Transcript Oxidation-Reduction Reactions

OXIDATION-REDUCTION
REACTIONS
Settle in, this is going to take a while…
What is redox?
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Reaction where there is a transfer of electrons
between reactants
Oxidation involves the loss of electrons (OIL)
 Oxidation
number/state of the element increases
 Oxidized element is the reducing agent
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Reduction involves the gain of electrons (RIG)
 Oxidation
number/state of the element decreases
 Reduced element is the oxidizing agent
Example
Complete Reaction:
Mg + Zn(NO3)2  Mg(NO3)2 + Zn
Net-ionic Reaction:
Mg + Zn2+  Mg2+ + Zn
The magnesium metal was oxidized by the zinc and
the zinc was reduced by the magnesium.
Do what?!?!
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The oxidation state of the magnesium changed from
0 to +2
 Oxidation
state increased = oxidation
 Because magnesium gave its electrons away, it is the
reducing agent
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The oxidation state of zinc changed from +2 to 0
 Oxidation
state decreased = reduction
 Because zinc took the electrons, it is the oxidizing agent
How do you know oxidation states?
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The oxidation number for any pure element is zero.
Group 1 metals form +1 ions, group 2 metals form
+2 ions, group 13 metals form +3 ions.
Transition metals can be all kinds of oxidation
numbers (ranging from +1 to +7)
Transition metal oxidation states can be determined
based on the nonmetal(s) it’s bonded to…
Nonmetal oxidation states
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Fluoride is ALWAYS -1, the other halides are usually
-1.
Oxide is usually -2, except when it’s in the peroxide
ion (-1) or bonded to fluorine (+2)
Hydrogen is +1, unless it is the hydride ion (-1)
Putting it all together
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The total charge on a compound is zero, so all
oxidation numbers must cancel out.
The total charge of elements in a polyatomic ion
must add to the charge on the ion
Practice
What is the oxidation number of each element in the
following compounds?
1. Zn(NO3)2
2. H2SO4
3. KMnO4
4. N2O4
5. PCl3
What’s the point?
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When an element gains electrons, another element
must accept those electrons (Newton’s 3rd law).
If you separate the reaction into half-reactions, you
can exploit this electron transfer to generate
electricity.
The study of this is electrochemistry, but more on
that later…
Half-Reactions?
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You can separate a redox reaction into the
reduction reaction and the oxidation reaction.
First you have to identify which element is oxidized
and which is reduced.
So let’s practice identification first:
Practice
Determine the oxidation states of all elements in the
following reactions and then identify which element
is oxidized and which is reduced.
N2 + 3H2  2NH3
2MnO2 + Zn + 2H2O  2MnO(OH) + Zn(OH)2
AgNO3 + Cu  Cu(NO3)2 + Ag
N2 + 3H2  2NH3
2MnO2 + Zn + 2H2O  2MnO(OH) + Zn(OH)2
AgNO3 + Cu  Cu(NO3)2 + Ag
Separating reactions
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Once the oxidized and reduced elements have
been identified, separate the reactions.
Use net ionic reactions instead of complete reactions
2AgNO3 + Cu  Cu(NO3)2 + 2Ag
2Ag+1 + Cu  Cu2+ + 2Ag
+1
2Ag +
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Cu 
2+
Cu +
2Ag
The silver is reduced, so that is the reduction
reaction:
2Ag+1  2Ag
The masses are balanced, but the charges are not,
so add the electrons being transferred:
2Ag+1 + 2e-  2Ag
Notice that the reduction half reaction has electrons
as reactants
+1
2Ag +
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Cu 
2+
Cu +
2Ag
The copper is oxidized, so that is the oxidation
reaction:
Cu  Cu2+
The masses are balanced, but the charges are not,
so add the electrons being transferred:
Cu  Cu2+ + 2eNotice that the oxidation half reaction has electrons
as products
+1
2Ag +
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Cu 
2+
Cu +
2Ag
When you put the 2 half-reactions together they
equal the complete reaction:
2Ag+1 + 2e-  2Ag
Cu  Cu2+ + 2ethe electrons cancel out to make the net ionic
reaction
More about balancing
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Sometimes you need more than electrons to balance
a half reaction.
Oxygens and hydrogens can be balanced by the
addition of H+, OH-, and H2O depending on if the
solution is acidic or basic.
Balancing Acidic Redox
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Balance all elements that are not oxygen or
hydrogen
Balance O by adding H2O where needed
Balance H by adding H+ where needed
Balance charges by adding e- to the more positive
side
Balancing Basic Redox
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Follow the steps for acidic balancing first, then…
Count the H+ used to balance the reaction and add
the same number of OH- to both sides.
Combine OH- and H+ to make water.
Combine/cancel water molecules as needed.
A LOT of practice
Balance the following equation in both an acidic and
a basic solution:
HOCl + NO(g)  Cl2(g) + NO3-
More practice
Balance this reaction in an alkaline environment:
MnO2(s) + Zn(s) + H2O  2MnO(OH)(s) + Zn(OH)2(s)
Homework Problems
Pick a few homework problems to work through
together 