Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Definition and purpose
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. The process converts electrical energy into chemical change and can produce elements, coat objects or transfer metals.
Why ions must be mobile
An ionic solid contains charged ions but they are fixed in a rigid lattice. When the compound is molten or dissolved, the ions can move through the liquid and carry charge. A covalent liquid without ions is generally not an electrolyte.
Direct current
A direct-current supply keeps one electrode positive and the other negative. This fixed polarity makes cations move consistently to the cathode and anions to the anode. Alternating current would repeatedly reverse the directions.
Evidence of decomposition
Products may appear as gas bubbles, a metal deposit, a colour change or loss of electrode mass. The observations must be connected to the identity of the species discharged at each electrode.
Worked example
Molten sodium chloride contains only Na+ and Cl- ions. Sodium forms at the cathode and chlorine forms at the anode because the two ions move to oppositely charged electrodes and exchange electrons.
Practical or data skill
Compare conductivity of an ionic solid, its solution and distilled water with a safe low-voltage conductivity tester.
Examination tip
A substance is not an electrolyte merely because it is liquid; it must contain mobile ions.
Review questions and suggested answers
Question 1
Define electrolysis.
Suggested answer
The decomposition of an ionic compound, molten or aqueous, by passage of electric current.
Question 2
Why does solid sodium chloride not conduct?
Suggested answer
Its ions are fixed in the lattice and cannot carry charge.
Question 3
What kind of electrical supply is used?
Suggested answer
A direct-current supply.
