1/4/2010
It Takes Two: Oxidation-Reduction
•In all reduction-oxidation (redox) reactions, one species is reduced
at the same time as another is oxidized.
•Oxidizing Agent:
Predicting Redox
Reactions
Chapter 9.3
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 oxidized and H in HCl is reduced.
•Note the change in oxidation state for these species:
•
Mg0  Mg+2 in MgCl2
•
&
•
H+ in HCl  H0 in H2
The Activity Series
•Metals can be
placed in order
of their
tendencies for
losing
electrons. This
is called the
activity series.
•The species which causes oxidation is called the oxidizing agent.
The substance which is oxidized 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 oxidized
Redox Rxn with Salt
•It is possible for metals to be oxidized with salt:
•Fe(s) + Ni(N03)2 (aq)  Fe(N03)2 (aq) + Ni (s) .
•Molecular Equation
•The net ionic equation shows the redox chemistry well:
•Fe(s) + Ni+2(aq)  Fe2+(aq) + Ni (s)
•Net ionic Equation
•In this reaction iron has been oxidized to Fe 2+ while the Ni+2 has been
reduced to Ni0.
•What determines whether the reaction occurs ?
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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1/4/2010
Reading Activity Table
•A metal in the activity
series can only be oxidized
by a metal ion below it.
•In our example, Na
•is oxidized 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.
•
•
•
Example: Redox Reaction
Based on the activity series, what is the outcome of the
following reaction ?
a) Ag(s) + Pb(NO3)2 (aq)  ?b) Cr (s) + NiSO4 (aq)  ?
c) H2 (g) + CuCl2 (aq)  ? d) Ba (s) + H2O (l)  ?
a) Ag vs. Pb , Pb is more active, rxn not occurs
b) Cr vs. Ni , Cr is more active, rxn occurs
Cr (s) + NiSO4 (aq) Ni (s) + CrSO4 (aq)
c) H2 vs. Cu , H2 is more active, rxn occurs
H2 (g) + CuCl2 (aq)
2HCl (aq) + Cu (s)
d) Ba vs. H2O , Ba is more active, rxn occurs
Ba (s) + 2H2O (l)
2H2 (g) + Ba(OH)2 (aq)
Example: Silver and Copper
•If we place Cu into a solution of Ag+ ions, will
copper go into 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
•
Cu 2+ ions will be formed because Cu
is above Ag in the activity series.
Copper
Silver
Cu  Cu2+ + 2 eAg  Ag+ + e-
Summary
•Redox - Oxidation/Reduction reaction
•OxidationLose electron (LEO)
•ReductionGain electron (GER)
•Activity Series- Table showing elements’ relative
ease of oxidation.
•Active Metal- Metal which prefers to lose e- and
there fore prefer the oxidized form.
•Noble Metal- Metal which do not lose e- and
therefore prefers the zeroth state.
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