Enthalpy and Hess's Law — HSC Chemistry — 1st Paper

Enthalpy and Exothermic / Endothermic Reactions

Enthalpy (H) is a measure of the total heat content of a system at constant pressure. A reaction that releases heat (ΔH < 0) is exothermic — the products are more stable than the reactants. A reaction that absorbs heat (ΔH > 0) is endothermic. Combustion, neutralisation, and cellular respiration are all exothermic.

Standard Enthalpy Change

ΔH°rxn = ΣΔH°f (products) − ΣΔH°f (reactants) [Hess's Law]

Heat Equation

q = mcΔT where m = mass (g), c = specific heat capacity (J g⁻¹ K⁻¹), ΔT = temperature change (K)

Entropy and Spontaneity

Entropy (S) measures the degree of disorder in a system. The Second Law of Thermodynamics states that for a spontaneous process, the total entropy of the universe increases (ΔS_universe > 0). Gibbs free energy combines enthalpy and entropy: ΔG = ΔH − TΔS. Spontaneous reactions have ΔG < 0.

Gibbs Free Energy

ΔG = ΔH − TΔS (ΔG < 0 → spontaneous; ΔG = 0 → equilibrium; ΔG > 0 → non-spontaneous)

HSC Exam Focus

For HSC, know that exothermic + entropy increase = always spontaneous. Endothermic + entropy decrease = never spontaneous. The tricky cases depend on temperature: ΔG = ΔH − TΔS. High T favours the TΔS term.

Biochemistry Bridge

ATP hydrolysis (ATP → ADP + Pᵢ) has ΔG° = −30.5 kJ/mol — spontaneous and exergonic. Cells couple non-spontaneous reactions (protein synthesis, active transport) to ATP hydrolysis to make them thermodynamically feasible. This is why mitochondria are critical: they regenerate ATP.