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Electrolysis for Edexcel GCSE Chemistry

Electrolysis

This page covers the following topics:

1. Electrolysis setup
2. Electrolysis of molten compounds
3. Electrolysis of solutions

Electrolysis is the passing of a current through an electrolyte, which moves ions towards respective electrodes, and two separate reactions happen at each of them. The anode, the positive electrode, attracts and oxidises negative ions. The cathode, the negative electrode, attracts and reduces positive ions.

During many of the electrolysis reactions, pure elements are produced. For example:
2AlBrโ‚ƒ โ†’ 2Al + 3Brโ‚‚
The overall reaction can be split to represent processes happening at specific electrodes by using half-equations, such as:
Alยณโบ + 3eโป โ†’ Al
2Brโป โ†’ Brโ‚‚ + 2eโป

Electrolysis setup

Some compounds during their electrolysis produce metals that are highly reactive. If these metals are exposed to water, they react with it and it is therefore not possible to obtain pure metals. Electrolysis of molten compounds can be used to address this issue and separate substances that produce reactive metals.

Platinum or graphite electrodes are commonly used for electrolysis due to being relatively unreactive. Graphite is a less expensive option; however, over the time it can react with oxygen to produce carbon dioxide.

Electrolysis of molten compounds

During the electrolysis of aqueous ionic solutions, multiple positive and negative ions compete to react at relevant electrodes. A more reactive ion participates in a relevant reaction involving it.

Metal ions follow the reactivity series, just in an opposite way, meaning the most reactive metals have the least reactive ions and vice versa. For negative ions, in many common reactions it is reasonable to assume that the larger oxygen-containing ions like sulphate are less reactive than the single-atom ions like chloride.

In some cases of electrolysis of ionic solutions, the reactivity of ions results in hydrogen being produced instead of a metal at the cathode (2Hโบ + 2eโป โ†’ Hโ‚‚). Similarly, sometimes oxygen is produced at the anode during such electrolysis (4OHโป โ†’ 2Hโ‚‚O + 4eโป + Oโ‚‚).

Electrolysis of solutions

1

Which of the following ions reaches the anode first during electrolysis?

SOโ‚„ยฒโป, Clโป, Liโบ, Hโบ, OHโป, POโ‚„ยณโป

The anode, the positive electrode, attracts and oxidises negative ions. For negative ions, in many common reactions it is reasonable to assume that the larger oxygen-containing ions like sulphate are less reactive than the single-atom ions like chloride.

Clโป

Which of the following ions reaches the anode first during electrolysis?

SOโ‚„ยฒโป, Clโป, Liโบ, Hโบ, OHโป, POโ‚„ยณโป

2

Provide a balanced chemical reaction for the electrolysis of a molten compound shown in the image.

During many of the electrolysis reactions, pure elements are produced. Since sodium chloride contains sodium and chloride elements, sodium metal and chloride gas are produced during this reaction.

2NaCl โ†’ 2Na + Clโ‚‚

Provide a balanced chemical reaction for the electrolysis of a molten compound shown in the image.

3

Provide a chemical equation for the electrolysis of copper(II) chloride.

During many of the electrolysis reactions, pure elements are produced. Since copper chloride contains copper and chlorine elements, copper metal and chlorine gas are produced during this reaction.

CuClโ‚‚ โ†’ Cu + Clโ‚‚

Provide a chemical equation for the electrolysis of copper(II) chloride.

4

What are the formulae of ions participating in oxidation and reduction reactions at relevant electrodes during the electrolysis of potassium bromide solution?

During the electrolysis of ionic solutions, multiple positive and negative ions compete to react at relevant electrodes. A more reactive ion participates in a relevant reaction involving it.

Metal ions follow the reactivity series, just in an opposite way, meaning the most reactive metals have the least reactive ions and vice versa. For negative ions, in many common reactions it is reasonable to assume that the larger oxygen-containing ions like sulphate are less reactive than the single-atom ions like chloride.

In a potassium bromide solution there are Kโบ, Brโป, Hโบ, OHโป ions. Hโบ and Brโป are the more reactive ones that participate in redox equations at the electrodes.

Hโบ, Brโป

What are the formulae of ions participating in oxidation and reduction reactions at relevant electrodes during the electrolysis of potassium bromide solution?

5

What two materials are commonly used for electrodes during the electrolysis of molten compounds?

Platinum or graphite electrodes are commonly used for electrolysis due to being relatively unreactive.

platinum, graphite

What two materials are commonly used for electrodes during the electrolysis of molten compounds?

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