Electrolysis — Part 2: Water, copper sulphate and applications: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Electrolysis at work

Part 2 — the same rules explain what happens to water, to copper sulphate solution, and in the plating and refining industries.

In this lesson you will

Explain the electrolysis of acidified water and why hydrogen and oxygen form in the ratio 2 : 1 by volume
Compare copper(II) sulphate electrolysis with copper electrodes and with platinum electrodes
Describe nickel and silver plating and the conditions for a good coat
Describe the electrorefining of copper

Table of four applications of electrolysis and their uses
Process	Use
Electrolysis of acidified water	Hydrogen and oxygen
Electroplating with nickel or chromium	Shiny, rust-resistant taps and cycle parts
Silver plating	Attractive cutlery and ornaments
Electrorefining	Very pure copper for electric wires

Diagram

Pure water is a very weak electrolyte (very few ions), so it conducts very poorly. A little dilute sulphuric acid is added to provide ions and increase conductivity.
Platinum electrodes are used: they are inert and are not attacked by oxygen.
Hydrogen collects at the cathode and oxygen at the anode, in the ratio 2 : 1 by volume.
The acid is not used up; only water is decomposed: 2H₂O → 2H₂ + O₂.

Worked example

Problem. Write the electrode equations for the electrolysis of acidified water and explain the 2 : 1 ratio of volumes.

Ions present. $\ce{H2SO4 -> 2H+ + SO4^2-}$ and $\ce{H2O <=> H+ + OH-}$ .

Cathode (reduction). $\ce{4H+ + 4e- -> 2H2}$ .

Anode (oxidation). OH⁻ is discharged in preference to SO₄²⁻: $\ce{4OH- -> 2H2O + O2 + 4e-}$ .

Ratio. The same 4 electrons give 2 molecules of hydrogen but only 1 molecule of oxygen. By Avogadro's law, the volumes are in the ratio 2 : 1.

Table of the ions attracted and discharged at each electrode in the electrolysis of acidified water
Electrode	Ions attracted	Ion discharged	Product
Cathode (−)	H⁺	H⁺	Hydrogen, 2 volumes
Anode (+)	OH⁻, SO₄²⁻	OH⁻	Oxygen, 1 volume