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 Oxidation
is the loss of electrons
 Reduction
is the gain of electrons

The total reaction = redox reaction
2Mg(s) + O2(g)  2MgO(s)
 The half reactions = oxidation and/or reduction
reactions
Mg  Mg+2 + 2eO2 + 4e-  2O-2
Mg(s) + O2(g)  MgO(s)
Mg  Mg+2 + 2e-
O2 + 4e-  2O-2
Mg loses 2 electrons to become Mg+2
 O gains 2 electrons to become O-2
( we have O2 molecule that contains 2 oxygen
atom, both atoms are gaining electrons : O2 gains 4
electrons to become 2O-2)

OXIDATION half reaction
 Mg  Mg+2 + 2eMg loses electrons, so it is oxidized
REDUCTION half reaction
 O2 + 4e-  2O-2
Oxygen gains electrons, so it is reduced
An overall redox reaction consist of two half
reactions
# of electrons lost = # of electrons gained

2Mg  2Mg+2 + 4e- (multiply by 2 to get 4)
O2 + 4e-  2O-2

2Mg + O2 + 4e-  2Mg+2 + 2O-2 + 4e2Mg + O2  2Mg+2 + 2O-2

Overall balanced equation:
2Mg(s) + O2(g)  2MgO(s)

Ex: For the rxn between sodium and chlorine,
write the chemical equation and the ionic
equation:
Na + Cl2→ NaCl
Sodium loses an electron – oxidized
 Chlorine gains an electron – reduced

Can also write the oxidation and reduction half
reactions:
 Na → Na+ + 1eoxidation half rxn
 Cl2 + 2e- → 2Clreduction half rxn


When combining half reactions make sure # egained = #e- lost.
2Na → 2Na+ + 2e- half rxn ×2
 Cl2 + 2e- → 2Cl
Total redox rxn
 2Na+ Cl2 + 2e- → 2Cl- + 2Na+ + 2e 2Na+ Cl2 → 2Cl- + 2Na+
 2Na+ Cl2 → 2NaCl

1.
2.
3.
4.
Fe + Br2  FeBr3
Ni + HgCl2  Hg + NiCl2
Sn+2 + Cu+2  Sn+4 + Cu
CO + I2O5  CO2 + I2
Oxidation and reduction reaction = redox rxn
 Oxidation is loss of electrons and reduction is gain
of electrons = transfer of electrons
 Those 2 reactions are occurring simultaneously

0
0
+2 -2
2Mg(s) + O2(g)  2MgO(s)

Mg loses 2 e- to become Mg+2: it is oxidized
› 2Mg

2Mg2+ + 4e-
(ox. # increases)
O gains 2 e- to become O-2: it is reduced
› O2 + 4e-
2 O-2
(ox. # decreases)
0
0
+2 -2
2Mg(s) + O2(g)  2MgO(s)
Oxidation half-reaction:
2Mg → 2Mg2+ + 4e-
(ox. # increases)

2Mg loses 4 e- to become 2Mg+2: it is oxidized

Mg was oxidized by of O2. Mg gave those e- to
oxygen. Therefore, oxygen is OXIDAZING
AGENT
Reduction half-reaction:
O2 + 4e- → 2 O-2
(ox. # decreases)

O gains 2 e- to become O-2: it is reduced

O2 was reduced by Mg. O2 accepted e- from
magnesium. Therefore, Mg is REDUCING
AGENT.

In a redox reaction, the reducing agent is
oxidized and the oxidizing agent is reduced.
Loss of electrons
0
0
+2 -2
2Mg(s) + O2(g)  2MgO(s)
Gain of electrons
Substance oxidized
Reducing agent
Substance reduced
Oxidizing Agent

The terms oxidizing and reducing agents tell you
what effect has a substance on its environment
and what happens to it.

For the following reaction:
Ni + HgCl2 → Hg + NiCl2
A) Draw a diagram showing the loss and gain of
electrons.
B) Identify the substance oxidized, the substance
reduced , the oxidizing agent and the reducing
agent.
C) Write the oxidation and reduction half reactions.

For following reactions:
5CO + I2O5 → 5 CO2 + I2
2 Na + 2 H2O → 2NaOH + H2
Cl2 + 2NaBr → Br2 + 2NaCl
4NH3 + 7O2 → 4NO2 + 6H2O