Energetics

This lesson covers enthalpy changes, calorimetry, Hess's law, Born-Haber cycles, enthalpy of solution and hydration, entropy, and Gibbs free energy. Energetics is a fundamental topic that links thermodynamics to chemical reactions and helps predict whether reactions are feasible. The content here covers both AS and A2 material for AQA and OCR A specifications.

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Enthalpy Changes

Key Definition: The enthalpy change (ΔH) of a reaction is the heat energy transferred between a system and its surroundings at constant pressure, measured in kJ mol⁻¹.

• Exothermic reactions release heat to the surroundings: ΔH is negative. The temperature of the surroundings increases.
• Endothermic reactions absorb heat from the surroundings: ΔH is positive. The temperature of the surroundings decreases.

Energy Level Diagrams

An energy level diagram (enthalpy profile diagram) has enthalpy on the vertical axis and progress of reaction on the horizontal axis. For an exothermic reaction, the products are drawn at a lower energy level than the reactants, and the downward arrow between them represents ΔH (negative). The hump above the reactant level represents the activation energy (Ea) — the minimum energy that colliding particles must have for a reaction to occur.