4.5 Energy changes — notes on one page

Every AQA GCSE Chemistry 8462 specification point in 4.5, read straight through: 5 points with definitions, explanations, mark-scheme wording and worked examples. Practice questions are in the printable question pack.

Checked against AQA 8462 section 4.5. Review basis: the qualification registry sourced from the AQA GCSE Chemistry (8462) specification; registry verification recorded 17 July 2026.

How this checking works

4.5.1.1 · Energy transfer during exothermic and endothermic reactions

Definition. An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases. An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.

Energy is conserved in chemical reactions: the energy in the universe after a reaction equals the energy before it. If a reaction transfers energy to the surroundings, the products must store less energy than the reactants, by exactly the amount transferred.
Exothermic reactions include combustion, many oxidation reactions and neutralisation, and are used in self-heating cans and hand warmers.
Endothermic reactions include thermal decompositions and the reaction of citric acid with sodium hydrogencarbonate; some sports injury packs work this way.
At GCSE this point is limited to measuring a temperature change — calculating delta H is not required.
Required practical 4 investigates the variables affecting temperature changes in reacting solutions, and carries recurring marks: a polystyrene cup is used rather than a glass beaker because polystyrene insulates better, so less energy reaches the surroundings; a lid reduces losses further; and consistent energy loss is a systematic rather than random error, because it shifts every result the same way.

An exothermic reaction transfers energy out to the surroundings, so their temperature rises; an endothermic reaction takes energy in, so their temperature falls.

Worked example

A student compares two neutralisation reactions. For each test, the student uses the same cup and the same total volume of solution. Give three other features of the method that should be kept the same or carried out consistently so the temperature changes can be compared fairly.

1.Choose controls that could otherwise alter the measured temperature change: the initial temperature, solution concentrations, mixing or stirring, and the measuring instrument or rule for selecting the maximum temperature. Any three valid consistent features score.

Answer: Use solutions with the same starting temperature and stated concentrations, stir in the same way, and record the maximum temperature using the same thermometer or temperature probe.

Mark-scheme wording to use

“energy is taken in from the surroundings, so the reaction is endothermic” — AQA 8462, June 2024 Paper 1F, Q03.8
“polystyrene is a better thermal insulator than glass. 1 mark: less energy is transferred/lost to the surroundings. Allow heat for energy” — AQA 8462, June 2024 Paper 1H, Q04.3
“temperature decreased. Ignore correct references to energy transfer” — AQA 8462, June 2023 Paper 1H, Q02.4

What the examiner wants. The temperature of the SURROUNDINGS, stated in the direction the data shows. When the question says 'how do the results show', quote the temperature change itself — an energy-transfer sentence with no temperature is ignored.

Common mistakes

Don't fall into the trap of describing the reacting chemicals, rather than the surroundings, as getting hotter or colder.
Don't fall into the trap of using the final temperature alone instead of calculating and comparing the temperature change.

Exam tip. For required practical 4, state how temperature change is measured and identify the quantities and apparatus kept constant.

4.5.1.2 · Reaction profiles

A reaction profile plots energy against progress of reaction; the curved line rises to a maximum before falling or rising to the products' energy level. Activation energy is the minimum energy that colliding particles must have for a reaction to occur, shown from the reactants' energy level to the peak.
Products below reactants indicate an exothermic reaction and a negative overall energy change; products above reactants indicate an endothermic reaction and a positive change.
A common error is to draw activation energy from the vertical axis or from the products.
For the forward reaction, measure it from the reactants' level to the peak.
The sign of the overall energy change follows products minus reactants, whereas the activation-energy arrow always rises to the peak.

An exothermic reaction profile with products below reactants and activation energy measured to the peak.

Worked example

A reaction profile places the reactants at $74\,\text{kJ mol}^{-1}$ , the peak at $139\,\text{kJ mol}^{-1}$ and the products at $102\,\text{kJ mol}^{-1}$ . Calculate the activation energy and the overall energy change, then identify whether the reaction is exothermic or endothermic.

1.The forward activation energy is peak minus reactants: $139-74=65\,\text{kJ mol}^{-1}$ . The overall change is products minus reactants: $102-74=+28\,\text{kJ mol}^{-1}$ . The positive change, with products above reactants, identifies an endothermic reaction.

Answer: Activation energy $=65\,\text{kJ mol}^{-1}$ ; overall energy change $=+28\,\text{kJ mol}^{-1}$ ; the reaction is endothermic.

Common mistakes

Don't fall into the trap of drawing activation energy from the vertical axis or from the products instead of from the reactants' level to the peak.
Don't fall into the trap of reversing exothermic and endothermic profiles by ignoring the relative product and reactant energy levels.

Exam tip. Label reactants, products, activation energy and overall energy change; activation energy starts at the reactant level.

4.5.1.3 · The energy change of reactions (HT only)

Higher tier: breaking bonds in reactants requires energy, whereas forming bonds in products releases energy. Calculate the overall energy change using $\Delta E=\sum E(\text{bonds broken})-\sum E(\text{bonds formed})$ and include every bond shown by the balanced equation.
If bond formation releases more energy than bond breaking requires, $\Delta E$ is negative and the reaction is exothermic; the reverse balance gives a positive, endothermic change.
A common error is to reverse the subtraction or count molecules instead of bonds.
Multiply each bond energy by the number of that bond broken or formed.
Higher tier: a negative result means more energy is released during bond formation than is supplied for bond breaking.

Worked example

Higher tier: use bond energies H-H $=436$ , Cl-Cl $=243$ and H-Cl $=431\,\text{kJ mol}^{-1}$ to calculate the energy change for $\mathrm{H_2+Cl_2\rightarrow2HCl}$ .

1.Bonds broken: one H-H and one Cl-Cl, so $436+243=679\,\text{kJ mol}^{-1}$ .
2.Bonds formed: two H-Cl, so $2\times431=862\,\text{kJ mol}^{-1}$ .
3.Calculate $\Delta E=679-862=-183\,\text{kJ mol}^{-1}$ .

Answer: $-183\,\text{kJ mol}^{-1}$ ; the reaction is exothermic.

Common mistakes

Don't fall into the trap of reversing the subtraction or counting molecules instead of every bond broken and formed.
Don't fall into the trap of using the number of molecules as the bond count without multiplying by every bond within each molecule.

Exam tip. Higher tier: tabulate every bond broken and formed, total each column, then calculate broken minus formed with the sign.

4.5.2.1 · Cells and batteries (chemistry only)

A simple cell uses two different metals in contact with an electrolyte; chemical reactions transfer energy electrically and produce a potential difference. The voltage depends on the electrode materials and the electrolyte.
Data about relative metal reactivity can be used to compare or predict cell voltages. Cells connected in series have their voltages added; a battery contains two or more cells connected together in series to provide a greater voltage.
Non-rechargeable cells stop when a reactant is used up.
Rechargeable cells use an external current to reverse the reactions; a common error is to claim that recharging creates new reactants from nothing.
A larger reactivity difference between suitable electrodes generally produces a larger potential difference, but the electrolyte also matters.

Worked example

A cell made from zinc and copper produces $1.08\,\text{V}$ . Three identical cells are connected in series. Calculate the battery voltage and explain why copper-copper electrodes would not make the same cell.

1.Series cell voltages add, so $V=3\times1.08=3.24\,\text{V}$ . A simple cell requires different electrode materials; using copper for both removes that difference and would not reproduce the zinc-copper potential difference.

Answer: $3.24\,\text{V}$ ; identical copper electrodes do not provide the required difference between electrode materials.

Common mistakes

Don't fall into the trap of claiming that recharging creates new reactants from nothing instead of reversing the chemical reactions.
Don't fall into the trap of assuming the more reactive metal must always be labelled positive without using the stated cell arrangement or data.

Exam tip. For a series battery, add cell voltages with their directions; for evaluation, link lifetime and recharging to the chemical reactions.

4.5.2.2 · Fuel cells (chemistry only)

A fuel cell receives a continuous external supply of fuel and oxygen or air; the fuel is oxidised electrochemically to produce a potential difference. In a hydrogen fuel cell, hydrogen is oxidised and the overall reaction forms water: $\mathrm{2H_2+O_2\rightarrow2H_2O}$ .
Hydrogen fuel cells can operate while reactants are supplied, whereas rechargeable cells store reactants and must be recharged; comparisons should include storage, refuelling, lifetime and environmental effects. Higher tier: alkaline-cell half-equations may be written $\mathrm{2H_2+4OH^-\rightarrow4H_2O+4e^-}$ and $\mathrm{O_2+2H_2O+4e^-\rightarrow4OH^-}$ ; atoms and charge must balance.
Do not call hydrogen automatically pollution-free without considering its production.
Higher tier: the electrode half-equations must cancel electrons and hydroxide ions to give the overall formation of water.
Higher tier: AQA restricts "write the half equations for the electrode reactions in the hydrogen fuel cell" to Higher tier; evaluating fuel cells against rechargeable cells is examined on both tiers.

Worked example

Give two differences between a hydrogen fuel cell and a rechargeable cell when each is used to power a vehicle.

1.Make paired comparisons rather than listing isolated facts. Contrast external fuel supply and refuelling with stored reactants and electrical recharging, then give a valid consequence such as operating emissions, infrastructure, mass or recharge time.

Answer: A fuel cell needs continuing supplies of hydrogen and oxygen and can be refuelled, whereas a rechargeable cell stores its reactants and needs an external current to reverse its reactions. The fuel cell produces water during use, but the overall environmental impact depends on hydrogen production and storage.

Common mistakes

Don't fall into the trap of calling hydrogen automatically pollution-free without considering the source of hydrogen and energy used in its production.
Don't fall into the trap of treating a fuel cell as a rechargeable battery that stores a fixed quantity of reactants inside.

Exam tip. Higher tier: balance atoms and total charge in each alkaline hydrogen-fuel-cell half-equation before combining them.