Equilibrium and Le Chatelier's Principle
Equilibrium and Le Chatelier's Principle
This domain tests dynamic equilibrium, equilibrium constants and how systems respond to changes. The exam may ask you to write Kc, calculate it, or predict the direction of shift and explain the effect on yield.
Core rules
- At equilibrium, forward and reverse rates are equal and concentrations remain constant, but the reaction is still occurring.
- For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d/([A]^a[B]^b). Pure solids and liquids are omitted.
- Le Chatelier: if a system at equilibrium is disturbed, it shifts to oppose the change.
- Increasing concentration of a reactant shifts forward. Increasing pressure shifts toward the side with fewer gas moles. Increasing temperature shifts in the endothermic direction; decreasing temperature shifts in the exothermic direction.
- A catalyst speeds both directions equally and does not change Kc or equilibrium position.
Worked example
For H2(g) + I2(g) ⇌ 2HI(g), equilibrium concentrations are [H2] = 0.20 M, [I2] = 0.20 M and [HI] = 1.60 M. Kc = (1.60)^2/(0.20 × 0.20) = 2.56/0.040 = 64. The large value indicates the equilibrium lies toward HI at this temperature. Changing pressure has no effect because there are two gas moles on each side.
Traps
Only temperature changes Kc; concentration and pressure changes shift position but leave Kc unchanged at a given temperature. Do not include solids such as CaCO3 or liquids such as water in Kc expressions. Pressure changes matter only when gas moles differ between sides.