OxfordAQA International GCSE Chemistry: Exothermic and endothermic reactions, and calculating and explaining energy change (9202) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Written for OxfordAQA International GCSE Chemistry (9202), this set follows the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. It covers sections 3.9.1 Exothermic and endothermic reactions (statements a to d) and 3.9.2 Calculating and explaining energy change (statements a to h); no required practical is attached to these sections. The qualification is untiered, so every question suits every candidate. Unless a question says otherwise, use c = 4.2 J/g °C for water and dilute solutions, and take 1 cm³ of solution to have a mass of 1 g.

Links: energy change study guide · energy change revision notes · course hub · checklist · free diagnostics

Questions

1. This question is about exothermic reactions.

(a) State what is meant by an exothermic reaction. [1] (b) Give two types of reaction that are exothermic. [2]

2. The table gives the enthalpy change for four reactions, W to Z.

Reaction	ΔH (kJ/mol)
W	−890
X	+178
Y	−57
Z	+34

(a) Identify the endothermic reactions. [1] (b) Identify the reaction that transfers the most energy to the surroundings per mole. [1] (c) Convert 3600 J into kJ, and 0.85 kJ into J. [2]

3. A student heats some blue hydrated copper sulfate crystals in a boiling tube.

(a) Describe what the student sees. [2] (b) After cooling, the student adds a few drops of water to the solid left in the tube. Give two observations. [2] (c) The change in (a) takes in 78 kJ per mole of hydrated copper sulfate (use this value). State the energy change, with its sign, for the change in (b) per mole of anhydrous copper sulfate. [1]

4. Two products use chemical reactions to change temperature.

(a) A sports injury pack goes cold when it is squeezed. State the type of reaction or process inside it, and explain why it makes the pack cold. [2] (b) A self-heating can of coffee contains calcium oxide and a separate pouch of water. Suggest why calcium oxide reacting with water is chosen for this product. [1]

5. A student compares two fuels by burning each one under a copper can holding 120 g of water.

Fuel	Mass of fuel burned	Temperature rise of water
Methanol (Mr = 32)	0.84 g	25.0 °C
Paraffin wax	0.35 g	18.5 °C

(a) Calculate the energy transferred to the water when methanol burns. [2] (b) Calculate the energy released per gram of methanol, in kJ/g. [1] (c) Calculate the energy released per mole of methanol, in kJ/mol. [2] (d) Use a calculation to decide which fuel releases more energy per gram. [2] (e) The student’s values are lower than the true values for these fuels. Give two reasons. [2]

6. 30.0 cm³ of 2.00 mol/dm³ hydrochloric acid is mixed with 30.0 cm³ of 2.00 mol/dm³ sodium hydroxide solution in a polystyrene cup. The temperature rises by 13.4 °C.

HCl + NaOH → NaCl + H₂O

(a) Calculate the energy transferred to the solution. [2] (b) Calculate the amount, in moles, of water formed. [1] (c) Calculate ΔH for the reaction in kJ/mol. Include the sign. [2] (d) Suggest why a lid is placed on the cup. [1]

7. 4.00 g of ammonium nitrate (Mr = 80) is dissolved in 40.0 cm³ of water. The temperature falls from 22.0 °C to 14.4 °C.

(a) State whether the process is exothermic or endothermic, giving a reason. [1] (b) Calculate the energy transferred. [2] (c) Calculate the energy change in kJ per mole of ammonium nitrate. Include the sign. [2]

8. On an energy level diagram for a reaction, the reactants are at 160 kJ, the top of the curve is at 290 kJ and the products are at 25 kJ. With a catalyst, the top of the curve is at 220 kJ.

(a) Calculate the activation energy without a catalyst. [1] (b) Calculate the overall energy change. [1] (c) Is the reaction exothermic or endothermic? Explain how the diagram shows this. [1] (d) Calculate the activation energy with the catalyst. [1] (e) Describe how the curve for the catalysed reaction differs from the uncatalysed one, and state one feature that stays the same. [2]

9. Hydrogen reacts with oxygen to form hydrogen peroxide, H–O–O–H.

H₂ + O₂ → H₂O₂ ΔH = −140 kJ/mol

Take O–H as 464, H–H as 436 and O=O as 498 kJ/mol. Calculate the bond dissociation energy of the O–O bond. [3]

10. A fuel does not burn until a flame or spark is applied, but once lit it keeps burning and releases energy.

(a) Explain, in terms of bonds, why the combustion of the fuel is exothermic. [2] (b) Explain why a flame or spark is needed to start the reaction. [2]

11. Hydrogen is made industrially by reacting methane with steam at a high temperature over a catalyst.

CH₄ + H₂O → CO + 3H₂

Use these bond dissociation energies (kJ/mol): C–H 413, O–H 464, C≡O 1077, H–H 436.

(a) Work out the energy change for one mole of methane reacting, and use its sign to classify the reaction. [4] (b) Use your answer to explain why the reactor must be heated all the time. [1] (c) Describe the energy level diagram for this reaction, with and without the catalyst. [3]

Answers

1. (a) A reaction that transfers energy to the surroundings (so the surroundings get hotter) [1] (b) Any two of: combustion [1]; oxidation or neutralisation [1] Examiner insight: “Makes heat” or “contains heat” describes the chemicals wrongly and risks losing the mark.

2. (a) X and Z (both needed) [1] (b) W [1] (c) 3600 ÷ 1000 = 3.6 kJ [1]; 0.85 × 1000 = 850 J [1] Examiner insight: A one-mark “identify” asking for reactions in the plural usually needs every correct letter and no wrong ones.

3. (a) The blue crystals turn white [1]; droplets of liquid (water) form near the top of the tube, or steam is seen [1] (b) The white solid turns blue [1]; the tube gets hot (or steam is seen) [1] (c) −78 kJ/mol (78 kJ released per mole) [1] Examiner insight: For (c), 78 kJ with no sign or “released” does not show the reverse change is exothermic.

4. (a) It is endothermic (ammonium nitrate dissolving, for example) [1]; it takes in energy from the surroundings, so the pack and the injury cool down [1] (b) The reaction is exothermic, transferring energy to the coffee, and it only starts when the water is released (any one) [1] Examiner insight: For “explain why it makes the pack cold”, the direction of energy transfer (from the surroundings into the reaction) is what earns the second mark.

5. (a) Q = 120 × 4.2 × 25.0 [1] = 12 600 J (12.6 kJ) [1] (b) 12.6 ÷ 0.84 = 15.0 kJ/g [1] (c) Moles = 0.84 ÷ 32 = 0.02625 mol [1]; 12.6 ÷ 0.02625 = 480 kJ/mol [1] (d) Wax: Q = 120 × 4.2 × 18.5 = 9324 J = 9.32 kJ, and 9.32 ÷ 0.35 = 26.6 kJ/g [1]; so paraffin wax releases more energy per gram [1] (e) Any two of: energy heats the can and surrounding air rather than the water [1]; incomplete combustion (soot) [1]; fuel evaporates from the wick; the can was too far from the flame Examiner insight: In (d) the comparison mark depends on a per-gram value for both fuels; comparing temperature rises alone ignores the different masses burned.

6. (a) Q = 60.0 × 4.2 × 13.4 [1] = 3377 J (3.38 kJ) [1] (b) 2.00 × (30.0 ÷ 1000) = 0.0600 mol [1] (c) 3.377 ÷ 0.0600 = 56.3 kJ/mol [1]; ΔH = −56.3 kJ/mol (negative because the temperature rose) [1] (d) To reduce energy (heat) loss to the surroundings [1] Examiner insight: Using 30 g instead of the total 60 g in (a) halves every later answer; the mass in Q = mcΔT is the whole mixture that warmed up.

7. (a) Endothermic, because the temperature fell [1] (b) ΔT = 22.0 − 14.4 = 7.6 °C; Q = 40.0 × 4.2 × 7.6 [1] = 1277 J (1.28 kJ) [1] (c) Moles = 4.00 ÷ 80 = 0.0500 mol [1]; 1.277 ÷ 0.0500 = +25.5 kJ/mol [1] Examiner insight: The positive sign carries meaning here; a bare 25.5 kJ/mol does not show that energy was taken in.

8. (a) 290 − 160 = 130 kJ [1] (b) 25 − 160 = −135 kJ [1] (c) Exothermic, because the products are at a lower energy than the reactants (ΔH negative) [1] (d) 220 − 160 = 60 kJ [1] (e) The catalysed curve has a lower peak (lower activation energy) [1]; it starts and ends at the same levels, so ΔH is unchanged [1] Examiner insight: Activation energy is always measured from the reactant level to the peak; measuring from the product level is a common slip.

9. Bonds broken = 436 + 498 = 934 kJ [1] Bonds formed = (2 × 464) + O–O = 928 + O–O; so −140 = 934 − (928 + O–O) [1] O–O = 934 + 140 − 928 = 146 kJ/mol [1] Examiner insight: Write bonds broken and bonds formed out separately before rearranging, so your method can be credited.

10. (a) Energy is needed to break bonds in the fuel and oxygen [1]; more energy is released forming the new bonds (in carbon dioxide and water) than is taken in [1] (b) Some bonds must break before new ones can form, which needs the activation energy [1]; the flame or spark supplies this energy, then energy released by the reaction keeps it going [1] Examiner insight: “Breaking bonds releases energy” contradicts 3.9.2 e and usually costs the mark even if the rest of the answer is correct.

11. (a) Broken: (4 × 413) + (2 × 464) = 1652 + 928 = 2580 kJ [1] Formed: 1077 + (3 × 436) = 1077 + 1308 = 2385 kJ [1] Energy change = 2580 − 2385 [1] = +195 kJ/mol, endothermic [1] (b) The reaction takes in energy, so energy must be supplied continuously to keep the temperature up and the reaction going [1] (c) Products drawn at a higher energy than the reactants [1]; activation energy labelled from the reactants to the peak and ΔH from reactants up to products [1]; catalysed curve with a lower peak between the same two levels [1] Examiner insight: In (a), a positive answer labelled “exothermic” is inconsistent and is unlikely to gain the final mark.

Where marks are usually lost

• Writing ΔH as positive for a reaction where the temperature rose.
• Using the mass of fuel, not of water, as m in Q = mcΔT.
• Forgetting to add both solution volumes together when mixing.
• Dividing joules by moles and calling the answer kJ/mol.
• Comparing fuels by temperature rise without dividing by mass or moles.
• Measuring activation energy from the product level.
• Leaving out a coefficient (such as the 3 in 3H₂) when totalling bonds.
• Saying a catalyst changes ΔH.

Next steps

• Recap fast with the energy change revision notes.
• Re-learn anything you missed in the energy change study guide.
• Practise moles with the quantitative chemistry practice set.
• See the whole course on the OxfordAQA IGCSE Chemistry hub and tick off topics on the printable checklist.
• Try one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. This practice set covers section 3.9.1 Exothermic and endothermic reactions and section 3.9.2 Calculating and explaining energy change.