Skip to notes
wise otter.Chemistry
Contents
H2 Chemistry

Chemistry study notes

Chemical Energetics

Track energy through bonds and cycles, then combine enthalpy and entropy to assess thermodynamic feasibility.

A-Level 9476 (2026-2027)

Start learningQuick revisionRead whole chapter

Choose a topic

5 topics

  1. Enthalpy changes and reaction profiles

    Keep heat flow, activation energy and the defined process separate.

  2. Calorimetry

    Convert the surroundings temperature change into energy per mole of reaction.

  3. Hess cycles and bond energies

    Choose a common endpoint and follow every sign and coefficient.

  4. Entropy and accessible arrangements

    Predict changes from temperature, phase and particle numbers.

  5. Gibbs energy, temperature and limits

    Decide whether a process is favourable under the stated conditions.

Scope and references

Learning outcomes and sources

7. Chemical Energetics: Thermochemistry and Thermodynamics. Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 7(a) Explain heat changes through bond breaking and formation.

    • Exothermic: negative delta H
    • Endothermic: positive delta H
    • Breaking absorbs and forming releases energy

    Breaking bonds costs energy; forming bonds releases it

  2. 7(b) Construct and interpret reaction energy profiles.

    • Reaction enthalpy
    • Forward/reverse activation energies
    • Reaction progress is not time

    Breaking bonds costs energy; forming bonds releases it

  3. 7(c) Define the specified enthalpy terms and standard conditions.

    • (i) Reaction, formation, combustion, hydration, solution, neutralisation, atomisation
    • (ii) Positive bond-breaking energy
    • (iii) Negative lattice energy for gaseous ions forming solid

    An enthalpy value belongs to an exact equation and set of statesBuild an ionic solid through gaseous atoms and ions

  4. 7(d) Calculate enthalpy from experimental heat measurements.

    • q = mc delta T
    • Reaction amount and sign
    • Appropriate heat-capacity and measurement assumptions

    The thermometer measures the surroundings, not the reaction directly

  5. 7(e) Explain lattice-energy magnitudes qualitatively.

    • Ionic charges
    • Ionic radii
    • Limits of simple electrostatic comparison

    Build an ionic solid through gaseous atoms and ions

  6. 7(f) Construct Hess and Born-Haber cycles and calculate energies.

    • Ionisation energies and electron affinities
    • (i) Indirect enthalpies, including formation from combustion
    • (ii) Simple ionic solid formation and aqueous solution
    • (iii) Average bond energies

    Hess Law compares routes with identical endpointsBuild an ionic solid through gaseous atoms and ions

  7. 7(g) Explain entropy.

    • Energy dispersal and accessible microscopic arrangements

    Entropy describes how widely energy and particles can be distributed

  8. 7(h) Explain qualitative effects on system entropy.

    • (i) Temperature
    • (ii) Phase
    • (iii) Particle numbers, especially gases
    • No quantitative microstate treatment required

    Entropy describes how widely energy and particles can be distributed

  9. 7(i) Predict the sign of an entropy change.

    • Processes and reactions
    • Dominant phase/gas changes and limits of simple rules

    Entropy describes how widely energy and particles can be distributed

  10. 7(j) Use the standard Gibbs-energy equation.

    • delta G = delta H - T delta S
    • Kelvin and consistent units
    • No calculation of reaction delta S from absolute standard entropies required

    Free energy combines the enthalpy and entropy contributions

  11. 7(k) Infer spontaneity from the sign of Gibbs energy.

    • Negative, zero and positive signs
    • Standard-state scope

    Free energy combines the enthalpy and entropy contributions

  12. 7(l) Explain limits of standard Gibbs-energy predictions.

    • Rate versus feasibility
    • Non-standard conditions and equilibrium extent
    • Temperature/phase assumptions

    Spontaneous does not mean rapid, complete or condition-independent

  13. 7(m) Predict how temperature changes spontaneity.

    • All four enthalpy/entropy sign combinations
    • Crossover temperature with stated approximations

    Spontaneous does not mean rapid, complete or condition-independent

  • SEAB H2 Chemistry 9476, examination 2026

    Topic 7, printed pages 18-19. All 13 lettered outcomes, named enthalpies, three Hess-cycle applications, entropy variables and stated exclusions inspected.

  • Grail: RI Chemical Energetics I, 2022

    Pages 4-5 visually inspected for energy-level and profile conventions. The numerical construction example is original. This older resource informs presentation only; current 9476 defines scope and feasibility language.