Equilibrium constants · Grade 11
At equilibrium the forward and backward reactions go at the same rate, so the concentrations stay the same. The equilibrium constant, Kc, compares the concentrations of products and reactants at equilibrium; for gases, Kp does the same with partial pressures. Its value depends only on the temperature.
Key facts
- For aA + bB ⇌ cC + dD: Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ), using equilibrium concentrations in mol dm⁻³.
- Units: put the units into the expression and cancel. If the powers on the top and the bottom add up to the same number, Kc has no units.
- To find Kc from starting amounts, work out the moles of everything at equilibrium (initial, change, equilibrium), then divide by the volume.
- The partial pressure of a gas = mole fraction × total pressure, where mole fraction = moles of that gas ÷ total moles of gas. Kp uses partial pressures in place of concentrations.
- Only temperature changes Kc or Kp. For an exothermic forward reaction, raising the temperature makes K smaller; for an endothermic one, it makes K bigger.
- Changing a concentration or the pressure moves the position of equilibrium but leaves K the same; a catalyst changes neither.