The exercise

Consider a cell made with zinc and iron strips and dilute sulphuric acid. Which statement about it is true?

1. The conventional current of the cell flows along the wire from the zinc to the iron.
2. Gas bubbles come off the iron electrode.
3. The iron electrode wears away.
4. The iron electrode is the negative terminal of the cell.

Statement 2: gas bubbles come off the iron.

Zinc is the more active metal, so it is the anode (negative) and wears away; iron is the cathode (positive), where hydrogen forms. Conventional current flows in the wire from the iron to the zinc.

Consider a cell made by dipping iron and copper electrodes in dilute sulphuric acid. Which is the anode reaction of the cell?

1. Cu (s) → Cu²⁺ (aq) + 2e
2. Fe²⁺ (aq) + 2e → Fe (s)
3. Fe (s) → Fe²⁺ (aq) + 2e
4. 2H⁺ (aq) + 2e → H₂ (g)

Option 3: iron is oxidised to Fe²⁺.

Iron is above copper, so iron is the anode, and the anode reaction is an oxidation. The hydrogen equation is the cathode reaction.

Which of these is essential for iron to corrode?

1. Water
2. Atmospheric carbon dioxide
3. Acids
4. Bases

Option 1: water.

Activity 12.3.3 showed nails in dry air do not rust. Acids speed rusting and bases slow it, but neither is essential.

Which of these is a factor that makes iron corrode faster?

1. Atmospheric water vapour
2. Atmospheric carbon dioxide
3. Lime water
4. Grease

Option 2: atmospheric carbon dioxide.

Carbon dioxide dissolves in the water on the iron to form a weak acid, carbonic acid, and acids speed rusting. Water vapour is needed for rusting rather than a speeder-up; lime water is a base and grease a barrier, and both slow it.

Iron nails are joined to other metals in four containers: A Fe/Cu, B Fe/Zn, C Fe/Sn, D Fe/Mg. In which containers do the nails corrode?

1. The nails in A and B
2. The nails in B and C
3. The nails in A and C
4. The nails in B and D

Option 3: A and C.

Copper and tin are below iron, so the iron is the anode and corrodes. Zinc and magnesium are above iron and protect it.

Choose the false statement.

1. On an iron spoon, the area where vinegar was smeared has rusted more.
2. Galvanised iron wire rusts quickly where the coating is scratched.
3. A tin-plated can rusts quickly where the coating is scratched.
4. Coating iron with magnesium can prevent the iron rusting.

Statement 2 is false.

Zinc is above iron, so even at a scratch the zinc corrodes in place of the iron. Tin is below iron, so a scratched tin can does rust quickly.

Consider the electrolysis of aqueous sodium chloride solution with carbon electrodes. In this chemical process —

1. hydrogen gas comes off the positive terminal.
2. sodium hydroxide forms in the solution.
3. chlorine gas comes off the cathode.
4. the anode dissolves.

Option 2: sodium hydroxide forms.

Hydrogen comes off the cathode (negative) and chlorine the anode (positive); the carbon anode does not dissolve. Na⁺ and OH^- are left behind.

When copper sulphate solution is electrolysed with carbon electrodes —

1. copper is deposited on the cathode.
2. copper is deposited on the anode.
3. oxygen bubbles form at the negative electrode.
4. the blue colour of the solution stays the same.

Option 1: copper on the cathode.

Cu²⁺ is reduced at the cathode; oxygen forms at the positive anode; and the blue fades as Cu²⁺ leaves the solution.

Which of these substances is not an electrolyte?

1. Aqueous sodium hydroxide
2. Acidified water
3. Solid sodium chloride
4. Aqueous sodium chloride

Option 3: solid sodium chloride.

Its ions are fixed in the crystal and cannot move. Dissolved or molten, it would conduct.

In the electrolysis of acidified water with carbon electrodes —

1. hydrogen gas comes off the anode.
2. oxygen gas comes off the cathode.
3. hydroxide ions are oxidised at the anode.
4. the anode dissolves.

Option 3: hydroxide ions are oxidised at the anode.

They give oxygen there; hydrogen comes off the cathode. A carbon anode does not dissolve.

Which of these is not an industrial use of electrolysis?

1. Nickel-plating an iron spoon
2. Extracting aluminium metal
3. Galvanising iron nails
4. Extracting sodium from molten sodium chloride

Option 3: galvanising iron nails.

Galvanising is done by dipping the iron in molten zinc, not by electrolysis. The other three are electroplating and two extractions.

1. Magnesium metal becoming Mg²⁺ ions
2. Aluminium metal becoming Al³⁺ ions
3. Sodium metal becoming Na⁺ ions
4. H⁺ ions forming hydrogen gas

1. Mg (s) → Mg²⁺ (aq) + 2e — oxidation
2. Al (s) → Al³⁺ (aq) + 3e — oxidation
3. Na (s) → Na⁺ (aq) + e — oxidation
4. 2H⁺ (aq) + 2e → H₂ (g) — reduction

1. Name the anode and the cathode.
2. Name the positive and negative terminals.
3. Write the anode and cathode reactions.
4. Name which electrode reaction is the oxidation and which the reduction.
5. Write the overall cell reaction.
6. Write the changes that can be seen at the electrodes.

1. Zinc is the anode; lead is the cathode.
2. Zinc is the negative terminal; lead the positive.
3. Anode: Zn (s) → Zn²⁺ (aq) + 2e. Cathode: 2H⁺ (aq) + 2e → H₂ (g)
4. The anode (zinc) reaction is the oxidation; the cathode (lead) reaction is the reduction.
5. Zn (s) + 2H⁺ (aq) → Zn²⁺ (aq) + H₂ (g)
6. The zinc slowly dissolves; gas bubbles come off the lead.