OCR GCSE Chemistry A: Energetics (J248) – Revision Notes

Here is the whole of sub-topic C3.2 Energetics, cut down for last-minute recall, from the OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification (Version 4.0, August 2026). The notes cover learning outcomes C3.2a–C3.2d, mathematical learning outcomes CM3.2i and CM3.2ii, and the temperature-change practical in PAG C8. C3.2a, C3.2b and C3.2c are assessed at both tiers; C3.2d (bond energy calculations) is Higher Tier only. For full explanations and longer worked examples, read the Energetics study guide first.

When you are ready for exam-style problems, use the Energetics practice questions. The OCR GCSE Chemistry hub links every J248 topic, and the printable checklist lets you track which statements you have revised. A free 10-minute diagnostic is a quick way to find out what to revise next.

Core definitions

Term	Meaning
Surroundings	Everything outside the reacting chemicals: solvent, container, thermometer, air
Exothermic	Energy transferred from the chemicals to the surroundings; surroundings’ temperature rises
Endothermic	Energy transferred from the surroundings to the chemicals; surroundings’ temperature falls
Reaction profile	Graph of energy against progress of reaction, from reactants to products
Activation energy	The energy needed for a reaction to occur (the minimum energy colliding particles need)
Energy change	Difference in energy between products and reactants
Bond energy (Higher Tier only)	Energy, in kJ/mol, taken in to break one mole of a bond, and released when one mole of it forms

Key idea from the specification: energy is transferred, never lost or used up.

C3.2a: classify from temperature change

Method in steps

1. Temperature change = final temperature − starting temperature.
2. Positive (rise) → surroundings gained energy → exothermic.
3. Negative (fall) → surroundings lost energy to the chemicals → endothermic.
4. No change → no measurable energy transfer (or no reaction).

Typical examples

Exothermic	Endothermic
Combustion of fuels	Thermal decomposition of a carbonate
Neutralisation (acid + alkali)	Hydrogencarbonate + citric or ethanoic acid
Many metals + dilute acids	Reactions that need continuous heating to keep going

PAG C8: measuring the temperature change

• Polystyrene cup, inside a beaker for stability, with a lid.
• Fixed volume of solution measured with a measuring cylinder.
• Thermometer or temperature probe; record the starting temperature, then the highest (or lowest) after mixing and stirring.
• Repeat, discard anomalous results, take the mean.
• Systematic error: heat transfer to or from the room makes every measured change too small. Insulation and a lid reduce it.
• Random error: misreading the scale, missing the peak. Repeats and a mean reduce its effect.
• Uncertainty estimate: ± half the range of the repeats that are kept.
• Precise = repeats close together. Repeatable = same person, same method, similar results. Reproducible = another person or method gives similar results.

C3.2b and CM3.2i: reaction profiles

Must label: reactants, products, activation energy, energy change.

Feature	Exothermic	Endothermic
Products vs reactants	Products lower	Products higher
Energy change arrow	Points down from reactants to products	Points up from reactants to products
Sign of energy change	Negative	Positive
Activation energy arrow	Starts at the reactants, ends at the peak	Same: reactants to peak

Reading a profile (M4a)

• Activation energy = peak level − reactants level
• Energy change = products level − reactants level

Small reminder: reactants 140 kJ/mol, peak 200 kJ/mol, products 90 kJ/mol. Activation energy = 200 − 140 = 60 kJ/mol. Energy change = 90 − 140 = −50 kJ/mol, so exothermic.

C3.2c: activation energy

• It is the energy needed for a reaction to occur: colliding particles need at least this energy for bonds to begin breaking.
• A spark or flame supplies the activation energy to start many exothermic reactions. Once started, the energy they release keeps them going.
• Needing a flame to start does not make a reaction endothermic. The overall energy change decides that.
• Higher peak on the profile = larger activation energy.

C3.2d and CM3.2ii: bond energy calculations (Higher Tier only)

Rules

• Breaking bonds takes in energy (endothermic step).
• Making bonds releases energy (exothermic step).
• Energy change = bonds broken − bonds made.
• Negative → exothermic. Positive → endothermic.

Method in steps

1. Draw displayed formulae so every bond is visible.
2. Multiply each bond energy by the number of that bond, using the equation’s numbers.
3. Total for bonds broken; total for bonds made.
4. Broken − made, with a sign and the unit kJ/mol.

Worked reminder: decomposition of hydrogen peroxide

2H₂O₂ → 2H₂O + O₂

Given (kJ/mol): O–H 464, O–O 146, O=O 498.

Each H–O–O–H has two O–H bonds and one O–O bond. Each water has two O–H bonds.

• Broken: 4 × O–H + 2 × O–O = 1856 + 292 = 2148 kJ
• Made: 4 × O–H + 1 × O=O = 1856 + 498 = 2354 kJ
• Energy change = 2148 − 2354 = −206 kJ/mol, exothermic

Shortcut: the four O–H bonds appear on both sides, so they cancel. Bonds that change: 292 broken, 498 made, giving −206 kJ/mol again. Either approach is fine, as long as you treat both sides the same way.

Why calculated values differ from measured ones: bond energies are average values from many compounds, so they only give an estimate.

Must-know distinctions

• Temperature of the surroundings vs energy of the chemicals. In an endothermic reaction the thermometer falls because the chemicals are gaining energy.
• Activation energy vs energy change. Activation energy goes from reactants to the peak; energy change goes from reactants to products.
• Bond breaking vs bond making. Breaking takes energy in; making gives energy out. Never the reverse.
• Broken − made (bond energies) vs products − reactants (profile levels). Both give the same sign convention, but use the right one for the data you have.
• Exothermic vs needing heat to start. A candle is exothermic even though it needs a match.

Quick self-test

1. The temperature of a solution goes from 21.0 °C to 14.5 °C during a reaction. Give the temperature change and classify the reaction.
2. State what is meant by activation energy.
3. A profile has reactants at 85 kJ/mol, the peak at 230 kJ/mol and products at 150 kJ/mol. Find the activation energy and the energy change, and classify the reaction.
4. A student says burning a candle is endothermic because you need a match to light it. Explain why the student is wrong.
5. On an exothermic profile, which way does the energy change arrow point, and where does it start?
6. Why is a lid put on the polystyrene cup when measuring a temperature change?
7. Repeat temperature rises are 4.1, 4.7 and 4.4 °C. Calculate the mean.
8. Is bond breaking exothermic or endothermic? (Higher Tier only)
9. Bonds broken total 1850 kJ and bonds made total 1720 kJ. Find the energy change and classify the reaction. (Higher Tier only)
10. Calculate the energy change for 2CO + O₂ → 2CO₂. Use C≡O 1077, O=O 498, C=O 805 kJ/mol. (Higher Tier only)
11. Calculate the energy change for CH₂=CH₂ + H–O–H → CH₃CH₂OH. Use C=C 612, O–H 464, C–C 348, C–H 413, C–O 358 kJ/mol. (Higher Tier only)

Answers

1. 14.5 − 21.0 = −6.5 °C. The surroundings cooled, so the reaction is endothermic.
2. The energy needed for a reaction to occur.
3. Activation energy = 230 − 85 = 145 kJ/mol. Energy change = 150 − 85 = +65 kJ/mol, so endothermic.
4. The match only supplies the activation energy. Overall, more energy is released to the surroundings than is taken in, so burning is exothermic.
5. Downwards, starting at the reactants line and ending at the products line.
6. To reduce heat transfer to or from the air, which would make the measured change too small.
7. (4.1 + 4.7 + 4.4) ÷ 3 = 13.2 ÷ 3 = 4.4 °C.
8. Endothermic: energy is taken in to break a bond.
9. 1850 − 1720 = +130 kJ/mol, endothermic.
10. Broken: 2 × 1077 + 498 = 2652 kJ. Made: 4 × 805 = 3220 kJ. Energy change = 2652 − 3220 = −568 kJ/mol, exothermic.
11. Bonds that change. Broken: C=C + one O–H = 612 + 464 = 1076 kJ. Made: C–C + one C–H + C–O = 348 + 413 + 358 = 1119 kJ. Energy change = 1076 − 1119 = −43 kJ/mol, exothermic.

Where marks are usually lost

• Saying an endothermic reaction “gives out cold”. The correct description is energy moving from the surroundings into the chemicals.
• Classifying from the size of the temperature change instead of its direction.
• Drawing the activation energy arrow from the products, the axis, or only halfway to the peak.
• Leaving labels off the profile: all four of reactants, products, activation energy and energy change are expected.
• Putting products above reactants on an exothermic profile (or below on an endothermic one).
• Stating that energy is released when bonds break.
• Missing multipliers from the equation, such as the 2 in 2CO or counting one C=O in CO₂.
• Subtracting the wrong way round (made − broken) and getting the sign flipped.
• Giving a final answer with no sign or no unit.
• Including an anomalous repeat in the mean.

Official syllabus

OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification, Version 4.0 (August 2026), for first assessment in 2018, published by OCR. Sub-topic C3.2 Energetics, learning outcomes C3.2a–C3.2d, mathematical learning outcomes CM3.2i–CM3.2ii and the linked practical suggestion (PAG C8).