Equilibrium constants Kc and KpEdexcel International A Level Chemistry: Revision notes
Section 1
The equilibrium constant Kc
For a reversible reaction aA + bB ⇌ cC + dD at equilibrium, Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ), with square brackets meaning equilibrium concentration in mol dm⁻³.
- Products go on top and reactants underneath.
- Each coefficient becomes a power.
Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) gives Kc = [NH₃]² / ([N₂][H₂]³).
Kc has a fixed value at a given temperature.
Multiplying by the coefficient instead of raising to the power, or inverting the fraction.
Section 2
Heterogeneous equilibria
A homogeneous equilibrium has all species in the same phase. A heterogeneous equilibrium has species in more than one phase.
Pure solids and pure liquids are left out of the expression, as their concentrations and pressures are constant.
- CaCO₃(s) ⇌ CaO(s) + CO₂(g): Kp = p(CO₂)
- C(s) + CO₂(g) ⇌ 2CO(g): Kp = p(CO)² / p(CO₂)
Section 3
Partial pressures and Kp
For gas equilibria Kp is written using partial pressures, in atm for this course.
Mole fraction of A = moles of A ÷ total moles of gas. Partial pressure p(A) = mole fraction of A × total pressure.
Kp = p(C)ᶜ p(D)ᵈ / (p(A)ᵃ p(B)ᵇ)
Solids do not appear. The partial pressures add up to the total pressure.
Section 4
Calculating Kc and Kp from data
- Work out equilibrium amounts from initial amounts and the amount reacted, using the equation's ratio.
- Convert to concentrations (÷ volume in dm³) for Kc, or to partial pressures for Kp.
- Substitute and calculate.
Worked example: 0.250 mol N₂O₄ in 5.00 dm³ with 0.050 mol left at equilibrium. NO₂ formed = 2 × 0.200 = 0.400 mol. [N₂O₄] = 0.0100 and [NO₂] = 0.0800 mol dm⁻³. Kc = 0.0800² / 0.0100 = 0.64 mol dm⁻³.
Write a small table of initial, change and equilibrium amounts. It stops ratio mistakes.
Section 5
Units
Work out the units by substituting the units into the expression and cancelling.
- N₂ + 3H₂ ⇌ 2NH₃: Kc units = (mol dm⁻³)² ÷ (mol dm⁻³)⁴ = mol⁻² dm⁶; Kp units = atm⁻².
- N₂O₄ ⇌ 2NO₂: Kc units mol dm⁻³; Kp units atm.
- H₂ + I₂ ⇌ 2HI: the same number of moles on each side, so no units.
A value without its units, where units are needed, loses a mark.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Equilibrium constants Kc and Kp
- In the Haber process nitrogen and hydrogen react reversibly: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). A sealed vessel is held at 700 K until equilibrium is reached. The equilibrium concentrations are: N₂ 0.40 mol dm⁻³, H₂ 1.20 mol dm⁻³ and NH₃ 0.60 mol dm⁻³.Calculate the value of Kc at 700 K, including units.2 marks
- Carbon dioxide reacts with hot carbon: C(s) + CO₂(g) ⇌ 2CO(g). Excess solid carbon is held in a closed vessel at 1000 K and the total pressure at equilibrium is 2.0 atm. The gas mixture at equilibrium contains 0.30 mol of CO₂ and 0.90 mol of CO.Calculate Kp at 1000 K, including units.2 marks
- Dinitrogen tetraoxide dissociates in a sealed flask: N₂O₄(g) ⇌ 2NO₂(g). A student places 0.250 mol of N₂O₄ in a 5.00 dm³ flask at 373 K. At equilibrium 0.050 mol of N₂O₄ remains and the total pressure in the flask is 2.75 atm.Calculate Kc at 373 K, including units.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).