Chemistry Content: Thermochemistry, Kinetics, and Equilibrium
This domain covers energy changes, calorimetry, reaction rates, collision theory, equilibrium expressions, and Le Chatelier’s principle. Questions often combine a calculation with a conceptual prediction about how a change shifts a system.
Core rules and concepts
- q = mcΔT calculates heat gained or lost; ΔH is negative for exothermic reactions and positive for endothermic reactions.
- Hess’s law: if a reaction is the sum of steps, ΔH is the sum of the step enthalpies. Reversing a step changes the sign; multiplying a step multiplies ΔH.
- Rate depends on concentration, temperature, surface area, and catalysts. A catalyst lowers activation energy and speeds both forward and reverse rates but does not change ΔH or K.
- For aA + bB ⇌ cC + dD, K = [C]^c[D]^d / ([A]^a[B]^b), omitting pure solids and liquids.
- Le Chatelier’s principle: a stress shifts equilibrium to partially relieve it. Increasing temperature favors the endothermic direction; increasing pressure favors the side with fewer gas moles.
Worked example
Given A → B with ΔH = +50 kJ and B → C with ΔH = −120 kJ, find ΔH for A → C. Add the steps: A → B → C, so ΔH = +50 + (−120) = −70 kJ. The overall reaction is exothermic even though the first step is endothermic.
Traps that cost marks
Do not reverse the sign of ΔH unless you reverse the reaction. Do not include solids or liquids in K. A catalyst does not shift equilibrium or change K. Rate and equilibrium extent are different: a fast reaction can still favor reactants. In calorimetry, use the mass of the solution when appropriate and account for the calorimeter if stated.