Electrolysis of Aqueous Solution

• Aqueous solution : solution produced when ionic compounds dissolved in water.

• It contains :

(a) cations & anions of the compound

(b) H+ ions & OH− ions ( partial dissociation of H2O molecules ).

H2O(l) H+(aq) + OH−(aq)

• Only 1 type of ion will be discharged at the cathode & anode respectively.

• The selective discharge of ions depends on 3 factors :

(a) position of ions in the electrochemical series

(b) concentration of ions

(c) types of electrodes used

(a) Position of ions in electrochemical series

Cation Anion

K+ F−

Na+ SO42−

Mg2+ NO3−

Al3+ Cl−

Zn2+ Br−

Fe2+ I−

Sn2+ OH−

Pb2+

H+

Cu2+

Ag+ Ease of discharge increases

• Ions in the upper position of the series are not selectively discharged to form atoms or molecules because they have stronger tendency to exist as ions than atoms or molecules.

Electrolysis of sodium chloride solutionElectrolysis of sodium chloride solution

+ −

OH−

OH−

Cl−

Cl−

H+

H+

H+

Na+

Na+

e−

e− e−

e−

e− e−

CurrentCurrent

• At cathode,

Half equation : 2H+(aq) + 2e− → H2(g)

• At anode,

Half equation : 4OH−(aq) → 2H2O(l) + O2(g) + 4e−

Overall equation :

• 4H+(aq) + 4OH−(aq) → 2H2O(l) + O2(g) + 2H2(g)

O2 : H2 = 1 : 2

Electrolyte Observation

Cathode Anode Change in solution

Copper (II) sulphate solution

Brown solid deposited.

Gas bubbles released. Colourless gas relights glowing splinter produced.

Intensity of blue colour of electrolyte decreases.

CuSO4 solution : Cu2+, SO42−, H+, OH−

Cu2+ , H+ → cathode ( terminal negative ) SO4

2−, OH− → anode ( terminal positive )

Cathode : Cu2+(aq) + 2e− → Cu(s)

Anode : 4OH−(aq) → 2H2O(l) + O2(g) + 4e−

(b) Concentration of ions in solutionIf the concentration of a particular ion is high, it may be discharged in preference to the one below it in the electrochemical series.

Electrolysis of 2 mol dm−3 HClAt anode, Cl− ions are selectively discharged because of their higher concentration even though Cl− ions is higher than OH− ions in electrochemical series.

Electrolysis of 0.001 mol dm−3 HClAt anode, OH− ions are selectively discharged because of its lower position in electrochemical series than Cl− ions.

* If the 2 ions are placed very far apart in the electrochemical series, concentration factor is unimportant.

Electrolysis of 2 mol dm−3 KNO3

At anode, OH− ions are selectively discharged even though their concentrations are very low.

Electrolysis concentrated NaCl ?

(c) Types of electrodes

Electrolysis of 0.1mol dm−3 copper (II) sulphate solution

The active anode ionises.

Electrode Observation

Cathode Anode Electrolyte

Carbon Brown solid deposited.

Gas bubbles released. Gas relights glowing wooden splinter produced.

Intensity of blue colour of electrolyte decreases.

Copper Brown solid deposited.

Cathode becomes thicker.

Anode becomes thinner.

Intensity of blue colour of electrolyte remains unchanged.

• Electrode carbon (graphite),Cathode : Cu2+(aq) + 2e− → Cu(s)

Anode : 4OH−(aq) → 2H2O(l) + O2(g) + 4e−

• Electrode copper,Cathode : Cu2+(aq) + 2e− → Cu(s)

Anode : Cu(s) → Cu2+(aq) + 2e−

• Electrolysis of saturated NaCl using graphite (anode) & mercury (cathode) :

saturated NaCl : Na+, Cl−, H+, OH−

Na+, H+ → cathode (terminal negative)

Cl−, OH− → anode (terminal positive)Cathode : Na+(aq) + e− → Na(l)

Na(l) + Hg(l) → NaHg(l) (amalgam)

Anode : 2Cl−(aq) → Cl2 + 2e−

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