Electrolysis

Rossita Radzak SASER

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ELECTROLYSIS OF AQUEOUS SOLUTIONS

Factors that affect the electrolysis of an aqueous solution:

(a) Position of ions in the electrochemical series (ECS).

(b) Concentration of ions / anion in the solution / electrolytes.

(c) Type of electrodes used in the electrolysis.

a. Position of ions in the electrochemical series (ES).

Cation K+ Na+

Ca2+ Mg2+ Al3+ Zn2+ Fe2+

Sn2+

Pb2+

H+ Cu2+

Ag+

Anion F- SO4

2- NO3

- Cl- Br-

I-

OH-

Ease of discharge increases

Electrochemistry Series


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Figure 2 Electrolysis of potassium nitrate solution

b. Concentration of ions in the solution / electrolytes.

Cathode: K+ and H+

Anode: NO3

- and OH-

KNO3 solution consists of K+ , H+, NO3- and OH-

H+ is selected for discharged as it is lower than K+ in ECS.

OH- is selected for discharged as it is lower than NO3

- in ECS.

2H+ + 2e H2 Hydrogen gas is collected at the cathode. Hydrogen gas 2 volume.

4OH- 2H2O + O2 + 4e Oxygen gas is collected at the anode. Oxygen gas 1 volume.

Hydrogen gas Potassium nitrate solution, 0.5 mol dm-3

Oxygen gas Carbon electrodes


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(b) Concentration of ions in the solution / electrolytes.

Figure 3 Electrolysis of sodium chloride solution

Sodium chloride solution, 1.0 mol dm-3

Carbon electrodes

From sodium chloride Na+, Cl- From water H+ , OH-

CATHODE Na+ , H+

Both types of cations are attracted to the cathode, H+ ions are selected for discharged as they are lower in the ECS.

ANODE: Cl- , OH-

Both anions are attracted to the anode, Chloride ions are discharged because the concentration of Cl- ions are higher than of OH- ions.

2H+ + 2e H2 (g) Gas bubbles are formed. When a lighted wooden splinter is place near the mouth of the test tube, a `pop` sound is produced. Hydrogen gas is evolved.

2Cl- Cl2 (g) + 2e Bubbles pungent yellowish-green gas is produced. The gas changed the damp blue litmus paper to red and bleached it. Chlorine gas is formed.

Solution becomes alkaline by presence of sodium hydroxide. NaOH(aq)


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(c) Type of electrodes used in the electrolysis.

Figure 4 Electrolysis of copper (II) sulphate solution using copper electrodes

Copper (II) sulphate solution 0.5 mol dm-3

Copper electrodes

From copper (II) sulphate Cu2+ , SO42-

From water H+ , OH-

CATHODE:

Cu2+ ions and H+ ions move to the cathode. Cu2+ ion which is at a lower position than H+ ion in the ECS is selected to be discharged.

ANODE:

OH- ions and SO42- ions move to the anode.

Copper electrode dissolves /copper atom ionizes and releases 2 electrons to form Cu2+ ions.

Cu2+ (aq) + 2e Cu (s) Brown deposit/ solid is formed. The mass of the cathode increases and the cathode becomes thicker.

Cu Cu2+ + 2e The mass of the anode decreases and the anode becomes thinner.

The colour intensity of blue solution dose not change because the concentration of Cu2+ ions in the solution remains unchanged. This is because the rate of formation of copper (II) ions, at the anode is the same as the rate of discharge of copper (II) ions, at the cathode.

Education

Electrolysis