The mechanism of electrolysis.

Mechanism of electrolysis

When an electrolyte is dissolved in water, it dissociates to form ions. Thus, the aqueous solution of an electrolyte contains both positive and negative ions.

Example of strong electrolytes

1. H2SO4 (Sulphuric acid)
2. HNO3 (Nitric acid)
3. NaCl (Sodium chloride)

Example of weak electrolytes

1. CH3COOH (Acetic acid)
2. H2CO3 (Carbonic acid)
3. NH3 (Ammonia solution)

Ions

An ion is a charged particle. It can either be positively charged or negatively charged:

1. Anions – Negatively charged ions
2. Cations – Positively charged ions

The electrodes

Electrodes are the two poles of metal rods or carbon by which the electrons leave or enter the electrolyte. There are two types of electrodes:

1. Anode (+): The positive electrode where electrons leave the electrolyte.
2. Cathode (-): The negative electrode where electrons enter the electrolyte.

When an electric current passes through an electrolyte, the ions move to the oppositely charged electrodes:

1. Cations (positive ions) move to the cathode
2. Anions (negative ions) move to the anode

Electrode reactions

Anode reaction

Anions carry negative charges due to extra electrons they have. On arrival at the surface of the anode, they surrender the extra electron, thereby becoming discharged. Any process involving the loss of electrons is called oxidation.

Cathode reaction

Cations have positive charges because they have a deficiency of electrons. On arrival at the surface of the cathode, they receive electrons. Any process involving the gain of electrons is called reduction.

Preferential discharge of ions

When two or more ions of similar charge are present in a solution under similar conditions, one of them is preferentially selected for discharge. The selection depends on:

1. Position of the metal or radical in the electrochemical series
2. Concentration
3. Nature of the electrode

Position of the metal/group in electrochemical series

The electrochemical series arranges metal ions in the order of their ability to be reduced (discharged). Those that are difficult to discharge are placed at the top of the series (e.g., K^+^, Ca^2+^, Na^+^), while ions like Ag^+^ and Cu^2+^ can easily accept electrons.

Example: Electrolysis of dilute hydrochloric acid using platinum electrode

1. Cathode reaction: H^+^ will migrate to the cathode and discharge. Hydrogen gas (H2) will evolve at the cathode.
2. Anode reaction: Cl^-^ and OH^-^ will migrate to the anode. OH^-^ will be preferentially discharged because it is lower than Cl^-^ in the electrochemical series. Oxygen gas (O2) will evolve at the anode.

Electrolysis of dilute sulphuric acid (using platinum electrode)

1. Anode reaction: SO4^2-^ and OH^-^ will migrate to the anode, and OH^-^ will be preferentially selected to discharge. Oxygen gas (O2) will evolve at the anode.
2. Cathode reaction: H^+^ will migrate to the cathode and discharge. Hydrogen gas (H2) will evolve at the cathode.
3. Overall effect: The concentration of the electrolyte will increase as the elements of water come out as gases.

Concentration of the ions

An increase in the concentration of an ion(s) tends to promote its discharge.

Example: Electrolysis of concentrated hydrochloric acid

1. Anode reaction: Cl^-^ and OH^-^ will migrate to the anode. Cl^-^ will be discharged in preference to OH^-^ because it is available in higher concentration. Chlorine gas (Cl2) will evolve at the anode.
2. Cathode reaction: Hydrogen gas (H2) will evolve at the cathode.
3. Overall effect: The concentration of the electrolyte will decrease as elements of HCl move out as gases.

Nature of the electrode

The type of electrode used can determine the products of electrolysis. Here are some examples:

Electrolysis of sodium chloride using carbon electrode

1. Cathode reaction: H^+^ will be selected to discharge in preference to Na^+^ because Na^+^ is difficult to discharge due to its position in the electrochemical series.
2. Anode reaction: Cl^-^ will be preferentially discharged due to the concentration factor. Chlorine gas (Cl2) will evolve at the anode.
3. Overall effect: Hydrogen gas (H2) will evolve at the cathode and chlorine gas (Cl2) will evolve at the anode.

Electrolysis of copper sulphate (CuSO4) using carbon/platinum electrode

1. Cathode reaction: Cu^2+^ ions will be reduced to form copper metal, which will be deposited at the cathode.
2. Anode reaction: Oxygen gas (O2) will be produced at the anode.
3. Overall effect: Copper metal is deposited at the cathode, and oxygen is released at the anode. The concentration of CuSO4 decreases, and the blue color of the solution fades.

Electrolysis of copper sulphate (CuSO4) using copper electrodes

1. Cathode reaction: Copper metal is deposited at the cathode as Cu^2+^ ions are reduced.
2. Anode reaction: Copper atoms from the anode are converted to Cu^2+^ ions, which are released into the solution.
3. Overall effect: Copper metal is deposited at the cathode, and copper ions are released into the solution. The concentration of CuSO4 remains constant, and the anode decreases in weight.