science form 4- chapter 5
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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.
5.1 PHYSICAL AND CHEMICAL CHANGES
- 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
5.1 PHYSICAL AND CHEMICAL CHANGES
Melting of ice Evaporation of water
5.1 PHYSICAL AND CHEMICAL CHANGES
Dissolving of sugar in water Heating iodine crystals
5.1 PHYSICAL AND CHEMICAL CHANGES
(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
5.1 PHYSICAL AND CHEMICAL CHANGES
(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
oxide
5.1 PHYSICAL AND CHEMICAL CHANGES
(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
5.1 PHYSICAL AND CHEMICAL CHANGES
(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
5.1 PHYSICAL AND CHEMICAL CHANGES
Frying an egg Browning of an apple
5.1 PHYSICAL AND CHEMICAL CHANGES
Heating a mixture of iron filings and sulphur powder
Reaction of zinc with copper sulphate solution
5.1 PHYSICAL AND CHEMICAL CHANGES
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.
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
(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.
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
Breaking an old bond absorbs heat energy Forming a new bond releases heat energy
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
• 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.
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
• 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.
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
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
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
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
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
• 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
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
Iron catalyst
Reactor
Coolant out
Coolant in
Condenser
Leftover H2 + N2
Liquid ammonia
H2 + N2
Haber process
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
• 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.
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
• 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
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
• Step 4: The oleum is diluted with water to
produce concentrated sulphuric
acid.
Oleum + Water Concentrated
sulphuric acid
5.2 HEAT CHANGE IN CHEMICAL REACTIONS
Sulphur
Dry air
Furnace
SO2 + O2 SO3
Converter
Concentrated sulphuric acid
Waste gas
Oleum
Concentrated sulphuric acid
Water
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.
5.3 THE REACTIVITY SERIES OF METALS
• 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
5.3 THE REACTIVITY SERIES OF METALS
• 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
5.3 THE REACTIVITY SERIES OF METALS
• 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.
5.3 THE REACTIVITY SERIES OF METALS
• 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.
5.4 APPLICATION OF REACTIVITY SERIES OF METALS
Ore Composition
Bauxite Aluminium oxide
Cassiterite Tin(IV) oxide
Haematite Iron(III) oxide
Sphaletite Zinc sulphide
Sedimentary rock Calcium carbonate
5.4 APPLICATION OF REACTIVITY SERIES OF METALS
• 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.
5.4 APPLICATION OF REACTIVITY SERIES OF METALS
• 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.
5.4 APPLICATION OF REACTIVITY SERIES OF METALS
• 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.
5.4 APPLICATION OF REACTIVITY SERIES OF METALS
Extraction of tin in a blast furnace
Molten iron
Blast furnace
Tin(IV) oxide, coke and limestone
Slag
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
metal
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
anode
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.
5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS
A simple cell
Copper sulphate solution
Magnesium
Copper
5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS
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.
5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS
• 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.
5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS
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
5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS
Type of cell Uses Advantages Disadvantages
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
5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS
Type of cell Uses Advantages Disadvantages
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
A. X, Y, Z
B. Y, X, Z
C. Z, X, Y
D. Y, Z, X
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
7.
Brown gas
Grey solid
Molten lead
bromide
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
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
CHAPTER 5: ENERGY AND CHEMICAL CHANGES QUIZ
10. Photosensitive chemicals are stored in
dark bottles because they are sensitive
to
A. heat
B. light
C. water
D. oxygen