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Topic 5 Practice Paper: Chemical energetics — mark scheme

QuizQuipPractice Series
QQ/IGCSE-CHEM/T5

IGCSE · Cambridge IGCSE Chemistry (Extended (Core + Supplement))

Mark scheme

Chemical energetics

Topic 5 Practice Paper · Maximum mark: 30

Notes for markers

  • Award one mark for each point marked [1], up to the marks shown for the part.
  • Accept answers worded differently that show the same understanding.
  • Units are required where the answer line shows one. ecf: allow an error carried forward.

Section A: Multiple-choice questions[10 marks]

QuestionAnswer
1BAn exothermic reaction transfers thermal energy to the surroundings, so the surroundings become hotter.1
2DThe reaction absorbs energy from its surroundings, so the surroundings lose thermal energy and become cooler.1
3AEnergy must be supplied to pull bonded atoms apart, while making a new bond gives energy back out, so breaking is endothermic and forming is exothermic.1
4CIt is the energy barrier at the top of the pathway that a collision must clear before a reaction can occur.1
5BEnergy has been transferred out of the reacting system, so the finishing level lies below the starting level.1
6DTwo H–H bonds (2 × 436 = 872) plus one O=O bond (498) gives 872 + 498 = 1370 kJ.1
7AHeating gives the particles more kinetic energy, so a larger share of their collisions carries at least the energy needed to break the existing bonds.1
8CThe temperature of the mixture went up, which means the reaction released thermal energy into its surroundings.1
9BA negative value means the products lie at a lower energy than the reactants, so energy leaves the system and the change is exothermic.1
10DThere is a net release of thermal energy because the products are held together more strongly overall, so the surroundings get hotter.1

Section B: Structured questions[20 marks]

QuestionAnswer and marking pointsMarks
11(a)

The reaction is exothermic. The temperature of the mixture rose from 21 °C to 34 °C, so thermal energy was transferred to the surroundings.

  • exothermic [1]
  • temperature rose / energy transferred to the surroundings [1]
2
11(b)

Use a polystyrene cup instead of the beaker (or add a lid, or lag the beaker).

  • use a polystyrene cup / lag the beaker / add a lid [1]
1
11(c)

The temperature rise is smaller, because less magnesium reacts and so less energy is transferred to the surroundings.

  • smaller temperature rise / less energy transferred [1]
1
Total for Question 114
12(a)

The products have less energy than the reactants.

  • products at a lower energy than the reactants [1]
1
12(b)

Energy is taken in to break the bonds in methane and oxygen. Energy is released when the new bonds in carbon dioxide and water form. More energy is released than is taken in, so the reaction gives energy out overall.

  • energy taken in to break bonds [1]
  • energy released when new bonds form [1]
  • more released than taken in / net energy given out [1]
3
12(c)

The particles must have at least the activation energy before they can react, so the mixture has to be heated to a high enough temperature first.

  • particles need the activation energy / ignition temperature before reacting [1]
1
Total for Question 125
13(a)

945 + (3 × 436) = 945 + 1308 = 2253 kJ.

  • 945 + 3 × 436 [1]
  • 2253 kJ [1]
2
13(b)

There are six N–H bonds, so 6 × 391 = 2346 kJ. [1]

1
13(c)

2253 − 2346 = −93 kJ per mol. The value is negative, so the reaction is exothermic.

  • 2253 − 2346 = −93 kJ per mol [1]
  • exothermic [1]
2
Total for Question 135
14(a)

P and R are exothermic, because in each case the final temperature is higher than the starting temperature, so energy was transferred to the surroundings.

  • P and R [1]
  • final temperature higher / energy transferred to the surroundings [1]
2
14(b)

The reaction takes in thermal energy from the surroundings, so the surroundings lose energy and their temperature falls.

  • energy taken in from the surroundings [1]
  • so the surroundings become cooler / lose thermal energy [1]
2
14(c)

The temperature change is 27 − 20 = +7 °C. The mixture got hotter, so energy was given out and the products are at a lower energy than the reactants.

  • +7 °C / a 7 °C rise [1]
  • products at a lower energy [1]
2
Total for Question 146

— End of mark scheme —