AQA GCSE Chemistry 8462: Energy changes – Revision Notes

These notes condense section 4.5 Energy changes of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards and exams in 2018 onwards (version 1.1). They cover spec points 4.5.1.1 to 4.5.2.2 and required practical activity 4. The topic is examined on Paper 1 at Foundation and Higher Tier; Higher tier only content is labelled. For full explanations and worked examples, use the Energy changes study guide.

Test yourself afterwards with the Energy changes practice questions. The course hub is AQA GCSE Chemistry, the printable checklist lists every spec point, and the free 10-minute diagnostics show where to focus.

Key definitions

Term	Definition to learn
Conservation of energy	The energy in the universe is the same after a reaction as before it
Exothermic reaction	Transfers energy to the surroundings, so the temperature of the surroundings increases
Endothermic reaction	Takes in energy from the surroundings, so the temperature of the surroundings decreases
Activation energy	The minimum amount of energy that particles must have to react
Reaction profile	Diagram showing relative energies of reactants and products, the activation energy and the overall energy change
Cell	Contains chemicals which react to produce electricity
Battery	Two or more cells connected in series
Fuel cell	Supplied with fuel (for example hydrogen) and oxygen or air; the fuel is oxidised electrochemically to produce a potential difference

4.5.1.1 Exothermic and endothermic – the facts

• If a reaction transfers energy to the surroundings, the products have less energy than the reactants, by the amount transferred.
• Exothermic examples: combustion, many oxidation reactions, neutralisation. Uses: self-heating cans, hand warmers.
• Endothermic examples: thermal decompositions, citric acid + sodium hydrogencarbonate. Use: some sports injury packs.
• Classify only by the temperature change of the surroundings.
• Calculating energy changes or ΔH from temperature data is not required. You measure temperature change.

Required practical 4 – method in steps

Aim: investigate a variable that affects the temperature change in a reacting solution (acid + metal, acid + carbonate, neutralisation or metal displacement).

1. Polystyrene cup inside a beaker; measure a fixed volume of solution with a measuring cylinder.
2. Record the starting temperature.
3. Add the other reactant, stir, fit a lid.
4. Record the highest (or lowest) temperature.
5. Temperature change = final − initial. Repeat, spot anomalies, take a mean.

Variable type	Example
Independent	Mass of metal powder, concentration of acid, type of metal
Dependent	Temperature change
Control	Volume of solution, starting temperature, type of cup, stirring

Improving accuracy: use a lid, insulate the cup, stir, use a thermometer or probe with a finer scale.

4.5.1.2 Reaction profiles

Feature	Exothermic	Endothermic
Products compared with reactants	Lower	Higher
Overall energy change arrow	Points down	Points up
Activation energy arrow	From reactants up to the peak	From reactants up to the peak

Draw: two labelled horizontal levels, a curved line joining them over a hump, the axes labelled “energy” and “progress of reaction”.

Worked reminder. Reactants 80 kJ, peak 250 kJ, products 150 kJ. Activation energy = 250 − 80 = 170 kJ. Overall change = 150 − 80 = +70 kJ, so endothermic.

4.5.1.3 Bond energies (Higher tier only)

Process	Energy
Breaking bonds in reactants	Energy supplied (taken in)
Forming bonds in products	Energy released
Exothermic overall	Energy released forming bonds > energy needed to break bonds
Endothermic overall	Energy needed to break bonds > energy released forming bonds

Method in steps

1. Balanced equation; draw every bond.
2. Bonds broken = Σ (number × bond energy) for reactants.
3. Bonds formed = Σ (number × bond energy) for products.
4. Energy change = broken − formed, in kJ/mol. Negative = exothermic; positive = endothermic.

Worked reminder. N₂ + 3H₂ → 2NH₃. Bond energies (kJ/mol): N≡N 945, H–H 436, N–H 391.

broken: 945 + (3 × 436)   = 2253 kJ/mol
formed: 2 × 3 × 391       = 2346 kJ/mol   (each NH₃ has three N–H bonds)
change: 2253 − 2346       = −93 kJ/mol   → exothermic

4.5.2.1 Cells and batteries

• Simple cell: two different metals in contact with an electrolyte.
• Voltage depends on factors including the type of electrode and the electrolyte.
• The bigger the difference in reactivity between the two metals, the bigger the voltage.
• Batteries: cells in series add their voltages. Four 1.2 V cells → 4 × 1.2 = 4.8 V.
• Non-rechargeable (for example alkaline batteries): reactions stop when one reactant is used up.
• Rechargeable: reactions are reversed when an external electrical current is supplied.

4.5.2.2 Fuel cells

• Hydrogen fuel cell: hydrogen is oxidised to water. Overall: 2H₂ + O₂ → 2H₂O.
• Potential alternative to rechargeable cells and batteries.

For hydrogen fuel cells	Against hydrogen fuel cells
Only product is water	Hydrogen is a gas – hard to store, needs large or pressurised tanks
No need to recharge; works while fuel is supplied	Hydrogen is flammable and can be explosive
No limit from charge cycles	Making hydrogen often uses fossil fuels or electricity
Less toxic waste on disposal	Needs a supply network for hydrogen

Half equations (Higher tier only), acidic electrolyte:

negative electrode:  2H₂ → 4H⁺ + 4e⁻          (oxidation)
positive electrode:  O₂ + 4H⁺ + 4e⁻ → 2H₂O     (reduction)

Alkaline electrolyte: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻ and O₂ + 2H₂O + 4e⁻ → 4OH⁻.

Reading cell data – method in steps

1. Find the voltage of each metal paired with the same reference metal.
2. Rank the metals: the bigger the voltage, the more reactive the metal.
3. To predict a cell made from two of those metals, subtract the smaller voltage from the larger one.
4. State the answer as approximate: the real value also depends on the electrolyte.

Worked reminder. Paired with silver, nickel gives 1.0 V and lead gives 0.9 V. Nickel is more reactive than lead, and a nickel–lead cell gives about 1.0 − 0.9 = 0.1 V – a small voltage because the two metals are close in reactivity.

Evaluating uses – checklist

When a question gives you data about a product and asks you to evaluate it, cover:

• size of the temperature change and how long it lasts
• single use or reusable
• safety of the chemicals if the pack splits
• cost, convenience and waste
• a final judgement for the stated use

Must-know distinctions

• Exothermic vs endothermic: temperature of surroundings up vs down. Not “hot vs cold reactants”.
• Activation energy vs overall energy change: activation energy is reactants to peak; overall change is reactants to products.
• Cell vs battery: one cell vs two or more cells in series.
• Non-rechargeable vs rechargeable: reactants used up vs reactions reversed by an external current.
• Fuel cell vs battery: fuel supplied from outside vs chemicals stored inside.
• Bond breaking vs bond forming: energy in vs energy out.

Quick self-test

1. The temperature of a solution falls from 19.5 °C to 12.0 °C. Give the temperature change and the type of reaction.
2. Name two types of exothermic reaction from the specification.
3. Give one everyday use of an endothermic reaction.
4. Define activation energy.
5. On a profile, which arrow runs from the reactant level to the top of the curve?
6. Temperature rises in three repeats are 3.4 °C, 3.8 °C and 3.6 °C. Calculate the mean.
7. (Higher tier only) Is bond breaking exothermic or endothermic?
8. (Higher tier only) Bonds broken = 1200 kJ/mol, bonds formed = 1450 kJ/mol. Give the energy change and type.
9. How many volts do four 1.2 V cells in series give?
10. Why do the reactions stop in an alkaline battery?
11. What is the only product of a hydrogen fuel cell?
12. (Higher tier only) Write the half equation at the negative electrode of a hydrogen fuel cell with an acidic electrolyte.

Answers

1. 12.0 − 19.5 = −7.5 °C; the surroundings got colder, so endothermic.
2. Any two of: combustion, many oxidation reactions, neutralisation.
3. Some sports injury packs.
4. The minimum amount of energy that particles must have to react.
5. The activation energy arrow.
6. (3.4 + 3.8 + 3.6) ÷ 3 = 3.6 °C.
7. Endothermic – energy must be supplied.
8. 1200 − 1450 = −250 kJ/mol; exothermic.
9. 4.8 V.
10. One of the reactants has been used up.
11. Water.
12. 2H₂ → 4H⁺ + 4e⁻.

Where marks are usually lost

• Writing “the reaction gets hot” without linking it to the temperature of the surroundings increasing.
• Using ΔH = mcΔT calculations that the 8462 spec does not ask for, instead of reporting the temperature change.
• Activation energy arrows drawn from the products, or double-headed and floating.
• Missing labels on a reaction profile: both levels, both arrows and the axes are all needed.
• Forgetting that each molecule may contain several identical bonds (CH₄ has four C–H; H₂O has two O–H).
• Leaving off the minus sign or the unit kJ/mol on a bond energy answer.
• Evaluations that list points for one side only, or give no conclusion.
• Describing a battery as a single cell, or saying cells are joined “in parallel” to raise the voltage.
• Saying a rechargeable battery is “refilled” rather than that its reactions are reversed by an external current.
• Half equations where the electrons or charges do not balance.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards, exams in 2018 onwards, version 1.1 (4 October 2019), published by AQA – section 4.5 Energy changes.