Energy changes
50 exam-style questions with answers for 4.5 Energy changes in AQA GCSE Chemistry (spec 8462). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official AQA material.
All 50 questions and answers
Question 1 · Spec 4.5.1.1 · Exothermic
What is an exothermic reaction? [2 marks]
Answer:A reaction that transfers energy to the surroundings (1); so the temperature of the surroundings increases (1).Question 2 · Spec 4.5.1.1 · Endothermic
What is an endothermic reaction? [2 marks]
Answer:A reaction that takes in energy from the surroundings (1); so the temperature of the surroundings decreases (1).Question 3 · Spec 4.5.1.1 · Exothermic
Give three examples of exothermic reactions. [3 marks]
Answer:Combustion (1); many oxidation reactions (1); neutralisation (1).Question 4 · Spec 4.5.1.1 · Endothermic
Give two examples of endothermic reactions. [2 marks]
Answer:Any two from: thermal decomposition (1); citric acid and sodium hydrogencarbonate (1); photosynthesis (1).Question 5 · Spec 4.5.1.1 · Uses
Give one everyday use of an exothermic and one of an endothermic reaction. [2 marks]
Answer:Exothermic: self-heating cans or hand warmers (1); endothermic: sports injury packs (1).Question 6 · Spec 4.5.1.1 · Required practical 4
Describe how to investigate the temperature change when an acid reacts with an alkali. [3 marks]
Answer:Measure a set volume of acid into a polystyrene cup with a lid and record the temperature (1); add a measured volume of alkali and stir (1); record the highest temperature reached and repeat with different volumes (1).Question 7 · Spec 4.5.1.1 · Required practical 4
Why is a polystyrene cup used? [1 mark]
Answer:It is a good insulator, reducing energy transfer to the surroundings.Question 8 · Spec 4.5.1.1 · Required practical 4
Why is the reaction mixture stirred? [1 mark]
Answer:To make sure the temperature is the same throughout and the reactants mix fully.Question 9 · Spec 4.5.1.1 · Conservation of energy
Why is energy conserved in a chemical reaction? [1 mark]
Answer:The total energy of the reactants and surroundings stays the same; energy is only transferred.Question 10 · Spec 4.5.1.2 · Activation energy
Define activation energy. [1 mark]
Answer:The minimum amount of energy that particles must have to react.Question 11 · Spec 4.5.1.2 · Reaction profiles
How does the reaction profile of an exothermic reaction show the energy change? [1 mark]
Answer:The products are at a lower energy level than the reactants.Question 12 · Spec 4.5.1.2 · Reaction profiles
How does the reaction profile of an endothermic reaction show the energy change? [1 mark]
Answer:The products are at a higher energy level than the reactants.Question 13 · Spec 4.5.1.2 · Reaction profiles
What does the hump on a reaction profile show? [1 mark]
Answer:The activation energy.Question 14 · Spec 4.5.1.3 · Bond energies · Higher only
Explain in terms of bonds why a reaction is exothermic. [2 marks]
Answer:Energy is released when new bonds form in the products (1); more energy is released than is needed to break bonds in the reactants (1).Question 15 · Spec 4.5.1.3 · Bond energies · Higher only
Is bond breaking exothermic or endothermic? [1 mark]
Answer:Endothermic.Question 16 · Spec 4.5.1.3 · Bond energies · Higher only
Calculate the energy change for H₂ + Cl₂ → 2HCl. Bond energies: H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol. [3 marks]
Answer:Bonds broken = 436 + 242 = 678 kJ (1); bonds made = 2 × 431 = 862 kJ (1); energy change = 678 − 862 = −184 kJ/mol (1).Question 17 · Spec 4.5.1.3 · Bond energies · Higher only
What does a negative overall energy change mean? [1 mark]
Answer:The reaction is exothermic.Question 18 · Spec 4.5.1.3 · Bond energies · Higher only
Calculate the energy change for 2H₂ + O₂ → 2H₂O. Bond energies: H–H 436, O=O 498, O–H 464 kJ/mol. [3 marks]
Answer:Broken: (2 × 436) + 498 = 1370 kJ (1); made: 4 × 464 = 1856 kJ (1); change = 1370 − 1856 = −486 kJ/mol (1).Question 19 · Spec 4.5.2.1 · Cells
How does a simple chemical cell produce electricity? [2 marks]
Answer:Two different metals are placed in an electrolyte (1); the chemical reactions produce a potential difference between them (1).Question 20 · Spec 4.5.2.1 · Cells
How can the voltage of a simple cell be increased? [1 mark]
Answer:Use two metals that are further apart in the reactivity series.Question 21 · Spec 4.5.2.1 · Batteries
What is a battery? [1 mark]
Answer:Two or more cells connected in series to give a greater voltage.Question 22 · Spec 4.5.2.1 · Non-rechargeable cells
Why do non-rechargeable cells stop producing electricity? [1 mark]
Answer:One of the reactants is used up.Question 23 · Spec 4.5.2.1 · Rechargeable cells
How can a rechargeable cell be recharged? [1 mark]
Answer:An external electrical current reverses the chemical reactions.Question 24 · Spec 4.5.2.2 · Fuel cells
What is a hydrogen fuel cell? [2 marks]
Answer:A cell supplied with hydrogen and oxygen (1); which react to produce electricity, with water as the only product (1).Question 25 · Spec 4.5.2.2 · Fuel cells
State two advantages of hydrogen fuel cells over rechargeable batteries. [2 marks]
Answer:Any two from: they do not need to be recharged (1); they only produce water (1); they do not wear out in the same way (1).Question 26 · Spec 4.5.2.2 · Fuel cells
State two disadvantages of hydrogen fuel cells. [2 marks]
Answer:Any two from: hydrogen is a flammable gas that is hard to store (1); hydrogen is often made using fossil fuels (1); fuel cells are expensive (1).Question 27 · Spec 4.5.2.2 · Fuel cells · Higher only
Write the half equation at the cathode of a hydrogen fuel cell (acid electrolyte). [1 mark]
Answer:O₂ + 4H⁺ + 4e⁻ → 2H₂O.Question 28 · Spec 4.5.2.2 · Fuel cells · Higher only
Write the half equation at the anode of a hydrogen fuel cell (acid electrolyte). [1 mark]
Answer:2H₂ → 4H⁺ + 4e⁻.Question 29 · Spec 4.5.1.1 · Temperature change
The temperature drops from 21 °C to 15 °C. Is the reaction exothermic or endothermic? [1 mark]
Answer:Endothermic.Question 30 · Spec 4.5.1.1 · Temperature change
What happens to the temperature of the surroundings in an exothermic reaction? [1 mark]
Answer:It increases.Question 31 · Spec 4.5.1.2 · Catalysts
How does a catalyst affect the reaction profile? [1 mark]
Answer:It lowers the activation energy.Question 32 · Spec 4.5.1.3 · Bond energies · Higher only
What is a bond energy? [1 mark]
Answer:The energy needed to break one mole of a particular bond.Question 33 · Spec 4.5.1.1 · Required practical 4
Name the independent and dependent variables in the neutralisation temperature practical. [2 marks]
Answer:Independent: volume of alkali added (1); dependent: temperature change (1).Question 34 · Spec 4.5.1.1 · Required practical 4
Why is a lid placed on the cup? [1 mark]
Answer:To reduce energy lost to the air.Question 35 · Spec 4.5.1.1 · Thermal decomposition
Why is thermal decomposition endothermic? [1 mark]
Answer:Energy must be supplied to break down the compound.Question 36 · Spec 4.5.2.1 · Cells
Why does a cell stop when the electrolyte is replaced by water? [1 mark]
Answer:Pure water does not contain enough ions to conduct and allow the reactions.Question 37 · Spec 4.5.2.2 · Fuel cells
Write the overall equation for a hydrogen fuel cell. [1 mark]
Answer:2H₂ + O₂ → 2H₂O.Question 38 · Spec 4.5.1.2 · Reaction profiles
Why do reactions need activation energy? [1 mark]
Answer:Particles must collide with enough energy to break bonds before new bonds can form.Question 39 · Spec 4.5.1.1 · Exothermic
Why are hand warmers based on an exothermic reaction? [1 mark]
Answer:They release energy to the surroundings, warming the hands.Question 40 · Spec 4.5.1.1 · Endothermic
Why do cold packs feel cold? [1 mark]
Answer:The reaction inside takes in energy from the surroundings, including the skin.Question 41 · Spec 4.5.1.3 · Bond energies · Higher only
Why is the reaction for burning methane exothermic overall? [1 mark]
Answer:More energy is released when the C=O and O–H bonds form than is needed to break the C–H and O=O bonds.Question 42 · Spec 4.5.2.1 · Cells
Give one disadvantage of non-rechargeable batteries. [1 mark]
Answer:They must be thrown away when used up, creating waste.Question 43 · Spec 4.5.2.2 · Fuel cells
Why are hydrogen fuel cells described as 'clean'? [1 mark]
Answer:The only product is water, so no carbon dioxide is released when they are used.Question 44 · Spec 4.5.1.1 · Combustion
Write the word equation for the complete combustion of methane. [1 mark]
Answer:Methane + oxygen → carbon dioxide + water.Question 45 · Spec 4.5.1.2 · Reaction profiles
What does the difference in height between reactants and products show on a profile? [1 mark]
Answer:The overall energy change of the reaction.Question 46 · Spec 4.5.1.1 · Neutralisation
Is neutralisation exothermic or endothermic? [1 mark]
Answer:Exothermic.Question 47 · Spec 4.5.2.1 · Cells
What is an electrolyte? [1 mark]
Answer:A liquid or solution containing ions that can conduct electricity.Question 48 · Spec 4.5.1.3 · Bond energies · Higher only
Is bond making exothermic or endothermic? [1 mark]
Answer:Exothermic.Question 49 · Spec 4.5.1.1 · Temperature change
The temperature rises from 18.5 °C to 26.0 °C. Calculate the temperature change and state the type of reaction. [2 marks]
Answer:Temperature change = 7.5 °C (1); the temperature rose, so the reaction is exothermic (1).Question 50 · Spec 4.5.1.1 · Required practical 4
Explain why the temperature stops rising once neutralisation is complete. [2 marks]
Answer:All the acid has reacted, so no more energy is released (1); adding more alkali at room temperature only cools the mixture (1).