Chemical Equilibrium
This domain tests dynamic equilibrium, equilibrium constants and the response of systems to change. STPM expects quantitative use of Kc and qualitative use of Le Chatelier’s principle.
Dynamic equilibrium and K
At equilibrium, forward and reverse rates are equal and concentrations remain constant. For aA + bB ⇌ cC + dD, Kc = [C]c[D]d/[A]a[B]b. Pure solids and pure liquids are omitted because their concentrations are effectively constant. Kp applies to gaseous systems. The reaction quotient Q has the same form as K; if Q < K the forward reaction is favoured, if Q > K the reverse is favoured.
Le Chatelier’s principle
A system at equilibrium shifts to oppose a change. Increasing reactant concentration shifts forward. Increasing pressure favours the side with fewer moles of gas. Increasing temperature favours the endothermic direction. A catalyst speeds attainment of equilibrium but does not change K or the position of equilibrium.
Worked example
H2 + I2 ⇌ 2HI. Initially 1.00 mol H2 and 1.00 mol I2 are placed in a 1.00 dm3 vessel. At equilibrium [HI] = 1.50 mol dm−3. The change in HI is +1.50, so H2 and I2 each decrease by 0.75. Equilibrium [H2] = [I2] = 0.25 mol dm−3. Kc = (1.50)2/(0.25 × 0.25) = 2.25/0.0625 = 36.0.
Traps that cost marks
Use equilibrium concentrations, not initial concentrations. Do not include solids or pure liquids in K. Pressure changes have no effect when gas moles are equal on both sides. Only temperature changes K; a catalyst does not. Check the units of Kc.