Quick answer: At chemical equilibrium the forward and reverse reaction rates are equal, so concentrations stay constant. This AP Chemistry worksheet on chemical equilibrium gives 20 problems from easy to hard: writing K expressions, what the size of K says and which way does it go?. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These twenty problems cover the core of AP Chemistry Unit 7: writing K, comparing Q with K, solving ICE tables and predicting shifts with Le Chatelier’s principle. They run from expressions and short predictions to quadratic solutions, graphs and multi-step stress problems. Show your ICE tables and always state units where K has them.
Level: AP Chemistry · Curriculum: AP Chem Unit 7
Problem 1 Writing K expressions ★★★
Write the equilibrium constant expression for each.1. N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g), Kc
2. CaCO₃(s) ⇌ CaO(s) + CO₂(g), Kc
3. 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g), Kc
Problem 2 What the size of K says ★★★
Three reactions have K = 3.0 × 10¹², 1.0 and 2.0 × 10⁻²⁰ at the same temperature.1. Identify which one mostly goes to products.
2. Then identify which one has comparable amounts of reactants and products.
3. Finally, identify which one hardly reacts.
Problem 3 Which way does it go? ★★★
For 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g), Kc = 25. A mixture has [SO₂] = 0.10 M, [O₂] = 0.10 M and [SO₃] = 0.10 M.1. Calculate Q.
2. Then predict the direction of change.
Problem 4 Le Chatelier directions ★★★
N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g), ΔH < 0. State the direction of shift (right, left or none) for each.1. Add N₂.
2. Then remove NH₃.
3. Next, compress the mixture.
4. After that, raise the temperature.
5. Finally, add a catalyst.
Problem 5 Reading an equilibrium graph ★★★
The graph shows A ⇌ B.2. Then read the equilibrium concentrations.
3. Next, calculate K.
4. Finally, state whether the reactions have stopped.
Problem 6 Changing the equation ★★★
For A ⇌ B, K = 5.0.1. Calculate K for B ⇌ A.
2. Then calculate K for 2 A ⇌ 2 B.
3. Finally, calculate K for ½A ⇌ ½B.
Problem 7 K from measurements ★★★
At equilibrium H₂(g) + I₂(g) ⇌ 2 HI(g) has [H₂] = 0.050 M, [I₂] = 0.050 M and [HI] = 0.40 M.1. Write the expression for Kc.
2. Then calculate Kc.
Problem 8 Kp from Kc ★★★
N₂O₄(g) ⇌ 2 NO₂(g) has Kc = 0.20 at 350 K (R = 0.08206 L atm mol⁻¹ K⁻¹).1. State Δn.
2. Then calculate Kp.
3. Finally, calculate Kp for 2 NO₂ ⇌ N₂O₄.
Problem 9 A perfect-square ICE table ★★★
H₂(g) + I₂(g) ⇌ 2 HI(g), Kc = 64. Aarav in Pune starts with 0.50 M H₂ and 0.50 M I₂.1. Set up an ICE table.
2. Then solve for x.
3. Finally, state all equilibrium concentrations.
Problem 10 Finding K from an ICE table ★★★
CO(g) + 2 H₂(g) ⇌ CH₃OH(g) starts with 1.00 M CO and 2.00 M H₂. At equilibrium [CH₃OH] = 0.30 M.1. Calculate [CO] and [H₂] at equilibrium.
2. Then calculate Kc with units.
Problem 11 Starting from a mixture ★★★
N₂O₄(g) ⇌ 2 NO₂(g), Kc = 0.20. A flask holds 0.10 M N₂O₄ and 0.30 M NO₂.1. Calculate Q and predict the direction.
2. Then calculate the equilibrium concentrations.
Problem 12 Temperature and colour ★★★
2 NO₂(g) (brown) ⇌ N₂O₄(g) (colourless), ΔH = −57 kJ. Emma in Leeds puts a sealed tube of the mixture in hot water.1. Predict the colour change.
2. Then state what happens to K.
3. Finally, explain using Q and K.
Problem 13 Compression and Q ★★★
2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g) is at equilibrium with [SO₂] = 0.20 M, [O₂] = 0.10 M and [SO₃] = 0.40 M.1. Calculate Kc.
2. The volume is halved. Calculate the new Q.
3. Finally, predict the shift and compare with the number of moles of gas.
Problem 14 A heterogeneous equilibrium ★★★
C(s) + CO₂(g) ⇌ 2 CO(g). At equilibrium P(CO₂) = 0.20 atm and P(CO) = 0.80 atm.1. Write Kp and calculate it.
2. Then state the effect of adding more solid carbon.
3. P(CO₂) is raised to 0.50 atm with P(CO) = 0.80 atm; calculate Q and predict the shift.
Problem 15 Adding equilibria ★★★
(1) N₂(g) + O₂(g) ⇌ 2 NO(g), K1 = 4.0 × 10⁻³¹. (2) 2 NO(g) + O₂(g) ⇌ 2 NO₂(g), K2 = 6.0 × 10¹².1. Write the sum of the two reactions.
2. Then calculate its equilibrium constant.
Problem 16 A quadratic ICE table ★★★
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), Kc = 0.0500. Wanjiru in Nairobi starts with 0.200 M PCl₅ only.1. Set up the equation in x.
2. Then solve for x.
3. Finally, calculate the equilibrium concentrations and the percentage of PCl₅ that dissociates.
Problem 17 The small-x approximation ★★★
N₂(g) + O₂(g) ⇌ 2 NO(g), Kc = 1.0 × 10⁻⁴. Start with 0.80 M N₂ and 0.20 M O₂.1. Write the ICE equation.
2. Then use the approximation to solve for [NO].
3. Finally, check that the approximation is valid.
Problem 18 A stress and a new equilibrium ★★★
CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), Kc = 4.0. Start with 1.0 M CO and 1.0 M H₂O. At equilibrium [CO₂] = [H₂] = 0.667 M and [CO] = [H₂O] = 0.333 M. A further 1.0 M CO is then added.2. Then predict the shift.
3. Finally, calculate the new equilibrium concentrations.
Problem 19 Deducing ΔH from K ★★★
The graph shows K for a gas-phase reaction.2. Then predict how the yield changes if the temperature is raised.
3. The reaction has 3 mol of gas on the left and 2 mol on the right. Predict the effect of raising the pressure.
4. Finally, explain why a catalyst is still used in industry.
Problem 20 Cobalt chloride equilibrium ★★★
[Co(H₂O)₆]²⁺(aq) (pink) + 4 Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) (blue) + 6 H₂O(l), ΔH > 0.1. Write Kc.
2. Then predict the colour change when you add concentrated HCl, add water, and heat the solution.
3. For [CoCl₄]²⁻ = 0.0020 M, [Co(H₂O)₆]²⁺ = 0.010 M and [Cl⁻] = 0.50 M, Kc = 5.0 M⁻³. Calculate Q and predict the direction.
