ELECTROLYSIS OF AN AQUEOUS

SOLUTION

In the electrolysis of aqueous solutions only one ion is involved in the selective discharge of ions at each electrode during the electrolysis and their ions are free to move.

Their ions are cation and anion.

Aqueous Solution

Salt Ionization

Water Ionization

Cation Anion

NaCl(aq) NaCl Na+ + OH-

H2O H+ + OH-

Na+ , H+

Cl-, OH-

CuSO4(aq) CuSO4 Cu2+ + SO4

2-

H2O H+ + OH-

Cu2+ , H+ SO42-, OH-

HNO3(aq) HNO3 H+ + NO3

-

H2O H+ + OH-

H+NO3

- , OH-

The positive ions are attracted to the negative electrode

The negative ions are attracted to the positive electrode

Na+

Na+

Na+

OH-

OH-OH-

NaCl Solution

Carbon Electrode

The ion which is selected for discharge at an electrode depends on a number of factors, that is:

1. Position of the ions in the electrochemical series

2. Nature of the electrode

3. Concentration of the ions in solution

Position of the ions in the Position of the ions in the electrochemical serieselectrochemical series

The figure below shows an idealized drawing of a cell in which an aqueous solution of sodium sulphate is electrolyzed

Na2SO4

OH-

O2 (g)

SO42-

H+

Na+

Observation Conclusion + Chemical Equation

Anode(+):

Ion OH-

• OH- ion much easier to discharge than SO4

2- ion in the electrochemical series.The only product formed is oxygen gas.

4OH-(aq) H2O(aq) + O2(g) + 4e-

Cathode(-):

Ion H+

H+ ion much easier to discharge than Na+ ion, the only product formed at the anode is hydrogen gas.

2H+(aq)+2e- H2(g)

Electrochemical SeriesElectrochemical SeriesLi+

Na+

Mg2+

Al3+

Zn2+

Fe2+

Sn2+

Pb2+

H+

Cu2+

Hg+

Ag+

F-SO4

2-NO3-Cl-Br-I-OH-

The tendency of cation to discharge increases

The tendency of anion to discharge increases

Nature of the ElectrodeNature of the ElectrodeElectrolysis of an aqueous solution of Electrolysis of an aqueous solution of Cuprum Sulphate: Carbon ElectrodeCuprum Sulphate: Carbon Electrode

Cuprum Sulphate SolutionCarbon Electrode

Anode Cathode

Observation Conclusion + Chemical Equation

Anode(+):

Ion OH-

• OH- ion much easier to discharge than SO4

2- ion in the electrochemical series. The only product formed is oxygen gas.

4OH-(aq) H2O(aq) + O2(g) 4e-

Cathode(-):

Ion Cu2+

Cu2+ much easier to discharge than H+ ion, the only product formed at the cathode is precipitate of brown colored.

Metal copper formed. Cu2+

(aq) + 2e- Cu(s)

Electrolysis of an aqueous solution Electrolysis of an aqueous solution of Cuprum Sulphate: Cuprum of Cuprum Sulphate: Cuprum

ElectrodeElectrodeCuprum sulphate

SolutionCuprum Electrode

cathodeAnode

Observation Conclusion + Chemical Equation

Anode(+):Transfer of copper metal from the anode to the cathode during electrolysis. The copper sulphate is ionized in aqueous solution.

Each copper atom loses two electrons to become copper ions, which go into solution.

Cu(p) Cu2+(aq) + 2e-

Cathode(-):• The positively charged copper ions migrate to the cathode.

Where each gains two electrons to become copper atoms that are deposited on the cathode.

Cu2+(aq) + 2e- Cu(p)

Concentration of the Ions in Concentration of the Ions in SolutionSolution

KI dilute

O2(g)

Electrode CarbonElectrode Carbon

OH-

Electrolysis of an aqueous solution of Dilute Electrolysis of an aqueous solution of Dilute Potassium IodidePotassium Iodide

K+I-

H+

Observation Conclusion + Chemical Equation

Anode(+):

Ion OH-

OH- ion much easier to discharge than I- ion. The only product formed at the anode is oxygen gas.

4OH-(aq) H2O(aq) + O2(g) 4e-

Cathode(-):

Ion H+

H+ ion much easier to discharge than K+ ion. The only product formed at the anode is hydrogen gas.

2H+(aq)+ 2e- H2(g)

Electrolysis of an aqueous solution of Electrolysis of an aqueous solution of Concentrated Potassium IodideConcentrated Potassium Iodide

Electrode CarbonElectrode Carbon

KI

I-

I2 (g)

H+

OH- K+

Observation Conclusion + Chemical Equation

Anode(+):

Ion I-

I- ion much easier to discharge than OH- ion because concentration of iodide ion is high.

Iodine gas formed. 2I-(aq) I2(g) + 2e-

Cathode(-):

Ion H+

H+ ion much easier to discharge than K+ ion. The only product formed at the anode is hydrogen gas.

2H+(aq)+2e- H2(g)

Activity 1Activity 1

Anode Cathode

Electrolyte Electrode Attracted Ion

Ionized Ion

Product Attracted Ion

Ionized Ion

Product

NaCl (aq)

Dilute

Carbon Ions Cl-, OH- Ion OH- O2 gas Ions Na+, H+

Ion H+ H2 gas

KCL (aq)

Concentrated

Carbon Ions Cl-, OH- Ion Cl- Cl2 gas Ions K+, H+

Ion H+ H2 gas

CuBr2 (aq)

Dilute

Carbon Ions Br-, OH- Ion OH- O2 gas Ions Cu2+ , H+

Ion Cu2+ Copper metal

AgBr2 (aq)

Concentrated

Carbon Ions Br-, OH- Ion Br- Br2 gas Ion Ag+ , H+

Ion Ag+ Argentum metal

NiCl2 (aq)

Dilute

Nikel Ions Cl-, OH- Metal nikel erode

Ion Ni2+ Ion Ni2+, H+

Ion H+ H2 gas