Chemical Equilibria
Chemical Equilibria
The exam tests the meaning of dynamic equilibrium, calculation of Kc and Kp, prediction of shifts using Le Chatelier's principle, and reasoning behind industrial conditions such as the Haber and Contact processes.
The equilibrium constant
For aA + bB <=> cC + dD at equilibrium, Kc = [C]c[D]d / ([A]a[B]b), with each concentration in mol dm-3 raised to its stoichiometric power. Pure solids and pure liquids are omitted because their concentration is effectively constant. Kc depends only on temperature; changing concentration or pressure shifts the position but not the value. A large Kc means the equilibrium lies to the right. For gases, Kp uses partial pressures, where partial pressure = mole fraction x total pressure.
Le Chatelier's principle
When a system at equilibrium is disturbed, the position shifts to oppose the disturbance. Adding a reactant shifts right; removing a product shifts right. Increasing pressure shifts towards the side with fewer moles of gas; if the mole counts are equal there is no shift. Increasing temperature shifts in the endothermic direction and changes K: K increases for endothermic reactions, decreases for exothermic ones. A catalyst speeds attainment but does not change the position or K.
Worked example
In a 1.00 dm3 vessel, H2(g) + I2(g) <=> 2HI(g) reaches equilibrium with 0.20 mol H2, 0.20 mol I2 and 1.60 mol HI. Then Kc = (1.60)2 / (0.20 x 0.20) = 2.56 / 0.040 = 64. Units: mol2 dm-6 divided by mol2 dm-6, so Kc has no units here. Note the squaring of HI because of its coefficient, and division by volume is unnecessary only because V = 1.00 dm3.
Traps
Including a solid such as CaCO3 or a liquid such as water in Kc; using moles instead of concentrations when the volume is not 1 dm3; forgetting that K changes with temperature alone; claiming a catalyst raises yield; and predicting a pressure shift when gas moles are equal on both sides.
Key ideas
- Kc uses equilibrium concentrations raised to stoichiometric powers.
- Only temperature changes the numerical value of K.
- Pressure affects equilibrium only when gas mole numbers differ.
- Catalysts change the rate of approach, not the position.
- For gases, partial pressure = mole fraction x total pressure.