Question

A high school chemistry student wishes to demonstrate how water can be separated into hydrogen and oxygen by electrolysis. She knows that the reaction will proceed more rapidly if an electrolyte is added to the water, and has 2.00 M solutions of these reagents available: H2SO4, HCl, NaCl, Na2SO4, Na2CO3, and CaCO3.

Which one(s) would you use?

Answer 1

The electrolysis of water leads to the formation of hydrogen gas and oxygen gas.

Electrolysis is basically a process that drives a reaction for which thermodynamics

is forbidden by using electrical energy.

In electrolysis of water following oxidation and reduction reaction occurs at

electrolytic cells.

2H,00)+2e > H,(g)\Reduction 2H,O0)H(aq ) +0,g)+4e (Oxidation

During electrolysis of water, hydrogen will appear at cathode and oxygen

will appear at anode.

Certain electrolytes are added to increase the process of electrolysis.

On addition of electrolytes in water the desired products (Hydrogen and Oxygen gas)

obtained would depend on the nature of electrolytes.

Tendency of ions to oxidize or reduce depends on the standard reduction potentials

of those ions. An ion with a large positive value of reduction potential would be

more easily reduced as compared with a ion having low reduction potential.

Similarly an ion with a more negative value of reduction potential is easily

oxidized as compared to an ion with less negative or positive value of reduction

potential.

Any ion present in electrolyte which has more tendency to oxidize than

Oxygen that is there reduction potential is lower then that ion would be

oxidized in spite of oxygen.

Similarly if any ion has more tendency to reduce than Hydrogen that is the

reduction potential of that ion is higher than that of Hydrogen so it will be

reduced in spite of hydrogen. So addition of Na₂SO₄, H₂SO_4, Na₂CO₃ as the

ions present in these electrolytes do not have ample value of reduction potential

to either oxidize or reduce than H₂ and O_2.. so the desired products of electrolysis

would be formed.

CaCO₃ is associated with solubility issues in water so this would not increase

electrolysis as it is insoluble but if you can make it dissolve then it would also

give the desired product. So addition of Na₂SO₄, H₂SO_4, Na₂CO₃ will increase

electrolysis of water.

When we add Hcl&NaCl, chlorine is oxidized to chlorine as chlorine has

lower reduction potential than oxygen. So addition of these electrolytes

would not give the desired product of electrolysis.