Kinetics and Equilibrium
Kinetics and equilibrium tests how reaction rates are controlled and how reversible systems respond to change, including collision theory, Boltzmann distributions, Le Chatelier's principle and Kc.
Core rules
- Rate rises with concentration, pressure (for gases), temperature, surface area and a catalyst.
- Collision theory: particles must collide with energy at least Ea and correct orientation. A catalyst lowers Ea by providing an alternative pathway; it increases rate but does not change ΔH or the equilibrium position.
- Raising temperature increases both collision frequency and, more importantly, the fraction of molecules with E ≥ Ea, so rate rises sharply.
- Dynamic equilibrium in a closed system has equal forward and reverse rates and constant concentrations, not equal amounts.
- Le Chatelier: a system at equilibrium shifts to oppose a change in concentration, pressure or temperature. Pressure affects only gaseous species; Kc changes only with temperature.
Worked example
For N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH is negative. Increasing pressure shifts the position to the right because 4 mol of gas become 2 mol, reducing pressure. Increasing temperature shifts it left because the reverse reaction is endothermic. Removing NH3 shifts it right. Adding a catalyst makes equilibrium be reached faster but leaves the yield unchanged.
Traps
Saying a catalyst increases yield; thinking pressure changes Kc; ignoring the number of gaseous moles when predicting pressure effects; assuming equilibrium means equal concentrations; forgetting that Kc expressions exclude solids and pure liquids; confusing rate with position of equilibrium.