Exothermic & endothermic reactions, reaction profiles & bond energies (RP4) – GCSE Chemistry exam questions with mark scheme

1Grade 1–33 marks

(a) In an exothermic reaction, what happens to the temperature of the surroundings? [1]

(b) Give one example of an exothermic reaction. [1]

(a) it increases allow gets hotter / rises(1)
(b) any one from: combustion / burning (a named fuel); neutralisation; (most) oxidation reactions; respiration; reaction in a hand warmer / self-heating can allow any other correct exothermic reaction, e.g. a metal reacting with an acid / a displacement reaction(1)
(c) any one from: thermal decomposition (e.g. of calcium carbonate); citric acid + sodium hydrogencarbonate; photosynthesis; electrolysis; reaction in a sports injury cold pack allow dissolving ammonium nitrate / ammonium chloride in water ignore other physical changes such as melting, boiling or evaporating(1)
Answer: (a) increases (b) e.g. combustion (c) e.g. thermal decomposition or electrolysis
Examiner tip: Exothermic = energy EXits to the surroundings, so the surroundings get hotter.

2Grade 4–53 marks

The diagram shows the reaction profile for a reaction.

(a) What does arrow X represent? [1]

(b) What does arrow Y represent? [1]

(a) activation energy(1)
(b) (overall) energy change (of the reaction) allow energy released / energy given out / ΔH(1)
(c) exothermic and the products have less energy than the reactants / energy is released to the surroundings (both needed for the mark)(1)
Answer: (a) activation energy (b) overall energy change (c) exothermic — products lower than reactants
Examiner tip: Activation energy is measured from the reactants to the peak, not from the products.

3Grade 4–54 marks

A student investigated the temperature change when hydrochloric acid reacts with sodium hydroxide solution. She used a polystyrene cup with a lid.

(a) Give one reason for using a polystyrene cup with a lid. [1]

(b) The starting temperature was 21.0 °C. The highest temperature reached was 27.5 °C. Calculate the temperature change. [1]

(c) The student repeated the experiment with different volumes of sodium hydroxide. Give the independent variable and one control variable. [2]

(a) to reduce energy (heat) loss to the surroundings allow insulates(1)
(b) 6.5 °C(1)
(c) independent: volume of sodium hydroxide(1)
(c) control, any one from: volume of acid; concentration of acid / of sodium hydroxide; starting temperature; type of cup(1)
Answer: (a) reduce heat loss (b) 6.5 °C (c) IV volume of NaOH; control e.g. volume of acid
Examiner tip: The independent variable is the one you change on purpose — the temperature is the dependent variable.

4Grade 6–7Higher4 marks

Hydrogen reacts with chlorine: $\text{H}{-}\text{H} + \text{Cl}{-}\text{Cl} \rightarrow 2\,\text{H}{-}\text{Cl}$

Bond	H–H	Cl–Cl	H–Cl
Bond energy in kJ/mol	436	243	432

(a) Calculate the overall energy change for the reaction. [3]

(b) Explain, in terms of bond breaking and bond making, why the reaction is exothermic. [1]

(a) energy to break bonds $= 436 + 243 = 679$ kJ/mol(1)
(a) energy released making bonds $= 2 \times 432 = 864$ kJ/mol(1)
(a) $679 - 864 = -185$ kJ/mol allow 185 kJ/mol released / given out; an answer of +185 (or 185 with no sign or direction) scores the first two marks only; correct answer with no working scores 3; allow ecf from incorrect totals(1)
(b) more energy is released making (new) bonds than is needed to break (existing) bonds(1)
Answer: (a) −185 kJ/mol (b) more energy released forming bonds than used breaking bonds
Examiner tip: Do “broken − made”; a negative answer means exothermic — the sign is part of the mark.

5Grade 8–9Higher4 marks

Ammonia is made from nitrogen and hydrogen.

$$\text{N}{\equiv}\text{N} + 3\,\text{H}{-}\text{H} \rightarrow 2\,\text{NH}_3$$

The overall energy change is −92 kJ/mol. Bond energies: N≡N = 945 kJ/mol, H–H = 436 kJ/mol.

Calculate the bond energy of the N–H bond. Give your answer to 3 significant figures. [4]

energy to break bonds $= 945 + (3 \times 436) = 2253$ kJ/mol(1)
6 N–H bonds are made: $2253 - 6x = -92$(1)
$6x = 2345$, so $x = 390.8\ldots$(1)
391 kJ/mol(1)
Answer: 391 kJ/mol
Examiner tip: Each NH₃ has three N–H bonds and there are two molecules: divide by 6, not 3.

6Grade 1–34 marks

A student carried out four reactions. She measured the temperature of each reaction mixture at the start and at the end.

Reaction	Reactants	Temperature at start in °C	Temperature at end in °C
A	magnesium + hydrochloric acid	19.5	33.5
B	citric acid + sodium hydrogencarbonate solution	20.0	14.5
C	sodium hydroxide solution + nitric acid	20.5	28.5
D	barium hydroxide + ammonium chloride	21.0	9.5

(a) Which two reactions are endothermic? [1]

(b) Calculate the temperature change for reaction C. [1]

Which reaction in the table would be best for a sports injury pack? Give a reason for your answer. [2]

(a) B and D (both needed; any additional letter scores 0)(1)
(b) 8.0 (°C) allow 8 / +8.0 / increase of 8 ignore units(1)
(c) D(1)
(c) (because it has) the largest temperature decrease / fall / drop allow temperature falls by 11.5 °C allow it takes in the most energy from the surroundings do not award reason mark if D not chosen(1)
Answer: (a) B and D (b) 8.0 °C (c) D, it gives the biggest temperature fall (11.5 °C)
Examiner tip: Endothermic means the temperature of the mixture goes down; students who pick the largest number without checking whether it is a rise or a fall choose A and lose both marks in (c).

7Grade 4–54 marks

When calcium carbonate is heated strongly it decomposes into calcium oxide and carbon dioxide.

$$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$

This reaction is endothermic.

(a) What is meant by the activation energy of a reaction? [1]

(b) The diagram shows the energy level of the reactant.

Complete the reaction profile for this reaction. You should:

draw the energy level of the products and label it
show the shape of the profile during the reaction
label the activation energy and the overall energy change using arrows.

[3]

(a) the minimum (amount of) energy that particles must have to react (when they collide) allow minimum energy needed to start a reaction do not accept 'energy needed for a reaction' without 'minimum'(1)
(b) products level drawn higher than the CaCO₃ (reactant) level and labelled products / CaO + CO₂(1)
(b) curve rises from the reactant level to a peak that is higher than the products level, then falls to the products level(1)
(b) activation energy arrow from the reactant level up to the peak and overall energy change arrow between the reactant level and the products level, both labelled allow double-headed arrows or lines with labels do not accept a single-headed overall energy change arrow pointing downwards (from products to reactants)(1)
Answer: (a) minimum energy particles need to react (b) products above reactants; hump above both; Ea from reactants to peak; energy change from reactants to products
Examiner tip: Many students draw the products below the reactants out of habit; for an endothermic reaction the products must finish higher.

8Grade 6–76 marks

When a metal powder is added to copper sulfate solution, a displacement reaction takes place and the temperature of the solution increases.

A student wants to find out how the type of metal affects the temperature change.

She is given zinc, iron and magnesium powders, copper sulfate solution, a polystyrene cup and a thermometer.

Plan an investigation to find out how the type of metal affects the temperature change.

Your plan should give valid results. [6]

measure a fixed volume of copper sulfate solution (e.g. 25 cm³) using a measuring cylinder(1)
pour it into the polystyrene cup, placed inside a beaker to stop it falling over(1)
measure and record the starting temperature of the solution with the thermometer(1)
add a fixed mass of one metal powder (e.g. 1.0 g) weighed on a balance(1)
stir the mixture (with the thermometer)(1)
record the highest temperature reached(1)
calculate the temperature change = highest temperature − starting temperature(1)
repeat with each of the other metals using fresh copper sulfate solution(1)
control variables: same volume and same concentration of copper sulfate solution, same mass of metal, same form of metal (all powders), same starting temperature(1)
put a lid on the cup to reduce energy transfer to the surroundings(1)
repeat each test (at least) three times and calculate a mean temperature change(1)
make sure the metal is in excess / enough metal so all the copper sulfate reacts(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, logically ordered method that could be followed by another person to produce valid results. It covers measuring the starting temperature, adding the metal, recording the highest temperature and finding the change for each metal, and identifies at least two control variables with the quantities kept the same. Safety statements are not required for any level and are ignored unless linked to the method.
Level 2	3–4	The main steps of a workable method are present but may be out of order or miss a detail, such as how the temperature change is found. At least one control variable is identified.
Level 1	1–2	Some relevant steps are given, such as adding a metal to copper sulfate and measuring a temperature, but the method could not be followed to give valid results.
No relevant content	0	Nothing worthy of credit.

Answer: Level 3 (5–6): measure same volume and concentration of CuSO₄ into a lidded polystyrene cup, take start temperature, add same mass of metal powder, stir, record highest temperature, find the change; repeat for each metal and repeat to get means.
Examiner tip: Students often forget to say which temperature they record; 'record the temperature' is not enough — say 'the highest temperature reached'.

9Grade 6–76 marks

A student investigated the reaction between hydrochloric acid and sodium hydroxide solution.

She put 25.0 cm³ of sodium hydroxide solution into a polystyrene cup.
She added hydrochloric acid 5.0 cm³ at a time and recorded the temperature after each addition.

The graph shows her results with two lines of best fit.

(a) Use the graph to find the volume of hydrochloric acid needed to exactly neutralise the sodium hydroxide solution. [1]

(b) Determine the maximum temperature rise for this reaction. Use the lines of best fit. [2]

Explain why. [2]

(d) The temperature the student measured at 25 cm³ was lower than the value shown where the lines of best fit cross.

Suggest one reason why. [1]

(a) 25 cm³ allow 24–26 cm³(1)
(b) maximum temperature read from where the lines cross = 26.5 (°C) allow 26.4–26.6(1)
(b) temperature rise = 26.5 − 19.0 = 7.5 (°C) allow ecf from their maximum temperature − 19.0(1)
(c) all the sodium hydroxide has reacted / been used up / no more neutralisation happens, so no more energy is released(1)
(c) the extra acid added is cooler (than the mixture) so it absorbs energy / cools the mixture or energy is transferred (lost) to the surroundings(1)
(d) energy is lost to the surroundings (through the cup or the open top) allow the reading was taken before the highest temperature was reached / the mixture was not stirred enough ignore 'human error' / 'thermometer error' alone(1)
Answer: (a) 25 cm³ (b) 26.5 − 19.0 = 7.5 °C (c) no more reaction so no energy released; cooler acid / heat loss lowers temperature (d) e.g. energy lost to surroundings
Examiner tip: In (b) students read the highest plotted point (26.1 °C) instead of where the lines of best fit cross, giving 7.1 °C.

10Grade 8–9Higher5 marks

Hydrogen peroxide decomposes to form water and oxygen.

$$2\,\text{H}{-}\text{O}{-}\text{O}{-}\text{H} \rightarrow 2\,\text{H}{-}\text{O}{-}\text{H} + \text{O}{=}\text{O}$$

Bond	O–H	O–O	O=O
Bond energy in kJ/mol	464	146	498

(a) Calculate the overall energy change for the reaction shown in the equation. [3]

(b) A bottle contains 3.4 g of hydrogen peroxide (H₂O₂).

Calculate the energy released when all of this hydrogen peroxide decomposes.

Relative atomic masses ($A_r$): H = 1 O = 16 [2]

(a) energy to break bonds = 2 × [(2 × 464) + 146] = 2148 (kJ)(1)
(a) energy released making bonds = (4 × 464) + 498 = 2354 (kJ)(1)
(a) overall energy change = 2148 − 2354 = −206 (kJ/mol) allow 206 kJ/mol released / given out allow ecf from their two totals allow −206 with 3 marks if bonds broken and made are cancelled correctly (only O–O broken, O=O made: 2(146) − 498 = −206)(1)
(b) moles of H₂O₂ = 3.4 ÷ 34 = 0.10 (mol)(1)
(b) energy released = 0.10 × (206 ÷ 2) = 10.3 (kJ) allow ecf from (a): 0.10 × (their value ÷ 2) allow 10 kJ do not accept 20.6 kJ (did not divide by 2) for this mark(1)
Answer: (a) −206 kJ/mol (b) 10.3 kJ
Examiner tip: The equation shows 2 mol of H₂O₂, so the energy change per mole of H₂O₂ is half of 206 kJ; most students lose the final mark by forgetting this.

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