Short on time? This page is the recap. For explanations, a Q-time graph and worked examples, use the full study guide.
Recap
- The reaction quotient, Q, describes the relative amounts of products and reactants in a mixture at any moment.
- As a reaction runs, Q changes and moves towards a fixed value. That value is the equilibrium constant, K. At equilibrium, Qc = Kc and Qp = Kp.
- Kc and Qc use concentrations in mol L⁻¹. Kp and Qp use partial pressures of gases, usually in atm. They are usually different numbers. You do not convert between them, but you must use the one the question asks for.
- Pure solids and pure liquids (including water as the solvent) are left out, because their “concentration” does not depend on how much is present.
- At a fixed temperature, K is constant. Changing amounts changes Q, not K.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| Qc = [C]^c[D]^d / ([A]^a[B]^b) | for aA + bB ⇌ cC + dD, concentrations at any moment | describe any mixture in solution or gas |
| Qp = (P_C)^c(P_D)^d / ((P_A)^a(P_B)^b) | same shape, partial pressures of gases | describe a gas mixture |
| At equilibrium: Qc = Kc, Qp = Kp | Q stops changing once rates are equal | recognise or test for equilibrium |
Assumptions behind the expression
- The equation is balanced; the powers come from its coefficients.
- The temperature is fixed. A different temperature gives a different K.
- Values put into K are equilibrium values. Any other values give Q.
- Questions where a dissolved substance is in equilibrium with the same substance as a gas are outside this course.
Mistakes to avoid
- Coefficients as multipliers. 2NO gives [NO]², not 2[NO].
- Upside down. Products go on top for the reaction as written.
- Adding instead of multiplying the terms.
- Including (s) or (l) species. Keep H₂O(g); leave out H₂O(l).
- Mixing types. No pressures in Kc, no concentrations in Kp.
- Calling any Q value “K”. Only equilibrium values give K.
Quick self-check
- Write Kp for CaCO₃(s) ⇌ CaO(s) + CO₂(g). (Kp = P_CO₂)
- Write Kc for Cu²⁺(aq) + 4NH₃(aq) ⇌ Cu(NH₃)₄²⁺(aq). (Kc = [Cu(NH₃)₄²⁺] / ([Cu²⁺][NH₃]⁴))
- For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), a mixture has P_SO₂ = 0.20 atm, P_O₂ = 0.50 atm and P_SO₃ = 0.40 atm. What is Qp? (0.40² ÷ (0.20² × 0.50) = 8.0)
- A mixture has Qc = 3.0 and Kc = 3.0 at the same temperature. What can you say? (It is at equilibrium; the concentrations stay constant.)
Next: practice questions.