Electrolysis

Electrolysis Basics

The anode is the positive electrode and cathode is the negative

Therefore negative ions are attracted to the anode (anions) and positive ions are attracted to the cathode (cations)

The electrolyte solution must be ether a solution or molten so it can conduct electricity

The cation will always be the metal since it gains electrons to make it’s full outer shell

Always connect the electrodes to a power source e.g. a battery.

Electrolysis with Aqueous Solutions

H₂O ⇌ H⁺ + OH^–

At the anode:

• Negative OH ions and non-metal ions are attracted here

• If halide ions are present then it gets discharged, loses electrons, and forms a halogen

• If there are none then OH^– is discharged, loses electrons and forms oxygen

At the cathode:

• Either hydrogen gas or the metal is produced

• The least reactive ion is discharged: hydrogen will be seen bubbling at the cathode IF IT’S DISCHARGED

Inert and Non-Inert Electrodes

Inert electrodes are un-reactive

Inert electrodes are used (graphite) because they cannot partake in the reaction

Non-inert electrodes are used to purify or electroplate copper

Purifying is necessary because copper extracted from ore is not pure enough to use

To purify copper:

• Both electrodes are copper

• The anode is impure (+) and the cathode is pure (-)

• Electrolyte is copper(III) sulphate solution

• Impure copper anode loses copper atoms — oxidation

• Impurities fall into the electrolyte solution from the anode.

• Copper ions from the solution are reduced at the pure cathode

• This forms a layer of pure copper

To electroplate copper:

• Anode is the coating material.

• Metal atoms lose electrons at the anode to form ions

• These ions move to the cathode through the solution and gain electrons, before discharging on the metal that needs coating

Redox Reactions

Oxidation is loss, reduction is gain (of electrons)

As ions make contact with the electrodes, they either lose or gain an electron and become neutral

At the anode, metal ions lose electrons and are thus oxidised

At the cathode, non-metal ions gain electrons and are thus reduced

Half Equations

Half equations show the transfer of electrons

For example:

• Lead ions are discharged at the cathode and gain electrons (reduction), so their half equation is…

• Pb²⁺ + 2e^– ⟶ Pb

• Bromide ions are discharged at the anode and lose electrons (oxidation), so their half equation is…

• 2Br^- – 2e^– ⟶ Br₂

• OR

• 2Br^- ⟶ Br₂ + 2e^–