic acid instead.

(B) To compare the relative strength of alkalis by measuring pH values.

12. Steps 9 to 10 were repeated using 0.1 M sodium hydroxide solution instead.

13. Steps 9 to 10 were repeated using 0.1 M ammonia solution instead.Observation

Approach 1: To compare the relative strength of acids and of alkalis by measuring


1. (Reference to Steps 1 to 6): Equal concentration of hydrochloric acid (0.1 M) and ethanoic

acid

(0.1 M) show different electrical conductivities. Hydrochloric acid shows a higher electrical

conductivity than that of ethanoic acid.

2. (Reference to Steps 7 and 8): Equal concentration of sodium hydroxide solution (0.1 M) and

ammonia solution (0.1 M) show different electrical conductivities. Sodium hydroxide

solution shows a higher electrical conductivity than that of ammonia solution.



values

1. (Reference to Steps 9 to 11): Equal concentration of hydrochloric acid (0.1 M) and ethanoic

acid (0.1 M) show different pH values. Hydrochloric acid shows a lower pH value than that

of ethanoic acid.

2. (Reference to Steps 12 and 13): Equal concentration of sodium hydroxide solution (0.1 M)

and ammonia solution (0.1 M) show different pH values. Sodium hydroxide solution shows a

higher pH value than that of ammonia solution.

InterpretationApproach 1: To compare the relative strength of acids and of alkalis by measuring


1. Equal concentration of hydrochloric acid and ethanoic acid means that they have the same

concentration of solute in the solution. However, hydrochloric acid is a strong acid, which

will ionize completely in aqueous solution. Ethanoic acid is a weak acid which will only

slightly/partially/incompletely ionize in aqueous solution. As a result, hydrochloric acid will

have a higher ionic concentration and conduct electricity better.

HCl(aq) H+(aq) + Cl−(aq) (complete ionization)

CH3COOH(aq) CH3COO2(aq) + H+(aq) (slight ionization)

2. Equal concentration of sodium hydroxide solution and ammonia solution means that they

have the same concentration of solute in the solution. However, sodium hydroxide solution is

a strong alkali which will dissociate completely in aqueous solution. Ammonia solution is a

weak alkali which will only slightly/partially/incompletely ionize in aqueous solution. As a

result, sodium hydroxide solution will have a higher ionic concentration and conduct

electricity better.

NaOH(s) + water Na+(aq) + OH−(aq) (complete dissociation)

NH3(aq) + H2O(l) NH4+(aq) + OH−(aq) (slight ionization)


values

3. Equal concentration of hydrochloric acid and ethanoic acid means that they have the same

concentration of solute in the solution. However, hydrochloric acid is a strong acid, which

will ionize, completely in aqueous solution. Ethanoic acid is a weak acid which will only

slightly/partially/ incompletely ionize in aqueous solution. As a result, hydrochloric acid will

have a higher concentration of hydrogen ions, and thus a lower pH value.

HCl(aq) H−(aq) + Cl−(aq) (complete ionization)

CH3COOH(aq) CH3COO−(aq) + H+(aq) (slight ionization)

4. Equal concentration of sodium hydroxide solution and ammonia solution means that they

have the same concentration of solute in the solution. However, sodium hydroxide solution is


a strong alkali which will dissociate completely in aqueous solution. Ammonia solution is a

weak alkali which will only slightly/partially/incompletely ionize in aqueous solution. As a

result, sodium hydroxide solution will have a higher concentration of hydroxide ions, and

thus a higher pH value.

NaOH(s) + water Na+(aq) + OH−(aq) (complete dissociation)

NH3(aq) + H2O(l) NH4+(aq) + OH−(aq) (slight ionization)

Discussion1. For a fair comparison, the strong acid and weak acid used should be of equal concentration.

The difference in electrical conductivities is thus a result of the difference in the degree of

ionization of the two acids in aqueous solution.

2. The digital multimeter is an ideal instrument for measuring electric current here. The readings

are quite accurate and can be easily read on the LCD screen.

3. Always remember to connect the negative pole of the power supply to the negative (black)

terminal of the multimeter.

4. Electrolysis of solution will take place, liberating gases at electrodes. The gas bubbles on the

surface of electrodes increase the resistance of the circuit, causing the current reading to drop

with time.

Conclusion1. Hydrochloric acid is a stronger acid than ethanoic acid of the same concentrations?

2. Sodium hydroxide solution is a stronger alkali than ammonia solution of the same

concentrations?

Answers to questions for further thought1. Citric acid, ethanoic acid, ascorbic acid, etc.

2. No, it is because for a fair comparison, all factors such as temperature and concentration of

solute in aqueous solution should be the same, except that different acids or alkalis are used.

So when we are measuring the electrical conductivities, the difference is a direct reflection of

the degree of ionization (or dissociation) of the acids or alkalis. Thus, the strength of acids

and of alkalis can be compared.

3. (a) water, ammonia (the most plentiful), ammonium ions, hydroxide ions, hydrogen ions

(b) water, sodium ions, hydroxide ions (the most plentiful), hydrogen ions


Experiment 19.1 To investigate the temperature change associated

with a neutralization reaction

4. Given out.

Exothermic

7. Roughly the same.

H+(aq); OH−(aq); Twice; given out; heat up; rises; roughly the same

11. The temperature rise is about half of that in Part B.

12. Exothermic.

13. Yes.

14. very little heat; bad; heat; heat; conduction; heat losses; bad; heat


Experiment 19.2 To prepare sodium sulphate crystals from an

acid-alkali titration

5. Yellow.

9. To obtain the salt free from any indicator.

10. b. If all the water were driven away, the sodium sulphate obtained would be a powder, not

crystals.

12. Colourless crystals.


Experiment 21.1 To prepare standard ethanedioic acid solutions

4. 9.51

(12.0 + 16.0 × 2 + 1.0) × 2 + 2 × (1.0 × 2 + 16.0)

126.0

250.0

9.51/126.0

250.0/1000

0.302

11. 0.302

25.0

250.0

Yes.

This is because its concentration (molarity) is accurately known.


0.302 0.0302

Experiment 21.2 To find the molarity of a given hydrochloric acid

5. Yellow.

6. b. The end point has been reached.

9. 0.102

26.1

10. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

0.102

25.0

1000

1:1

26.1; 2.55 × 10−3

2.55 × 10−3

26.1/1000


0.0977

0.102 2.55 × 10−3

Experiment 21.3 To determine the concentration of ethanoic

acid in commercial vinegar

2. c. Colourless.

6. 0.102

25.2

7. CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)

25.2; 0.102

25.2

1000

1:1

2.57 × 10−3

a. 2.57 × 10−3

25.0/1000

b. 250.0

25.0


2.57 × 10−30.102

0.103

1.030.103

Experiment 22.1 To investigate the effect of concentration on

the rate of reaction

8. The curve is steep at first as the reaction rate is the fastest, but becomes less steep with time.

Finally, it becomes horizontal, indicating the finish of the reaction.

Quick and accurate results can be obtained by using data-logger. Besides, continuous

monitoring is possible.

However, using data-logger is more complicated than traditional methods and it is less easy

to be handled.

9. increase; increase



Title: To investigate the effect of surface area on rate of reaction

Purpose

To investigate the effect of surface area on rate of reaction by using data-logger.

Apparatus and chemicals used• Suction flask (250 cm3) • Dry lumps of calcium carbonate, 0.5 g

• Measuring cylinder (25 cm3) • Dry powdered calcium carbonate, 0.5 g

• Plastic sample bottle, large enough to hold • Dilute HCl(aq) (1 M)

about 25 cm3 of 1 M hydrochloric acid

• Stand, boss and clamp

• Data-logger connected to a computer with

pre-installed software, absolute pressure sensor

• Rubber tubing (8 inches) for connecting the

suction flask and the data logger

• Electronic balance (accurate to 0.01 g)

• Vacuum sealant and adhesive

Chemical reactions involvedCaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

Procedure1. 0.5 g of lumps of calcium carbonate was weighed out and put inside a dry suction flask.

(Figure 1)


2. 25 cm3 of 1 M hydrochloric acid was measured by a measuring cylinder and transferred into a

plastic sample bottle (Figure 2).

3. The plastic sample bottle with hydrochloric acid was put into the suction flask carefully.

4. The suction flask was stoppered carefully and sealed to avoid air leakage. A piece of rubber

tubing was used to connect the data-logger and the suction flask for pressure measurement.

5. The data logging software was started and set properly for data collection.

6. The suction flask was inverted to mix the reactants thoroughly and the data collection process

was started immediately. See Figures 3 and 4.

pressure sensor data-loggerto computer

Figure 4


7. When the reaction finished, the data collection process was stopped.

8. The results (data and graph) were saved and printed out to hand in together with the report.

9. Steps 1 − 8 were repeated, but 0.5 g of powdered calcium carbonate was used instead.

Observation1. (Reference to Step 6): Effervescence occurred in the reaction mixture.

2. (Reference to Step 6): The pressure inside the suction flask increased steadily and very

slowly with lumps of calcium carbonate added (more than 2.5 minutes) but very rapidly (less

than 0.5 minute) with powdered calcium carbonate added.

3. The results were shown below:

With lumps of calcium carbonate: (Specimen results)


With powdered calcium carbonate: (Specimen results)

3

Pressure vs Time

Time (s)

Interpretation1. Effervescence occurred in the reaction mixtures, due to the formation of carbon dioxide gas.

CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

2. As more and more carbon dioxide was produced, the pressure inside the suction flask also

increased.

Discussion1. In case the calcium carbonate lumps are covered with powder, they should be washed

beforehand in dilute HCl and then in water to remove the powder. They should then be

blotted.

2. Since particle size of calcium carbonate solids would change as reaction proceeds, only the

initial rates of the two experiments should be compared.

3. Theoretically, the two graphs should become flat at the same value on the Y-axis, i.e. same

final pressure. But in practice, there may be some deviations as a result of experimental errors


(e.g. masses used not the same). Actually, the final pressure cannot be reached as it takes

quite a long time for the calcium carbonate lumps to react completely.

ConclusionThe reaction rate increases with larger surface area of solid reactant.

Answers to questions for further thought1. Hydrochloric acid HCl. The hydrochloric acid should be in excess so that the concentration of

it does not change a lot.

2. Reaction rate was the greatest at the start and decreased gradually to zero. At the start, the

concentration of the reactants, were the highest (hence fastest reaction rate). As the reaction

proceeded, the concentrations of the reactants decreased (hence slower reaction rate), until

one of the reactants (calcium carbonate CaCO3 in this case) was used up. Reaction rate

became zero as the reaction stopped.

3. Chewing can break the food into small pieces, so that there can be a large increase in surface

area of the food for the digestive juices to work on, and thus facilitates digestion.



Title: Effect of temperature on the rate of reaction

PurposeTo investigate the effect of temperature on the rate of reaction.

Apparatus and chemicals used• Beakers (100 cm3) • Dilute HCl(aq) (2.0 M) (about 40 cm3)

• Measuring cylinder (50 cm3) • Na2S2O3 solution (~0.05 M) (about 80 cm3)

• Measuring cylinder (10 cm3) • Deionized water

• Stopwatch

• White tile

• Black/blue 'Wytebord' markers

• Bunsen burner

• Tripod and gauze

• Heat-resistant mat

• Thermometer (−0 110 °C, stirring rod type)

• Tissue paper or blotting paper

(to wipe beakers dry)

Chemical reaction involvedS2O32−(aq) + 2H+(aq) SO2(g) + S(s) + H2O(l)

Procedure1. A thick cross with the size just smaller than the base of a small beaker was marked with a

black/blue 'Wytebord' marker on a white tile placed on the bench.

2. (a) 5 cm3 of sodium thiosulphate solution was mixed with 45 cm3 of water in a 50 cm3

measuring cylinder.

(b) The mixture was added to the small beaker placed on the marked white tile.

(c) 5.0 cm3 of hydrochloric acid was poured into the beaker quickly. The stopwatch was

started at the same time (Figure 1a). The mixture was stirred gently with the

thermometer and the temperature of the reaction mixture was taken (Figure 1b).


(d) The cross was observed by looking vertically down through the solution. The time

taken for the cross to be just 'blotted out' completely was recorded (Figure 1c).

3. Another 5 cm3 of thiosulphate solution was mixed with 45 cm3 of water in a 50 cm3

measuring cylinder. The mixture was added to another clean small beaker. The beaker was

heated until the temperature of the reaction mixture was just above 35°C (Figure 2a).

(a) The beaker was placed on the marked white tile. 5.0 cm3 of hydrochloric acid was

poured into the beaker quickly. The stopwatch was started at the same time. The

mixture was stirred gently with the thermometer and the temperature of the mixture was

taken once it became


(b) The time taken for the cross to be just 'blotted out' completely was recorded.

4. Step 3 was repeated at about 45°C, 55°C, 60°C, 65°C and 70°C.

5. The results obtained in the experiment were tabulated and a graph of time

1 against

temperature was

plotted.

Observation1. At the start of the experiment, the cross was seen clearly. As reaction proceeded, sulphur

concentration continued to rise. The cross became fainter but could still be seen. After a

certain time, sulphur concentration became just sufficient to 'blot out' the cross completely.

2. The results were tabulated as below:

Temperature of reactionmixture (°C)

Time for cross to be just'blotted out' (s)

22 154.035 63.545 41.355 27.460 23.065 18.069 14.9

(Specimen results)

3. Time

1 was worked out in each case:

Temperature of reactionmixture (°C)

Time for cross to be just'blotted out' (s)

1/Time (s−1)

22 154.0 6.5 × 10−3

35 63.5 15.7 × 10−3

45 41.3 24.2 × 10−3

55 27.4 36.5 × 10−3

60 23.0 43.5 × 10−3

65 18.0 55.6 × 10−3

69 14.9 67.1 × 10−3


A graph of Time

1 against temperature was plotted below:

0

10

20

30

40

50

60

70

10 20 30 40 50 60 70

Interpretation1. Sodium thiosulphate solution and hydrochloric acid reacted according to the equation:

S2O32−(aq) + 2H+(aq) SO2(g) + S(s) + H2O(l)

As sulphur formed, the mixture became cloudy white, and then cloudy yellow. The degree of


cloudiness of the reaction mixture at any moment indicated the extent of the reaction. In this

experiment, the beaker containing the reaction mixture was placed over a cross marked on a

piece of white tile. The cross was observed directly from above. The time (t) taken for the

cross just to be 'blotted out' was recorded. This corresponded to the time needed for the

formation of a certain definite amount of sulphur.

2. As temperature of the reactants increased, the time to 'blot out' the cross became shorter.

Since reaction rate is inversely proportional to the time taken to 'blot out' the cross, a shorter

time means a faster reaction. From the graph, it could be seen that reaction rate increased

with an increase in temperature.

Discussion1. For a more accurate comparison, the same cross mark must be used throughout the

experiment. So it is necessary to keep the cross dry and clean. The same person is to observe

the cross at the same distance from above the cross. He should also be the timer.

2. Small amount of SO2 with choking smell is produced during the experiment. Although the

gas is poisonous, it does not matter much at such low concentration in a well-ventilated

laboratory.

3. The beaker inside should be cleaned and dried, otherwise, it will dilute the solution in the

beaker.

4. When heating Na2S2O3 solution in the beaker, the thermometer may be used as a stirring rod

(provided that care is taken). It is advisable to place the thermometer in the warm

thiosulphate solution before pouring the acid in. The temperature will only drop by 1 to

several degrees (depending on the starting temperature of the thiosulphate solution), and thus

the thermometer can quickly adjust to register the temperature after mixing. This practice is

particularly advantageous for experiments carried out at high temperatures (e.g. 70°C) the

reaction rate is so fast that we cannot wait too long for the mercury thread of thermometer to

go up from room temperature to the final high temperature.

ConclusionReaction rate increases with increasing temperature of the reaction mixture.

Answers to questions for further thought1. No. If the reaction rate is directly proportional to temperature, the shape of the graph should

be a straight line, which is not the case in this experiment. From the graph, it seems that

reaction rate rises exponentially with temperature. It is a rule of thumb that for every 10°C

rise in temperature, reaction rate will be doubled.

2. To investigate the effect of a certain factor (temperature in this case) on reaction rate, all other

factors should be made the same at the start.


3. Fresh food can be kept fresh for a longer time in a refrigerator. Low temperatures would slow

down reactions that deteriorate the food.


Experiment 23.1 To compare the service life of a zinc-carbon

cell with an alkaline manganese cell of the

same size (T)

2. c

3.


4. (a) 13.5

52.5

(b) Alkaline manganese cell. About 4 times.

(c) Alkaline manganese cell.


Experiment 24.1 To make simple chemical cells and construct

part of the Electrochemical Series

5.

Metal couple in cell Voltage of cell (volt)

Mg/Cu +1.41

Fe/Cu +0.20

Zn/Cu +0.79

Cu/Cu 0

Ag/Cu −0.22

8.


Mg/Cu +1.41

Fe/Cu +0.20

Zn/Cu +0.83

Cu/Cu 0

Ag/Cu −0.23

11.


Mg/Cu +1.84

Fe/Cu +0.49

Zn/Cu +0.92

Cu/Cu 0

Ag/Cu −0.17


12. a. Mg/Cu cell, Fe/Cu cell, Zn/Cu cell.

b. From the other metal to copper.

c. The other metal.

d. Mg, Zn, Fe, Cu.

13. a. Ag/Cu cell.

b. From copper to silver.

c. Copper.

14. Mg, Zn, Fe, Cu, Ag.

15. a. Yes.

b. Metals react by losing electrons. The higher the tendency of a metal to form ions (the

higher its position in the E.C.S.), the more reactive it would be (the higher its position

in the metal reactivity series).

16. electrodes; electrolyte; positive ions; greater; voltage


Experiment 26.1 To construct a simple chemical cell using

inert electrodes

2. b. 0.98

3. b. Yes.

Yellow.

Iodine.

c. 2I−(aq) → I2(aq) + 2e−

Oxidation.

4. a. MnO4−(aq) + 8H+(aq) + 5e− → Mn2+(aq) + 4H2O(l)

Reduction.

b. 2MnO4−(aq) + 16H+(aq) + 10I−(aq) → 2Mn2+(aq) + 5I2(aq) + 8H2O(l)

c. Y.

d. From X to Y.

X.

Y.

Cathode.

5. Zero volt.

To complete the circuit by allowing ions to move between the two half-cells.


Experiment 26.2 To examine and compare the internal structures

of a well-used and a new zinc-carbon cell

1. a. The zinc cup of the used cell is thinner than that of the new cell. This shows that some

zinc metal has dissolved in the used cell.

b. A strong smell of ammonia is detected in the used cell, but not in the new cell. This

shows that some ammonia has formed in the used cell.

c. In the used cell, a large area around the graphite rod looks wet. In the new cell, the

ammonium chloride paste looks only very slightly wet. This shows that some liquid

(probably water) has formed in the used cell.

2. a. Zn(s) → Zn2+(aq) + 2e−

b. 2NH4+(aq) + 2e− → 2NH3(g) + H2(g)

2MnO2(s) + H2(g) → Mn2O3(s) + H2O(l)

3. (1) The metal casing makes the cell leakproof, preventing the electrolyte from leaking

out through the worn-out zinc cup. (Any electrolyte leaking out would damage the electrical

appliance in which the cell is put.)

(2) The cell with a metal casing has a longer life.


Experiment 27.1 Electrolysis of dilute sulphuric acid (T)

3. b. Small colourless gas bubbles are evolved continuously.

Very small colourless gas bubbles are evolved continuously.

4. c. 2 : 1 (approx.)

6. The glowing splint is relighted.

Oxygen.

4OH−(aq) → O2(g) + 2H2O(l) + 4e−

7. A ‘pop’ sound is heard.

Hydrogen.

2H+(aq) + 2e− → H2(g)

8. 2H2O(l) → 2H2(g) + O2(g)

Water.

No. Distilled water is a very poor conductor of electricity. Although sulphuric acid itself is not

electrolysed, it can increase the electrical conductivity of water.

9. hydrogen H+(aq); hydrogen gas; hydroxide OH−(aq); oxygen gas; decrease;

remains unchanged; increases


Experiment 27.2 Effect of concentration on preferential

discharge of ions

4. At cathode Colourless gas bubbles are evolved. The solution around the cathode turns purple

(in a few seconds).

At anode Colourless gas bubbles are evolved. The solution around the anode turns red (in a

few seconds).

5. c. A 'pop' sound is heard.

The gas is hydrogen.

8. At cathode Colourless gas bubbles are evolved. The solution around the cathode turns purple

(in a few seconds). The solution becomes bleached after some time.

At anode Colourless gas bubbles are evolved. The solution around the anode turns red (in a

few seconds). The solution becomes bleached within a short time (less than half a

minute).

9. A 'pop' sound is heard.

Hydrogen.

10. Bleaching solution or 'swimming pool'.

Chlorine.


11.

0.1 M NaCl 2 M NaCl

C

A

T

H

O

D

E

Cations present Na+(aq), H+(aq) Na+(aq), H+(aq)

Is the solution around

the electrode acidic,

neutral or alkaline?

alkaline alkaline

Main product hydrogen hydrogen

Ionic half-equation 2H+(aq) + 2e− → H2(g) 2H+(aq) + 2e− → H2(g)

A

N

O

D

E

Anions present Cl−(aq), OH−(aq) Cl−(aq), OH−(aq)




acidic acidic

Main product oxygen chlorine

Ionic half-equation 4OH−(aq) → O2(g) + 2H2O(l) + 4e− 2Cl−(aq) → Cl2(g) + 2e−

12. hydrogen H+(aq); hydrogen gas; hydroxide OH−(aq); oxygen gas; chloride Cl−(aq) ;

concentration effect; chlorine gas


15.

2 M NaBr 2 M NaI

C

A

T

H

O

D

E

Cations present Na+(aq), H+(aq) Na+(aq), H+(aq)




alkaline alkaline

Main product hydrogen hydrogen

Ionic half-equation 2H+(aq) + 2e− → H2(g) 2H+(aq) + 2e− → H2(g)

A

N

O

D

E

Anions present Br−(aq), OH−(aq) I−(aq), OH−(aq)

Colour of solution

around the electrode

brown colour

(bleached after some time)

deep brown colour

Main product bromine iodine

Ionic half-equation 2Br−(aq) → Br2(aq) + 2e− 2I−(aq) → I2(aq) + 2e−

16. hydrogen

hydroxide OH−(aq); reducing; halide; oxygen gas; halide; halide ions; hydroxide OH−(aq); halogen


Experiment 27.3 Effect of electrodes on products of electrolysis

3. A reddish brown solid is deposited.

Cu2+(aq) + 2e− → Cu(s)

Colourless gas bubbles are evolved.

4OH−(aq) → O2(g) + 2H2O(l) + 4e−

4. The cathode is now electrode Y (graphite); the anode is electrode X (copper coated on

graphite).

5. Reddish brown copper is deposited.

Cu2+(aq) + 2e− → Cu(s)

The reddish brown solid (copper) dissolves gradually. (Some copper may fall to the bottom of

the beaker.) When there is little or no copper on the anode, colourless gas bubbles are

liberated there.

At first, Cu(s) → Cu2+(aq) + 2e−;

when there is little or no copper left,

4OH−(aq) → O2(g) + 2H2O(l) + 4e−.

6. copper; oxygen; copper dissolves



Title: Electroplating

PurposeTo design and perform an experiment to electroplate a metal object with nickel.

Apparatus and chemicals used• 6 V d.c. supply • Propanone (2 cm3), kept inside the fume

• 3 connecting wires with crocodile cupboard

clips at both ends • Nickel foil (5 cm × 2 cm)

• 6 V bulb in holder • Copper foil (5 cm × 2 cm) /brass key/

• Sand paper (5 cm × 5 cm), 2 pieces 50¢ coin

• Beaker (100 cm3) • Nickel plating solution (an aqueous

• Crucible tongs nickel(II) salt solution), 60 cm3

• Electrode foil holder • Tissue paper/ cotton wool

• Forceps (placed beside propanone,

kept in the fume cupboard)

Chemical reactions involvedAt the nickel anode: Ni(s) Ni2+(aq) + 2e−

At the cathode (object to be electroplated): Ni2+(aq) + 2e− Ni(s)

Procedure1. (a) A copper foil was cleaned with sandpaper (Figure 1a).


(b) The foil was washed in running tap water (Figure 1b).

(c) The foil was degreased (grease was removed) by rubbing the foil with a piece of cotton

wool (soaked in propanone) held by forceps (Figure 1c).

(d) The foil was washed again under tap water (Figure 1d).

(e) The copper foil was put on a piece of tissue paper (Figure 1e) to keep the foil clean.

2. (a) 60 cm3 of the nickel plating solution was poured into a beaker.

(b) The circuit was connected as shown in Figure 2. The copper was made the cathode

(connected to the negative terminal of the battery); nickel was made the anode

(connected to the positive terminal).

(c) A current was allowed to flow for 10 minutes. When half the time had passed, the

cathode was taken out. The sides were reversed and it was put back into the plating


solution. (This was to ensure even plating.)

3. (a) The cathode was removed (the copper foil was newly coated with nickel).

(b) It was washed in running tap water.

(c) It was allowed to dry on a tissue paper.

(d) The appearance of the electroplated copper was noted.

Observation1. (Reference to Step 1): The copper foil looked shiny and clean.

2. (Reference to Step 2): The electroplated copper became silvery grey in appearance.

3. (Reference to Step 3): A silvery grey coating was electroplated on the copper foil.

Interpretation1. Propanone is a very good solvent to dissolve grease, thus the copper metal looks shiny.

2. At the cathode, Ni 2+(aq) ions are discharged to form Ni(s) on the copper surface.

Ni2+(aq) + 2e− Ni(s)

Discussion1. Alternatively, the copper foil can be degreased by dipping it in concentrated detergent

solution for a minute, or rubbing with cotton wool soaked in dilute ammonia solution. Warm

dilute sodium hydroxide can also remove grease. However, the solution is corrosive.

2. Stirring of the plating solution during electroplating may result in a good coating. This can be

done by using the magnetic stirrer.

3. The light bulb indicated that an electric current was flowing and limited this to a small

current density suitable for electroplating. If the bulb is not used, a variable resistor and an

ammeter can be used to obtain good results.

4. The metal foils can be conveniently clamped by an electrode foil holder which consists of a

plastic strip fitted with two crocodile clips. The advantages are:

(a) The metal foils will not touch each other and the circuit will not be shorted.

(b) The foils can be kept at a fixed distance apart.

(c) The crocodile clips will not be wetted by the solution so easily. So that the clips will not

rust so easily.

ConclusionWhen an object is to be electroplated, it is made the cathode of an electrolytic cell. The plating

metal is usually made the anode. The plating solution (electrolyte) is a solution of one of the salts


of the plating metal. Simple electroplating experiment can be done in the school laboratory

conveniently.

Answers to questions for further thought

1. (i) The object to be plated should be clean and free from grease.

(ii) Keep the electric current small.

(iii) The electrolyte should be maintained at a certain constant acidity or alkalinity.

2. Copper, silver, gold, tin or chromium. They are low in the Electrochemical Series.

3. These objects are first sprayed with a layer of powdered graphite or metal and then

electroplated in the usual way.

4. The nickel anode ionizes to form Ni2+(aq) ions, replacing those removed from the solution at

the cathode. Thus the concentration of the electrolyte can be kept constant at a desired level.


Experiment 28.1 To investigate the oxidizing property of chlorine

water

1. d.

Halide (a) Colour of solution after

addition of chlorine water

(b) Colour of heptane

layer

Bromide yellow red-orange

Iodide brown purple

4. oxidizing; bromine; yellow; iodine; brown; heptane; red-orange; purple

5. Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq);

Cl2(aq) + 2I−(aq) → 2Cl−(aq) + I2(aq)

Redox (or displacement).

6. oxidizing



Title: To make chlorine bleach

PurposeTo make chlorine bleach in the school laboratory.

Apparatus and chemicals used• Electrolysis cell • Stand, boss and clamp

• Rubber bands • Dropper

• Rimless test tubes • Boiling tube (150 × 24 mm) in rack

• Stopper to fit small test tube • Stopper to fit 24 mm diameter test tube

• Wooden piece (longer than diameter • Red litmus paper

of wide glass tube) • Blue litmus paper

• Connecting wires fitted with crocodile clips • Brine (saturated NaCl solution)

• 6 V battery

Chemical reactions involvedAt the cathode: 2H+(aq) + 2e− H2(g)

At the anode: 2Cl− (aq) Cl2(g) + 2e−

Cl2(g) + 2NaOH(aq) NaOCl(aq) + NaCl(aq) + H2O(l)

sodium hypochlorite

ProcedureIn this experiment, brine (saturated solution of sodium chloride) was electrolysed. The chlorine gas

collected from the anode was allowed to react with the resulting alkaline solution.

1. Brine was added to an electrolysis cell, until the liquid level was slightly above the

electrodes. See Figure 1.


2. (a) A dropper

was used

to fill two

small

rimless test

tubes with

brine

(Figure 2).

(b) The tubes

were

inverted

over the

electrodes

(Figure 3).

(Before the

tubes were

inverted,

an empty

beaker

should be

placed

under the

electrolysis

cell. This

would

prevent

possible

spillage of

liquid on

the bench.)

(c) The electrolysis cell was clamped.

(d) The electrodes were connected to a 6 V d.c. source.

3. Any observation around the

electrodes and in the small

test tubes A and B was

recorded.

4. (a) Electrolysis was allowed to continue. When the small


tubeA

Figure 3

tube above the anode (tube B) was full of gas, the power supply was disconnected.

(b) The small tubes were taken out of the solution and tube B was quickly stoppered.

(c) The rubber bands were loosen to release tube B.

5. A piece of red litmus paper was used to test the resulting solution in the electrolysis cell. The

change in colour of the litmus paper was recorded.

6. (a) The resulting solution from the electrolysis cell was poured into a boiling tube to a

depth of

3 cm.

(b) Tube B was inverted. The stopper was quickly removed and tube B was dropped into

the boiling tube (Figure 4a).

(Caution! It must be very careful when tube B was dropped into the boiling tube. It is

afraid that the glass might be broken and the solution might be spilled out.)

(c) The boiling tube was stoppered tightly immediately (Figure 4b).

(d) The boiling tube was inverted gently and the contents were swirled (Figure 4c)

(e) The boiling tube was inverted back and it was swirled again. Steps (d) and (e) were

repeated for about 5 minutes.

7. A chlorine bleach had been prepared. It should contain sodium hypochlorite and show

bleaching action.

(a) A piece of blue litmus paper and a piece of red litmus paper were dropped into the

solution in the boiling tube.

(b) The changes in colours of the blue and the red litmus papers were recorded.


Observation1. (Reference to Step 3): Colourless gas bubbles were evolved at cathode. The gas was collected

and water level in tube A dropped.

Greenish-yellow gas bubbles were evolved at anode. The gas was collected and water level in

tube B dropped.

2. (Reference to Step 5): The red litmus paper turned blue.

3. (Reference to Step 7): The blue litmus paper was bleached (from blue to white) immediately.

The red litmus paper was bleached (from red to white) immediately.

Interpretation1. Hydrogen was formed at the cathode and collected in tube A.

2H+(aq) + 2e− H2(g)

Chlorine was formed at the anode and collected in tube B.

2Cl−(aq) Cl2(g) + 2e−

2. The resulting solution was alkaline. It should contain sodium hydroxide.

3. The chlorine bleach formed bleached the dyes in the red and blue litmus paper.

Cl2(g) + 2NaOH(aq) NaOCl(aq) + NaCl(aq) + H2O(l)

OCl−(aq) + dye(aq) Cl2(aq) + (dye + O)

coloured colourlesss

Discussion1. The small tubes should not cover the electrodes completely, otherwise electrical resistance

would be increased and the rate of electrolysis would be reduced considerably.

2. The time needed to collect a full tube of hydrogen gas is shorter than that of chlorine.

This is because hydrogen is insoluble in water, while chlorine is fairly soluble.

3. The resulting solution is mainly a mixture of sodium hydroxide and sodium chloride, with a

little chlorine dissolved. It is alkaline. A longer electrolysis results in a higher alkalinity and

stronger corrosiveness of the resulting solution.

Conclusion1. Chlorine can be prepared by the electrolysis of brine.

2. Chlorine gas formed at the anode can react with the resulting sodium hydroxide solution in

the electrolysis cell. Chlorine bleach, containing sodium hypochlorite as the active ingredient,

is formed.

3. The chlorine bleach can bleach the dyes in the litmus paper.

Answers to questions for further thought1. The theoretical volume ratio of H2 to Cl2 should be 1:1. Smaller volume of chlorine gas is

collected because chlorine is fairly soluble in water.


2. At room temperature, chlorine can be liquefied under pressure. It is stored and transported in

liquid state in steel cylinders.

3. (i) Chlorine bleach should be stored away from sunlight or excess heat. It is because

sunlight and excess heat can speed up the decomposition of chlorine bleach.

(ii) Chlorine bleach should be kept out of reach of children because it is toxic.

(iii) When chlorine bleach is used, the room must be well ventilated because a little chlorine

gas is given off, which is toxic.

(iv) When using chlorine bleach, wear plastic gloves because it is irritant to skin.

(v) Never mix chlorine bleach with acidic substances or other cleaners. It is because acidic

substances react with chlorine bleach to liberate the toxic chlorine gas.

OCl−(aq) + Cl− (aq) + 2H+(aq) Cl2(g) + H2O(l)


Experiment 28.3 Action of chlorine bleach on coloured substances

1. Sodium hypochlorite (usually 5.25%).

3. A faint choking smell (like that of swimming pool).

4. b. It turns white (is bleached) immediately.

It turns white (is bleached) immediately.

5. i. The colour becomes paler.

ii. The stain is bleached.

iii. The stain is bleached.

iv. The stain is bleached.

6. sodium hypochlorite; oxidation


Experiment 28.4 Action of acid on chlorine bleach

1. a. Colourless.

b. The solution changes from colourless to pale greenish yellow, a stream of small bubbles

being evolved.

2. The gas has a characteristic pungent, choking smell (like that of swimming pool).

Chlorine.

3. b. It becomes pink first (in about 5 seconds) and then white, i.e. bleached (in about 10

seconds).

Cl−(aq); OCl−(aq); 2; Cl2(g)

5. mineral acid; smell; blue litmus paper


conc.

H2SO4

Experiment 29.1 To investigate properties of concentrated

sulphuric acid (S/T)

1. c. It turns from blue granular crystals to a pale blue powder.

CuSO4 5H‧ 2O(l) CuSO4(s) + 5H2O(l)

2. b. The sugar turns brown. A little white steam is given out.

d. The sugar chars (turns black). A lot of white steam is given out. A black spongy mass

rises up the beaker.

Sugar charcoal (carbon).

12C(s) + 11H2O(g)

3. d. All the words appear (black words on white paper).

On heating, water is driven away from dilute sulphuric acid. The dilute sulphuric acid

becomes concentrated. The concentrated sulphuric acid formed dehydrates the cellulose

in paper, leaving black carbon.

Dehydrating property.

4. c. No signs of reaction no matter the acid is cold or hot.

Yes. A dilute aqueous solution of a typical acid does not react with copper and those

metals below copper in the metal reactivity series.

5. a. No.

c. It changes from light orange to green.

Yes.


e. Blue.

Cu2+(aq).

Cu(s) + 2H2SO4(l) → CuSO4(s) + SO2(g) + 2H2O(l)

No. A typical acid reacts with those metals which are above copper in the reactivity

series. In such cases, the gas liberated is hydrogen.

Redox reaction.

An oxidizing agent. Concentrated sulphuric acid is reduced, as the oxidation number of S

decreases from +6 in H2SO4(l) to +4 in SO2(g).

6. dehydrating; oxidizing


Experiment 29.2 To dilute concentrated sulphuric acid (S/T)

7. lighter; heat; heating; boiling; steam; acid spray; heat; break



Title: To prepare sulphur dioxide and test for its properties

Purpose(A) To prepare sulphur dioxide gas in the school laboratory.

(B) To investigate the properties of sulphur dioxide gas.

Apparatus and chemicals used• Anti-bumping granules • 5 test tubes (4 fitted with rubber stoppers)

• Blue litmus paper/pH paper • Test tube rack

• Dilute sulphuric acid • Bunsen burner and matches

• Sodium sulphite solid • Heat-resistant mat

• Potassium dichromate solution • Stand, boss and clamp

• Boiling tube (fitted with a rubber stopper • Beaker (250 cm3)

carrying a bent delivery tube)

Chemical reactions involvedSO3

2−(aq) + 2H+(aq) SO2(g) + H2O(l)

SO2(aq) + H2O(l) H2SO3(aq)

sulphurous acid

3SO2(aq) + Cr2O72−(aq) + 2H+(aq) 3SO4

2−(aq) + 2Cr3+(aq) + H2O(l)

Procedure(A) To prepare sulphur dioxide gas in the school laboratory

1. (a) 3 spatula measures of sodium sulphite solid was put in a boiling tube.

(b) Dilute sulphuric acid was added to the boiling tube to a depth of 3 cm.

(c) A few anti-bumping granules were added to the boiling tube.

2. The boiling tube was clamped as shown in Figure 1.


3. (a) The reaction mixture in the boiling tube was warmed gently. The flame was moved

about continuously to ensure uniform heating

(b) Record the observation when the reaction mixture was heated.

4. (a) The gas liberated was collected in a test tube by download delivery (upward

displacement of air). The test tube was stoppered once it was full of the gas. (As the gas

generated was misty, one could see when a tube was full of the gas.) The stoppered test

tube was put in a test tube rack.

(b) 3 more tubes of sulphur dioxide gas were collected. The tubes of gas would be used for

Part B of the experiment.

5. Heating was stopped. With a towel to protect the hand, the boiling tube was taken to the fume

cupboard, to be cleaned later after cooling.

(B) To investigate the properties of sulphur dioxide gas

6. Test for smell

(a) A tube of sulphur dioxide gas was taken. The stopper was lifted up slightly to leave a

small opening.

(b) The gas that escaped out from the tube was smelled carefully. This was done by

'fanning' a little of the gas towards the nose.

(c) The stopper was put in place immediately.

(d) The smell of sulphur dioxide gas was recorded.

7. Test for water solubility and acidic property

(a) A beaker was filled with water to half-full.

(b) A tube of sulphur dioxide was inverted and immersed under water in the beaker (Figure

2).


(c) The stopper was removed under water.

(d) The water was stirred gently with the inverted tube for 4 minutes.

(e) A piece of blue litmus paper was dropped into the solution in the beaker.

(f) Any change in water level inside the tube and the colour change of the litmus paper

were recorded.

8. Test for bleaching property

(a) Inside the fume cupboard, the stopper of a tube of sulphur dioxide was removed. A

piece of moist blue litmus paper was quickly put into the tube.

(b) The stopper was immediately put in place again.

(c) The stoppered tube was taken back to the students' bench.

(d) The tube was placed in the test tube rack and allowed to stand for 10 minutes.

(e) Any colour change of the blue litmus paper was recorded.

9. Test for reducing property

(a) 1 cm3 of potassium dichromate solution and 1 cm3 of dilute sulphuric acid were put in a

test tube. (The resulting solution is called 'acidified potassium dichromate solution'.)

(b) Inside the fume cupboard, the acidified potassium dichromate solution was added

quickly to a tube of sulphur dioxide gas.

(c) The stopper of the tube was immediately replaced and the tube was shaken a few times.

(d) Any colour change of the acidified potassium dichromate solution was recorded.

Observation1. (Reference to Step 3): Effervescence occurred in the reaction mixture. A misty gas was

formed.

2. (Reference to Step 6): Sulphur dioxide gas had an irritating choking smell of burning sulphur.

3. (Reference to Step 7): Water level rose well up inside the test tube. The blue litmus paper

turned red.

4. (Reference to Step 8): The blue litmus paper turned red and then white (or very pale red).

5. (Reference to Step 9): The orange colour of acidified potassium dichromate solution changed


to dark green.

Interpretation1. Effervescence occurred in the reaction mixture, due to sulphur dioxide gas formed.

SO32−(aq) + 2H+(aq) SO2(g) + H2O(l)

The sulphur dioxide gas liberated was misty because it contained moisture in it.

2. Water level rose well up inside the test tube. This shows that a lot of sulphur dioxide had

dissolved in water, hence the gas is very soluble in water.

The litmus paper turned red, showing that the solution was acidic. Sulphur dioxide

dissolves in water to form sulphurous acid:


sulphurous acid

3. The blue litmus paper turned red first. This shows that sulphur dioxide is an acidic gas. The

litmus paper then turned white (or very pale red), showing that sulphur dioxide is a bleaching

agent.

4. The orange colour of acidified potassium dichromate solution changed to dark green,

showing that sulphur dioxide gas is a reducing agent. The orange dichromate ions were

reduced to dark green chromium(III) ions:

3SO2 (aq) + Cr2O72−(aq) + 2H+(aq) SO4

2−(aq) + 2Cr3+(aq) + H2O(l)

orange dark green

Discussion1. Sulphur dioxide gas is much denser than air. Thus it can be collected by downward delivery,

as in this experiment. However, the gas collected would have air mixed with it.

2. Inevitably, some sulphur dioxide escaped into the laboratory in this experiment. It would be

better if the whole experiment had been carried out inside the fume cupboard.

3. The sulphur dioxide gas collected in this experiment (by downward delivery) was moist and

mixed with air. To prepare pure dry sulphur dioxide, the gas generated should be first passed

into concentrated sulphuric acid for drying; the dried gas can then be collected using a gas

syringe. Using a gas syringe can also minimize the sulphur dioxide gas escaped into the

laboratory.

Conclusion1. Sulphur dioxide can be prepared by warming a sulphite with a dilute acid.

2. Sulphur dioxide has an irritating choking smell of burning sulphur.

3. Sulphur dioxide is acidic.

4. Sulphur dioxide is a bleaching agent.

5. Sulphur dioxide is a reducing agent. It turns acidified potassium dichromate solution from


orange to dark green.

Answers to questions for further thought1. Sulphur dioxide cannot be collected by displacement of water, as it is very soluble in water.

2. It is not advisable to test for a gas by its smell because the gas may be poisonous. A chemical

test would be preferred.

3. Sulphur dioxide emitted into the atmosphere is formed by the burning of sulphur-containing

fuels. Almost all of the gas comes from industrial sources ¾ electric power stations, factories

and incinerators. Sulphur dioxide dissolves in rainwater to form sulphurous acid, one of the

causes of acid rain:


sulphurous acid


Experiment 29.4 To bleach coloured papers and flower petals with

sulphur dioxide (S/T)

1. A colourless gas (or a misty gas, due to presence of impurity).

3. d. The blue litmus paper becomes white (or very pale red).

The red litmus paper becomes white (or very pale red).

The blue (or red) flower petals become paler in colour.

4. Yes.

Reduction.


Experiment M1 Electrolysis using a microscale Hoffman

apparatus

11. Small colourless bubbles are evolved continuously.

Very small colourless gas bubbles are evolved continuously

12. Test the gas evolved at the anode with a glowing splint, the glowing splint is relighted,

indicating that the gas is oxygen.

Test the gas evolved at the cathode with a burning splint, the burning splint gives a 'pop' sound,

indicating that the gas is hydrogen.


wb 2 suggested ans

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