2. INTRODUCTION
• Electrochemistry is a branch of Chemistry, which deals with
the interaction of electricity and chemical reactions. The
chemical reactions involved are called Redox reactions.
• Either:-
Redox reactions occur to generate electric current as in
Electrochemical/ Voltaic/Galvanic cell. Galvanic cells
convert chemical energy into electrical energy
Electric current causes redox reactions to occur as in
Elelectrolytic cells. Electrolytic cells convert electrical
energy into chemical
energy.
5. REDUCTION AND OXIDATION (REDOX)
REACTIONS
Redox reactions are reactions in which both oxidation
and reduction reactions occurs simultaneously. Redox
reactions can be defined in terms of
• Oxygen transfer
• Hydrogen transfer, or
• electron transfer.
In this unit, electron transfer will be considered.
6. In teams of oxygen transfer:
• Reduction is removal of oxygen; Oxidation is
addition of oxygen; Redox is simultaneous
addition and removal of oxygen.
• e.g. Hydrogen reduces hot copper(II)oxide to
copper metal, when passed over it and itself
oxidised to water as in the equation below:
• CuO (s) + H2 (g) Cu (s) + H2O (l)
7. In terms of hydrogen transfer:
• Oxidation is the removal of hydrogen;
Reduction is the addition of hydrogen; Redox
is simultaneous addition and removal of
hydrogen.
• e.g. Hydrogen sulphide gas is oxidized (loose
the hydrogen) to sulphur when mixed with
chlorine gas The chlorine is reduced (gain
hydrogen) to hydrogen chlorine gas.
• Cl2 (g) + H2S (g) S(S) + 2HCl (g)
8. In terms of electron transfer
• Oxidation is loss/ removal of electrons.
Recall: OIL for Oxidation Is Loss of electrons.
O - Oxidation
I - Is
L - Loss of electrons
• Metals ionizes by loss of electrons to form
positively charged ions.
Na(s) → Na+(aq) + e-
• Non-metallic ions also lose electrons to form
neutral elements as shown below.
2O2-(aq) → O2 (g) + 4e-
9. In terms of electron transfer
• Reduction is gain/ addition of electrons.
Recall: RIG for Reduction Is Gain of electrons
R - Reduction
I - Is
G - Gain of electrons
• Non-metals gain electrons to form anions
Non-metals: I2 (g) + 2e- → 2I- (aq)
• Metallic cations will gain electrons to form metals
Cations: Ca2+(aq) + 2e- → Ca(s)
10. In terms of electron transfer
• Redox is the simultaneous gaining and losing of
electrons in a chemical process. Example
CuO (s) + H2 (g) → Cu (s) + H2O (l)
• CuO is undergoing reduction. Cu in CuO occurs as
Cu2+ and will gain electrons (RIG) hence undergo
reduction.
Cu2+(s) + 2e- → Cu (s)
• Hydrogen gas is undergoing oxidation. H2 is losing
electrons to form H+ ions hence oxidation.
H2 (g) → 2H+(l) + 2e-
• Therefore, in redox reactions, both reduction and
oxidation reactions occur simultaneously.
11. Reducing and Oxidising Agents
• Reducing agent is the species that undergoes
oxidation and therefore it loses electrons. e.g.
as in above H2.
• Oxidizing agent is the species that undergoes
reduction and therefore it gains electrons. e.g.
CuO.
CuO (s) + H2 (g) → Cu (s) + H2O (l)
Oxidizing agent Reducing agent
13. DEISPLACEMENT REACTIONS
• A displacement reaction is a type of reaction in
which part of one reactant is replaced by another
reactant. They are sometimes referred to as
replacement reaction and include:
• Metal- Acid reaction- Displacement of hydrogen
with metal above it in reactivity series.
• Metal- Metal ion Reaction- Displacement of a
cation with a metal above it in reactivity series.
• Halogen-Halogen ion Displacement-
Displacement of anionic halogen with a halogen
above it in reactivity series.
15. Displacement of Hydrogen from Acid
• Magnesium metal reacts with hydrochloric acid to
produce hydrogen gas. Magnesium replaces
hydrogen ions in acid to form a salt of that acid.
Chemical Equation:
Mg(s)+2HCl (aq) MgCl2 (aq) + H2 (g)
Ionic equation:
Mg(s) + 2H+ (aq) Mg2+ (aq) + H2 (g)
• Redox reaction: Mg is oxidized from Mg (0) to
Mg2+(+2) by loss of electrons. Hydrogen is reduced
from H+(+) to H2(0) by gain of electrons.
• Metals like Cu, Hg, Ag, and Au are below hydrogen
on reactivity series hence cannot displace it.
17. Displacement of Metals from Solution
• Experiment
• When about 1g copper, zinc and magnesium
powders are added into three separate test
tubes each containing Iron (II) sulphate
displacement reactions occur in some.
Metal added to Iron (II)
sulphate solution
Colour changes
Copper Solution remains green
Zinc Green colour fades
Magnesium Green colour fades
18. Results of Displacenment
• A less reactive metal does not displace a more
reactive metal from its solution.
• Thus Cu cannot displace Fe2+ ions from its
solution.
• A more reactive metal displaces a less reactive
metal from its solution.
• Zn and Mg are more reactive than Fe hence
displace Fe2+ from its solution.
FeSO4 (aq) + Zn(s) ZnSO4 (aq) + Fe(s)
FeSO4 (aq) + Mg(s) MgSO4 (aq) + Fe(s)
19. Displacement in terms of electron transfer
• The more reactive metal undergoes oxidation by loss of
electrons to form ions. It is a reducing agent or reductant.
• The less reactive metal ions undergoes reduction by gain of
electrons to form the metal. It is an oxidizing agent or
oxidant.
• Displacement of metals involves electron transfer from a
more reactive metal to ions of another less reactive metal.
Zn(s) Zn2+(aq) + 2e (oxidation)
Fe2+(aq) + 2e- Fe(s) (reduction)
Fe2+(aq) + Zn(s) Zn2+(aq) + Fe(s) (redox reaction)
Mg(s) Mg2+(aq) + 2e (oxidation)
Fe2+(aq) + 2e Fe(s) (reduction)
Fe2+(aq) + Mg(s) Mg2+(aq) + Fe(s) (redox reaction)
20. Summary of Displacement of Metals
M Mn+ + ne-
• The forward reaction is oxidation
• The backward reaction is reduction
Mn+ gains e- (Reduction)
M losses e- ( Oxidation)
21. Displacement of Halides by Halogens
• Halogen solutions are prepared by dissolving them in
water to form halogen water (eg chlorine water).
X2(aq) + H2O(l) → HX(aq) + HOX(aq)
Halogen water
• Chlorine water is added to KCl, KBr and KI solutions,
but there will be no reaction with KCl. However, Cl2
displaces Br- and I- in KBr and KI respectively, because
it is higher on reactivity series hence more reactive.
• Bromine water is added to KCl, KBr and KI solutions,
but there will be no reaction with KCl and KBr.
However, Br2 displaces I- in KI, because it is higher on
reactivity series than it hence more reactive.
• iodine water is added to KCl, KBr and KI solutions, but
there will be no reaction. Iodine is least reactive, hence
cannot displace halides above it from their solutions.
22. Displacement of Halides by Halogens
Halogen Colour after shaking
with CCl4
Reactio
n with
KCl
solution
Reaction with
KBr solution
Reaction with
KI solution
Chlorine
water
Aqueous layer: pale
yellow-green
Hydrocarbon layer:
pale yellow-green
No
reaction
The yellow-
orange colour
of bromine
appears
The brown
colour of iodine
appears
Bromine
water
Aqueous layer:
yellow-orange
Hydrocarbon layer:
pale yellow-orange
No
reaction
No reaction Colour darkens
from yellow-
orange to
brown
Iodine
solution
Aqueous layer: brown
Hydrocarbon layer:
Purple
No
reaction
No reaction No reaction
23. Displacement of Halides by Halogens
• Iodine colour change is clear, from brown in water to
purple in the hydrocarbon layer.
• Iodine is the least soluble in water but more soluble in KI
solution, so the ‘iodine solution’ here is actually iodine in
KI solution.
• Chlorine displaces both bromine and iodine from their
halides and bromine displaces iodine from iodide:
• Cl2(aq) + 2KI(aq) → I2(aq) + 2KCl(aq)
Cl2(aq) + 2I- (aq) → I2(aq) + 2Cl- (aq) (Redox)
Cl2(aq) + 2e- (aq) → 2Cl- (aq) (Reduction)
2I- (aq) → I2(aq) + 2e- (aq) (Oxidation)
• Cl2(aq) + 2Br- (aq) → Br2(aq) + 2Cl- (aq)
• Br2(aq) + 2I- (aq) → I2(aq) + 2Br- (aq)
25. Displacement of Halides by Halogens
• The order of reactivity is
therefore
Cl2 > Br2 > I2.
• Therefore, Cl2 is an oxidizing
agent with both KI and KBr
while Br2 is an oxidizing agent
for KI. The respective halides
are reducing agents.
• Cl2(aq) + 2e- → + 2Cl- (aq)
• 2I- (aq) → I2(aq) + 2e- (aq)
