Electrolysis Revision

Electrolysis

Electrolytes are ionic substances that are molten (melted) or dissolved in water.

During electrolysis, positively charged cations migrate to the negatively charged cathode and negatively charged anions migrate to the positively charged anode.

Oxidation means loss of electrons and reduction means gain of electrons.

Reduction occurs at the cathode and oxidation occurs at the anode.

A half equation shows the reaction at one electrode only. Electrons are shown by ‘e-’. For example:

Cu²⁺ + 2e^- à Cu

 

The full equation for the electrolysis of molten sodium chloride is NaCl à Na + Cl₂

The half equations are:

At the cathode: Na⁺ + e^- à Na

At the anode: 2Cl^- à Cl₂ + 2e^-

 

The electrolysis of solutions can give products from ions in water rather than from ions in the dissolved solid. We can predict the products of electrolysis using the following rules.

At the cathode:

·         If the metal is low in the reactivity series (e.g. copper, silver, lead), the metal is deposited on the cathode.

·         If the metal is high in the reactivity series (e.g. sodium, potassium), then hydrogen is given off.

At the anode:

·         If the non-metal in the sale is a halide ion (e.g. chloride, bromide, iodide), then the halogen (e.g. chlorine gas) is released.

·         If the non-metal is not a halogen, oxygen from the water is given off.

 

Sodium chloride solution is sodium chloride dissolved in water. The electrolysis of sodium chloride solution is: NaCl + 2H₂O à NaOH + Cl₂ + H₂.

In this case, hydrogen gas is formed at the cathode from the water instead of sodium. This is because hydrogen more easily gains an electron than sodium. Chlorine gas is formed at the anode. Na⁺ and OH^- ions are left in the water, and this means the solution is NaOH (sodium hydroxide).

 

Sodium and chlorine are both useful. Sodium is used in street lamps and as a coolant in some nuclear reactors. Chlorine is a toxic gas that can be used in disinfectants.

 

Write half equations for the electrolysis reactions for these ionic substances:

1.       Copper chloride solution CuCl₂

2.       Copper sulphate solution CuSO₄

3.       Sodium sulphate solution Na₂SO₄

4.       Molten lead bromide PbBr₂

 

Electrolysis is used for electroplating metals (adding a thin coating of one metal on the surface of another). This is useful because it can improve resistance to corrosion and improve the appearance of the metal.

 

Copper can be purified by electrolysis: the copper ions from the impure anode dissolve in the water and impurities sink to the bottom. The copper ions migrate to the pure copper cathode, so increasing its mass.