Quick answer: Chemical equilibrium is reached when forward and reverse reactions occur at the same rate, so concentrations stay constant. This Chemistry 11-12 worksheet on chemical equilibrium gives 20 problems from easy to hard: equilibrium words, writing Kc and solids in the expression. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These 20 problems cover writing and calculating Kc and Kp, ICE tables, the reaction quotient, and predicting shifts from concentration, pressure and temperature changes. They begin with expressions and qualitative shifts and finish with quadratic equilibria and the effect of temperature on K.
The level suits IB Chemistry, A Level, CBSE Class 11-12 and WAEC/NECO senior students. Always write the expression first and check the units.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.3
Problem 1 Equilibrium words ★★★
Aarav in Pune is revising.1. Define dynamic equilibrium.
2. State whether this is true or false and correct it if false: at equilibrium the reaction has stopped.
3. State why a closed system is needed.
Problem 2 Writing Kc ★★★
Write the Kc expression and the units of Kc for each.1. N₂(g) + O₂(g) ⇌ 2NO(g)
2. 2NO₂(g) ⇌ N₂O₄(g)
3. CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g)
4. H₂(g) + Br₂(g) ⇌ 2HBr(g)
Problem 3 Solids in the expression ★★★
Write Kc and Kp for each equilibrium.1. CaCO₃(s) ⇌ CaO(s) + CO₂(g)
2. C(s) + CO₂(g) ⇌ 2CO(g)
3. Explain why solids do not appear.
Problem 4 Reading equilibrium concentrations ★★★
The graph shows 2NO(g) + Cl₂(g) ⇌ 2NOCl(g) approaching equilibrium.1. Read the equilibrium concentrations.
2. Write the Kc expression.
3. Calculate Kc with units.
Problem 5 Size of K ★★★
Chiamaka in Lagos compares three reactions at 298 K: (A) K = 2 × 10⁻²⁵, (B) K = 1.0, (C) K = 6 × 10³⁰.1. State which reaction gives mostly products at equilibrium.
2. State which gives mostly reactants.
3. State what the mixture in B looks like.
Problem 6 Predicting the shift ★★★
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) has ΔH = +93 kJ/mol. State the shift (left, right or none) for each change.1. Chlorine is added.
2. The pressure is increased by compression.
3. The temperature is raised.
4. A catalyst is added.
5. PCl₃ is removed as it forms.
Problem 7 Does K change? ★★★
Wanjiru in Nairobi studies an equilibrium. State whether Kc changes for each action and give a reason.1. Adding more reactant
2. Compressing the gas mixture
3. Raising the temperature
4. Adding a catalyst
Problem 8 Kp from partial pressures ★★★
In 2NO₂(g) ⇌ N₂O₄(g) the equilibrium partial pressures are p(N₂O₄) = 30 kPa and p(NO₂) = 12 kPa.1. Write the Kp expression.
2. Calculate Kp with units.
3. Calculate the total pressure and the mole fraction of NO₂.
Problem 9 Decomposition of PCl₅ ★★★
The graph shows 0.100 mol/dm³ of PCl₅ decomposing in a closed vessel.1. Read the equilibrium concentrations.
2. Write the Kc expression.
3. Calculate Kc with units.
Problem 10 Reaction quotient ★★★
CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g) has Kc = 6.0 at a certain temperature.1. Mixture 1: [CO] = [H₂O] = 0.50, [CO₂] = [H₂] = 1.5 mol/dm³. Calculate Q and state the direction of change.
2. Mixture 2: [CO] = [H₂O] = 0.20, [CO₂] = [H₂] = 0.30 mol/dm³. Calculate Q and state the direction of change.
Problem 11 Finding a concentration ★★★
For 2HI(g) ⇌ H₂(g) + I₂(g), Kc = 0.020 at a given temperature. At equilibrium [HI] = 0.50 mol/dm³ and [H₂] = [I₂].1. Write the Kc expression.
2. Calculate [H₂].
3. Calculate Kc for the reverse reaction H₂(g) + I₂(g) ⇌ 2HI(g) at the same temperature.
Problem 12 Temperature and K ★★★
For a reaction X the equilibrium constant is 8.0 × 10³ at 300 K, 1.2 × 10² at 400 K and 4.5 at 500 K.1. Deduce whether the forward reaction is exothermic or endothermic.
2. Calculate how many times K falls from 300 K to 500 K.
3. Explain, in terms of Le Chatelier’s principle, why the yield falls as the temperature rises.
Problem 13 Methanol yield ★★★
The graph shows the equilibrium yield of methanol from CO(g) + 2H₂(g) ⇌ CH₃OH(g) at 500 K and 600 K.1. State and explain the effect of pressure on the yield.
2. State the effect of temperature and deduce the sign of ΔH.
3. Read the yield at 10 MPa for each temperature.
Problem 14 Iron and thiocyanate colours ★★★
Fe³⁺(aq) (pale yellow) + SCN⁻(aq) (colourless) ⇌ [Fe(SCN)]²⁺(aq) (red). Predict the colour change and explain it for:1. adding more iron(III) nitrate solution
2. adding silver nitrate, which removes SCN⁻ as a precipitate of AgSCN
3. adding an equal volume of water
Problem 15 Kp from mole amounts ★★★
At 150 kPa total pressure, an equilibrium mixture of COCl₂(g) ⇌ CO(g) + Cl₂(g) contains 0.60 mol COCl₂, 0.30 mol CO and 0.30 mol Cl₂.1. Calculate the mole fraction of each gas.
2. Calculate the partial pressures.
3. Calculate Kp with units.
Problem 16 Kp from Kc ★★★
For PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), Kc = 0.0267 mol/dm³ at 500 K. Use Kp = Kc(RT)^Δn with R = 8.314 kPa dm³ mol⁻¹ K⁻¹.1. State Δn, the change in moles of gas.
2. Calculate RT.
3. Calculate Kp with units.
Problem 17 Esterification ★★★
Emma in Leeds mixes 2.00 mol of ethanol with 1.00 mol of ethanoic acid. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O has Kc = 4.0 (no units).1. Write the Kc expression.
2. Let x be the moles of ester formed. Show that 3x² − 12x + 8 = 0.
3. Calculate the moles of ester at equilibrium.
Problem 18 Water gas shift ★★★
Diego in Texas mixes 1.00 mol CO and 1.00 mol H₂O(g) in a 5.0 dm³ vessel. CO + H₂O ⇌ CO₂ + H₂ has Kc = 6.0.1. Draw an ICE table in moles (let x mol react).
2. Show that x / (1 − x) = √6.
3. Calculate the equilibrium concentration of H₂.
Problem 19 K at a new temperature ★★★
N₂O₄(g) ⇌ 2NO₂(g) has ΔH = +57.2 kJ/mol and Kc = 4.6 × 10⁻³ mol/dm³ at 298 K. Use ln(K₂/K₁) = (−ΔH/R)(1/T₂ − 1/T₁).1. Calculate 1/298 − 1/350.
2. Calculate ln(K₂/K₁) for 350 K.
3. Calculate Kc at 350 K and state the direction of the change.
Problem 20 Spot the errors ★★★
Find and correct the error in each statement.1. For CaCO₃(s) ⇌ CaO(s) + CO₂(g), Kc = [CaO][CO₂] / [CaCO₃].
2. Adding a catalyst increases the value of Kc.
3. Increasing the pressure shifts the equilibrium towards the side with more moles of gas.
4. For 2A ⇌ B, Kc = [B] / 2[A].
