Chemical Energetics
Enthalpy changes, Hess's law, and bond energies
Enthalpy change (ΔH)** = heat energy change at constant pressure. **Exothermic: ΔH negative (heat released, products lower energy). **Endothermic:** ΔH positive (heat absorbed).
Standard enthalpy changes:
Hess's Law: Total enthalpy change is independent of the route taken (conservation of energy). Use enthalpy cycles to calculate unknown ΔH values.
Bond energies: Energy needed to break 1 mol of bonds. ΔH = Σ(bonds broken) − Σ(bonds formed). If more energy released forming bonds than breaking → exothermic.
Calorimetry: q = mcΔT where m = mass of water, c = 4.18 J g⁻¹ K⁻¹, ΔT = temperature change. Then ΔH = −q/n.
Key Points to Remember
- 1Exothermic: ΔH negative; Endothermic: ΔH positive
- 2Hess's Law: enthalpy independent of route
- 3ΔH = bonds broken − bonds formed
- 4Calorimetry: q = mcΔT
Pakistan Example
Natural Gas Combustion — Heating Every Pakistani Home
Pakistan's Sui Northern and Sui Southern gas companies supply methane (CH₄) for cooking and heating. The combustion of methane (ΔH°c = −890 kJ/mol) is highly exothermic. Using Hess's Law with bond energies: breaking 4 C-H bonds and 2 O=O bonds requires less energy than forming 2 C=O and 4 O-H bonds — explaining why it releases heat.