Reduction Oxidation
Red-ox
An introduction to electron transfer reactions
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Oxidation-Reduction: A Reaction
Oxidation: When a substances loses electrons.
Reduction: When a substance gains electrons.
Consider:
Ca(s) + 2H+(aq)  Ca2+(aq) + H2(g).
The neutral Ca(s) has lost two e- to 2 H+ to become Ca2+.
We say Ca has been oxidised to Ca2+
At the same time 2 electrons are gained by 2 H+ to form H2 .
We say H+ is reduced to H2 .
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Redox Reaction with Air
Consider the reaction of Ca with O2:
2Ca(s) O2(g)  2CaO(s)
Ca is easily oxidised in air.
On the left we see freshly cut Ca.
On the right we see Ca with a dull surface.
On the surface, there is a coat of CaO.
Again, Ca(s) gains electron and is oxidised to Ca+2
And the neutral O2 has gained electrons from the Ca to
become O2- in CaO.
We say O2 has been reduced to O2-.
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Electron Transfer and Terminology
Lose electrons: Oxidation
Gain electrons: Reduction.
GER
Leo
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It Takes Two: Oxidation-Reduction
In all reduction-oxidation (redox) reactions, one species is
reduced at the same time as another is oxidised.
Oxidizing Agent:
The species which causes oxidation is called the oxidizing agent.
The substance which is oxidised loses electrons to the other.
The oxidizing agent is always reduced
Reducing Agent:
The species which causes reduction is called the reducing agent.
The substance which is reduces gains electrons from the other.
The Reducing agent is always oxidised
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Oxidation of Metals with Acids
It is common for metal to produce hydrogen gas when
they react with acids. For example, the reaction
between Mg and HCl:
Mg(s) + 2HCI(aq)  MgCl2(aq) + H2(g) .
In this rxn, Mg is oxidised and H in HCl is reduced.
Note the change in oxidation state for these specie:
Mg0  Mg+2 in MgCl2
&
H+ in HCl  H0 in H2
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Redox Rxn with Acid
It is possible for metals to be oxidised with salt:
Fe(s) + Ni(N03)2 (aq)  Fe(N03)2 (aq) + Ni (s) .
Molecular Equation
The overall ionic equation shows the redox chemistry:
Fe(s) + Ni+2(aq)  Fe2+(aq) + Ni (s)
Net ionic Equation
In this reaction iron has been oxidised to Fe2+ while the Ni+2
has been reduced to Ni0.
What determines whether the reaction occurs ?
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The Activity Series
Metals can be
placed in order
of their
tendencies for
losing electrons.
This is called
the activity
series.
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Competition For e- Transfer
Consider: Na, Mg, Al,
Metallic character decreases left to right.
Metal tend to give up electrons.
Now consider the reaction:
Na + AlCl3  ??? (NaCl + Al)
To determine if the reaction occurs, the question is to determine which
metal has a greater affinity for electrons (or which is willing to lose e- ).
Na is more willing to lose e- than Al
Al is more willing to accept e- (less metallic)
Conclude: The reaction occurs.
3Na + AlCl3  3NaCl + Al
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Reading Activity Table
A metal in the activity
series can only be
oxidised by a metal ion
below it.
In our example, Na
is oxidised by Al.
The metals at the top of
the activity series are
called active metals.
The metals at the bottom
of the activity series are
called noble metals.
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Example: Silver and Copper
If we place Cu into a solution of Ag+ ions, will
copper plate out of solution ?
Cu(s) + 2AgNO3(aq)  ? [Cu(NO3)2(aq) + 2Ag (s)]
or Cu (s) + 2Ag+ (aq)  ? [Cu2+(aq) + 2Ag (s)]
Which metal is active? Which is noble ?
 Cu
 Ag
\ Therefore, Cu 2+ ions is be formed because
Cu is above Ag in the activity series.
Copper
Silver
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Cu  Cu2+ + 2 eAg  Ag+ + e-
Example: Redox Reaction
B&L 4.47: Based on the activity series, what is the outcome of the
following reaction ?
b) Ag(s) + PbNO3 (aq)  ?
c) Cr (s) + NiSO4 (aq)  ?
e) H2 (g) + CuCl2 (aq)  ?
f) Ba (s) + H2O (l)  ?
b) Ag vs. Pb , Pb is more active, rxn not occurs
c) Cr vs. Ni , Cr is more active, rxn occurs
Cr (s) + NiSO4 (aq)  Ni (s) + CrSO4 (aq)
d) H2 vs. Cu , H2 is more active, rxn occurs
H2 (g) + CuCl2 (aq)  2HCl (aq) + Cu (s)
e) Ba vs. H2 , Ba is more active, rxn occurs
Ba (s) + H2O (l)  H2 (g) + Ba(OH)2 (aq)
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Summary
Redox OxidationReduction-
Oxidation/Reduction reaction
Lose electron (LEO)
Gain electron (GER)
Activity Series- Table showing elements’ relative
ease of oxidation.
Active MMetal which prefers to lose e- and
there fore prefer the oxidised form.
Noble MMetal which do not lose e- and
therefore prefers the zeroth state.
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