Thermochemistry and Chemical Kinetics
Thermochemistry and Chemical Kinetics
This domain tests enthalpy changes, calorimetry, Hess's law, bond energies and reaction rates. You must distinguish thermodynamic favourability from kinetic rate, and handle signs correctly.
Core rules
- Exothermic reactions release heat: ΔH is negative. Endothermic reactions absorb heat: ΔH is positive.
- Calorimetry: q = mcΔT, where m is the mass of the solution being heated, c is its specific heat capacity (4.18 J g^-1 K^-1 for water) and ΔT is the temperature change.
- Hess's law: the enthalpy change of a reaction is independent of route. Add equations and their ΔH values, reversing signs when reversing equations.
- Bond breaking is endothermic; bond forming is exothermic.
- Rate increases with higher concentration, higher surface area, higher temperature and a catalyst. A catalyst lowers activation energy without changing ΔH.
Worked example
Calculate ΔH for C(s) + ½O2(g) → CO(g), given: C(s) + O2(g) → CO2(g), ΔH = −393 kJ mol^-1 CO(g) + ½O2(g) → CO2(g), ΔH = −283 kJ mol^-1. Reverse the second equation: CO2(g) → CO(g) + ½O2(g), ΔH = +283 kJ mol^-1. Add to the first: C(s) + O2(g) + CO2(g) → CO2(g) + CO(g) + ½O2(g). Cancelling gives C(s) + ½O2(g) → CO(g), ΔH = −393 + 283 = −110 kJ mol^-1.
Traps
Do not omit the sign or confuse q with ΔH per mole. In calorimetry, the mass is usually the total solution mass, not just the solute. A catalyst does not shift equilibrium or change ΔH; it only speeds attainment of equilibrium.