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.

A few crystals of potassium chromate (K₂CrO₄) are dissolved in water in a test tube. The solution is yellow in colour. With addition of a little amount of concentrated acid the solution changes to orange. The following equilibrium reaction occurs: 2CrO₄²⁻(aq) (yellow) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) (orange) + H₂O(ℓ) ΔH < 0. 6.1 State Le Chatelier's principle. 6.2 How will the colour of the equilibrium mixture be affected in the following? Choose from BECOMES YELLOW, STAYS ORANGE or NO CHANGE. 6.2.1 The test tube is placed in ice-cold water for a few minutes. 6.2.2 If the pH of the solution is increased. 6.2.3 Explain the answer to QUESTION 6.2.2 by referring to Le Chatelier's principle. 6.3 Consider the following hypothetical reaction that takes place in a 2 dm³ container: aP(g) + bQ(g) ⇌ cR(g) + dS(g). The graph below represents the change in mole of the substances.

Determine the values of a, b, c and d. The temperature of the equilibrium mixture is increased. Is the forward reaction EXOTHERMIC or ENDOTHERMIC? Fully explain the answer. Calculate the equilibrium constant Kc.

During the contact process, sulphur dioxide (SO₂) reacts with oxygen (O₂) to form sulphur trioxide (SO₃) according to the following balanced chemical equation: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). Initially, X mol SO₂(g) and 6 mol O₂(g) are sealed in a 2 dm³ container, and the system is allowed to reach its first equilibrium at 450 °C. It is found that the concentration of O₂ at this equilibrium is 1,5 mol·dm⁻³. 6.1 Explain why chemical equilibrium is considered dynamic. 6.2 The pressure of the system is increased by decreasing the volume of the container, while the temperature is kept constant. 6.2.1 Which reaction will be favoured as a result of the pressure increase? Write FORWARD or REVERSE. Give a reason. 6.2.2 How will the Kc value be affected by the increase in pressure? Write INCREASES, DECREASES, or REMAINS THE SAME. 6.3 A new equilibrium is reached at the same temperature of 450 °C. The second equilibrium concentrations are as follows: [SO₂] = 1,7888 mol·dm⁻³; [O₂] = 1,2 mol·dm⁻³ and [SO₃] = 2,4 mol·dm⁻³. Calculate the number of moles of SO₂, X, that was initially placed in the container. 6.4 When the temperature is decreased, the equilibrium constant (Kc) increases. 6.4.1 State Le Chatelier's principle. 6.4.2 Is the forward reaction EXOTHERMIC or ENDOTHERMIC? Explain your answer in terms of Le Chatelier's principle.

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.

1.4 What is the IUPAC NAME of the major product that will be formed when pentan-2-ol is heated in the presence of an acid catalyst? A. Pentane. B. Pent-2-ene. C. Pent-1-ene. D. Pentanal. 1.5 The activated complex can best be described as the … A. energy absorbed or released in a chemical reaction. B. minimum energy needed for a reaction to take place. C. unstable transition state from reactants to products. D. increase in the kinetic energy of reactants. 1.6 Consider the following equilibrium: COBr₂(g) ⇌ CO(g) + Br₂(g) ΔH > 0. The pressure of the equilibrium mixture is increased. Which ONE of the following combinations are CORRECT at the new equilibrium?

1.4 A compound CH₃(CH₂)₅CH₃ undergoes the following reaction: CH₃(CH₂)₅CH₃ → R + CH₃(CH₂)₂CH₃. Which ONE of the following combinations CORRECTLY describes the type of reaction and the IUPAC name of compound R? A. Elimination; propane. B. Addition; propene. C. Cracking; propane. D. Cracking; propene. 1.5 Which ONE of the following graphs shows the relationship between activation energy (Ea) of a reaction and temperature? 1.6 A reaction reaches equilibrium at 25 °C in a flask according to the following balanced equation: Co(H₂O)₆²⁺(aq) (pink) + 4Cl⁻(aq) ⇌ CoCl₄²⁻(aq) (blue) + 6H₂O(ℓ) ΔH > 0. Which ONE of the following will change the colour of the mixture from pink to blue? A. Adding NH₄Cl(aq). B. Adding water. C. Adding NaOH(aq). D. Cooling the flask.