Chemical Equilibrium
What this domain tests
Writing equilibrium constants, calculating equilibrium compositions, and predicting the effect of changing conditions.
Core rules
- Dynamic equilibrium requires a closed system and equal forward and reverse rates; concentrations remain constant, not equal.
- For aA + bB ⇌ cC + dD, Kc = [C]c[D]d / [A]a[B]b. Pure solids and liquids are omitted because their activity is constant.
- K depends only on temperature. Changing concentration or pressure shifts the position of equilibrium but not the value of K.
- Le Chatelier: raising pressure favours the side with fewer gas moles; raising temperature favours the endothermic direction.
- Catalysts speed up both directions equally and do not change the equilibrium yield.
Worked example
1.00 mol H2 and 1.00 mol I2 are sealed in a 1.00 dm3 vessel and reach equilibrium with [HI] = 1.60 mol dm−3. From the stoichiometry, 0.80 mol of each reactant has reacted, leaving 0.20 mol dm−3 of H2 and of I2. Kc = (1.60)2 / (0.20 × 0.20) = 2.56 / 0.040 = 64. Kc has no units here because the powers cancel.
Traps
Including a solid in a Kc expression; thinking a catalyst raises yield; assuming K changes when pressure changes; substituting initial rather than equilibrium concentrations; forgetting the units of Kc depend on the overall power of the expression.