3.7 Electrolysis 3 - Aqueous Solutions

Question 1

In aqueous solutions, as well as the ions from the ionic compound, which other ions will be present?

Answer 1

Hydrogen ions and hydroxide ions

Question 2

Where do the hydrogen and hydroxide ions come from in aqueous solutions?

Answer 2

The water

Question 3

What are the rules of electrolysis of aqueous solutions? (2)

Answer 3

1. The negatively charged cathode attracts the positive ions so the metal ion from the compound and the hydrogen ion from the water. Importantly, it will only discharge one of these ions. The ion of the least reactive element will be discharged. Hydrogen gas will be produced if the metal is more reactive than hydrogen. If the metal is less reactive than hydrogen then a solid layer of the pure metal will be produced instead
2. The positively charged anode attracts the negative ions so as well as the negative ion from the compound which could be something like a nitrate ion, the hydroxide ion from the water will also be attracted. If there’s a concentrated solution of chloride ions, chlorine molecules will be formed at the anode. If no chloride ions (or other halides) are present, then the hydroxide ions are discharged and oxygen will be formed. If a halide is present, it is discharged. If a halide is not present, it is always the hydroxide ion that gets discharged.

Question 4

At the cathode, what rule decides whether the metal ions or hydrogen ions are discharged?

Answer 4

Whichever ion is less reactive is discharged

Question 5

At the cathode, if the metal is more reactive than hydrogen, what is produced?

Answer 5

Hydrogen gas

Question 6

At the cathode, if the metal is less reactive than hydrogen (e.g. copper or silver), what will be produced?

Answer 6

A solid layer of the pure metal will be produced instead of

Question 7

At the anode, if theres a concentrated solution of chloride ions, what will form?

Answer 7

Chlorine molecules

Question 8

At the anode, if no chloride ions (or other halides) are present, what happens?

Answer 8

The hydroxide ions are discharged and oxygen will be formed

Question 9

A solution of copper (II) sulfate contains four different ions. What are these?

Answer 9

Copper, Sulfur, Hydrogen, Hydroxide

Question 10

In the case of a solution of copper (II) sulphate, what is discharged and formed at the cathode?

Answer 10

The copper ions - they will gain 2 electrons each from the cathode and form pure copper metal. This coats the electrode.

Question 11

In the case of a solution of copper (II) sulphate, why is copper discharged at the cathode?

Answer 11

It is less reactive than hydrogen

Question 12

In the case of a solution of copper (II) sulphate, what is discharged and produced at the anode?

Answer 12

The hydroxide ions - they give up their extra electron to the anode and form oxygen and water. The oxygen can be seen as bubbles.

Question 13

In the electrolysis of aqueous solutions, what is the rule for determining which ion will be oxidised (lose electrons) at the anode (positive electrode)?

Answer 13

It will always be the hydroxide unless there is a halide ion (such as chlorine or bromine) present

Question 14

In the electrolysis of aqueous solutions, what is the rule for determining which ion will be reduced (gain electrons) at the cathode (negative electrode)?

Answer 14

It is always the ion of the least reactive element

Question 15

In the electrolysis of aqueous sodium chloride, NaCl (aq), which of the four ions will be oxidised at the anode (positive electrode)?

Answer 15

Chloride ions

Question 16

In the electrolysis of aqueous sodium chloride, NaCl (aq), which of the four ions will be reduced at the cathode (negative electrode)?

Answer 16

Hydrogen ions