science form 4- chapter 5

SCIENCE FORM 4

CHAPTER 5: ENERGY AND

CHEMICAL CHANGES

5.1 PHYSICAL AND CHEMICAL CHANGES

• Many things undergo changes in our lives. • There of two types of changes:

(i) Physical changes

- Affects the physical properties of a

substance, such as its size, shape

and state.

- Usually reversible.

- No new substance is formed.

- Examples involving physical changes:

(a) Melting of ice

(b) Evaporation of water

(c) Dissolving sugar in water

(d) Crystallisation of sodium chloride

from its saturated solution

(e) Heating of iodine crystals

(f) Heating of wax

Melting of ice Evaporation of water

Dissolving of sugar in water Heating iodine crystals

(ii) Chemical change

- Produces new substances that have

properties different from those of the

original substance.

- Usually irreversible.

- Examples involving chemical changes:

(a) Burning of a paper

(b) Colour of a peeled apple changes

when it is exposed to air

(c) Frying an egg

(d) Combustion of fuel

(e) When iron filings are heated with sulphur

powder, the mixture glows brightly. A

black solid, iron sulphide is formed.

Iron + Sulphur Iron sulphide

(f) When magnesium ribbon is heated,

it burns with a bright flame. A white solid,

magnesium oxide is formed.

Magnesium + Oxygen Magnesium

(g) Zinc reacts with blue copper sulphate

solution to form a colourless zinc

sulphate solution and brown copper.

Zinc + Copper sulphate Zinc sulphate

+ Copper

(h) Iron nail rust when water and oxygen are

present. Rust is brown.

Iron + Oxygen + Water Rust

(i) Heating green copper carbonate

produces black copper oxide and

releases carbon dioxide.

Copper carbonate Copper oxide +

Carbon dioxide

(j) Mixing potassium iodide and lead nitrate

solution forms lead iodide, a yellow

precipitate.

Potassium + Lead Potassium + Lead

iodide nitrate nitrate iodide

Frying an egg Browning of an apple

Heating a mixture of iron filings and sulphur powder

Reaction of zinc with copper sulphate solution

Aspect Physical changes Chemical changes

Formation of new substance No Yes

Reversibility Reversible Difficult to reverse

Energy needed Less energy needed More energy needed

5.2 HEAT CHANGE IN CHEMICAL REACTIONS

• There are two types of chemical reactions:

(i) Exothermic reactions

- Reactions which release heat to the

surroundings are called exothermic

reactions.

- Temperature of the surroundings

increases.

(ii) Endothermic reactions

- Reactions which absorb heat from

the surroundings.

- Temperature of the surroundings

decreases.• During chemical reactions, old bonds in

the reactants are broken and new bonds in the products are formed.

• The breaking down of old bonds absorbs heat energy whereas the formation of new bonds releases heat energy.

Breaking an old bond absorbs heat energy Forming a new bond releases heat energy

• In exothermic reactions, the heat energy absorbed to break the old bonds in the reactants is less than the heat energy released when new bonds are formed in the products. There is a net of heat energy loss.

• The temperature of the surroundings increases due to the heat energy that is released.

• In endothermic reactions, the heat energy absorbed to break the old bonds in the reactants is greater than the heat energy

released when the new bonds are formed in the products. There is a net of heat energy gain.

• The temperature of the surroundings decreases due to heat energy is absorbed from the surroundings.

In exothermic reactions, the heat energy absorbed to break the old bonds in the reactants is less than the heat energy released when the new bonds are formed in the

products

In endothermic reactions, the heat energy absorbed to break the old bonds in the reactants is greater than the heat energy released when the new bonds are formed in

the products

• Haber process is used to manufacture ammonia in the industry. Ammonia is an important material for making nitrogenous fertilisers.

• In the Haber process, nitrogen and hydrogen gases are mixed together. The mixture is passed over an iron catalyst.

Iron catalyst Nitrogen + Hydrogen Ammonia 450 °C 200 atm

Iron catalyst

Reactor

Coolant out

Coolant in

Condenser

Leftover H2 + N2

Liquid ammonia

H2 + N2

Haber process

• Contact process is used to manufacture

sulphuric acid in the industry. • Step 1: Sulphur is burnt in air to produce

sulphur dioxide.

Sulphur + Oxygen Sulphur

dioxide• Step 2: A mixture of sulphur dioxide and

air are passed over vanadium(V)

oxide catalyst at 450 °C to produce

sulphur trioxide.

• Step 3: Sulphur trioxide is dissolved in

concentrated sulphuric acid to

produce oleum.

Sulphur + Concentrated Oleum

trioxide sulphuric acid

Vanadium(V) oxide catalystSulphur dioxide + Oxygen Sulphur trioxide

450 °C

• Step 4: The oleum is diluted with water to

produce concentrated sulphuric

Oleum + Water Concentrated

sulphuric acid

Sulphur

Dry air

Furnace

SO2 + O2 SO3

Converter

Concentrated sulphuric acid

Waste gas

Concentrated sulphuric acid

Absorber

Vanadium(V)oxide

catalyst

Contact process

5.3 THE REACTIVITY SERIES OF METALS

• Why do gold, silver and platinum exist as elements in the Earth’s crust and whereas metals such as sodium and potassium do not exist as elements in the Earth’s crust?

• Some metals are more reactive than others. Different metals show different

reactivity with water, acids and oxygen.

• The reactions involving metals:

(i) Reaction of reactive metal with water to

produce alkali and hydrogen.

Metal + Water Alkali + Hydrogen

(ii) Reaction of reactive metal with dilute

acid to produce a salt and hydrogen.

Metal + Dilute acid Salt + Hydrogen

(iii) Reaction of metal with oxygen to form

metal oxide

Metal + Oxygen Metal oxide

• Based on their reactivity with oxygen, metals and carbon (non-metal) can be arranged in a reactivity series as follows:

PotassiumSodium Calcium

MagnesiumAluminium

CarbonZinc Iron Tin

LeadCopperSilverGold

PotatoSoup Can

Make All

Cruel Zoos

In The Land Change

So Good

• The position of carbon in the reactivity series is determined by comparing its attraction for oxygen with other metals.

• When carbon is heated with a metal oxide, carbon can remove oxygen from the metal oxide if carbon is more reactive than the metal.

• When zinc oxide and iron oxide are heated with carbon, the oxygen in the metal oxides are removed by carbon. Therefore, carbon is more reactive than zinc and iron.

• When carbon is heated with aluminium oxide, the carbon cannot remove oxygen from aluminium oxide.

• This means that carbon is more reactive than zinc and iron but less reactive than aluminium. Therefore, carbon is positioned between aluminium and zinc in the reactivity series.

5.4 APPLICATION OF REACTIVITY SERIES OF METALS

• Most metals in the Earth’s crust are reactive. They react readily with other elements to form compounds such as oxides, sulphides and carbonates. These compounds are called ores.

Ore Composition

Bauxite Aluminium oxide

Cassiterite Tin(IV) oxide

Haematite Iron(III) oxide

Sphaletite Zinc sulphide

Sedimentary rock Calcium carbonate

• The method of extracting a metal from its ore depends on its position in the reactivity series.

• Metals below carbon in the reactivity series can be extracted from their oxides with using carbon because they are less reactive than carbon.

• Carbon is used for extraction because it is cheap and readily available.

• Metals above carbon in the reactivity series cannot be extracted using carbon because they are more reactive than carbon. They are extracted using electrolysis.

• Tin ore or cassiterite is tin(IV) oxide. Tin is extracted by heating cassiterite with carbon (coke) and limestone at high temperature in a blast furnace.

• Carbon removes the oxygen from tin(IV) oxide because it is more reactive than tin.

Tin(IV) oxide + Carbon Tin + Carbon

dioxide• Limestone is added to react with the

impurities in the tin ore. Slag is formed. • Two products are collected at the bottom of

the blast furnace. Molten slag floats on the molten tin. This allow the two products to

flow separately.

Extraction of tin in a blast furnace

Molten iron

Blast furnace

Tin(IV) oxide, coke and limestone

Blast of hot air

5.5 ELECTROLYSIS

• Electrolysis is the decomposition of an electrolyte by electricity.

• An electrolyte is a liquid or solution which

contains free-moving ions that can conduct electricity.

• The electrodes are conductors which carry electricity into or out of an electrolyte.

• The electrode joined to the positive terminal of the dry cell is called the anode whereas the electrode joined to the negative terminal is called the cathode.

5.5 ELECTROLYSIS

• During electrolysis, the positively-charged ions (cations) are attracted to the cathode and receive electrons. The negatively-charged ions (anions) are attracted to the anode and release electrons.

5.5 ELECTROLYSIS

Bromine gas Lead

Molten lead bromide

Electrolysis

5.5 ELECTROLYSIS

Refer to the diagram above:• In the complete circuit, the bulb lights up.

This is because molten lead bromide conducts electricity.

• At the anode, the bromide ions release electrons to become bromine atoms. The bromine atoms combine together to form bromine gas (brown colour).

• At the cathode, lead ions receive electrons to become lead atoms. Lead metal is formed (grey solid).

5.5 ELECTROLYSIS

• Uses of electrolysis in industry:

(i) Electroplating

- To prevent iron objects from corrosion,

they are electroplated with a thin layer

of unreactive metals such as

copper, silver and chromium.

- Electroplating make these objects

resistant to corrosion and more

attractive.

5.5 ELECTROLYSIS

Electroplating an iron spoon with copper

Copper as

Iron spoon

as cathode

Copper sulphate solution

Copper ions

Rules for electroplating:(i) Anode : Plating metal(ii) Cathode: Object to be electroplated(iii) Electrolyte: Solution of the plating metal

Rheostat

5.5 ELECTROLYSIS

- At the anode, the copper dissolves to

form positively-charged copper ions.

- At the cathode, the copper ions receive

electrons to form a coat of copper on

the iron spoon.

5.5 ELECTROLYSIS

- A rheostat is used to control the current

flow in the circuit so that a small

current is used and the object to be

electroplated must be cleaned with a

sandpaper before electrolysis. These

steps are to obtain good results.

5.5 ELECTROLYSIS

Iron spoon electroplated with copper

5.5 ELECTROLYSIS

(ii) Extraction of reactive metals

- Metals which are more reactive than

carbon are extracted from their ores by

electrolysis.

- For example, aluminium can be

extracted from its ore, bauxite

(aluminium oxide).

- Bauxite is first purified and then dissolved

in cryolite. This is to lower the melting

point of aluminium oxide.

5.5 ELECTROLYSIS

- When aluminium oxide melts, aluminium

ions and oxide ions are free to move.

- When electricity is passed through the

electrolyte, the positively-charged

aluminium ions are attracted to the

cathode. They receive electrons and

become aluminium atoms.

- The molten aluminium formed is

channelled into moulds.

5.5 ELECTROLYSIS

- At the anode, the oxide ions lose

electrons to become oxygen atoms.

The oxygen atoms combine together to

form oxygen gas.

Aluminium is extracted from bauxite by electrolysis

5.5 ELECTROLYSIS

(iii) Purification of metals

- When electricity is passed through the

electrolyte, the copper anode dissolves

to form copper ions. At the same, the

impurities settle to the bottom.

- These positively-charged ions are

attracted to the cathode. They receive

electrons and form copper atoms and

causes copper to be deposited on the

pure copper.

5.5 ELECTROLYSIS

Purifying copper by electrolysis

5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS

• A simple cell consists of two electrodes and an electrolyte.

• The more reactive metal is the negative terminal which releases electrons whereas the less reactive metal acts as a positive terminal which receives electrons.

A simple cell

Copper sulphate solution

Magnesium

Copper

Refer to the diagram above:• Magnesium and copper strips are used as

electrodes with copper sulphate solution as the electrolyte.

• Magnesium (negative terminal) is more reactive than copper (positive terminal).

Therefore, magnesium atoms release electrons to form magnesium ions which move into the electrolyte. As a result,

the magnesium strip becomes thinner.

• The electrons flow to the copper strip through the wire and this produces electricity.

• Positively-charged copper ions from the electrolyte receive the electrons and form copper atoms. This causes copper to be deposited on the copper strip. Therefore, the copper strip becomes thicker. At the same time, the blue colour of the copper sulphate solution fades.

Type of cell Uses Advantages Disadvantages

Dry cells Used in portable devices such as radios, cassette players, cameras and toys

• Light • Small • Portable• Cheap

• Non-rechargeable• Not long-lasting• Leakage may happen when the zinc case becomes thinner

Lead-acid accumulators

Used in vehicles • High voltage• Rechargeable• Long-lasting if well taken care of

• Heavy• Expensive• Acid might spill• Distilled water needs to be added

Alkaline batteries

Used in radios, torch lights and toys which need large electric current for long-lasting periods

• Long-lasting• Large current• Constant voltage

• Non-rechargeable

Mercury cell Used in watches, calculators, hearing aides and measuring instruments

• Small• Portable• Steady voltage• Constant current •Long-lasting

• Expensive• Non-rechargeable

Nickel-cadmium batteries

Used in electronic devices such as digital cameras

• Rechargeable• Long-lasting

• Expensive

5.7 CHEMICAL REACTIONS THAT OCCUR IN THE PRESENCE OF LIGHT

• During photosynthesis, chlorophyll absorbs light energy to split water molecules into hydrogen and oxygen.

• The oxygen is released into the atmosphere

whereas the hydrogen atoms react with

carbon dioxide to form glucose.

Carbon + Water Light Glucose + Oxygen

dioxide Chlorophyll

5.7 CHEMICAL REACTIONS THAT OCCUR IN THE PRESENCE OF LIGHT

Photosynthesis requires light

5.8 INNOVATIVE EFFORTS IN THE DESIGN OF EQUIPMENT USING CHEMICAL REACTIONS AS SOURCES OF ENERGY

• We should use electric cells efficiently to prevent wastage. Turn off all the electrical devices when they are not in use.

• After using, we should dispose them wisely to reduce environmental pollution.

5.8 INNOVATIVE EFFORTS IN THE DESIGN OF EQUIPMENT USING CHEMICAL REACTIONS AS SOURCES OF ENERGY

• Fuel cells are electric cells which does not run down or require recharging.

• A hydrogen fuel cell uses hydrogen as fuel

to react with oxygen to produce energy.

The product is water which will not pollute the environment.

CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ

1. Which of the following involve chemical

changes?

I. Lighting a match

II. Hard-boiling an egg

III. Melting tin to cast tin bars

A. I and II

B. I and III

C. II and III

D. I, II and III

2. Which of the following statements about a

chemical which undergoes physical changes is not true?

A. Its size is changed

B. Its state is changed

C. Its shape is changed

D. Its chemical properties are changed

3. Why does the temperature rise in a chemical reaction?

A. The reaction is fast

B. The reaction is slow

C. The reaction absorbs heat energy

D. The reaction releases heat energy

4. The table below shows the result of an

experiment to study the reactivity of

metals X, Y and Z with water and steam.

Based on the observation, put these metals in order of reactivity. Start with the

most reactive metal.

Metal Reaction with

Water Steam

X No Yes

Y No No

Z Yes Yes

A. X, Y, Z

B. Y, X, Z

C. Z, X, Y

D. Y, Z, X

5. Which of the following metals can be

extracted from their ores by heating their

oxides with carbon?

I. Tin

II. Iron

III. Zinc

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

6. Lead can be obtained by heating a

mixture of lead oxide and carbon. This

reaction takes place because carbon

A. is a fuel

B. acts as a catalyst

C. is more reactive than lead

D. is a non-metal whereas lead is a metal

Brown gas

Grey solid

Molten lead

bromide

When the switch is turned on, what can be

observed?

I. The bulb lights up

II. Brown gas is given off at the anode

III. A grey solid is deposited at the cathode

A. I only

B. I and II only

C. II and III only

D. I, II and III only

8. Which of the following is true about

electroplating an iron key with silver?

A. The key is used as the anode

B. A small current is used

C. A silver plate is used as the cathode

D. Copper sulphate solution is used as the

electrolyte

9. An endothermic reaction is one in which

A. heat is absorbed from the surrounding

B. heat is released to the surroundings

C. heat is neither released nor absorbed

D. temperature of the surroundings

increases

10. Photosensitive chemicals are stored in

dark bottles because they are sensitive

A. heat

B. light

C. water

D. oxygen

science form 4- chapter 5

chemical reactionsbreaking

chemical reactionsthere

chemical changesexamples

types of chemical reactions

chemical changesg zinc

physical properties

heat change

chemical changesmany

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