Kinetics and Equilibrium
What to know
This domain links reaction rate to collision theory and the factors that affect it (concentration, temperature, surface area, catalysts), then extends to equilibrium: the equilibrium constant, Le Chatelier's principle, and the relationship between kinetics and equilibrium.
Know that increasing temperature raises rate by increasing the fraction of collisions with sufficient energy, not merely collision frequency. Catalysts lower activation energy without changing ΔH or the equilibrium position. For equilibrium, write K expressions with products over reactants, each raised to its coefficient, excluding pure solids and liquids. Use Le Chatelier's principle to predict shifts from concentration, pressure, and temperature changes.
Traps
- Thinking a catalyst shifts equilibrium; it only speeds approach to equilibrium.
- Including solids or pure liquids in the equilibrium expression.
- Confusing rate with equilibrium position; a fast reaction is not necessarily product-favored.
- Misapplying pressure changes: only gas-phase mole changes matter.
- Forgetting that temperature is the only stress that changes the value of K.
Worked example
For N2(g) + 3H2(g) ⇌ 2NH3(g), K = [NH3]^2 / ([N2][H2]^3). Increasing pressure shifts toward fewer gas moles (right, toward NH3). Raising temperature shifts the equilibrium in the endothermic direction, which changes K itself.
Rule of thumb: count gas moles on each side to predict pressure effects.