Chemical Equilibrium
Unit 7 tests reversible reactions, the law of mass action, equilibrium constants and the qualitative predictions of Le Chatelier's principle.
Core rules
- At equilibrium, forward and reverse rates are equal and concentrations remain constant, though the reaction has not stopped.
- For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d/[A]^a[B]^b; pure solids and pure liquids are omitted.
- Kp = Kc(RT)^Δn, where Δn is the change in moles of gaseous species.
- If Q < Kc the reaction moves forward; if Q > Kc it moves backward; if Q = Kc it is at equilibrium.
- Le Chatelier's principle: increasing concentration of a reactant shifts the equilibrium forward; increasing pressure favours the side with fewer gas moles; a catalyst changes neither position nor K.
- Only temperature changes the value of K.
Worked example
H₂ + I₂ ⇌ 2HI. Initially 1.0 mol each of H₂ and I₂ are placed in a 1 L vessel. At equilibrium, HI = 1.5 mol, so 0.75 mol of each reactant has reacted, leaving 0.25 mol of H₂ and 0.25 mol of I₂. Kc = (1.5)²/(0.25 × 0.25) = 2.25/0.0625 = 36.
Traps
- Count only gaseous moles when calculating Δn.
- Adding an inert gas at constant volume does not shift the equilibrium.
- A catalyst speeds up both directions equally and does not alter K.
- For exothermic reactions, raising temperature decreases K.
- Do not include water as a pure liquid in Kc, but do include it as a gas.