Also, this chemistry grades 11-12 lesson on chemical equilibrium notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on chemical equilibrium and check yourself with the quiz.
Quick answer: Chemical equilibrium is reached when forward and reverse reactions occur at the same rate, so concentrations stay constant. Moreover, the equilibrium constant Kc, products over reactants raised to their coefficients, depends only on temperature. Le Chatelier’s principle predicts the direction of shift when concentration, pressure or temperature changes.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.3
Chemical equilibrium notes: the key ideas
What is a dynamic equilibrium?
A dynamic equilibrium is the state of a reversible reaction in a closed system in which the forward and reverse reactions occur at equal rates, so the concentrations of all species stay constant. In addition, both reactions continue; nothing stops.
How do you write and use Kc?
for aA + bB ⇌ cC + dD: Kc = [C]c[D]d / ([A]a[B]b)
Concentrations are equilibrium values in mol/dm³; Kc depends only on temperature. Moreover, pure solids and liquids are left out of the expression.
| Equilibrium | Kc expression | Units of Kc |
|---|---|---|
| N₂O₄(g) ⇌ 2NO₂(g) | [NO₂]² / [N₂O₄] | mol dm⁻³ |
| H₂(g) + I₂(g) ⇌ 2HI(g) | [HI]² / ([H₂][I₂]) | none |
| 2NO(g) + O₂(g) ⇌ 2NO₂(g) | [NO₂]² / ([NO]²[O₂]) | dm³ mol⁻¹ |
| CaCO₃(s) ⇌ CaO(s) + CO₂(g) | [CO₂] | mol dm⁻³ |
1.00 mol of H₂ and 1.00 mol of I₂ react in a 2.0 dm³ vessel at 720 K. At equilibrium 1.56 mol of HI is present. Furthermore, calculate Kc.
HI formed = 1.56 mol, which used 0.78 mol of each reactant. remaining H₂ and I₂ = 0.22 mol each. Furthermore, concentrations: [HI] = 0.78, [H₂] = [I₂] = 0.11 mol/dm³. Kc = 0.78² / (0.11 × 0.11) = 0.6084 / 0.0121 = 50.3 (no units).
A large Kc (above 10³) means the equilibrium mixture is mostly products. A small Kc (below 10⁻³) means mostly reactants. Also, values near 1 give a mixture of both.
What is Kp for gas equilibria?
Kp has the same form as Kc but uses partial pressures. The partial pressure of a gas is its mole fraction times the total pressure: p(A) = x(A) × P.
1.00 mol of N₂O₄ is 20 % dissociated into NO₂ at a total pressure of 100 kPa. In addition, calculate Kp for N₂O₄ ⇌ 2NO₂.
moles: N₂O₄ = 0.80, NO₂ = 0.40, total 1.20. Moreover, mole fractions: 0.667 and 0.333. Partial pressures: 66.7 kPa and 33.3 kPa. Kp = p(NO₂)² / p(N₂O₄) = 33.3² / 66.7 = 16.7 kPa.
How does Le Chatelier’s principle predict shifts?
Le Chatelier’s principle: when a change is made to a system at equilibrium, the position of equilibrium shifts to oppose the change. The reaction quotient Q, calculated like Kc with current concentrations, tells you the direction: if Q is less than K the reaction goes right, and if Q is greater than K it goes left.
| Change | Shift of equilibrium | Effect on K |
|---|---|---|
| Add reactant or remove product | to the right | none |
| Increase pressure (gas) | to the side with fewer gas moles | none |
| Raise temperature, forward ΔH positive | to the right | K increases |
| Raise temperature, forward ΔH negative | to the left | K decreases |
| Add a catalyst | no shift | none |
Also, temperature is the only factor that changes the value of K. A rise in temperature favours the endothermic direction, so K increases for a positive forward ΔH and decreases for a negative one.
A catalyst does not change K or the position of equilibrium. It speeds up both reactions equally so equilibrium is reached sooner. Changing concentration or pressure shifts the position, but Kc stays the same.
Furthermore, use an ICE table: Initial, Change, Equilibrium. Write the change using the mole ratio from the balanced equation, then substitute the equilibrium line into Kc.
- In addition, at dynamic equilibrium the forward and reverse rates are equal and concentrations are constant.
- Kc = [products]^coefficients / [reactants]^coefficients; solids and pure liquids are omitted.
- Kp uses partial pressures: p = mole fraction × total pressure.
- Only a change in temperature changes the value of K; catalysts do not.
- If Q is less than K the reaction shifts right; if Q is greater than K it shifts left.
Equilibrium constants and shifting equilibria: frequently asked questions
What does the equilibrium constant Kc tell you?
Kc shows how far a reversible reaction goes at a given temperature. A large value means mostly products at equilibrium, and a small value means mostly reactants.
Why are solids not included in the Kc expression?
The concentration of a pure solid or liquid is constant, so it does not change with the amount present. Also, it is absorbed into the value of K and left out of the expression.
How does temperature change the value of K?
For an endothermic forward reaction K increases as temperature rises, and for an exothermic forward reaction K decreases. In addition, concentration, pressure and catalysts do not change K.
How do you find the direction of shift with Q?
Furthermore, calculate the reaction quotient Q with the current concentrations. If Q is less than K the system moves to the right to form more products, and if Q is greater than K it moves to the left.
What is the difference between Kc and Kp?
Kc uses equilibrium concentrations in mol/dm³, whereas Kp uses partial pressures of gases. Both are constant at a given temperature and have the same form.
