Answer Key For Teacher

1. C5 Chemical changes
Student Book answers
C5.1 The reactivity series
Question
number
Answer Marks Guidance
1 a lithium + water → lithium hydroxide + hydrogen
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
1
3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
1 b zinc + hydrochloric acid → zinc chloride + hydrogen
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
1
3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
2 a gas given off / fizzing, magnesium gets smaller then disappears,
thermal energy released / surroundings get warmer / solution and test
tube gets hot
1
1
2 b metal + acid → salt + hydrogen 1
2 c magnesium + sulfuric acid → magnesium sulfate + hydrogen
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
1
3 1 mark for correct reactants and products. 1
mark for correct state symbols.
3 a gold, silver, and platinum are very unreactive
will not tarnish not react with oxygen in the air so will stay bright for a
long time
1
1
3 b react with water vapour (and oxygen) in the air
oil keeps oxygen and water vapour away from the metals
1
1
3 c zinc reacts with acidic foods at room temperature
tin does not
1
1
4 Aluminium is protected by a tough / impervious layer of aluminium
oxide.
1
5 4.00 g 3
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2. C5 Chemical changes
Student Book answers
C5.2 Displacement reactions
Question
number
Answer Marks Guidance
1 a no reaction 1
1 b zinc + copper(II) sulfate → zinc sulfate + copper 1
1 c magnesium + iron(II) chloride → magnesium chloride + iron 1
2 Carbon is more reactive than zinc but less reactive than magnesium,
so displaces zinc but not magnesium.
1
1
3 a below hydrogen in reactivity series 1
3 b WO3 + 3H2 → W + 3H2O 1
4 a Zn(s) + Fe2+
(aq) → Zn2+
(aq) + Fe(s) 3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
4 b zinc atoms lose 2 electrons to form zinc(II) ions,
zinc atoms oxidised, as lose electrons,
iron(II) ions reduced,
as gain 2 electrons from zinc
1
1
1
1
4 c 0.05 mol zinc,
0.025 mol iron(II) sulfate,
so as they react 1 mol : 1mol, iron(II) sulfate is limiting reactant and
zinc is in excess
1
1
1
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3. C5 Chemical changes
Student Book answers
C5.3 Extracting metals
Question
number
Answer Marks Guidance
1 A metal ore is a rock that contains enough of a metal or metal
compound
to make it worth extracting the metal.
1
1
2 very unreactive 1
3 very low reactivity,
used for jewellery / electrodes in electrolysis
1
1
4 a zinc oxide + carbon → zinc + carbon monoxide
Oxidised: carbon
Reduced: zinc
1
1
1
4 b ZnO(s) + C(s) → Zn(l) + CO(g) 3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
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4. C5 Chemical changes
Student Book answers
C5.4 Salts from metals
Question
number
Answer Marks Guidance
1 acid + metal → salt + hydrogen 1
2 a copper metal does not react with dilute acid / copper not reactive
enough to displace hydrogen from acid
1
2 b potassium metal explodes in dilute acid 1
3 a Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g) 3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
3 b Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) 3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
4 a Zn(s) + 2H+
(aq) → Zn2+(aq) + H2(g) 3 1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
4 b Zn → Zn2+
+ 2e−
2H+
+ 2e−
→ H2
1
1
4 c each zinc atom loses two electrons
to two hydrogen ions
to form hydrogen gas
zinc atoms oxidised as lose electrons
hydrogen ions reduced as gain electrons
1
1
1
1
1
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5. C5 Chemical changes
Student Book answers
C5.5 Salts from insoluble bases
Question
number
Answer Marks Guidance
1 acid + base → salt + water 1
2 zinc oxide + hydrochloric acid → zinc chloride + water
ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
2
3
1 mark for correct reactants. 1 mark for correct
products.
1 mark for correct reactants. 1 mark for correct
products. 1 mark for correct state symbols.
3 a NaBr 1
3 b MgF2 1
3 c KNO3 1
3 d Al2(SO4)3 1
4 Heat copper sulfate solution in evaporating dish on a water bath,
some water evaporates from copper sulfate solution until point of
crystallisation when crystals appear at the edge of the solution,
solution then left at room temperature for the remaining water to
evaporate slowly,
leaving crystals in the dish. If small volume of solution remains on the
crystals dry by gently dabbing between two pieces of filter paper.
1
1
1
1
5 a Li2O(s) + H2SO4(aq) → Li2SO4(aq) + H2O(l) 3
5 b 3.3 g 3
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6. C5 Chemical changes
Student Book answers
C5.6 Making more salts
Question
number
Answer Marks Guidance
1 a acid + alkali → salt + water 1
1 b H+
+ OH−
→ H2O 1
1 c acid + carbonate → salt + water + carbon dioxide 1
2 a add indicator to measured volume of lithium hydroxide solution
add dilute hydrochloric acid until indicator just changes colour noting
volume of acid needed
repeat until results concordant then calculate mean
repeat using this mean volume of acid without indicator
evaporate water from LiCl solution by heating until point of
crystallisation
leave at room temperature for rest of water to evaporate; if necessary,
dry off any remaining solution with filter paper
1
1
1
1
1
1
2 b LiOH(aq) + HCl(aq) → LiCl(aq) +H2O(1) 3 1 mark for correct products. 1 mark for correct
reactants. 1 mark for correct state symbols
3 a BaCO3(s) + 2HNO3(aq) → Ba(NO3)2(aq) + H2O(l) + CO2(g) 3
3 b 2H+
(aq) + CO3
2−
(s) → H2O(l) + CO2(g) 3
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7. C5 Chemical changes
Student Book answers
C5.7 Neutralisation and the pH scale
Question
number
Answer Marks Guidance
1 a alkali dissolves in water 1
1 b produce hydroxide ions in water 1
1 c KOH(s) → K+
(aq) + OH−
(aq) 2
2 a H+
(aq) 1
2 b HBr(aq)/(g) → H+
(aq) + Br−
(aq) 2
3 distilled water pH = 7,
sodium hydroxide solution pH > 7, e.g., 14;
ethanoic acid pH < 7, e.g., 4
1
1
1
4 starts high and remains almost constant
until all alkali neutralised then falls very rapidly to low value,
continues almost level as excess acid added
1
1
1
5 any three from:
• pH sensor and data-logger more accurate,
• matching colours against pH chart by eye is subjective and
difficult to judge,
• pH sensor gives more repeatable measurements over narrower
range, giving more precise data
• than U.I. paper,
• also useful for monitoring changes in pH continuously over
time,
• if you only need a rough estimate of pH,
• U.I. paper is quicker and easier to use
3
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8. C5 Chemical changes
Student Book answers
C5.8 Strong and weak acids
Question
number
Answer Marks Guidance
1 fizzing / rate gas is given off is slower with ethanoic acid than nitric acid 1
2 a pH = 5.0 1
2 b 0.000 0001 mol/dm3
1 × 10−7
mol/dm3
1
1
3 Any four from:
• propanoic acid does not ionise completely when added to water
• reaction is reversible
• majority of molecules remain intact
• only a small fraction form H+
(aq) ions
• Therefore, propanoic acid does not produce as high a
concentration of H+
(aq) ions as a strong acid of equal
concentration
• Nitric acid is a strong acid, because its molecules ionise
completely in water
4
4 pH depends on concentration of H+
(aq) ions
so although weak acid does not ionise completely, it could have a
higher concentration of H+
(aq) ions if amount of strong acid per dm3
is
very, very small
Strong acid ionises completely
but in very dilute solution may produce fewer H+
(aq) ions in a given
volume of solution than a concentrated solution of the weak acid
1
1
1
1
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9. C5 Chemical changes
Student Book answers
C4.9 Volumes of gases
Question
number
Answer Marks Guidance
1 volume of gas occupied by 1 mole of gas at room temperature and
pressure
1
2 a i 1.5 mol 1
2 a ii 417 mol 1
2 b i 216 dm3
(or 216000 cm3
) 2 1 mark each for correct answer and correct unit
2 b ii 7.2 dm3
(or 7200 cm3
) 2 1 mark each for correct answer and correct unit
2 c 0.064 g 2 1 mark each for correct answer and correct unit
3 300 dm3
(or 300000 cm3
) 1
4 0.048 dm3
(or 48 cm3
) 3
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