2010 m33-34 ms -08 sk-amended

of 40

L.33/34

Pre-Leaving Certificate Examination, 2010

Chemistry

Marking Scheme

Ordinary Pg. 2

Higher Pg. 19

2010 L.33/34_MS 2/40 of 40

Dublin Examining Board


Chemistry

Ordinary Level Marking Scheme (400 marks)

Answer two questions from Section A and six questions from Section B. All questions carry equal marks (50).

Notes: (i) In calculations, 3 marks are deducted for a mathematical error but no further penalty is incurred if the problem, otherwise correct, is completed.

(ii) Alternative valid answers are, of course, acceptable.

SECTION A (100 marks)

Answer at least two questions from this section. 1. A sample of ethanoic acid (CH3COOH) was prepared in the laboratory by the oxidation of ethanol using

the apparatus shown. The reaction is exothermic. After refluxing for 20 minutes the apparatus was rearranged to remove the ethanoic acid.

(a) Name a suitable oxidising agent for this reaction (6)

Any 1: (6m) – sodium dichromate // – Na2Cr2O7 // (b) State one safety precaution to be taken when adding

the ethanol to the flask. (6)

Any 1: (6m) – add slowly // – keep flask in cold water // – shake after every addition (c) Why is it important to reflux the mixture? (6) Any 1: (6m) – to oxidise all of the ethanol // – to oxidise the ethanal // – to complete the reaction // – to increase the amount of product (d) Describe the colour change in the flask. (2 × 4m) (8) – from orange – to green

2010 L.33/34_MS 3/40 of 40

(e) Describe how the apparatus is rearranged to remove the ethanoic acid from the mixture. (2 × 6m) (18)

– connect the condenser to the side – heat flask with Bunsen burner or hot plate Name this process. (6m) – distillation (f) If a wine is kept exposed to the air the ethanol is oxidised to ethanoic acid. Name the product that is

formed from the wine. (6m) (6) – vinegar 2. A student prepared 1 L of a 0.2 M solution of sodium carbonate (Na2CO3) using anhydrous sodium

carbonate and distilled water. This solution was then used to find the concentration of a hydrochloric acid solution.

(a) Why is anhydrous sodium carbonate suitable for making a standard

solution for acid / base titrations? (6) Any 2: (2 × 3m) – it is stable // – it dissolves well in water // – it is pure // – it is a primary standard (b) What mass of sodium carbonate should be weighed out to make 1 L

of the 0.2 M solution? (6) Any 1: (6m) – 21.2 g // – 106 g (3m) × 0.2 (3m) (c) Name the piece of equipment A and outline fully the steps the student should follow to make the

sodium carbonate solution. (14) – A - volumetric flask (2m) Any 4 steps: (4 × 3m) – dissolve the weighed sodium carbonate in distilled water in beaker // – add to volumetric flask // – add washings from beaker // – add deionised water to bring bottom of meniscus to mark // – at eye level // – dropwise // – invert several times (d) Name the pieces of apparatus B and C. (2 × 3m) (6) – B - pipette – C - burette

2010 L.33/34_MS 4/40 of 40

(e) Outline how the student prepared C, which is clean and dry, for the titration between the sodium carbonate solution and hydrochloric acid solution. (9)

Any 3: (3 × 3m) – add hydrochloric acid // – open tap to fill jet // – remove funnel // – bring to 0 // – clamp vertically (f) It was found that 19.5 cm3 of the hydrochloric acid was required to neutralise 25 cm3 of the sodium

carbonate solution. The balanced equation for the reaction is: (9)

Na2CO3 + 2HCl 2NaCl + H2O + CO2

Calculate the concentration of the hydrochloric acid solution in moles per litre. (9)

– 2

M5.19 (3m) =

1

2.025 (3m)

– M = 0.51 M (3m)

2010 L.33/34_MS 5/40 of 40

3. The diagram shows the apparatus that was used to measure the heat of reaction (ΔH) for the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH). When 50 cm3 of 1.0 M hydrochloric acid solution was added to 50 cm3 of 1.0 M sodium hydroxide solution the amount of heat released was 2.6 kJ. The equation for the reaction is:

HCl + NaOH NaCl + H2O

(a) Is this reaction endothermic or exothermic? Explain your answer. (2 3m) (6)

– exothermic – it causes a temperature rise / gives out heat (b) Define heat of reaction. (2 3m) (6) – heat change when molar quantities react – according to a balanced equation (c) What is the advantage of using a polystyrene cup instead of a metal can? (6) Any 1: (6m) – insulation // – it does not absorb heat (d) Both of these substances are corrosive. Draw the symbol that signifies that a substance is corrosive. (5) ** Symbol for corrosive. (5m) (e) Outline two precautions you should observe in order to accurately measure the temperature change

for the reaction. (6) Any 2: (2 × 3m) – allow temperature equalisation (take average temperatures) // – use accurate thermometer (to 0.1 °C) // – stir // – measure temperature quickly // – record highest temperature (f) Calculate: (i) the number of moles of hydrochloric acid that was used, (6m) (6) – 0.05 moles (50/1000) (ii) the heat of reaction (ΔH) for the reaction. (9) – – (minus sign) (3m) – 52 kJ (2.6/0.05) (6m) (g) What term is used to describe the amount of energy in 100 g of a food? (6m) (6) – calorific value

2010 L.33/34_MS 6/40 of 40

SECTION B (300 marks)

4. Answer eight of the following items (a), (b), (c), etc. Award 1 bonus mark to the first two fully correct answers.

(a) How does the atomic radius vary across a period of the periodic table? (6m) (6) – it decreases (b) Name the scientist pictured on the right who discovered the nucleus. (6m) (6) – Rutherford (c) State Boyle’s Law. (2 × 3m) (6) – the volume of a fixed mass of gas is inversely proportional to the pressure – at constant temperature (d) What does an Arrhenius base form when it dissolves in water? (6m) (6) – OH– ions (e) Name the group of hydrocarbons that have a benzene ring. (6m) (6) – aromatic (f) Name the reagents that identify a nitrate ion in solution. (2 × 3m) (6) – iron (II) sulfate – concentrated sulfuric acid (g) Which ions cause water hardness? (2 × 3m) (6) – calcium – magnesium (h) The label on a bottle of beer says that the alcohol content is 4% (v/v). How many cm3 of ethanol are in a 330 cm3 bottle? (6)

– 100

3304 (3m)

– 13.2 cm3 (3m) (i) Name the gas that is released from refuse dumps and which can be used as a fuel. (6m) (6) – methane

2010 L.33/34_MS 7/40 of 40

(j) At which electrode in electrolysis does oxidation take place? (6m) (6) – anode (k) Answer part A or B. (6)

A Name two types of production processes in the chemical industry. Any 2: (2 × 3m) – batch // – continuous // – semi-continuous

or

B State two examples of macromolecular crystals. Any 2: (2 × 3m) – diamond // – graphite // – silica

2010 L.33/34_MS 8/40 of 40

5. (a) What is meant by (i) a covalent bond, (ii) an ionic bond? (8) (i) a shared pair of electrons (4m) (ii) attraction of oppositely charged ions (transfer of electrons) (4m) (b) What is meant by a polar bond? (3m) (6) – unequal sharing of electrons Explain why water is a polar molecule. (3m) – it has a positive and negative pole separated by a distance (c) Describe, using dot and cross diagrams, the bonding in a molecule of (i) ammonia (NH3) and (ii)

water (H2O). (24) ** Diagram of ammonia molecule. (9m) ** Diagram of water molecule. (9m) State the shape of each molecule. (2 × 3m) – ammonia - pyramidal – water - V-shaped / bent (d) Define electronegativity. (2 × 3m) (6)

– relative attraction

– for a shared pair of electrons (e) Use electronegativity values (Mathematical Tables, page 46) to predict the type of bond (polar,

ionic, polar covalent or non-polar) likely to be formed between each of the following pairs of elements: (6)

(i) potassium and fluorine (3m) – ionic (ii) hydrogen and chlorine (3m) – polar covalent

2010 L.33/34_MS 9/40 of 40

6. (a) The diagram shows the apparatus used to prepare ethyne in a school laboratory. (i) Identify X and Y, by name or formula, and draw the structural formula

of ethyne. (3 × 3m) (9)

– X - water / H2O – Y - calcium dicarbide / calcium carbide / CaC2

– ethyne - H−C≡C−H

(ii) What is meant by unsaturated? (2 × 3m) (12) – has a double / triple / multiple bond – between two C atoms Describe how to show that ethyne is unsaturated. (2 × 3m) – decolourises – bromine / acidified potassium permanganate

(iii) Describe the flame that is produced when ethyne is burned in air. (6m) (6) – smoke / soot / luminous flame

(iv) When the gas is mixed with oxygen it burns with a very hot flame that is used in welding. What is the common name for this mixture? (3m) (3) – oxyacetylene (b) The fractional distillation of crude oil in an oil refinery produces the following fractions: light petroleum naphtha kerosene gas oil residue From this list select a fraction (i) which is used as an aircraft fuel (4m) (4) – kerosene (ii) which is the main fraction used to make petrol. (4m) (4) – light petroleum (c) Hydrogen has been proposed as an alternative fuel for cars. (i) Give one advantage of using hydrogen instead of a hydrocarbon fuel. (6) Any 1: (6m) – no CO2 produced // – no air pollution (ii) Outline one method of producing hydrogen industrially. (6) Any 1: (6m) – electrolysis of water // – steam reforming of natural gas

2010 L.33/34_MS 10/40 of 40

7. The Periodic Table of the Elements has the elements arranged in order of atomic number. Mendeleev arranged the elements by atomic mass and sometimes had to reverse the order of certain pairs of elements.

(a) Give one other difference between the modern Table and Mendeleev’s Table. (6) Any 1: (6m) – no gaps // – more elements (b) Explain why the relative atomic mass number for an element is rarely a whole number. (6) Any 1: (6m) – isotopes // – different number of neutrons (c) Define atomic number. (2 × 3m) (6) – number of protons – in an atom (in nucleus) (d) State the arrangement of electrons in the main energy levels in an atom of aluminium. (6) – 2, 8, 3 (e) Explain why the activity of the alkali metals (Group I) increases going down

the group. (2 × 6m) (12)

– ease of losing an electron increases – due to increased atomic radius / increased shielding

(f) Some radioactive elements emit beta (β) particles. Explain how a nucleus changes when it emits a beta particle. (2 × 3m) (14)

– a neutron changes – into a proton and an electron State two properties of a beta particle. Any 2: (2 × 4m) – negative charge / charge of an electron // – attracted to a + charged plate // – deflected by a magnetic field // – can penetrate several metres of air or thin (5 mm) sheet of aluminium

(moderately penetrating)

2010 L.33/34_MS 11/40 of 40

8. Examine the reaction scheme and answer the questions that follow:

X Y C2H4 C2H6 C2H2Cl

A B C

(a) Name the compounds A, B and C. (3 × 3m) (9) – A - ethene – B - ethane – C - chloroethane (b) Which one of the compounds A, B or C has only planar carbon atoms? (4m) (4) – A - ethene (c) Classify (i) conversion X, and (ii) conversion Y as an addition, a substitution or

an elimination reaction. (2 × 6m) (12) (i) – X - addition (ii) – Y - subsititution (d) Which one of the compounds can undergo polymerisation? (9) – A - ethene (6m) Name the polymer that is formed. Any 1: (3m) – polyethene // – polythene (e) Compound A can be formed in the laboratory by passing an alcohol vapour over a

heated catalyst. Name the alcohol and the catalyst. (2 × 6m) (12) – alcohol - ethanol / C2H5OH – catalyst - aluminium oxide / Al2O3 (f) Identify the reagent that converts B to C. (4m) (4) – hydrogen chloride / HCl

2010 L.33/34_MS 12/40 of 40

9. The treatment of water for domestic use involves a number of stages including sedimentation, filtration and chlorination.

(a) Describe what change happens to the water in the process of sedimentation. (6m) (9) – the suspended solids in the (flocculated) water settle at the bottom of the tank Name a substance that is added to the water to improve this process. Any 1: (3m) – aluminium sulfate // – alum // – polyelectrolytes (b) Describe briefly how the water is filtered. (6m) (6) – passed through beds of sand / gravel (c) What is the purpose of chlorination? (6m) (9) – to kill bacteria / pathogens Name a substance that can be used for this purpose. Any 1: (3m) – chlorine // – Cl2 // – hypochlorite (d) What problem in the distribution of the water can arise if the pH is too low? (9) Any 1: (6m) – corrosion of metal pipes // – dissolving metals // – dissolving lead Name a substance that can be added to raise the pH. Any 1: (3m) – lime // – sodium hydroxide // – sodium carbonate (e) Outline how to measure the concentration of dissolved solids in a sample of tap water. (12) Any 4: (4 × 3m)

– measure mass of clean vessel / beaker // – measure definite volume with instrument // – of filtered water // – heat to dryness // – measure mass and subtract original

(f) A 100 cm3 sample of water was found to contain 0.21 g of suspended solids. Express this concentration in ppm. (5)

– 0.21 × 10 × 1000 mg/l (3m) – 2100 ppm (2m)

2010 L.33/34_MS 13/40 of 40

10. Answer any two of the parts (a), (b), and (c). (2 × 25)

(a) (i) Define pH. (7) Any 1: (7m) – log [H+] // – –log10 [H

+] (ii) Describe how you could measure the pH of a solution. (3 × 3m) (9)

– universal indicator paper / liquid – compare colour – with colour chart

(iii) Calculate the pH of a 0.12 M solution of hydrochloric acid (HCl). (3 × 3m) (9) – –log – –log 0.12 – 0.92 (b) (i) Explain briefly how paper chromatography works. (3 × 3m) (9) – different attraction / adsorption / affinity – between paper (stationary phase) – and mobile phase (ii) Describe how you would separate the dyes in an ink mixture using

paper chromatography. (4 × 3m) (12) – put small amount of solvent / eluent into test tube / tank – apply spot to paper with dropper / capillary tube – just above / 1-2 cm above / level of solvent – let solvent move up to top of paper (iii) Name the chromatography method that uses a pressurised solvent to move the sample

through a solid inside a column. (4m) (4) – HPLC / high-performance liquid chromatography

2010 L.33/34_MS 14/40 of 40

(c) The effect of concentration on the rate of the reaction between sodium thiosulfate and hydrochloric acid was investigated by a student using the apparatus shown. The student used the same volume of solutions of sodium thiosulfate at different concentrations and the same volume of hydrochloric acid to measure the time for an amount of yellow precipitate to form and obscure an X mark placed below the flask. The table shows the results.

Concentration of sodium thiosulfate solution (M)

Time taken for cross tobecome obscured (min)

1/time (min –1)

0.02 8.20 0.12 0.04 4.10 0.24 0.06 2.73 0.36 0.08 2.05 0.48 0.10 1.64 0.61

(i) Plot a graph of concentration (x-axis) against the rate of reaction (y-axis). (16) – axes correctly labelled with numbers (4m) – axes with correct units (4m) – four points correctly plotted (4m) – curve drawn (4m) (ii) From the graph find the time for the cross to become obscured if a solution of 0.05 M

was used. (2 × 3m) (6) – from graph - 0.30 – invert rate - 3.3 min (iii) Name the yellow precipitate formed. (3m) (3) – sulfur

2010 L.33/34_MS 15/40 of 40

11. Answer any two of the parts (a), (b), (c). (2 × 25)

(a) Flame tests are used to identify the metal present in a salt. (i) Describe how you would carry out a test to confirm the presence of potassium in

potassium chloride. (5 × 3m) (15) – with a clean platinum wire / nichrome / wooden splint – pick up some salt – hold in edge of blue flame – of bunsen burner – lilac colour produced (ii) Outline a test to identify the presence of chloride ions in aqueous solution. (2 × 3m + 4m)

(10) – dissolve chloride solution in distilled / deionised / water and add silver nitrate – a white precipitate forms in the presence of chloride ions – confirmed by adding ammonia to the test tube dissolving the precipitate (b) Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas according

to the balanced equation:

Mg + 2HCl MgCl2 + H2

In an experiment 2 g of magnesium were completely reacted with hydrochloric acid. (i) How many moles of magnesium were used? (6m) (6) – 2/24 = 0.083 moles (ii) How many moles of hydrochloric acid reacted? (6m) (6) – 0.167 moles (iii) What volume (measured at s.t.p.) of hydrogen gas was produced? (6m) (6) – 0.083 × 22.4 l = 1.867 l (iv) Write a balanced equation for the combustion of magnesium in air. (7) Any 1: (3m + 2m + 2m) – Mg + ½ O2 MgO // – 2Mg + O2 2MgO

2010 L.33/34_MS 16/40 of 40

(c) Answer part A or part B.

A

(i) Name the main product of the chemical industry on which you carried out a case study. Give the use of the product you have named. (6) ** Use must match the product named. Named product (3m), Use (3m) – ammonia // - to make nitric acid / fertiliser // – nitric acid // - to make nitrate / fertiliser // – magnesium oxide // - to make refractory bricks (ii) Name three of the raw materials that are used in the manufacturing process. (9) ** Raw materials named must match the product named above. Any 1: (3 × 3m) – ammonia // - methane - steam - air // – nitric acid // - ammonia - water - air // – magnesium oxide - limestone - water - sea water

(iii) Name a by-product or a waste product from the process. (6) ** By-product / waste product named must match the product named above. Any 1: (6m) – ammonia // - CO2 //

- urea // – nitric acid // - ammonium nitrate // – magnesium oxide - CO2 //

- calcium chloride (iv) State one factor that is important in choosing a location for a chemical industry. (4) Any 1: (4m) – transportation of raw materials or product // – space for operation or storage // – close to workforce // etc.

or B

(i) What is a polymer? (2 × 3m) (6) – large molecules created by – by combining many monomer molecules

2010 L.33/34_MS 17/40 of 40

(ii) Give the name by which poly(chloroethene) is commonly known and state one use for it. (6)

– PVC / polyvinyl chloride (3m)

Use Any 1: (3m)

– window frames // – gutters // – rain coats // – cable insulation // – hoses // etc. (iii) Describe one property of poly(phenylethene) that has been expanded with an inert gas and

name one use for it. (6) Property Any 1: (3m) – light // – good heat insulator Use Any 1: (3m) – packaging // – insulation // etc. (iv) What do the letters LDPE stand for? (3m) (3) – low density poly(ethene) (v) What is the main source for the raw materials for making polymers? (4) Any 1: (4m) – petroleum // – crude oil

2010 L.33/34_MS 18/40 of 40

2010 L.33/34_MS 19/40 of 40

Dublin Examining Board


Chemistry

Higher Level Marking Scheme (400 marks)

Answer two questions from Section A and six questions from Section B. All questions carry equal marks (50).

Notes: (i) In calculations, 3 marks are deducted for a mathematical error but no further penalty is incurred if the problem, otherwise correct, is completed.

(ii) Alternative valid answers are, of course, acceptable.

SECTION A (100 marks)

Answer at least two questions from this section.

1. An experiment was carried out to determine the concentration of sodium hypochlorite (NaClO) in a sample of household bleach. A 25 cm3 sample of bleach was diluted to 250 cm3 with deionised water. A 25 cm3 portion of the diluted bleach was placed in a conical flask and an excess of acidified potassium iodide was added. It was then titrated against a 0.21 M solution of sodium thiosulfate a number of times. The average titration volume was 19.6 cm3.

The equations for the reactions are:

ClO– + 2I

– + 2H+ Cl

– + I2 + H2O

2S2O2

3 + I2 S4O2

6 + 2I–

(a) Outline how to accurately dilute the bleach. (4 × 3m) (12) – with a pipette / burette – place in volumetric flask – add deionised water to the mark at eye level – stopper and invert many times (b) Why was an excess of potassium iodide used? (5m) (5) – to react with all of the hypochlorite (c) Why is potassium iodide used instead of iodine itself? (3m) (3) – potassium iodide is soluble in water (d) Name the reagent that is used as a primary standard in redox titrations. (6m) (6) – ammonium iron (II) sulfate / Mohr’s salt

2010 L.33/34_MS 20/40 of 40

(e) Name the indicator for this experiment and state when to add it. (2 × 3m) (6) – starch – when the iodine solution is pale yellow (f) Calculate the concentration of sodium hypochlorite (NaClO) in (i) the diluted bleach, (ii) the

original bleach, in moles per litre. Express the concentration in the original bleach in terms of % (w/v). (18)

(i) – 1

25 M =

2

21.06.19

M = 225

21.06.19

(3m)

– M = 0.082 M (3m) (ii) – original bleach = 0.082 M × 10 = 0.82 M (6m) – % w/v: 0.82 × 74.5 g/l = 61.32 g/l (3m) = 6.1% (3m)

2010 L.33/34_MS 21/40 of 40

2. A group of students prepared ethanal (CH3CHO) in the laboratory by slow addition of an aqueous solution of ethanol (C2H5OH) and sodium dichromate (VI) (Na2Cr2O7.2H2O) to a hot solution of sulfuric acid (H2SO4). The apparatus was set up as shown in the diagram. The reaction is described by the following equation:

3C2H5OH + Cr2O

27 + 8H+ 3CH3CHO + 2Cr3+ + 7H2O

(a) Why was the receiving vessel in ice-water? (4m) (4)

– ethanal has a low boiling point / is volatile (b) Why is it not necessary to heat the flask

during the preparation? (4m) (4) – the reaction is exothermic (c) Describe and explain the colour change that

occurred in the flask during the addition. (3 × 3m) (9) – from orange – to green – Cr2O

27 to Cr3+ / Cr6+ to Cr3+

(d) Describe how to test ethanal with ammoniacal silver nitrate (Tollens’ reagent) and

describe the result. (3 × 3m) (12) – pour a few cm3 of ethanal into a clean test tube – add ammonia solution and silver nitrate solution – heat gently Result (3m) – silver deposit on inside of test tube on completion of the reaction (e) How would you use the same apparatus if you were to prepare ethanoic acid? What change would

you make to the reagents? (9) Apparatus change (2 × 3m) – put condenser on top of flask – heat flask to boil mixture for refluxing Reagent change (3m) – have excess of sodium dichromate (f) Calculate the amount (in grams) of ethanal that could be expected from 25 cm3 of ethanol (density

0.8 g cm–3) if a 60% yield was obtained. (12) – mass of ethanol: 25 × 0.8 g (3m) – moles of ethanol: 25 × 0.8 / 46 = 0.43 moles (3m) – moles of ethanal: 0.43 × 0.6 = 0.258 moles (3m) – mass of ethanal: 0.258 × 44 = 11.35 g (3m)

2010 L.33/34_MS 22/40 of 40

3. A student carried out an experiment to measure the relative molecular mass of a volatile liquid. This involved measuring the mass of a certain amount of the vapour of the volatile liquid, and its temperature, pressure and volume. Calculations were made using the equation of state for an ideal gas, PV = nRT.

(a) Draw a labelled diagram of a suitable apparatus that could be used for this experiment. (8) ** Diagram (5m), Any 1 label (3m) Any 1 – apparatus 1 - flask + tinfoil with hole + immersed in beaker of water // – apparatus 2 - syringe + heating (b) Describe how to measure the mass of the vapour. (12) ** Method given should correspond to the apparatus drawn above. Any 1: (4 × 3m) Method 1 – measure mass of flask – allow liquid to completely vaporise – cool and reweigh – subtract Method 2 – measure mass of hypodermic syringe with liquid – inject liquid into gas syringe – reweigh syringe – subtract (c) Outline how to measure the (i) volume, (ii) pressure and (iii) temperature of the vapour. (12) ** Method given should correspond to the apparatus drawn above. (i) Volume Any 1: (3m) Method 1 – fill flask with water and measure volume of water in graduated (measuring) cylinder Method 2 – measure the volume of hot air in the gas syringe and subtract from the volume of liquid

measured in the gas syringe (ii) Pressure (2 × 3m) – measure atmospheric pressure – on barometer (iii) Temperature (3m) – measure temperature of hot water ** Note: temperature of water or steam cannot be assumed to be 100 °C

2010 L.33/34_MS 23/40 of 40

In an experiment to measure the relative molecular mass of a volatile liquid 0.82 g of the liquid was vaporised at a temperature of 99 °C. The volume occupied was found to be 255 cm3. The pressure was 100,000 N/m2 (Pa).

(d) Calculate the number of moles of the volatile liquid vaporised. (12) – n = PV/RT – V = 255/106 m3 (3m) – T = 273 + 99 K (3m) – n = 100,000 × 255 /106 × 8.31 × 372 (3m) – n = 0.00824 moles (3m) (e) Calculate the relative molecular mass of the volatile liquid. (6) – Mr = 0.82 / 0.00824 (3m) – Mr = 99.5 (3m)

2010 L.33/34_MS 24/40 of 40

SECTION B (300 marks)

4. Answer eight of the following items (a), (b), (c), etc. Award 1 bonus mark to the first two fully correct answers.

(a) Name an aromatic acid-base indicator. (6) Any 1: (6m) – phenolphthalein // – methyl orange (b) Name the scientist pictured on the right who discovered the neutron by bombarding beryllium with radiation.

What kind of radiation did he use? (2 × 3m) (6)

– Chadwick – alpha particles (c) Write the electronic configuration (s, p, etc.) of a chromium atom. (6m) (6) – 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5 (d) How many molecules of oxygen are present in 250 cm3, measured at s.t.p.? (6) – 250 / 22400 moles (3m) – 0.0111 × 6 × 1023 = 6.7 × 1021 (3m) (e) Define catalyst. (2 × 3m) (6)

– it alters the speed of a reaction – without being used up (f) When rain water passes through limestone rock it acquires a calcium compound that causes

temporary hardness. Show by an equation how this substance is formed. (6m) (6) – CaCO3 + CO2 + H2O (H2CO3) Ca(HCO3)2

(g) Name this ester: CH3COOCH3. (6m) (6) – methylethanoate (h) What two measurements can be found from the mass spectrum of an element? (2 × 3m) (6) – atomic mass – relative abundance (i) Name two plant nutrients that are removed in the tertiary treatment of sewage. (2 × 3m) (6) – nitrates – phosphates

2010 L.33/34_MS 25/40 of 40

(j) What is meant by the activation energy of a reaction? (6m) (6) – the minimum energy which colliding particles must have for a reaction to occur (k) Answer part A or part B.

A Place these atmospheric gases in order of their greenhouse effect, with the least effective first: carbon dioxide methane water (6m) (6) – water – carbon dioxide – methane or B Name two solid substances that are added to iron ore in a blast furnace to

extract the iron. (2 × 3m) (6) – coke (carbon) – limestone

2010 L.33/34_MS 26/40 of 40

5. (a) Define electronegativity. (3m + 2m) (5) – relative attraction – for a shared pair of electrons (b) Explain why the values of electronegativity increase across a period of the periodic table of the

elements. (2 × 3m) (6) – decreasing atomic radius – increasing nuclear charge

(c) Use electronegativity values (Mathematical Tables p 46) to predict the kinds of bonding in (i) ammonia, (ii) magnesium oxide. (2 × 3m) (6)

(i) – ammonia - polar covalent (ii) – magnesium oxide - ionic (d) Use a dot and cross diagram to show the bonding in magnesium oxide. Would you expect

magnesium oxide to dissolve in hexane? Explain your answer. (3m) (12) – diagram (2 × 3m) e.g. or Dissolve? (3m) – no Explain (3m) – hexane is non-polar

2010 L.33/34_MS 27/40 of 40

(e) Use a dot and cross diagram to show the bonding in a molecule of ammonia. (12) – diagram (6m) e.g. Explain why the bond angle is 107°. (2 × 3m) – tetrahedral bond angle is 109° – but repulsion from lone pair is stronger than from bond pair (f) Explain why ammonia has a much higher boiling point (–33 °C) than

methane (–161 °C). (3 × 3m) (9)

– ammonia has hydrogen bonds – methane has van der Waals bonds – hydrogen bonds stronger

2010 L.33/34_MS 28/40 of 40

6. (a) Define octane number of petrol. (5) Any 1: (5m) – the measurement of tendency to auto-ignite // – the measurement of tendency to resist auto-igniting (knocking) (b) Name one of the fractions produced in the oil refinery

that contains molecules suitable for petrol. (3) Any 1: (3m) – gasoline // – naphtha (c) Name and draw the molecular structure of a hydrocarbon with an octane number of 100. (6) – 2,2,4-trimethylpentane (3m) – diagram (3m) (d) Explain how (i) isomerisation and (ii) dehydrocyclisation raise the octane number of a fuel. (6)

(i) – makes more branched molecules (3m) (ii) – makes rings / cyclic / aromatic molecules (3m) (e) Apart from the effect on human health, why is lead unsuitable for improving the octane number of

petrol? (3m) (6) – poison to catalysts of catalytic converter Name a suitable oxygenate that raises the octane number. Any 1: (3m) – ethanol // – methanol // – MTBE // etc. (f) Name two components of LPG. (2 × 3m) (9) – propane – butane What is added to LPG and to natural gas to give it a strong odour? (3m) – mercaptans

2010 L.33/34_MS 29/40 of 40

(g) Write a balanced equation for the combustion of methane (CH4). If the heat of combustion of methane is –890 kJ mol–1 and the heats of formation of carbon dioxide and of water are –394 kJ mol–1 and –286 kJ mol–1 respectively, calculate the heat of formation of methane. (15)

– CH4 + 2O2 CO2 + 2H2O (3m) – –890 = –394 + 2(–286) – ΔHf of methane (6m) – ΔHf methane = –394 – 572 + 890 (3m) – ΔHf methane = –76 kJ mol–1 (3m)

2010 L.33/34_MS 30/40 of 40

7. Examine the reaction scheme and answer the questions that follow.

X Y Z C2H5OH C2H4 C2H6 C2H5Cl A B C D

(a) Which of the compounds is the most soluble in water? (3m) (6)

– A - C2H5OH

Explain your answer. (3m)

– it forms H bonds to the water molecules (b) For each of the conversions X, Y and Z, classify it as an addition, an elimination or a substitution

reaction. (3 × 3m) (9)

– X - elimination – Y - addition

– Z - substitution (c) What term describes the mechanism for conversion Z? (14)

– free radical substitution reaction (3m)

Name, or give the formula for the other reagent. (3m)

– chlorine / Cl2

What provides the energy to initiate the reaction? (3m)

– UV light

Name or give the formula for the compound that is formed in trace amounts in the conversion. (5m)

– butane / C4H10

(d) Conversion X can be carried out in a school laboratory. (9)

Name a catalyst for this reaction. (3m)

– aluminium oxide / Al2O3

Describe how to show that the product is unsaturated. (2 × 3m)

– decolourises – bromine solution / acidified KMnO4 (e) Describe what you would see if a small piece of sodium is added to compound A. (2 × 3m) (12) – it sinks – gas produced / fizzes Write a balanced equation for the reaction. (3m)

– C2H5OH + Na C2H5ONa + ½ H2 Explain why compound A has reacted as an acid in this reaction. (3m) – it releases a H+ ion

2010 L.33/34_MS 31/40 of 40

8. (a) Define (i) acid, (ii) conjugate pair, according to the Brønsted-Lowry theory. (2 × 4m) (8) (i) – an acid is a proton donor / H+ donor (ii) – a conjugate pair differs by a proton / H+ (b) Identify one acid and its conjugate base in this reaction: (6)

NH4 + CO 2

3 HCO3 + NH3

Any 1: (6m) – acid - HCO

3 - conjugate base - CO 23 //

– acid - NH4 - conjugate base - NH3

(c) Calculate the pH of a 0.1 M aqueous solution of NH3, if the Kb for NH3 is 1.8 × 10–5. (12) – p(OH) = –log square root 1.8 × 105 × 0.1 (3m) – = 2.87 (3m) – pH = 14 – pOH (3m) – = 11.13 (3m) (d) State two limitations to the pH scale. (6) Any 2: (2 × 3m)

– solution must be dilute // – in aqueous solutions //

– at 25 °C // – not useful outside the 0–14 range (e) If an indicator that is a weak acid is represented as HIn, explain how it works as

an indicator. (4 × 3m) (12) – HIn (colour 1) (+H2O) ↔ H+ / H3O

+ + In– (colour 2) – in acid, equilibrium shifts from right to left – colour 2 decreases – colour 1 predominates (f) Name a suitable indicator for the titration between ammonia solution and hydrochloric acid and

give a reason for your choice. (6)

– methyl orange (3m) Reason Any 1: (3m) – it is a strong acid vs. weak base // – it changes colour completely at neutralisation // – the colour change takes place within the vertical part of the titration curve / between pH 3 – 5 (acidic range)

2010 L.33/34_MS 32/40 of 40

9. (a) State Le Chatelier’s principle. (2 × 4m) (8)

– a system is at equilibrium – opposes a stress

(b) Cobalt chloride was dissolved in water to form a pink solution, according to the following equation:

CoCl 2

4 + 6H2O Co(H2O) 26 + 4Cl

The forward reaction is exothermic. (i) What change, if any, would you observe if concentrated hydrochloric

acid was added to the mixture? Explain your answer. (9) Observe (2 × 3m) – forms a blue colour – addition of Cl– ions Explain (3m) – pushes system to the left (ii) Would you heat or cool the pink solution to change the colour to blue?

Explain your answer. (6)

– heat it (3m) Why (3m) – reverse reaction takes in heat

(c) Hydrogen iodide decomposes to form hydrogen and iodine gases according to the following

equation: 2HI(g) H2(g) + I2(g) (i) Write the equilibrium constant (Kc) expression for the reaction. (6m) (6)

– 2

22

]HI[

][I]H[

2010 L.33/34_MS 33/40 of 40

(ii) Two moles of hydrogen iodide were placed in a vessel at 683 K and allowed to come to equilibrium. The Kc at 683 K is 0.0156. Calculate the number of moles of hydrogen iodide that is present at equilibrium. (15)

– Number of moles of H2 at equilibrium = x

HI H2 I2 Initially 2 0 0 At equilibrium 2 – 2x x x (3m)

222

]HI[

]I[]H[ = Kc

2]22[

][][

x

xx

= 0.0156 (3m)

x2 = 0.0156(2 – 2x)2 x2 = 0.0156(4 – 4x + 4x2) x2 = 0.0624 – 0.0624x + 0.0624x2 0.9376x2 + 0.0624x – 0.0624 = 0 9,376x2 + 624x – 624 = 0 586x2 + 39x – 39 = 0 (3m)

x = a

acbb

2

42

x = )586(2

)39)(586(4)39(39 2

= 172,1

91,416521,139

= 172,1

...304.85570339

= 172,1

...304.85570339

= 172,1

...265.855703

= 0.226839... 0.227 moles (3m)

– Number of moles of HI at equilibrium = 2 – 2(0.227) = 2 – 0.454 = 1.546 moles (3m)

(iii) Why is it not necessary to know the volume of the flask in this calculation? (6m) (6)

– because there was the same number of molecules on each side of equation

2010 L.33/34_MS 34/40 of 40

10. Answer any two of the parts (a), (b) and (c). (2 × 25)

(a) (i) Define the rate of a chemical reaction. (4m) (4) – change in concentration of a reactant (product) per unit of time (ii) Why does the rate of a chemical reaction generally decrease with time? (3m) (3) – reduction in the concentration of a reactant (iii) Explain why the reaction between ionic compounds in solution is generally instantaneous,

while reactions between covalent compounds are slower. (2 × 3m) (6) – ionic compounds dissociated – covalent compounds have bonds that have to be broken (iv) Explain why an increase in temperature generally increases the rate of a chemical reaction. (6) Any 2: (2 × 3m) – more frequent collisions // – more effective collisions // – more molecules with activation energy (v) Why can explosions occur in flour storage silos? (6) Any 2: (2 × 3m) – flour reacts with oxygen / air // – flour dust has very large surface area // – flour dust is dry and combustible (b) In 1922 Neils Bohr was awarded the Nobel Prize in physics for his theory of

atomic orbits, which he developed from a consideration of the emission spectrum of hydrogen.

(i) Distinguish between an atomic orbit and an atomic orbital. (4) Any 1: (4m) – Orbit – has definite position (location) or velocity for electron // – describes the path of an electron travelling at a certain velocity and

position from the nucleus // – Orbital – has uncertainty about position // – a region with high probability of locating an electron (ii) How is one atomic orbital different to another orbital? (3) Any 1: (3m) – amount of energy // – energy level // – distance from nucleus // – shape

2010 L.33/34_MS 35/40 of 40

(iii) Name the series of lines in the visible region of the hydrogen spectrum. (3m) (3) – Balmer (iv) Explain why these lines are emitted from hydrogen gas that is

conducting electricity. (5 × 3m) (15) – electrons are excited to higher energy levels – electrons fall to first energy level – difference in energy / E2 – E1 – is emitted as a line of colour / frequency – E2 – E1 = h × f (c) (i) Define oxidation in terms of electron transfer. (4m) (4) – loss of electrons (ii) What is the oxidation number of S in (a) SO 2

3 , (b) SO 24

? (6)

(a) – +4 (3m) (b) – +6 (3m) (iii) Outline how to use a solution of sodium sulfite to demonstrate that chlorine water is an

oxidising agent. (4 × 3m) (12) – add barium chloride to sodium sulfite and then hydrochloric acid – white precipitate formed goes clear – add chlorine water to sodium sulfite and repeat test – white precipitate remains (iv) Explain why the oxidising strength of halogens decreases down the group. (3) Any 1: (3m) – increasing atomic radius // – increased shielding // – decrease in electronegativity // – increase in number of shells

2010 L.33/34_MS 36/40 of 40

11. Answer any two of the parts (a), (b) and (c). (2 × 25)

(a) Calcium carbonate reacts with hydrochloric acid according to the balanced equation: CaCO3 + 2HCl CaCl2 + H2O + CO2

Carbon dioxide reacts with calcium hydroxide according to the balanced equation: 2CO2 + Ca(OH)2 Ca(HCO3)2 (i) What mass of carbon dioxide is formed when 10 g of calcium carbonate reacts completely

with hydrochloric acid? (9) – moles of calcium carbonate: 10/100 = 0.1 moles (3m) – moles of carbon dioxide = 0.1 (3m) – mass of carbon dioxide = 0.1 × 44 = 4.4 g (3m) (ii) What is the volume of that amount of gas at room temperature and pressure? (6) – 0.1 × 24.0 l (3m) – 2.4 l (3m) (iii) What mass of calcium hydrogencarbonate would be formed if that amount of carbon dioxide

reacted completely? (6) – moles of calcium hydrogencarbonate: 0.05 moles (3m) – mass of calcium hydrogencarbonate: 0.05 × 162 = 8.1 g (3m) (iv) Give the common name for a solution of Ca(OH)2. (4m) (4) – limewater (b) (i) Explain the principle upon which chromatography is based. (3 × 3m) (9) – difference in adsorbance / affinity / interactions – between a stationary phase – and a mobile phase (ii) Outline an experiment to separate the components of a mixture of dyes using paper or

thin-layer chromatography. (12) Any 4: (4 × 3m) – with dropper / capillary tube // – about 2 cm above bottom of paper / plate // – stand in tank of eluent (solvent) // – eluent (solvent) moves up // – dyes move to different positions (iii) Name the instrument that may be attached to a gas chromatograph to identify the molecular

mass of the components. (4m) (4) – mass spectrometer

2010 L.33/34_MS 37/40 of 40

(c) Answer part A or part B.

A

(i) What is meant by the term the greenhouse effect of the atmosphere? (2 × 3m) (6) – the sun’s heat passes through without heating – the earth’s heat is absorbed and reflected back CO2 is a greenhouse gas that is produced by human activity. (ii) Apart from the combustion of fossil fuels name an industrial process that releases large

quantities of CO2 into the atmosphere. (3m) (6) – manufacture of cement // etc. Name a natural process that removes CO2 from the atmosphere. (3m) – photosynthesis (iii) After CO2 dissolves in water it may be found in its free state or as two ions. Show by

equations how these ions are formed. (9) – H2CO3 (3m) ↔ 2H+ + CO 2

3 (3m)

– H2CO3 ↔ H+ + HCO 3 (3m)

(iv) Name the group of greenhouse gases that also cause ozone depletion. (4m) (4) – chlorofluorocarbons / CFCs or B

(i) Name the father and son team, pictured right, who pioneered a method of determining crystal structure. (6)

– Braggs (3m) What method did they develop? (3m) – X-ray crystallography

2010 L.33/34_MS 38/40 of 40

Diamond and graphite are different forms (allotropes) of carbon. (ii) What type of crystal do they possess? (3m) (12) – macromolecular Name the form that conducts electricity and explain how its crystal structure gives it this

physical property. Conducts electricity (3m) – graphite How structure conducts (2 × 3m) – has sheets of atoms / bonded to three other atoms – delocalised electrons between the sheets (iii) Name a substance that forms molecular crystals with Van der Waals forces as the binding

force between the lattice points. (7) Any 1: (3m) – I2 // – CO2 Give one physical property based on this crystal structure. Any 1: (4m) – low melting point // – insoluble in water

2010 L.33/34_MS 39/40 of 40

2010 L.33/34_MS 40/40 of 40

2010 m33-34 ms -08 sk-amended

Documents