Electrochemical reactions: Grade 12 Past Paper Questions

An electrolytic cell is set up for the electrolysis of a concentrated copper(II) chloride solution, CuCl₂(conc.). X and Y are carbon electrodes. 9.1 Define the term electrolysis. 9.2 Write down balanced equations for TWO reactions that will take place at the anode. 9.3 Write down the NAME or FORMULA of the substance produced in higher amount at electrode Y. 9.4 A solution of zinc iodide, ZnI₂, is added to the electrolyte of this cell. What product(s) will be formed at 9.4.1 Electrode X? 9.4.2 Electrode Y?

A group of learners set up a standard galvanic cell, using cadmium and chlorine gas electrodes, as shown below. 8.1 Define the term galvanic cell. 8.2 For this cell, write down the: 8.2.1 Cell notation. 8.2.2 Direction of the electron flow. Choose from P TO Q or Q TO P. 8.2.3 Reduction half-reaction. 8.3 How will the reading on the voltmeter be affected when the concentration of the chloride ions is increased? Choose from INCREASES, DECREASES or REMAINS THE SAME. Give a reason for the answer.

8.1 A piece of nickel (Ni) is placed in a beaker containing copper (Cu²⁺) solution as shown in the diagram below. It is observed that the reading on the thermometer increases which indicates that a spontaneous redox reaction had taken place. 8.1.1 Define the term reduction in terms of electron transfer. 8.1.2 Refer to the relative strengths of the reducing agents to explain why a redox reaction had taken place. 8.2 A galvanic cell is set up under standard conditions using Cu²⁺|Cu as a fixed half-cell. Half-cell B is changed and connected to the standard Cu²⁺|Cu half-cell and the initial voltmeter reading is recorded. The results are shown in the table below: experiment 1, Ni|Ni²⁺, 0,61 V; experiment 2, Fe|Fe²⁺, 0,78 V; experiment 3, half-cell X, 2,00 V. 8.2.1 State ONE standard condition that is applicable to these cells. 8.2.2 Will the concentration of Fe²⁺ in experiment 2 INCREASE, DECREASE or REMAIN THE SAME as the cell is in operation? Give a reason for the answer. 8.2.3 Write down the conclusion regarding the relative strength of the reducing agents and the initial voltmeter readings for experiments 1 and 2. 8.2.4 Identify X using relevant calculations.

The electrolytic cell below is set up to obtain pure copper from a piece of impure copper. The impure copper contains platinum and silver. The graph below, NOT drawn to scale, represents the changes in the mass of electrode P during electrolysis. 9.1 Is the power source AC or DC? Give a reason for the answer. 9.2 Which electrode, P or Q, contains the impure copper? Give a reason for the answer. 9.3 Write down the half-reaction that occurs at electrode P. 9.4 How will the concentration of CuSO₄ at the start of the process compare to that after 10 hours? Choose from INCREASES, DECREASES or REMAINS THE SAME. 9.5 Calculate the number of electrons transferred during the process.

The diagram below shows a galvanic cell that operates under standard conditions. A and B are carbon rods. 8.1 State the energy conversion taking place in this cell. 8.2 Write down ONE function of component X. 8.3 Which electrode, A or B, is the cathode? 8.4 For this cell, write down: 8.4.1 The formula of the reducing agent. 8.4.2 The reduction half-reaction. 8.4.3 The cell notation. 8.5 Calculate the cell potential of the above cell. 8.6 The concentration of Sn⁴⁺ ions is increased. How will this change affect the following? Write down INCREASES, DECREASES, or REMAINS THE SAME. 8.6.1 The size of electrode A. 8.6.2 The voltmeter reading.

Copper chloride solution (CuCl₂(aq)) is used as an electrolyte in the copper purification process as shown in the diagram below. 9.1 Define the term electrolysis. 9.2 Is the reaction taking place in this cell EXOTHERMIC or ENDOTHERMIC? 9.3 It is observed that the mass of electrode B decreases while the cell is in operation. 9.3.1 Which terminal, X or Y, is the negative terminal of the battery? 9.3.2 Which electrode is pure copper? Write down A or B. 9.3.3 Write down the half-reaction taking place at electrode A. 9.4 One major change is now made to this cell: the two electrodes, A and B, are replaced with two CARBON rods. 9.4.1 How will the concentration of copper ions in the electrolyte be affected while the cell is in operation? Write down INCREASES, DECREASES, or REMAINS THE SAME. 9.4.2 Write down the NAME or FORMULA of the gas that will be formed at the anode. 9.4.3 Write down a half-reaction to support your answer in question 9.4.2.

According to the Arrhenius theory, an acid is: A. a source of H⁺ ions in water. B. a source of OH⁻ ions in water. C. a substance that can accept a proton. D. a substance that can donate a proton. A property of sodium nitrate solution, NaNO₃(aq), is that it is: A. acidic. B. basic. C. ampholytic. D. neutral. An iron rod, Fe(s), is placed in a beaker containing chromium(III) ions, Cr³⁺(aq). Which ONE of the following is the CORRECT combination of products formed? A. Fe²⁺(aq) and Cr(s). B. Fe²⁺(aq) and Cr²⁺(aq). C. Fe³⁺(aq) and Cr²⁺(aq). D. Fe³⁺(aq) and Cr(aq).

1.9 Consider the balanced net ionic equation for the reaction that occurs in a galvanic cell: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻. Which ONE of the following is the CORRECT cell notation for the galvanic cell? 1.10 Consider the following statements regarding an electrolytic cell. I: Electrons flow from anode to cathode. II: The anode is the negative electrode. III: The oxidising agent is reduced at the cathode. Which of the above statement(s) is/are TRUE?

1.7 Dilute nitric acid is added to distilled water at 25 °C. How will this affect the hydronium ion concentration [H₃O⁺] and the ionisation constant (Kw) of water at 25 °C? A. [H₃O⁺] decreases; Kw increases. B. [H₃O⁺] increases; Kw remains the same. C. [H₃O⁺] remains the same; Kw increases. D. [H₃O⁺] increases; Kw decreases. 1.8 In a galvanic (voltaic) cell, electrons move from the … A. anode to the cathode in the external circuit. B. cathode to the anode in the external circuit. C. anode to the cathode through the salt bridge. D. cathode to the anode through the salt bridge. 1.9 Which ONE of the following redox reactions is SPONTANEOUS under standard conditions? A. Br₂(aq) + 2Cl⁻(aq) → 2Br⁻(aq) + Cl₂(g). B. 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(g). C. Ni²⁺(aq) + H₂(g) → Ni(s) + 2H⁺(aq). D. Cu(s) + Pb²⁺(aq) → Cu²⁺(aq) + Pb(s). 1.10 A concentrated sodium chloride solution (NaCl) undergoes electrolysis in an electrolytic cell. The product(s) formed at the cathode is/are: A. H₂ and OH⁻. B. O₂ and H⁺. C. Na. D. Cl₂.