Chemical energetics
Exothermic and endothermic reactions, enthalpy change, energy profiles, and bond breaking and bond making for G3 Pure and O-Level Chemistry.
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Before you begin
Chemical reactions involve energy transfers: breaking covalent bonds absorbs energy and making covalent bonds releases energy. The lessons classify exothermic and endothermic changes using Δ H and energy profiles, then explain the overall change by comparing bond breaking with bond making.
Be comfortable with: writing and balancing chemical equations and covalent bonding, because the energy change of a reaction comes from the bonds that break and form.
Learning goals
- Describe enthalpy change, with ∆H negative for exothermic and positive for endothermic reactions
- Draw energy profile diagrams showing the enthalpy change and activation energy of a reaction
- Describe bond breaking as endothermic and bond making as exothermic
- Explain the overall enthalpy change of a reaction from the energy needed to break bonds and released when bonds form
Syllabus statements covered
- describe the meaning of enthalpy change in terms of exothermic (∆H negative) and endothermic (∆H positive) reactions
- represent energy changes by energy profile diagrams, including reaction enthalpy changes and activation energies (see also 10(c), 10(d))
- describe bond breaking as an endothermic process and bond making as an exothermic process
- explain qualitatively overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds.
Lessons
Work through them in order.
Exothermic and endothermic evidence
- Energy changes and the sign of ΔHDefine exothermic and endothermic reactions from the reacting system's point of view and give the sign of ΔH.
- Exothermic reactionsRecognise exothermic reactions such as combustion and neutralisation, and explain their negative ΔH.
- Endothermic reactionsRecognise endothermic changes such as thermal decomposition, and explain their positive ΔH.
Reading and drawing energy profiles
Bond breaking and making
Beyond the syllabus
Practise and check
Or choose
Topic reference
Quick reference
| Item | Quick rule / reminder |
|---|---|
| Enthalpy change | Δ H = energy of products-energy of reactants (compare the levels qualitatively) |
| Exothermic | heat released to surroundings; Δ H < 0 |
| Endothermic | heat absorbed from surroundings; Δ H > 0 |
| Bond changes | bond breaking absorbs energy; bond making releases energy |
| Energy profile diagram | exothermic: products lower; endothermic: products higher |
| Activation energy (Eₐ) | energy needed to start the reaction; label from reactants → peak |
| Catalyst effect | lowers Eₐ (different pathway); does not change Δ H |
| Temperature clue | exothermic: temperature often rises; endothermic: temperature often falls |
Core knowledge to remember
- Exothermic reaction: releases heat to the surroundings (Δ H < 0).
- Endothermic reaction: absorbs heat from the surroundings (Δ H > 0).
- Enthalpy change (Δ H): heat transferred by the reacting system at constant pressure. A molar value, when supplied, refers to a mole of the reaction as written.
- Surroundings: what supplies or receives the transferred heat, such as the container, air and a non-reacting solvent.
- System: the reacting chemicals themselves.
- Bond breaking: requires energy (endothermic step).
- Bond forming: releases energy (exothermic step).
- Activation energy (Eₐ): minimum energy needed for a reaction to occur.
Common mistakes
- Sign of Δ H: exothermic is negative; endothermic is positive.
- System vs surroundings: a temperature rise is evidence of heat release when starting temperatures and other heat exchange are controlled.
- Bond changes: breaking bonds absorbs energy; making bonds releases energy. Compare the two effects to explain the overall enthalpy change.
- Δ H vs Eₐ: Eₐ is the height to the peak; Δ H compares reactants with products.
- Catalyst diagrams: a catalyst lowers Eₐ but does not change Δ H.