Chemical equilibrium: Grade 12 Past Paper Questions

Iodine gas, I₂(g), reacts with chlorine gas, Cl₂(g), in a sealed container at temperature T. Equilibrium is reached according to the balanced equation below. I₂(g) + 3Cl₂(g) ⇌ 2ICl₃(g), ΔH = -390 kJ·mol⁻¹. The graph below shows the masses of I₂(g), Cl₂(g) and ICl₃(g) versus time during the course of the reaction. 6.1 State Le Chatelier's principle. 6.2 Why are the masses constant after t₂? 6.3 At t₁, the pressure is adjusted by changing the volume of the container at constant temperature. Was the volume of the container INCREASED or DECREASED? Explain the answer. 6.4 What other change to the equilibrium system will produce a SIMILAR change to the masses at t₁? 6.5 The volume of the container is 2 dm³. At t₂ the concentrations of the gases are: [I₂] = 0,15 mol·dm⁻³, [Cl₂] = 0,36 mol·dm⁻³, [ICl₃] = 0,05 mol·dm⁻³. More I₂ is now added to the container. When a new equilibrium is reached, the concentration of ICl₃ is 0,09 mol·dm⁻³. If the equilibrium constant, Kc, is 0,357, calculate the number of moles of I₂ added.

6.4 The graph below shows the changes in the rate of the reaction after further changes were made to the equilibrium mixture above, showing the forward and reverse reaction rates against time. 6.4.1 The equilibrium was disturbed at 5 minutes due to a change in the concentration of CS₂. Was this concentration INCREASED or DECREASED? 6.4.2 At 15 minutes the temperature was changed. Use Le Chatelier's principle to determine whether the temperature has INCREASED or DECREASED. Explain the answer. 6.4.3 At what time does the system reach equilibrium after the temperature change?

The reaction between carbon disulfide, CS₂(g), and chlorine gas, Cl₂(g), reaches chemical equilibrium in a closed container at constant temperature. The products that form are carbon tetrachloride, CCl₄(g), and sulphur dichloride, S₂Cl₂(g). The balanced equation for this reaction is given below: CS₂(g) + 3Cl₂(g) ⇌ CCl₄(g) + S₂Cl₂(g), ΔH < 0. Initially, an unknown quantity of CS₂(g) and 5 moles of Cl₂ are placed in a 2 dm³ container and allowed to reach equilibrium. The equilibrium mixture contains 0,8 mol of CCl₄. The equilibrium constant, Kc, for this reaction is 0,36. 6.1 State Le Chatelier's principle. 6.2 Calculate the initial number of moles of CS₂(g) required. 6.3 How will each of the following affect the yield of S₂Cl₂(g) at equilibrium? Write INCREASE, DECREASE, or REMAIN THE SAME and give a reason in terms of the reaction which is favoured. 6.3.1 Carbon tetrachloride is removed from the system. 6.3.2 The volume of the container is increased.

1.5 The reaction below reaches equilibrium in a closed system. N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH < 0. A change is made to the system at time t₁. The reaction rate versus time graph is given below. Which ONE of the following changes was made to the system at time t₁? A. A catalyst was added. B. The pressure was decreased at constant temperature. C. The temperature was increased. D. The concentration of the H₂(g) was increased. 1.6 The equilibrium constant, Kc, for a hypothetical reaction is given by the following expression: Kc = [A]² / ([C]³[D₂]). The equation for this reaction is: A. 3C(g) + D₂(g) ⇌ 2A(g) + B(s). B. 2A(g) + B(s) ⇌ 3C(g) + D₂(g). C. 3C(g) + D₂(aq) ⇌ 2A(g) + B(s). D. 2A(l) + B(s) ⇌ 3C(g) + D₂(g).

The expression for the equilibrium constant (Kc) of a hypothetical reaction is given as Kc = [PQ₃]² / ([P₂][Q₂]³). Which of the following equations for a reaction at equilibrium matches this expression? A. P₂(g) + 3Q₂(g) ⇌ 2PQ₃(aq). B. 2PQ₃(l) ⇌ P₂(g) + 3Q₂(g). C. 2PQ₃(aq) ⇌ P₂(g) + 3Q₂(g). D. P₂(g) + 3Q₂(g) ⇌ 2PQ₃(g). Which statement best describes the difference between the endpoint and the equivalence point in a titration? A. The endpoint occurs when the acid or base has completely reacted with each other, while the equivalence point is when the indicator changes colour. B. The equivalence point occurs when the acid or base has completely reacted with each other, while the endpoint is when the indicator changes colour. C. The endpoint and equivalence point occur at different times and have no connection in a titration. D. The equivalence point and endpoint are always exactly the same in every titration.

Consider the energy profile graph for a REVERSIBLE REACTION shown below. The following statements are given: (i) a − c represents the activation energy for the reverse reaction; (ii) ΔH for the reverse reaction is a − b; (iii) a catalyst lowers the activation energy for both forward and reverse reactions. Identify the CORRECT statement(s): A. (ii) and (iii) only. B. (iii) only. C. (i), (ii) and (iii). D. (i) only. The reaction of an acid-base indicator (HIn), represented as HIn(aq), with H₂O(l), reaches equilibrium: HIn(aq) + H₂O(l) ⇌ H₃O⁺(aq) + In⁻(aq), ΔH < 0. Which of the following will change the colour of the solution from red to yellow? A. Increasing the concentration of H₃O⁺ ions. B. Increasing the temperature of the solution. C. Adding a base. D. Adding an acid.