Energetics — A-Level Chemistry Revision

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What is Energetics?

Energetics in A-Level Chemistry is the study of energy changes in chemical reactions, primarily focusing on enthalpy changes (ΔH). Key concepts include standard enthalpy changes of reaction, formation, combustion, and neutralisation. Hess's Law is a fundamental principle used to calculate enthalpy changes that cannot be measured directly, by using alternative reaction pathways. Bond enthalpies are also used to estimate enthalpy changes by considering the energy required to break bonds and the energy released when forming new ones.

Board notes: AQA, Edexcel, and OCR all cover Hess's Law and bond enthalpy calculations extensively. AQA may include more complex, multi-step problems involving Born-Haber cycles. Edexcel often links energetics to reaction feasibility by introducing concepts of entropy and Gibbs free energy. OCR questions frequently require students to draw and interpret enthalpy profile diagrams and reaction pathway diagrams.

Worked example

Common mistakes

• 1Sign errors in enthalpy calculations. Exothermic reactions have a negative ΔH (energy is released), while endothermic reactions have a positive ΔH (energy is taken in).
• 2Confusing enthalpy of formation and enthalpy of combustion. Formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states. Combustion is the enthalpy change when one mole of a substance is completely burned in oxygen.
• 3Incorrectly applying Hess's Law. Students often reverse the sign of an enthalpy change when reversing a reaction direction or forget to multiply the enthalpy change by the stoichiometric coefficient.

Energetics exam questions

Exam-style questions for Energetics with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is the difference between bond enthalpy and mean bond enthalpy?

  Bond enthalpy is the energy required to break one mole of a specific bond in a specific molecule in the gaseous state. Mean bond enthalpy is an average value for a particular type of bond, taken from a range of different compounds, as the actual bond enthalpy can vary slightly depending on the molecular environment.

• Why are bond enthalpy calculations less accurate than using Hess's Law with enthalpies of formation?

  Bond enthalpy calculations are an estimation because they use average bond enthalpies. Hess's Law calculations using standard enthalpies of formation are more accurate because they are based on specific, experimentally determined values for the actual compounds involved in the reaction.