Rate of reaction: Grade 12 Past Paper Questions

Pure powdered copper, Cu(s), is oxidised in EXCESS concentrated nitric acid, HNO₃(conc.), as shown by the balanced equation below. Cu(s) + 4HNO₃(conc.) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l), ΔH < 0. 5.1 Explain what is meant by 'IN EXCESS' in the statement above. 5.2 The volume of nitrogen dioxide gas produced is shown in the graph below. 5.2.1 Use the collision theory to explain the decrease in the rate of the reaction between t₃ and t₄. 5.2.2 How does the rate of the reaction at t₂ compare to that at t₁? Choose from HIGHER THAN, LOWER THAN or REMAINS THE SAME. 5.2.3 Give a reason for the answer to QUESTION 5.2.2 by referring to the graph. 5.2.4 Calculate the value of time t if the average rate for the production of 1,7 dm³ of NO₂(g) is 0,02 dm³·s⁻¹. 5.2.5 The initial concentration of the nitric acid is 12 mol·dm⁻³. If 20 cm³ of nitric acid is used, calculate the hydronium ion concentration after the reaction is completed. (Assume the volume of the acid does not change.) Take the molar gas volume to be 25 dm³·mol⁻¹. A change is made to the reaction conditions and the reaction is repeated. 5.2.6 Which ONE of the following changes will result in more NO₂ gas being produced when the reaction is completed? Choose from A, B or C. (Write down only the letter.) A. A piece of pure copper with the same mass is used. B. Impure copper with greater mass is used. C. Higher concentration HNO₃ in excess is used. 5.3 Determine the oxidation number of nitrogen in HNO₃.

EXPERIMENT III. 5.6 Experiment I is repeated, but a catalyst is added, and the following graph is obtained: a catalysed chemical reaction, showing energy against the progress of the reaction. 5.6.1 Is the reaction EXOTHERMIC or ENDOTHERMIC? 5.6.2 Which letter represents the activated complex? 5.6.3 Give the value of the activation energy for the forward catalysed reaction.

EXPERIMENT II. 5.5 Experiment I is repeated, but this time at 30 °C. The following Maxwell-Boltzmann distribution curve was produced, showing number of particles against kinetic energy for curves A and B. 5.5.1 Which graph, A or B, best represents Experiment II? 5.5.2 Explain the answer to QUESTION 5.5.1. 5.5.3 If a catalyst was added, would the line representing the activation energy (Ea) be drawn to the LEFT or the RIGHT of the current line?

Three different experiments are performed using the reaction between magnesium carbonate and hydrochloric acid: MgCO₃(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) + CO₂(g). EXPERIMENT I. In this experiment, magnesium carbonate reacts with EXCESS hydrochloric acid in a closed container at 23 °C. The reaction is monitored by measuring the volume of carbon dioxide gas produced over time. The molar gas volume at 23 °C is 24 dm³·mol⁻¹. The data collected is shown in a table of time against the volume of CO₂ produced. 5.1 Define the term reaction rate. 5.2 Use the collision theory to explain the effect on the reaction rate as time progresses. 5.3 Calculate the reaction rate for the production of CO₂(g) during the first 20 seconds, in dm³·s⁻¹. 5.4 Calculate the mass of MgCO₃ required for this experiment.

The reaction between sodium carbonate (Na₂CO₃) and EXCESS hydrochloric acid (HCl) is used to investigate factors that affect the rate of a chemical reaction. The balanced equation for the reaction is: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(ℓ) + CO₂(g). In the investigation 2,3 g of sodium carbonate is added to hydrochloric acid in all four experiments. The table below summarises the reaction conditions in each experiment: the initial concentration of HCl (mol·dm⁻³), the volume of HCl (cm³), the state of division of Na₂CO₃ and the temperature (°C). 5.1 Define the term reaction rate. 5.2 For the investigation between experiment 1 and 2, write down the: 5.2.1 Independent variable. 5.2.2 Dependent variable. 5.3 How will the volume of CO₂(g) produced in experiment 2 compare to that produced in experiment 3? Choose from GREATER THAN, SMALLER THAN or EQUAL TO. 5.4 Use the collision theory to explain why the reaction rate in experiment 3 will be higher than that in experiment 1.

5.5 The Maxwell-Boltzmann energy distribution curves for particles in each of experiments 3 and 4 are shown below. 5.5.1 Which experiment, 3 or 4, is represented by B? 5.5.2 Explain the answer to QUESTION 5.5.1. 5.6 Calculate the average rate at which hydrochloric acid (HCl) in experiment 3 reacts in mol·dm⁻³·s⁻¹ if the reaction takes 20 s to reach completion. Assume the volume remains constant.

An unknown mass of zinc granules is added to a dilute hydrochloric acid solution in a conical flask at room temperature. The balanced chemical equation of the reaction taking place is: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). The graph below shows the change in mass of the zinc granules as they react with the acid. The temperature of the reaction mixture increases as the reaction progresses. 5.1 Is the reaction above EXOTHERMIC or ENDOTHERMIC? Give a reason. 5.2 Why is the graph horizontal after 50 seconds? 5.3 Define reaction rate. 5.4 How will the rate of this reaction be affected if POWDERED zinc was used instead of zinc granules? Write down INCREASES, DECREASES, or REMAINS THE SAME. 5.5 Use the collision theory to explain your answer in Question 5.4. The volume of hydrogen gas collected after 50 seconds is 0,168 dm³. Take the molar volume of gas at room temperature as 24 dm³·mol⁻¹. 5.6 Calculate the value of Y shown on the graph. 5.7 Calculate the rate at which zinc reacts in g·s⁻¹.

Two learners want to investigate the factors affecting the rate of reaction between magnesium (Mg) and hydrochloric acid (HCl). The concentration of the acid remains the same. Which of the following changes to the experiment will produce the fastest rate of reaction? A. Powdered magnesium with hydrochloric acid at room temperature. B. Powdered magnesium with hydrochloric acid at a higher temperature. C. Magnesium ribbon with hydrochloric acid at a higher temperature. D. Magnesium ribbon with hydrochloric acid at room temperature.

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.

1.3 Consider the equation for the reaction below. R is the MAJOR PRODUCT. CH₃CH₂CH=C(CH₃)CH₃ + HBr → R. Which ONE of the following is the CORRECT combination for the type of reaction and the formula of R? A. Addition, CH₃CH₂CHBrCH(CH₃)CH₃. B. Substitution, CH₃CH₂CH₂CBr(CH₃)CH₃. C. Addition, CH₃CH₂CH₂CBr(CH₃)CH₃. D. Substitution, CH₃CH₂CHBrCH(CH₃)CH₃. 1.4 The Maxwell-Boltzmann distribution curve for a sample of a gas in a container is shown below. Which ONE of the following will change the area under the curve? A. The addition of a catalyst. B. A decrease in temperature. C. Gas particles escaping from the container. D. A decrease in concentration by increasing the volume of the container.

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.