4.4.3 - Electrolysis

Electrolysis

A way of breaking down a substance using electricity

What happens when an ionic compound is melted or dissolved in water?

The ions are able to move within the solution and can conduct electricity

Electrolyte

The substance being broken down by electrolysis

Diagram for electrolysis of sodium chloride

Where do positive ions go when an electric current is passed through an electrolyte?

Negative electrode (cathode)

Where do negative ions go when an electric current is passed through an electrolyte?

Positive electrode (anode)

What happens at the electrodes?

Ions are discharged to produce elements

What material are electrodes made from and why?

Platinum or graphite as they are inert and can conduct electricity

Half equations for the electrolysis of zinc chloride

2H+ + 2e- --> H2

2Cl- --> Cl2 + 2e- OR 2Cl- - 2e- --> Cl2

Is the metal or non-metal in a molten ionic compound produced at the cathode and why?

Metal as they form positive ions

Is the metal or non-metal in a molten ionic compound produced at the anode and why?

Non-metals as they form negative ions

When can metals be extracted using electrolysis?

When they are in molten compounds

When is electrolysis used to extract metals?

When the metal is too reactive to be extracted by reduction with carbon or if the metal reacts with carbon

Why is electrolysis expensive?

Lots of energy is required to melt compounds and produce the current

Aluminium extraction steps

1) Cryolite added to aluminium oxide to lower the melting point

2) The mixture is melted

3) Electrolysis is carried out on the molten mixture

What is the element used as the positive electrode (anode)?

Carbon

Why must the anode be replaced regularly?

The oxygen produced there reacts with the carbon in the anode to form CO2

When is hydrogen produced by electrolysis?

If the metal is more reactive than hydrogen (in an aqueous solution)

Where is hydrogen produced?

Cathode

When is oxygen produced by electrolysis?

If halide (halogen) ions aren't present in an aqueous solution

Where is oxygen produced?

Anode

Why can oxygen and hydrogen be produced by electrolysis?

Some water molecules break down into positive hydrogen ions (H+) and negative hydroxide ions (OH-) which are discharged

OILRIG

Oxidation is loss, reduction is gain (of electrons)

Aqueous solution: Copper sulphate

What are the positive and negative ions in the solution?

Positive: Cu2+, H+

Negative: SO42-, OH-

Aqueous solution: Copper sulphate

What elements are formed at the positive and negative electrodes?

Positive: Oxygen

Negative: Copper

Aqueous solution: Copper sulphate

Word equation

Copper sulphate solution --> Oxygen + Copper + Sulphuric acid

Aqueous solution: Copper sulphate

Half equation at positive and negative electrodes

Positive: 4OH- --> O2 + 2H2O + 4e-

Negative: Cu2+ + 2e- --> Cu

Aqueous solution: Copper II chloride

What are the positive and negative ions in the solution?

Pos: Cu2+, H+

Neg: Cl-, OH-

Aqueous solution: Copper II chloride

What elements are formed at the positive and negative electrodes?

Pos: Chlorine (gas)

Neg: Copper

Aqueous solution: Copper II chloride

Word equation

Copper chloride solution --> Copper + Chlorine + Water

Aqueous solution: Copper II chloride

Half equation at positive and negative electrodes

Pos: 2Cl- --> Cl2 + 2e-

Neg: Cu2+ + 2e- --> Cu

Is copper oxidised or reduced during the electrolysis of Copper II chloride and why?

Reduced as it gains electrons

Aqueous solution: Sodium chloride

What are the positive and negative ions in the solution?

Pos: Na+, H+

Neg: Cl-, OH-

Aqueous solution: Sodium chloride

What elements are formed at the positive and negative electrodes?

Pos: Chlorine (gas)

Neg: Hydrogen

Aqueous solution: Sodium chloride

Word equation

Sodium chloride solution --> Hydrogen + Chlorine + Sodium hydroxide

Aqueous solution: Sodium chloride

Half equation at positive and negative electrodes

Pos: 2Cl- --> Cl2 + 2e-

Neg: 2H+ + 2e- --> H2

Is chlorine oxidised or reduced during the electrolysis of sodium chloride and why?

Oxidised as it loses electrons