Redox Reactions, Chemical Cells and Electrolysis

Choose the best answer for each question.

1. Sulfur dioxide reacts with hydrogen sulfide:
   SO₂ + 2H₂S → 3S + 2H₂O
   Which statement is correct?
   1. Sulfur dioxide is oxidised.
   2. Hydrogen sulfide is reduced.
   3. Sulfur dioxide is the oxidising agent.
   4. The oxidation number of sulfur in both reactants increases.
2. Acidified permanganate ions oxidise iron(II) ions to iron(III) ions:
   MnO₄⁻ + H⁺ + Fe²⁺ → Mn²⁺ + H₂O + Fe³⁺
   What is the mole ratio of MnO₄⁻ to Fe²⁺ in the balanced equation?
   1. 1 : 2
   2. 1 : 5
   3. 2 : 5
   4. 5 : 1
3. Chlorine reacts with cold dilute sodium hydroxide:
   Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O
   Which statement is correct?
   1. Chlorine is oxidised only.
   2. Chlorine is reduced only.
   3. Chlorine is both oxidised and reduced.
   4. Hydroxide ions act as the oxidising agent.
4. The following electrochemical series is arranged in the DSE direction:

   Oxidised form	Electron change	Reduced form
   K+	+ e− ⇌	K
   Zn2+	+ 2e− ⇌	Zn
   Fe2+	+ 2e− ⇌	Fe
   2H+	+ 2e− ⇌	H2
   Cu2+	+ 2e− ⇌	Cu
   Cl2	+ 2e− ⇌	2Cl−
   F2	+ 2e− ⇌	2F−

   Which reaction occurs spontaneously?
   1. Cu + Zn²⁺ → Cu²⁺ + Zn
   2. Fe + Cu²⁺ → Fe²⁺ + Cu
   3. 2Cl⁻ + 2F⁻ → Cl₂ + F₂
   4. Cu + 2H⁺ → Cu²⁺ + H₂
5. A zinc–copper chemical cell consists of zinc–zinc ion and copper–copper ion half-cells connected by a salt bridge. Which statement is correct when the cell operates?
   1. The zinc electrode is the positive electrode.
   2. Electrons flow from copper to zinc.
   3. The concentration of zinc ions decreases.
   4. Anions in the salt bridge migrate towards the zinc half-cell.
6. A chemical cell contains these two half-cells:
   • Fe³⁺(aq)/Fe²⁺(aq), with a platinum electrode
   • I₂(aq)/I⁻(aq), with a platinum electrode
   2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
   Which statement is correct?
   1. Iron(III) ions are oxidised.
   2. Iodide ions are reduced.
   3. Electrons flow from the iodine half-cell to the iron half-cell.
   4. The platinum electrode in the iodine half-cell gains mass.
7. Concentrated sodium chloride solution is electrolysed using graphite electrodes. Which combination is correct?

   Option	Cathode product	Anode product	Main product remaining in solution
   A	Sodium	Chlorine	Hydrochloric acid
   B	Hydrogen	Chlorine	Sodium hydroxide
   C	Hydrogen	Oxygen	Sodium hydroxide
   D	Sodium	Oxygen	Sodium hydroxide

   1. Combination A
   2. Combination B
   3. Combination C
   4. Combination D
8. Dilute and concentrated sodium chloride solutions are separately electrolysed using inert electrodes. Which statement is correct?
   1. Sodium is formed at the cathode of the concentrated solution.
   2. Chlorine is always formed at the anode, regardless of concentration.
   3. Hydrogen is normally formed at the cathode in both experiments.
   4. Oxygen is formed at the cathode of the dilute solution.
9. Aqueous copper(II) sulfate is electrolysed in two experiments:
   • Experiment I uses graphite electrodes.
   • Experiment II uses copper electrodes.
   Which statement is correct?
   1. Copper is deposited at the cathode only in Experiment II.
   2. Oxygen is formed at the anode in both experiments.
   3. The blue colour becomes paler more readily in Experiment I.
   4. The copper anode gains mass in Experiment II.
10. A steel spoon is to be electroplated with copper. Which arrangement is correct?

    Option	Cathode	Anode	Electrolyte
    A	Steel spoon	Copper	Copper(II) sulfate solution
    B	Copper	Steel spoon	Copper(II) sulfate solution
    C	Steel spoon	Graphite	Sodium sulfate solution
    D	Copper	Graphite	Copper(II) sulfate solution

    1. Arrangement A
    2. Arrangement B
    3. Arrangement C
    4. Arrangement D

Write “True” or “False” for each statement. If a statement is false, rewrite it correctly.

1. Oxidation must involve the gain of oxygen.
2. A reducing agent causes another substance to be reduced and is itself oxidised.
3. In a polyatomic ion, the sum of the oxidation numbers of all the atoms is equal to the charge of the ion.
4. In a chemical cell that generates electricity, oxidation occurs at the negative electrode.
5. Electrons move through the salt bridge from one half-cell to the other.
6. A salt bridge completes the circuit and helps maintain electrical neutrality in the two half-cells.
7. An inert electrode conducts electrons but does not normally take part in the overall chemical reaction.
8. Oxidation always occurs at the anode, although the sign of the anode is different in a chemical cell and an electrolytic cell.
9. During the electrolysis of concentrated sodium chloride solution, sodium ions are preferentially discharged because their concentration is high.
10. During the electrolysis of copper(II) sulfate solution using copper electrodes, the copper(II) ion concentration remains approximately constant.

A student electrolyses aqueous copper(II) sulfate using graphite electrodes. The solution is initially blue.

1. Name the ions present in the solution before electrolysis. [2]
2. State which ions or molecules are preferentially discharged at:
   1. the cathode;
   2. the anode.
   Explain each answer. [4]
3. Write an ionic half-equation for the reaction at:
   1. the cathode;
   2. the anode.
   [2]
4. State two observations at the cathode. [2]
5. State one observation at the anode and describe a chemical test for the product formed. [3]
6. State and explain the change in:
   1. the intensity of the blue colour;
   2. the acidity of the solution.
   [4]
7. Write the overall equation for the electrolysis. [2]
8. The experiment is repeated using copper electrodes. State and explain:
   1. the change in mass of the cathode;
   2. the change in mass of the anode;
   3. the change in copper(II) ion concentration;
   4. whether a gas is formed at the anode.
   Support your answer with the relevant half-equations. [7]
9. A student claims:
   “Graphite and copper are both electrical conductors, so using them as anodes must give the same product.”
   Explain why this statement is incorrect. [2]

The relevant electrochemical series is shown below in the DSE direction.

Two sodium chloride solutions are electrolysed using graphite electrodes:

• Cell A contains dilute sodium chloride solution.
• Cell B contains concentrated sodium chloride solution.

1. For both cells:
   1. state the product formed at the cathode;
   2. write the cathodic half-equation;
   3. explain why sodium is not formed.
   [4]
2. For Cell A:
   1. state the expected product at the anode;
   2. write the anodic half-equation;
   3. describe a test for the product.
   [4]
3. For Cell B:
   1. state the expected product at the anode;
   2. write the anodic half-equation;
   3. describe two observations at the anode;
   4. describe a chemical test for the product.
   [6]
4. Explain why different products may be formed at the anodes of Cells A and B even though both electrolytes contain the same types of ions. [2]
5. State the main substance that accumulates in Cell B after electrolysis. Explain your answer using the ions remaining or formed in the solution. [3]
6. Explain why the gaseous products should be collected separately rather than allowed to mix. [2]

A student then wants to electroplate an iron key with copper using copper(II) sulfate solution.

7. State what should be used as:
   1. the cathode;
   2. the anode;
   3. the electrolyte.
   [3]
8. Before electroplating, the iron key is cleaned carefully. Explain why this is necessary. [2]
9. Write the half-equations at the two electrodes during electroplating. [2]
10. State and explain the expected change in:
    1. the mass of the iron key;
    2. the mass of the copper electrode;
    3. the intensity of the blue colour of the electrolyte.
    [5]
11. State one possible environmental problem caused by the improper disposal of waste from an electroplating factory. [1]