O-Level and SEC G3 Chemistry K324
C9: Chemical Energetics
Use energy-change ideas, bond-energy reasoning, and reaction profiles with system and sign conventions stated.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Chemical Energetics tracks energy transferred when bonds break and form. K324 uses signs of enthalpy change, energy profile diagrams, and qualitative bond reasoning. The system boundary and direction of energy transfer must be stated before interpreting whether a reaction is exothermic or endothermic.
Exothermic and endothermic change
An exothermic reaction transfers energy from the reacting system to the surroundings, so the surroundings warm and the reaction enthalpy change is negative. An endothermic reaction takes in energy from the surroundings, so the surroundings cool and the reaction enthalpy change is positive. Temperature change is evidence, not the definition by itself.
Examples must be classified from the stated process and data. Combustion and many neutralisations are exothermic, while thermal decomposition can be endothermic. Do not assume that every reaction releasing light or every dissolution has the same energy direction.
Energy profile diagrams
An energy profile shows reactant and product energy levels against reaction progress. Reaction enthalpy is the vertical difference between products and reactants. Activation energy is the minimum energy barrier from the reactant level to the peak. Label axes, levels, arrows, and signs.
For an exothermic profile, products lie below reactants and enthalpy change is negative. For an endothermic profile, products lie above reactants and enthalpy change is positive. A catalyst provides a lower-activation-energy pathway but does not change reactant energy, product energy, or overall enthalpy change.
Bond breaking and bond making
Breaking covalent bonds requires energy and is endothermic. Making covalent bonds releases energy and is exothermic. The overall enthalpy change depends on the balance between energy absorbed to break reactant bonds and energy released when product bonds form.
If more energy is released in bond formation than absorbed in bond breaking, the reaction is exothermic. If more is absorbed than released, it is endothermic. Keep these statements separate from intermolecular forces unless the question concerns a physical change.
Formulae and relationships
| Relationship | Use |
|---|---|
| Determine the sign of reaction enthalpy from energy levels. | |
| Explain overall change qualitatively from bond energies. |
Worked examples
Example 1: Reactants are at 120 kJ mol-1, products at 70 kJ mol-1, and the profile peak at 200 kJ mol-1. Find the enthalpy change and forward activation energy.
- Enthalpy change .
- Forward activation energy .
- The negative enthalpy change means the reaction is exothermic.
Answer: and activation energy .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Distinguish exothermic and endothermic change through system energy transfer and temperature evidence.
- Read reaction profiles for activation energy and overall energy change.
- Use bond breaking and bond forming with the correct energy signs and complete bond inventory.
Official outcome coverage
K324 C9: 4 mapped outcomes, references C9(a), C9(b), C9(c), C9(d). Check the official K324 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Measure temperature change with insulation, a fixed total volume or mass, stirring, and a stated method for reducing heat exchange.
Exam traps and retrieval check
Avoid these traps
- Using the profile peak as the enthalpy change.
- Saying bond breaking releases energy.
- Claiming a catalyst changes the overall enthalpy change.
Check from memory
What sign does an exothermic enthalpy change have?
Negative.
What does activation energy measure?
The minimum energy barrier for the reaction pathway.
When is a bond-based reaction exothermic?
When bond formation releases more energy than bond breaking absorbs.
Pure versus Combined scope
Combined Chemistry uses the same top-level topic label but a narrower outcome set. Use the Combined component checklist before removing or adding detail.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K324. Open the related IP explanation.