Electrolytic ProcessesEdexcel GCSE Chemistry: Revision notes

Section 1

What is electrolysis?

Electrolytes are ionic compounds in the molten state, or dissolved in water, that can conduct electricity because their ions are free to move.

Electrolysis is the process in which electrical energy, from a direct current (d.c.) supply, decomposes (breaks down) an electrolyte into its elements.

During electrolysis, ions move towards the electrode of opposite charge:

Positively charged cations migrate to the negatively charged cathode
Negatively charged anions migrate to the positively charged anode

Key termselectrolyteelectrolysiscathodeanode

Common mistake

Remember opposites attract: positive cations go to the negative cathode, negative anions go to the positive anode — a common exam slip is mixing these up.

Section 2

What products form in electrolysis of aqueous solutions?

Using inert electrodes, the products of electrolysis depend on the ions present and their relative reactivity:

Solution	At cathode	At anode
Copper chloride solution	Copper	Chlorine
Sodium chloride solution	Hydrogen	Chlorine
Sodium sulfate solution	Hydrogen	Oxygen
Water acidified with sulfuric acid	Hydrogen	Oxygen

General rules for aqueous solutions with inert electrodes: at the cathode, hydrogen is produced unless the metal is less reactive than hydrogen (then the metal forms instead). At the anode, oxygen is produced unless a halide ion is present (then the halogen forms instead).

For a molten binary ionic compound (e.g. molten lead bromide), there is no water present, so the metal forms at the cathode and the non-metal forms at the anode — for example, molten lead bromide gives lead at the cathode and bromine at the anode.

Key termsinert electrodes

Exam tip

To predict products of a molten binary ionic compound, simply split the compound into its metal (cathode) and non-metal (anode) elements.

Section 3

How are half equations written? (Higher tier)

Half equations show the reactions occurring at each electrode.

At the cathode (reduction — gain of electrons), e.g.: Cu2+ + 2e− → Cu

At the anode (oxidation — loss of electrons), e.g.: 2Cl− → Cl2 + 2e−

Oxidation is the loss of electrons; reduction is the gain of electrons (this can be remembered as 'OIL RIG'). In electrolysis:

Reduction occurs at the cathode (positive ions gain electrons)
Oxidation occurs at the anode (negative ions lose electrons)

Key termshalf equationoxidationreduction

Exam tip

Use 'OIL RIG' (Oxidation Is Loss, Reduction Is Gain) to remember which electrode does which in electrolysis.

Section 4

How is copper purified by electrolysis?

In the electrolysis of copper sulfate solution using copper electrodes (rather than inert electrodes), the products are different: at the anode, copper atoms lose electrons and dissolve into solution as Cu2+ ions (the anode gradually gets smaller); at the cathode, Cu2+ ions from the solution gain electrons and are deposited as pure copper (the cathode gradually gets bigger).

This is used industrially to purify copper: an impure copper anode dissolves, and pure copper is deposited on the cathode, while insoluble impurities fall to the bottom as 'anode sludge'.

Core practical: investigate the electrolysis of copper sulfate solution with both inert electrodes and copper electrodes to compare the different products.

Key termspurification

Example

With copper electrodes in copper sulfate solution: anode reaction Cu − 2e− → Cu2+ (dissolves); cathode reaction Cu2+ + 2e− → Cu (deposits).

Must Know

Electrolytes are ionic compounds molten or dissolved in water; electrolysis decomposes them using a d.c. supply
Cations move to the cathode (negative); anions move to the anode (positive)
With inert electrodes: cathode gives hydrogen unless the metal is less reactive than hydrogen; anode gives oxygen unless a halide is present
Molten binary compounds split into their elements: metal at cathode, non-metal at anode
(Higher) Reduction (gain of electrons) at cathode; oxidation (loss of electrons) at anode — write half equations for each
With copper electrodes in copper sulfate solution, the anode dissolves and pure copper deposits at the cathode — used to purify copper

That's the notes covered.

Carry on to the next subtopic.