Basic Electrolysis Questions

2019 O Level, MCQ 13

Four different conditions under which sodium chloride is electrolysed using inert electrodes are listed.

  1. concentrated aqueous sodium chloride
  2. dilute aqueous sodium chloride
  3. molten sodium chloride
  4. solid sodium chloride
Under which conditions is a green gas formed?

Answer: 1 and 3

  1. When concentrated chloride ions are present, chloride ions are in higher concentration than hydroxide ions, causing chloride ions to be preferentially discharged.
  2. However, in a dilute solution of sodium chloride, hydroxide ions give up electrons more readily than chloride ions. Therefore, hydroxide ions are preferentially discharged to form colourless, odourless oxygen gas.
  3. In molten sodium chloride, only chloride ions are present at the anode for discharge.
  4. Solid sodium chloride has no free-moving ions. Electrolysis does not occur and no products are formed.

2019 O Level, MCQ 14

Aqueous copper(II) sulfate is electrolysed using a positive carbon electrode and a negative copper electrode. The table below shows a list of possible products at the two electrodes.

positive electrode negative electrode
A electrode dissolves copper deposited
B hydrogen gas given off oxygen gas given off
C oxygen gas given off copper deposited
D oxygen gas given off hydrogen gas given off
Which row is correct?

Answer: C

At the positive electrode:

  1. The positive electrode is inert, and does not take part in the reaction.
  2. Negatively charged ions, namely hydroxide ions and sulfate ions, are attracted.
  3. Hydroxide ions are preferentially discharged, oxidised to form oxygen gas,

At the negative electrode:

  1. The negative electrode is theoretically reactive. However, copper cannot be reduced and is hence not reacted.
  2. Positively charged ions, namely copper(II) ions and hydrogen ions, are attracted.
  3. Copper(II) ions are preferentially discharged, reduced to form copper that is deposited on the electrode..

2018 O Level, MCQ 14

Four electrolytes are listed. Each is electrolysed using inert electrodes.

  1. aqueous copper(II) bromide
  2. concentrated aqueous sodium chloride
  3. dilute aqueous sodium chloride
  4. molten aluminium oxide
For which electrodes is a metal formed at the cathode (negative electrode)?

Answer: 1 and 4

  1. Both copper(II) and hydrogen cations are present. However, copper(II) ions gains electrons more readily than hydrogen ions, and hence copper(II) ions are preferentially discharged to form copper metal.
  2. Both sodium and hydrogen cations are present. While sodium ions exist in high concentration, they gain electrons way less readily than hydrogen ions. Thus, hydrogen ions are still preferentially discharged to form hydrogen gas instead.
  3. Both sodium and hydrogen cations are present. Hydrogen ions gain electrons more readily than sodium ions, and hence hydrogen ions are preferentially discharged to form hydrogen gas.
  4. Only aluminium cations are present. They will be discharged to form aluminium metal.

2018 O Level, MCQ 15

What happens to the ions that move to the negative electrode during the electrolysis of molten potassium iodide?

A. Ions are reduced.
B. Ions form molecules.
C. Ions increase in charge.
D. Ions release electrons.

Answer: A

Positive ions are attracted to the negative electrode for reduction.

Here, potassium ions are attracted. They gain electrons, reducing their positive charge to neutral, to form potassium. As potassium is a metal, it has a giant metallic lattice structure, and does not exist as molecules.

2018 O Level, Open-ended Question A2

During the electrolysis of concentrated aqueous copper(II) chloride using carbon electrodes, what product is formed at the positive electrode?

Chlorine gas.

The positive electrode attracts negatively charged ions: chloride and hydroxide ions. As the concentration of chloride ions are greater, they are preferentially discharged to form chlorine gas.

During the electrolysis of dilute aqueous copper(II) sulfate using copper electrodes, what product is formed at the negative electrode?

Copper metal.

The negative electrode is where reduction occurs, attracting positively charged ions: copper(II) and hydrogen ions. As copper(II) ions gain electrons more readily than hydrogen ions, copper(II) ions are reduced to form copper metal.

During the electrolysis of an unknown substance using platinum electrodes, chlorine and sodium are produced at the positive and negative electrodes respectively. Deduce the substance electrolysed.

Molten sodium chloride.

Chlorine and sodium came from the chloride ions and sodium ions of sodium chloride.

To preferentially discharge sodium ions, sodium chloride must be in the molten state. Otherwise, in the aqueous state, hydrogen ions will preferentially discharge instead.

Challenging Electrolysis Questions

2018 O Level, MCQ 13

In the diagram, each cell contains an aqueous solution of a single salt. All four electrodes are platinum.

Electrodes Q and S increase in mass during the electrolysis but no gas is given off at Q or S. The increase in mass of Q is greater than the increase in mass of S.

Which statement must be correct?

A. The cation in cell 1 is the same as in cell 2 but the solution in cell 1 is more concentrated than in cell 2.
B. The cation of the solution in cell 1 is different from the cation in the solution in cell 2.
C. The current flowing in cell 1 is greater than the current flowing in cell 2.
D. The loss of mass of electrode P is less than the loss of mass of electrode R.

Answer: B

Let’s answer this question by elimination. Option D is glaringly wrong as the platinum electrodes are inert. They are unreactive and do not participate in electrolysis.

In this question, the electrolytic cells are connected in series, whereby the same current flows through both cell 1 and cell 2. This means that option C is wrong.

The series connection means that the number of electrons gained or lost at each electrode is exactly the same. Comparing both cathodes Q and S, the same number of electrons are gained by the respective metals. However, the increase in mass is different. This means that the cations are different to form metals of different molar mass. Furthermore, the original charge of each cation may be different, such that the same number of electrons would reduce different number of cations of different charges. This means that option A is wrong, while option B is correct.

Simple Cell Questions

2019 O Level, Open-ended Question A7

Two metal electrodes and an electrolyte can be used to produce electrical energy.

The table shows the voltage produced by some cells when different metals are used.

metal 1 metal 2 voltage/ V
copper zinc 1.10
copper nickel 0.60
silver zinc 1.56
silver nickel 1.06
silver iron 1.25
Explain why different combinations of metals produce different voltage?

As metals have different reactivity, different combinations have varying differences in reactivity between the two metals. The greater the difference, the larger the voltage produced.

Suggest the voltage that will be produced when metal 1 is copper and metal 2 is iron.

0.85, or any value between 1.10 and 0.60. When metal 1 is silver, it produces the greatest potential difference with zinc, followed by iron and then nickel. This suggests that the reactivity of iron is between zinc and nickel. This trend is repeated when metal 1 is copper.

Suggest the voltage that will be produced when metal 1 is silver and metal 2 is magnesium.

2, or any reasonable value above 1.56. Because magnesium is more reactive than zinc, its voltage will be larger than when metal 2 is zinc.