Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- To predict equilibrium concentrations you need the balanced equation, the initial concentrations or partial pressures, and K.
- Step 1: calculate Q from the starting values and compare it with K. The mixture always changes so that Q moves towards K.
- Step 2: build an ICE table, substitute the Equilibrium row into K and solve for x.
- At equilibrium (Q = K) the forward and reverse rates are equal. Both reactions continue, but concentrations stay constant. This is dynamic equilibrium.
Key relationships
| Idea | Rule | Use it to |
|---|---|---|
| Q < K | net forward reaction; reactants fall, products rise | set the signs in the Change row |
| Q > K | net reverse reaction; products fall, reactants rise | set the signs in the Change row |
| Q = K | no net change; forward rate = reverse rate | recognise a mixture already at equilibrium |
| Change row | changes in the ratio of the coefficients (−x, −2x, +3x …) | write every species in terms of one unknown |
| c = n / V | mol L⁻¹ from moles and litres | get concentrations before using Kc |
| Small-x check | x < about 5% of the starting value | decide whether c − x ≈ c is acceptable |
Assumptions behind the numbers
- The temperature is constant; K only applies at the temperature it was measured at.
- Pure solids and liquids are left out of K and out of the ICE table.
- Kc uses concentrations; Kp uses partial pressures. Use the one the question gives. Converting between them is not assessed.
- In the small-x shortcut, you assume very little reacts. You must check this assumption afterwards.
Mistakes to avoid
- Putting initial values into K. They give Q, not K.
- Ignoring coefficients. A coefficient of 2 gives a change of 2x, squared in K as (2x)².
- Assuming a forward reaction. Compare Q with K first.
- Using moles in Kc. Divide by the volume.
- Forcing the small-x shortcut. If x is more than about 5% of the starting value, solve the quadratic.
- Keeping a root that gives a negative concentration. Only one root makes physical sense.
- Thinking equilibrium means equal amounts or no reaction. It means constant amounts and equal rates.
Quick self-check
- For A(g) ⇌ 2 B(g), Kc = 0.10. A mixture has [A] = 0.50 mol L⁻¹ and [B] = 0.30 mol L⁻¹. Which way does it react? (Q = 0.30² ÷ 0.50 = 0.18 > 0.10, so backwards: B falls, A rises)
- For 2 G ⇌ H going forwards, [G] falls by 0.30 mol L⁻¹. By how much does [H] rise? (0.15 mol L⁻¹)
- For Z ⇌ Y, Kc = 4.0. A flask starts with [Z] = 1.00 mol L⁻¹ and no Y. Find the equilibrium concentrations. (x ÷ (1.00 − x) = 4.0, so x = 0.80: [Y] = 0.80 mol L⁻¹, [Z] = 0.20 mol L⁻¹)
Next: practice questions.