MC test: Electrochemical cells (AHL)

Multiple choice test on 19.1 Electrochemical cells (AHL)

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet). You may find a calculator helpful to answer one of the questions.

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

A simple cell is made from a manganese half-cell and a copper half-cell.

Mn2+(aq) + 2e− ⇄ Mn(s) E⦵ = − 1.18 V

Cu2+(aq) + 2e− ⇄ Cu(s) E⦵ =  + 0.34 V

Which is a correct statement about this cell?

The electrons flow from the Mn(s)/Mn2+(aq) half-cell (more negative E⦵ value) to the Cu2+(aq)/Cu(s) half-cell, so the spontaneous reaction is Mn(s) + Cu2+(aq) → Cu(s) + Mn2+(aq) and E⦵cell = 1.52 V.

 

When a zinc half-cell is connected to a tin half-cell the electrons in the external circuit flow from zinc to tin and the EMF of the cell is 0.62 V.

When a tin half-cell is connected to a nickel half-cell the electrons in the external circuit flow from nickel to tin and the EMF of the cell is 0.12 V.

Zn2+(aq) + 2e− ⇄ Zn(s) E⦵ = − 0.76 V

What is the standard electrode potential for Ni2+(aq) + 2e− ⇄ Ni(s)?

Zn(s) + Sn2+(aq) → Zn2+(aq) + Sn(s) Ecell⦵ = 0.62 V so Sn2+(aq) + 2e− ⇄ Sn(s) E⦵ = 0.62 V − 0.76 V = − 0.14 V
Ni(s) + Sn2+(aq) → Ni2+(aq) + Sn(s) Ecell⦵ = 0.12 V so Ni2+(aq) + 2e− ⇄ Ni(s) E⦵ = − 0.12 V − 0.14 V = − 0.26 V

 

What will be observed when the electrolysis of copper(II) sulfate solution is performed using copper electrodes?

With copper electrodes the cathode increases in mass due to Cu2+(aq) + 2e− → Cu(s) and the anode decreases in mass due to Cu(s) → Cu2+(aq) + 2e−. This means [Cu2+(aq)] remains constant and no gases are evolved.

 

Which factors may affect the nature of the products formed during the electrolysis of aqueous solutions?

I. The amount of charge passed.

II. The concentration of the solution.

III. The material(s) used for the electrodes.

The amount of charge will affect the amount of products formed but will not affect their nature.

 

What will be formed at the cathode during the electrolysis of dilute sodium chloride solution?

Hydrogen is lower in the ECS than sodium so is preferentially discharged at the negative electrode (cathode).

 

Two electrolytic cells are connected in series. One contains a dilute solution of sulfuric acid and the other contains molten magnesium bromide. After a certain time the mass of magnesium metal formed = 0.486 g. What amount of oxygen gas will have been evolved during this period of time.

Mg2+(aq) + 2e− → Mg(s) and 4OH−(aq) → O2(g) + 2H2O(l) + 4e− (or 2H2O(l) → O2(g) + 4H+(aq) + 4e−) . Since the same charge passed through both cells, 1 mol of O2 is formed for every 2 mol of Mg.

 

What inert material is surrounded by hydrogen gas and hydrogen ions to make the electrode used in a standard hydrogen electrode?

A standard hydrogen electrode (SHE) consists of an inert platinum electrode in contact with 1 mol dm-3 solution of hydrogen ions and hydrogen gas at 100 kPa and 298 K.

 

What will be the Gibbs free energy change, ΔG⦵ for the reaction between iron metal and lead(II) ethanoate under standard conditions?

Fe2+(aq) + 2e− ⇄ Fe(s) E⦵ = − 0.45 V

Pb2+(aq) + 2e− ⇄ Pb(s) E⦵ = − 0.13 V

I Faraday = 9.65 x 104 C mol−1

Ecell⦵ = − 0.13 V − (− 0.45 V) = 0.32 V.
ΔG⦵ = −nFEcell⦵ = − 2 x 9.65 x 104 x 0.32 = − 61760 J = − 61.8 kJ

 

Which row correctly identifies the anode, cathode and electrolyte used in the electroplating of a steel spoon with silver?

Row Anode Cathode Electrolyte
1 silver steel spoon Fe(NO3)2(aq)
2 silver steel spoon AgNO3(aq)
3 steel spoon silver Fe(NO3)2(aq)
4 steel spoon silver AgNO3(aq)

At the anode Ag(s) → Ag+(aq) + e−; at the cathode Ag+(aq) + e− → Ag(s).

 

Why should hydrochloric acid not be used to acidify potassium chromate(VI) when it is used as an oxidizing agent with organic compounds?

Cr2O72−(aq) + 14H+(aq) + 6e− ⇄ 2Cr3+ + 7H2O(l) E⦵ = + 1.36 V

1/2Cl2(g) + e− ⇄ Cl−(aq) E⦵ = + 1.36 V

Ecell⦵ is only zero under standard conditions. If the conditions change and Ecell⦵ becomes positive for the reaction between dichromate(VI) and chloride ions then the reaction will become spontaneous (ΔG will be < 0) and the chloride ions in the hydrochloric acid may be oxidized by the dichromate ions instead of (or as well as) the organic compound.

 

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