ELECTROLYSIS OF AN AQUEOUS SOLUTION
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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 EquationAnode(+):
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 EquationAnode(+):
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 EquationAnode(+):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 EquationAnode(+):
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) Concentrate
d
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) Concentrate
d
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
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