How Electrolysis and Electrolytic Reactions Occur in a Cell

What is electrolysis? Electrolysis is the chemical change brought about by the passage of a direct current through an electrolyte via electrodes. Before the passage of current in electrolytic reactions, the ions move about randomly in the electrolyte. When electrolysis begins, the battery or generator of electric current pumps electrons from its negative terminal (anode) to the cathode of the electrolytic cell. The negatively charged cathode then attracts cations in the electrolyte  to itself. The cations accept electrons to become electrically neural and are eventually discharged. The positive terminal (cathode) of the battery draws electrons from the anode of the electrolytic cell. Anions in the electrically are then attracted to the positively charged anode, where they give up their electrons to became electrically neutral and are also finally discharged. Hence, and electric current passes through the complete circuit.   

 What Are Electrolytic Reactions?    

Electrolytic reactions are redox reactions since they involve the transfer of electrons. Oxidation occurs at the anode where the anions lose electrons. This reaction as the andoe is know as the anode half-reaction. The cathodic half-reaction which takes place simultaneously at the cathode is a reduction reaction since the cations gain electrons here. The overall reaction is obtained by the algebraic addition of the two half-reactions.

As a result of the chemical reactions that occur during electrolysis, certain products are formed at each electrode. In general, metals or hydrogen gas are discharged at the cathode while non-metals (except hydrogen) are discharged at the anode.

Preferential Discharge of Ions

During the electrolysis of a given electrolyte, the products formed at the electrodes depend on the nature of the electrolyte. Where the electrolyte is a solution, the electrolytic produce formed at the electrodes may vary because the solvent, which is usually water, will also ionize. In such a case, the cations and anions of both the electrolyte and the solvent will migrate to the cathode and the anode respectively where they will compete with one another to be discharged. The produce which is formed at an electrode will depend on which ions are preferentially discharged-the ions from the electrolyte or those from the solvent.

The discharge of ions is governed by three conditions, namely.

• The position of the ions in the electrochemical series;
• The concentration of the ions in the electrolyte;         
• The nature of the electrode.

1. Relative Position of Ions in the Series: The position of ions in the electrochemical series. If all other factors are constant, a cation which is lower in the series (less electropositive) will show a greater tendency to be discharged than another which is higher up (more electropositive). This is because the former gains electrons more readily form the cathode and so because discharged as a neutral atom while the latter tends to persist in solution as a positive ion. For example, K⁺, Na⁺ and Ca²⁺ are never discharged at all from aqueous solutions since H⁺ will always be preferentially discharged. Only when there is no competition with H+ or other ions will these strongly electropositive elements be discharged. Such as situation occurs during the electrolysis of the molten salts of these metals.

An anion which is higher in the series (less electronegative) is discharged in preference to another which is lower down the series (more electronegative) as the former loses electrons more readily. For example, SO₄² and NO₃ are never discharged from an aqueous solution due to the preferential discharge of OH.

2. Concentration of ions: If other conditions are equal, increasing the concentration of a given ion tends to promote its discharge from solution. The influence of concentration, however, is effective only when the two competing ions are closely positioned in the electrochemical series. The effect of concentration becomes less important as the positions of the competing ions because further apart in the series.

We can illustrate this by considering what happens during the electrolysis of a concentrated solution of sodium chloride. In this solution, there are four types of ions as shown.

At the cathode H⁺ are preferentially discharged although the concentration of Na+ is far greater. Here the great distance between the positions of Na⁺ and K⁺ in the series makes the effect of concentration less important.

At the anode C1^– are preferentially discharged. The effect of concentration is important in this case because the positions of C1^– and OH^– care close to one another in the series.

3. Nature of Electrodes: Electrodes which take no part in the electrolytic reaction are described as inert electrodes. Platinum and carbon (graphite) usually behave as inert electrodes, although platinum is attacked by liberated chlorine and carbon by liberated oxygen. Some electrodes which have strong affinity for certain ions may influence ionic discharge. For example, in the electrolysis of aqueous sodium chloride using platinum electrodes, H⁺ are preferentially discharged. If a mercury cathode is used, however, the mercury will tend to associate with the Na⁺ to form sodium amalgam, Na/Hg, so that the discharge of Na⁺ requires less energy than that of H+, and so occurs in preference.

          Na⁺(aq) + Hg(1) + e → Na/Hg(1)

        H⁺(aq) + e → H;   H + H → H₂(g)

Other electrodes many also influence the ionic discharge because they posses characteristics similar to those of the ion in the electrolyte. For example, if a copper anode is used in the electrolysis of a solution of copper(II) tetraoxosulphate(VI), neither the SO₄² nor the OH will be discharged. Instead, the copper atoms from the anode will go into solution as Cu²⁺ because these atoms will give up their electrons more use of in the extraction and purification of some metals.