Oxford Revise AQA GCSE Chemistry | Chapter C11 answers

C11: Energy changes

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Answers

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Mark

AO / Specification reference

01.1

thermal decomposition – endothermic

citric acid with sodium hydrogencarbonate – endothermic

neutralisation – exothermic

combustion – exothermic

1

1

1

1

AO1

4.5.1.1

01.2

it transfers energy to the surroundings

1

AO1

4.5.1.1

01.3

exothermic

1

AO2

4.5.1.1

02.1

curved line going from reactants to products, peaking above reactants

arrow from reactants line vertically down to level of products with arrow down

1

1

AO1

4.5.1.2

02.2

energy of products is lower than energy of reactants

showing that energy is transferred to the surroundings

1

1

AO1

4.5.1.1

4.5.1.2

02.3

energy needed to break bonds = 436 + 242 = 678

energy released on making bonds = 431 × 2 = 862

energy change = 678 – 862 = –184 kJ/mol

award three marks for correct answer if working is not shown

1

1

1

AO2

4.5.1.3

03.1

insulated container/cup (with lid)

reduce energy transfers to the surroundings

1

1

AO3

4.5.1.1

03.2

stir

to ensure that the solution is the same temperature throughout/make it dissolve faster

1

1

AO3

4.5.1.1

03.3

C

1

AO3

4.5.1.1

03.4

temperature increase would be less

1

AO3

4.5.1.1

04.1

2H2 + O2 → 2H2O

one mark for reactants

one mark for products

2

AO2

4.5.2.2

04.2

hydrogen

1

AO1

4.5.2.2

04.3

any four from:

advantages:

  • produces water/no pollutants produced
  • does not need to be electrically recharged

disadvantages:

  • hydrogen fuel cell cars do not perform as well as standard electric cars at present/runs for less miles before it needs to be recharged
  • difficult to obtain hydrogen
  • hydrogen is very flammable
  • hydrogen fuel cell cars are more expensive
  • release of water as a product may have currently unknown negative effect on environment

one mark for each correct answer

must have at least one advantage and one disadvantage to gain full marks

4

AO3

4.5.2.2

05.1

A (4 × 412) + (2 × 496) = +2640 (kJ/mol)

B (2 × 743) + (4 × 463) = –3338 (kJ/mol)

C + 2640 – 3338 = –698 (kJ/mol)

1

1

1

AO2

4.5.1.3

05.2

overall energy change of reaction

allow ‘enthalpy’ for ‘energy’

1

AO1

4.5.1.2

05.3

activation energy/the minimum amount of energy the particles must have in order to react

1

AO1

4.5.1.2

06.1

temperature change

1

AO2

4.5.1.1

06.2

any two from:

  • concentration of acid
  • volume of acid
  • amount of acid
  • amount of metal

one mark for each correct answer

accept other suitable answers

2

4.5.1.1

06.3

copper does not react with dilute hydrochloric acid

1

AO2

4.4.1.2

06.4

magnesium 17.7; zinc 6.1

both values required for the mark

1

AO2

06.5

magnesium

1

AO2

4.5.1.1

07.1

B and C

both answers required for the mark

1

AO3

4.5.1.2

07.2

B

1

AO3

4.5.1.2

07.3

C

1

AO3

4.5.1.2

08.1

CuCO3(s) + 2HCl(aq) → CuCl2(aq) + CO2(g) + H2O(l)

one mark for formulae and state symbols of reactants

one mark for formulae and state symbols of reactants

or

one mark for correct formulae

one mark for correct state symbols

one mark for balancing

3

AO2

4.4.2.3

08.2

Level 3: The description of the method is detailed and accurate. Apparatus and variables are named correctly. The description is clear and coherent.

5-6

AO1

4.5.1.1

Level 2: The description of the method is correct, although lacks detail. Apparatus or variables are named correctly. The description lacks some clarity and coherence

3-4

Level 1: The method is outlined correctly. The names of one or two pieces of apparatus are given or the names of one or two variables. The description overall lacks clarity and coherence.

1-2

No relevant content.

0

Indicative content

  • measure out a given volume of acid with a measuring cylinder and transfer to an insulated container with a lid
  • measure the temperature of the acid with a thermometer
  • add a given mass/amount of carbonate
  • stir the mixture
  • measure the temperature again
  • repeat with the other two metal carbonates
  • independent variable – metal carbonate
  • dependent variable – temperature change
  • control variables – amount/volume of acid, concentration of acid, mass of carbonate, type of acid

08.3

the reaction with the greatest increase in temperature is the most exothermic

1

AO3

4.5.5.1

09.1

iron

largest difference in reactivity with copper

1

1

AO1

4.5.2.1

09.2

too reactive/it would react with water/it would oxidise

1

AO2

4.5.2.1

09.3

Zn(s) → Zn2+(aq) + 2e

1

AO2

4.5.2.1

10.1

methane/CH4

1

AO2

4.5.1.1

10.2

C9H20 + 14O2 → 9CO2 + 10H2O

one mark for formulae of reactants

one mark for formulae of products

one mark for balancing

3

AO2

4.5.1.1

10.3

Level 3: The calculations are correct and each step is clearly explained. The analysis is correct and well-argued.

5-6

AO3

4.5.1.1

Level 2: The calculations are correct, but the steps are not clearly explained. The analysis has some merit, but is not clearly argued.

3-4

Level 1: The calculations are partly correct, but include mistakes. The steps are not explained. The analysis lacks clarity and detail and is not clearly argued.

1-2

No relevant content

0

Indicative content

  • energy transferred per gram of fuel:

methane:
\(
\begin{array}{l}
{\rm{methane:}}\frac{890}{{12 + (4\;\times\;1)}} = 55.63{\rm{\ kJ/g}}\\
{\rm{nonane:}}\frac{{{\rm{6125}}}}{{{\rm{(9\;\times\;12)\ }} + {\rm{\ (20\;\times\;1)}}}}\\
= 47.85{\rm{\ kJ/g}}
\end{array}
\)

so more energy transferred per gram for methane, indicating that a smaller mass of methane is needed for a given amount of energy to be transferred

  • energy transferred per gram of carbon dioxide made:

methane: 1 mol methane makes 1 mol of carbon dioxide

so transferring 890 kJ of energy makes 44 g of carbon dioxide

\(
{\rm{and\ }}\frac{{{\rm{890}}}}{{{\rm{44}}}} = 20.2{\rm{\ kJ\ energy\ produced\ per\ g\ of\ C}}{{\rm{O}}_2}
\)

nonane: 1 mol of nonane makes 9 mol of CO2

so transferring 6125 kJ makes (9 × 44) = 396 g of CO2

\(
{\rm{and\ }}\frac{{6125}}{{396}}\; = 15.5{\rm{\ kJ\ energy\ per\ g\ of\ C}}{{\rm{O}}_2}
\)
  • so more energy transferred per gram of CO2 produced for methane than for nonane
  • environmental impacts of methane are less on both measures

11.1

Level 3: The deductions are correct, comprehensive and clearly explained.

5-6

AO1 × 2

AO3 × 4

4.4.2.6

Level 2: The deductions are correct, but are not clearly explained and lack detail.

3-4

Level 1: One deduction has been made correctly. It is not clearly explained and lacks detail.

1-2

No relevant content.

0

Indicative content

  • pH of W and X is the same, so as HCl is the stronger acid, its concentration must be less than 1 mol / dm3
  • pH of Y is greater than W, so as the acid is the same in both cases, the concentration of acid W must be greater than the concentration of acid Y
  • as a decrease in pH of one unit indicates an increase in hydrogen ion concentration by a factor of 10, the concentration of Y must be one hundredth the concentration of W/concentration of Y = 0.01 mol/dm3

11.2

for a concentration of 1.0 mol/dm3 the pH of chloroethanoic acid is lower than that of ethanoic acid

this means that chloroethanoic acid has a greater degree of dissociation than ethanoic acid

1

1

AO3

4.4.2.6

11.3

H+ + OH → H2O

ignore state symbols

1

AO1

4.4.2.4

12.1

Level 3: A detailed and coherent comparison is given, demonstrating a sound knowledge and understanding of reactions of the Group 1 metals and magnesium with water.

5-6

AO1

4.1.2.5

4.4.1.2

Level 2: Correct descriptions of the reactions are given. Some comparisons are made, but not all are clearly articulated.

3-4

Level 1: Some correct points are made about the reactions. Few comparisons are made, and these are not clearly articulated.

1-2

No relevant content.

0

Indicative content

  • all react with cold water to make hydrogen gas and the metal hydroxide
  • potassium reacts most vigorously, and a purple/lilac flame is seen as the metal whizzes around on the surface of the water
  • magnesium reacts very slowly indeed, with small bubbles being observed on the surface of the magnesium
  • the reactivity of lithium is intermediate, with no flame seen but the metal whizzing around on the surface of the water

12.2

hydrogen and caesium hydroxide

both names required to achieve the mark

1

1

AO2

4.1.2.5

12.3

the tendency to form positive ions increases down the group

so reactivity increases down the group

1

1

AO2

4.1.2.5

4.4.1.2

13.1

lead oxide

1

AO1

4.4.1.3

13.2

PbSO4 + 2NaOH → Na2SO4 + Pb(OH)2

one mark for formulae of reactants

one mark for formulae of reactants

one mark for balancing

3

AO2

4.1.1.1

13.3

no SO2 made in method B

no raw material used

allow other suitable answers

2

AO3

4.4.1.3

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