During electrolysis, redox reactions occur where ions are reduced or oxidized at the electrodes. At the cathode, ions gain electrons through reduction reactions and at the anode, ions lose electrons through oxidation reactions. The overall reactions in electrolysis depend on the electrolyte, but generally involve the transfer of electrons between electrodes to split the electrolyte into separate elements or compounds.

2.  Redox (oxidation and reduction)reactions take place during electrolysis of
chemical compounds.
 One element gains electrons whilst one element loses electrons

3. An ionic compound conducts electricity when it is molten
or in solution. The current causes the ionic compound
to split up and form new substances.
Electrolysis has many uses, including:
 purifying copper
 plating metals with silver and gold
 extracting reactive metals, such as aluminium
 making chlorine, hydrogen and sodium hydroxide.
This process is called electrolysis, a word which comes from
Greek and means “splitting by electricity”.

4. heat
In electrolysis, the substance that the current passes through and splits up is called
the electrolyte.
Positive ions move
to the negative electrode
(cathode) and gain electrons.
This is reduction.
Negative ions move
to the positive electrode (anode) and
lose electrons.
This is oxidation.
The electrolyte contains positive and negative ions.
What happens to these ions during electrolysis?

5. When a dilute solution of sulfuric
acid is electrolysed, gases are
produced at both the anode and the
cathode.
The gas produced at
the cathode burns with a 'pop' when
a sample is lit with a lighted splint.
This shows that the gas is hydrogen.
The gas produced at
the anode relights a glowing splint
dipped into a sample of the gas. This
shows that the gas is oxygen.
At cathode,
2H⁺ + 2e−→H2
At anode,
4OH−→2H2O +O2 +4e−

6. What happens at the electrodes during the
electrolysis of dilute sulfuric acid?
What is the overall equation for the electrolysis of dilute sulfuric acid?
At the negative electrode:
2H+ + 2e-  H2 (reduction)
2H2O (l)  2H2 (g) + O2 (g)
Twice as much hydrogen forms as oxygen. Why is this?
At the positive electrode:
4OH-  2H2O + O2 + 4e- (oxidation)
In water, there are 2 hydrogen atoms for every oxygen atom, so the ratio
by volume, of H2 to O2, is 2:1.

7.  Electrolysis of aqueous NaCl results in hydrogen and chloride gas.
 When concentrated NaCl solution undergoes electrolysis, sodium hydroxide and
hydrogen are produced at the cathode while chlorine is produced at the anode.
 Cathode: 2H2O + 2e- H2 + 2OH⁻
 Anode: 2Cl ⁻ Cl2 + 2e-

8. The electrolysis of sodium chloride solution produces three very useful
products:
 Chlorine used for killing bacteria in water, for bleach and
making plastics like PVC.
 Hydrogen used for making margarine and fertilizers, and
for rocket fuel.
 Sodium hydroxide used in many chemical reactions,
such as making soap, neutralizing acids and making
paper.
Chlorine is expected as a product of this process but hydrogen and sodium
hydroxide are surprising products.

9. During the electrolysis of dilute NaCl
solution, Na+, Cl- and water are
present.
At the cathode
Cathode: 2H2O + 2e- H2 + 2OH(-)
At the anode
Anode: 2H2O O2 + 4H⁺ + 4e-

10. The negative cathode electrode attracts Cu2+ ions (from copper sulfate)
and H+ ions (from water).
Only the copper ion is discharged, being reduced to copper metal.
A brown copper deposit forms as the positive copper ions are attracted to
the negative electrode (cathode)
Cu 2⁺(aq) + 2e– Cu(s)
The blue colour fades as more and more copper is deposited, depleting the
concentration of the blue copper ion Cu2+ ions in solution.
Oxygen gas is formed at the positive electrode,
an oxidation reaction (electron loss).
The negative sulfate ions (SO4
2-) or the traces of hydroxide ions
(OH– from water) are attracted to the positive electrode.
Hydroxide ions or water molecules are discharged and
oxidised to form oxygen.
(i) 4OH–
(aq) 4e– 2H2O(l) + O2(g)
The negative hydroxide ion is oxidised by electron loss
or written as: 4OH–
(aq) 2H2O(l) + O2(g) + 4e–

11.  The negative cathode electrode attracts Cu2+ ions (from copper
sulphate) and H+ ions (from water).
 Only the copper ion is discharged, being reduced to copper metal.
 A reduction electrode reaction at the negative cathode
 Cu2+(aq) + 2e– Cu(s)
 A copper deposit forms, reduction of the copper ions, each gains 2
electrons.
 (ii) The positive anode reaction with a copper electrode
 Its the copper anode that is the crucial difference than
electrolysing copper sulphate solution with a inert
carbon/graphite/platinum electrode.
 An oxidation electrode reaction at the positive anode
 The copper dissolves, oxidation of the copper atoms, each losing 2
electrons to form blue Cu2+ ions
 or written as: Cu(s) Cu2+(aq) + 2e–

12. What redox processes occur at the electrodes
during the electrolysis of molten lead
bromide (PbBr2)?
What is the overall equation for the electrolysis of molten lead bromide ?
At the negative electrode:
Pb2+ + 2e-  Pb (reduction)
lead bromide  lead + bromine
PbBr2 (l)  Pb (l) + Br2 (g)
At the positive electrode:
2Br-  Br2 + 2e- (oxidation)

13.  https://www.middlesbrough.gov.uk › default › files. Chapter 6 Electrochemistry
 https://www.docbrown.info/page01/ExIndChem/electrochemistry04.htm
 Tindale, Ritchie et al, 2014, Chemistry for CSEC 2nd Edition, Nelson Thornes.
Chapter 10, p182-187