ELECTRODE REACTIONS

Cathode reactions (reduction)

(-) negative cathode where reduction of the attracted positive cations is by electron gain (reduction) to form metal atoms or hydrogen [from Mn+ or H+, n = numerical charge].

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The electrons come from the positive anode. Hydrogen ions are reduced to hydrogen gas molecules. Electrolysis of many dilute salts or acid solutions make hydrogen gas by reduction as shown.

2H⁺(aq) + 2e- H₂(g)

Copper (II) ions are reduced to copper atoms in the electrolytic purification or electroplating using copper (II) sulphate solution.

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Cu2⁺(aq) + 2e^– Cu(s)

Silver ions reduced to silver atoms in silver electroplating

Ag⁺(aq) + e- Ag(s)

Anode reactions (oxidation)

Positive anode is where the oxidation of the atom or anion is by electron loss. Non-metallic negative anions are attracted and may be oxidised to the free element.

For example, in the electrolysis of molten chloride salts or their concentrated aqueous solution or conc. hydrochloric acid ,chloride ion oxidised to chlorine gas molecules.

2Cl^–(l/aq) Cl₂(g) + 2e^– In the electrolysis of molten oxides eg anode reaction in the extraction of aluminium from molten bauxite, oxide ion oxidised to oxygen gas molecules. 2O₂^–(l) O₂(g) + 4e- The electrons released by this process travel round the circuit and are donated to the cations (reduction).

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Electrolysis of many salt solutions such as sulphates, sulphuric acid etc. gives oxygen.

Hydroxide ions oxidised to oxygen gas molecules.

4OH^–(aq) 2H₂O(l) + O₂(g) + 4e^–

Factors That Determine Products of Electrolysis

The ions that are successfully released (or discharged) at the electrodes depend on three factors:

The position of the ion in the electrochemical series
The concentration of the ion in the solution
The nature of the electrode
The position of the ion in the electrochemical series

1. This is probably better expressed as the position of the ions in the electrochemical series. The ions that are lower in the electrochemical series get discharged in preference to the ones above them. For e.g., if a solution has potassium ions and copper ions, the copper ions will accept electrons, and get discharged as copper atoms first. The potassium ions will not be affected.

The concentration of the ions

When two ions with similar reactivity are in competition then the relative concentration of the two ions becomes an important factor. If an electrolyte contains a higher concentration of ions, which are higher in the electrochemical series than those that are lower, then these ions get discharged in preference to the lower ones.

For e.g., a solution of sodium chloride in water contains two types of anions i.e., the chloride (Cl-) ions and the hydroxyl (OH-) ions.

Hydroxyl ions are lower in the electrochemical series than chloride ions.

But if the concentration of chloride ions is much higher than that of the hydroxyl ions then the chloride ions get discharged first.

Electrolysis of NaCl Solution at different Concentration

The nature of the electrode

Usually inert electrodes such as graphite or platinum are used for electrolysis. These electrodes do not interfere with the reactions occurring at the surface of the electrode they simply act as a point of connection between the electrical circuit and the solution.

However, if metal electrodes are used in same metal ion solutions, they can get involved in the reactions by dissolving as ions leaving their electrons behind. Example: Electrolysis of sodium chloride solution

The ions present in the solution are:

Na⁺, Cl^–, H⁺, OH^–

At the cathode

The positive ions are attracted to the negative cathode. There is competition between the sodium ions and the hydrogen ions. As the hydrogen ion hydrogen ion is lower in the electrochemical series than the sodium ion sodium, the hydrogen ions are preferentially reduced and hydrogen gas is produced at the electrode (bubbles are seen)

2H⁺ + 2e H₂

At the anode

There is competition between the negative ions at the positive anode. The chloride ions compete with the hydroxide ions to release their electrons to the anode. If the solution is fairly concentrated the chloride ions preferentially lose electrons to become chlorine atoms (and then molecules).

2Cl^– Cl₂ + 2e

Ions remaining in solution

The ions that are removed from the solution, then, are the hydrogen ions and the chloride ions. This means that the sodium ions and the hydroxide ions remain in the solution – i.e. sodium hydroxide is also produced.

Note: When the solution of chloride ions is dilute then OH, ions are preferentially released at the anode.

Electrolysis of dilute sulphuric acid (electrolysis of water)

Water is a poor conductor of electricity. However, it can be made to decompose if some dilute sulphuric acid is added. A Hofmann voltammeter below can be used to keep the gases produced separate.

At anode

4OH⁺(aq) 2H₂O(l) + O₂(g) + 4e^–

At cathode

2H⁺(l) + 2e- H₂(g)

After a while, the volume of gas in each arm can be measured and tested. Oxygen collects at the anode. The ratio of volumes is about 2:1 for hydrogen and oxygen respectively. Effectively, this experiment is the electrolysis of water.`

Example: Electrolysis of copper II sulphate solution

The ions present in the solution are:

Cu2⁺, SO₂^2-,H⁺, OH^–

At the cathode

The positive ions are attracted to the negative cathode. There is competition between the copper ions and the hydrogen ions. As the hydrogen ion appears higher in the electrochemical series than the copper ion, copper ions are preferentially reduced and copper metal is deposited at the electrode (pink layer is observed).

Cu2⁺ + 2e Cu

At the anode

There is competition between the negative ions at the anode.

The sulphate ions compete with the hydroxide ions to release their electrons to the anode.

The hydroxide ions are lower in the series and are preferentially released as oxygen gas (bubbles are seen) and water.

4OH- 2H₂O + O₂ +4e

OILRIG

Oxidation Is Loss, Reduction Is Gain (of electrons). Oxidising agents are easily reduced. Reducing agents are easily oxidized.

Ions remaining in solution

The ions that are removed from the solution, then, are the copper ions and the hydroxide ions, this means that the hydrogen ions and the sulphate ions remain in the solution – i.e. sulphuric acid is also produced. The solution changes colour from blue to colourless.