C7 Energy Changes

1. C7 Energy changes
Student Book answers
C7.1 Exothermic and endothermic reactions
Question
number
Answer Marks Guidance
1 a exothermic 1
1 b endothermic 1
1 c i any two from:
• oxidation,
• combustion,
• neutralisation,
• respiration
2
1 c ii any two from:
• any thermal decomposition,
• citric acid and sodium hydrogencarbonate,
• photosynthesis
2
2 beaker feels cold
dissolving process absorbs energy from surroundings, which
includes beaker and hand holding it, so energy transferred into
reaction mixture
1
1
3 energy stored in reactants greater than in products,
so difference transferred to surroundings as energy,
raising temperature of surroundings
1
1
1
4 a MgCO3(s) → MgO(s) + CO2(g) 3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
4 b 117 kJ taken in from surroundings 2
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2. C7 Energy changes
Student Book answers
C7.2 Using energy transfers from reactions
Question
number
Answer Marks Guidance
1 a treat injuries with cold packs,
chill drinks in cans
1
1
1 b dissolving ammonium nitrate in water 1
1 c i NH4NO3 1
1 c ii to chill drinks in cans 1
2 a calcium oxide 1
2 b CaO(s) + H2O(l) → Ca(OH)2(aq) 3
2 c It would form a harmful alkaline solution. 1
3 a hand warmer uses energy transferred to surroundings
in oxidation of iron,
forming hydrated iron(III) oxide in exothermic reaction,
NaCl catalyst
1
1
1
1
3 b Supersaturated solution
made to crystallise by pressing a small metal disc.
Crystals spread throughout solution, transferring energy to
surroundings.
Crystals are redissolved in hot water, ready to use again.
1
1
1
1
3 c disposable:
advantage: lasts longer when activated,
disadvantage: can only be used once.
reusable: opposite applies
1
1
3 d self-heating cans 1
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3. C7 Energy changes
Student Book answers
C7.3 Reaction profiles
Question
number
Answer Marks Guidance
1 a reactants H2(g) + Cl2(g) above products HCl(g),
arrow points from reactants to products / down,
arrow labelled ‘184kJ/mol of energy released’
1
1
1
1 b reactants H2(g) + I2(g) below products 2HI(g),
arrow points from reactants to products / up,
arrow labelled ‘26.5kJ/mol of energy absorbed’
1
1
1
2 Compare energy required to break bonds with energy
transferred to surroundings when new bonds form to get
overall energy change.
If energy transferred breaking bonds greater, then
endothermic,
if energy transferred to surroundings when new bonds are
made greater then exothermic.
1
1
1
3 a Energy transferred from the surroundings to break bonds
to overcome attraction between atoms,
so separated atoms have more energy stored than original
molecule.
1
1
1
3 b structural diagram of methane and oxygen, showing C–H
bonds being broken and carbon dioxide and water formed
2
3 c bonds broken: 4 C–H; 2 O=O
bonds made: 2 C=O; 4 O–H
2
2
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4. C7 Energy changes
Student Book answers
C7.4 Bond energy calculations
Question
number
Answer Marks Guidance
1 endothermic 1
2 energy required to break a specific bond 1
3 1.49kJ (to 3 sig. fig.) 2
4 a H2 + Cl2 → 2HCl
energy transferred to surroundings = 185kJ
1
5
4 b 2H2 + O2 → 2H2O
energy transferred to surroundings = 486kJ
1
5
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5. C7 Energy changes
Student Book answers
C7.5 Chemical cells and batteries
Question
number
Answer Marks Guidance
1 must be difference in reactivity between two different metals to
produce voltage
1
2 a diagram similar to C7.5, Figure 2 but with iron instead of
copper
3
2 b iron 1
2 c zinc, more reactive than iron,
greater tendency to lose electrons and form positive ions,
zinc atoms → Zn2+
ions at negative terminal, donating 2
electrons to iron(II) ions
Zn → Zn2+
+ 2e−
Fe2+
ions → iron atoms at positive terminal
Fe2+
(aq) + 2 e−
→ Fe(s)
1
1
1
1
1
1
3 dry cell: once one reactant is used up,
cell stops working and must be discarded as it cannot be
recharged,
prone to leakage
if zinc casing is used up (as it changes to zinc ions) and
dissolves releasing inner paste from outer casing
1
1
1
1
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6. C7 Energy changes
Student Book answers
C7.6 Fuel cells
Question
number
Answer Marks Guidance
1 a hydrogen and oxygen / air 1
1 b water 1
1 c hydrogen + oxygen → water 1
2 2H2(g) + 4OH−
(aq) → H2O(l) + 4e−
O2(g) + 2H2O(l) + 4e−
→ 4OH−
(aq)
2
2
3 Electricity to recharge cells from the mains might be produced
by burning / combustion
of fossil fuel.
However, if electricity from renewable source,
such as wind power or solar cells,
no carbon dioxide produced.
1
1
1
1
1
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