Chemistry 0620 · syllabus 5, 6.1
Cambridge IGCSE Chemistry 0620 — Chemical energetics topical past papers
Physical and chemical changes, exothermic and endothermic reactions, reaction pathway diagrams, enthalpy change, activation energy and bond energy calculations.
- Questions
- 651
- Years
- 2009–2026
- With mark scheme
- 98%
- Questions 2022–2026
- 238
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Chemical energetics covers
A chemical change makes new substances; a physical change, such as melting, boiling or dissolving, makes none and is usually easy to reverse. Most chemical reactions also transfer thermal energy. An exothermic reaction gives out thermal energy to the surroundings, so they get warmer, as when a fuel burns or an acid is neutralised; an endothermic reaction takes thermal energy in, so the surroundings cool, and thermal decomposition, which needs continuous heating, is endothermic too. Some physical changes transfer energy too — dissolving certain salts makes the water colder — so a temperature change alone does not prove a chemical change.
A reaction pathway diagram shows the energy of the reactants and of the products and the route between them: for an exothermic reaction the products finish lower than the reactants, for an endothermic one higher. On the Extended route the energy transferred is called the enthalpy change, ΔH, negative for exothermic and positive for endothermic reactions, and the activation energy, Ea, is the least energy colliding particles must have to react; you draw and label both on a diagram from information you are given.
Bond energies explain where ΔH comes from. Breaking bonds takes energy in and making bonds gives energy out, so a reaction is exothermic when the bonds formed in the products release more energy than is needed to break the bonds in the reactants. Extended candidates calculate ΔH as the energy taken in breaking bonds minus the energy given out making them. Temperature changes during reactions are also a regular context in the practical papers.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 5 Chemical energetics
- 5.1 Exothermic and endothermic reactions
- 6.1 Physical and chemical changes
What you should be able to do
- Identify physical and chemical changes and describe the differences between them
- Classify reactions as exothermic or endothermic from temperature changes, with examples of each
- Interpret reaction pathway diagrams, and on the Extended route draw and label them with reactants, products, ΔH and Ea
- State the sign of ΔH for exothermic and endothermic reactions (Extended)
- Explain an enthalpy change in terms of the bonds broken and the bonds made (Extended)
- Calculate ΔH from bond energies, counting every bond in the balanced equation (Extended)
Key ideas
- Chemical change: new substances form; physical change: no new substance, as in changes of state and dissolving
- Exothermic: thermal energy given out to the surroundings, temperature rises, ΔH negative
- Endothermic: thermal energy taken in from the surroundings, temperature falls, ΔH positive
- Activation energy, Ea: the minimum energy colliding particles need to react, shown from the reactants up to the top of the curve
- ΔH = energy taken in breaking bonds − energy given out making bonds, in kJ/mol
How Chemical energetics has been examined
From 2022 to 2026, Chemical energetics was on Paper 4 · Theory (Extended) in 13 of 14 exam series, worth about 18% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 81 | 6% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 89 | 7% | 14 of 14 |
| Paper 3 · Theory (Core) | 21 | 8% | 9 of 14 |
| Paper 4 · Theory (Extended) | 32 | 18% | 13 of 14 |
| Paper 6 · Alternative to Practical | 15 | 17% | 9 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Energy Changes and Reversible Reactions.
A worked example
Ethene reacts with hydrogen to form ethane: CH₂=CH₂ + H₂ → CH₃CH₃. Using the bond energies C=C 612, C–C 347, C–H 413 and H–H 436 kJ/mol, calculate the enthalpy change and say whether the reaction is exothermic or endothermic.
- Bonds broken: one C=C, four C–H and one H–H = 612 + (4 × 413) + 436 = 612 + 1652 + 436 = 2700 kJ
- Bonds made: one C–C and six C–H = 347 + (6 × 413) = 347 + 2478 = 2825 kJ
- ΔH = energy taken in − energy given out = 2700 − 2825 = −125 kJ/mol
- Check with only the bonds that change: broken C=C + H–H = 1048 kJ; made C–C + two C–H = 1173 kJ; 1048 − 1173 = −125 kJ/mol
- ΔH is negative because more energy is given out making bonds than is taken in breaking them
Answer: ΔH = −125 kJ/mol; the reaction is exothermic
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Giving an exothermic reaction a positive ΔH — energy leaves the reacting chemicals, so ΔH is negative even though the thermometer reading goes up
- Saying that breaking bonds releases energy — breaking bonds always takes energy in; it is making bonds that gives energy out
- Counting each type of bond only once in a bond energy calculation — CH₄ contains four C–H bonds and 2O₂ contains two O=O bonds, so multiply by the numbers in the balanced equation
- Subtracting the wrong way round — ΔH is bonds broken minus bonds made; reversing it flips the sign and turns an exothermic reaction into an endothermic one
- Measuring ΔH from the peak of a pathway diagram — ΔH is the gap between reactants and products, while the rise from the reactants to the peak is Ea
Exam technique
- In bond energy questions, list every bond in each molecule, total the bonds broken and the bonds made on separate lines, then subtract
- Always give ΔH a sign and units, such as −125 kJ/mol, and state whether that makes the reaction exothermic or endothermic
- On a pathway diagram, label the energy axis, put the reactants and products at the right levels, and draw ΔH and Ea as arrows between the correct points
- When explaining an energy change, compare the two totals: 'more energy is given out making the new bonds than is taken in breaking the old ones' explains an exothermic reaction
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.