Rates of Reaction and Chemical Equilibrium
Rates
Rate measures change in concentration per unit time. Collision theory requires sufficient energy and correct orientation; only collisions exceeding the activation energy react. Factors increasing rate include higher concentration or pressure, greater surface area, higher temperature and catalysts. Temperature increases both collision frequency and the fraction of collisions exceeding activation energy, so its effect is largest. Catalysts lower activation energy without altering ΔH or the position of equilibrium.
Equilibrium
A reversible reaction reaches dynamic equilibrium when forward and reverse rates are equal and concentrations remain constant. Le Chatelier's principle predicts the response to change: adding reactant shifts forward, increasing pressure shifts toward fewer gas moles, raising temperature shifts in the endothermic direction, and a catalyst changes nothing about position.
Equilibrium constant
Kc = [products]^coefficients / [reactants]^coefficients, with pure solids and liquids omitted. K depends only on temperature. A large K favours products.
Traps
- Saying equilibrium means equal concentrations.
- Claiming a catalyst increases yield.
- Forgetting that K changes with temperature but not with concentration or pressure.
- Confusing rate with extent of reaction.
- Including solids in Kc expressions.
Rule of thumb
For any disturbance, ask which direction the system can move to oppose the change, then state how the rates differ temporarily.
Worked example
N₂ + 3H₂ ⇌ 2NH₃ is exothermic. Raising temperature shifts the equilibrium left, lowering yield, but increases the rate. Industrial conditions therefore balance yield against speed.