Redox Reactions

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Redox Reactions
Oxidation Numbers Oxidation-Reduction
Mr. Shields Regents Chemistry U14 L01
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Oxidation Numbers
Weve talked about oxidation nos. before. What is
the Definition of an atoms oxidation
number? The number of electrons an atom tends to
give up or gain in a Chemical reaction
Where can we find an elements oxidation number?
What are the oxidation nos. for O and S in the
compound SO2 ? Looking at the reference table
what is oxygens oxidation no. If Oxygen is -2
Sulfur must be 4 (Why?)
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Oxidation Numbers
But what if we had looked up the oxidation no. of
Sulfur first? The table says sulfurs most
common oxidation no. is -2 so Oxygen would be 1
(why). But that cant be. There is no 1 for
oxygen. So how do we figure this out?
There are a few RULES we need to learn when
assigning Oxidation Numbers to atoms in
compounds.
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Ox. Number Rules
Rule 1 Uncombined atoms are always 0 Rule
2 Group 1 atoms are always 1 Rule 3
Group 2 atoms are always 2 Rule 4 Oxygen
is always - 2 exception Peroxides like
H2O2, Na2O2 oxygen is -1 In OF2
oxygen is (know these) 2 Rule 5 Fluorine
is always - 1 Rule 6 The other Halogens
are usually - 1 Rule 7 Hydrogen combined
with non-metals 1 example HBr Rule 8
Hydrogen combined with metals (hydrides) -
1 example CaH2 Rule 9 Total of the
charges for all atoms in a compound is zero
(0) Rule 10 Total of the charges for all atoms
in a polyatomic is equal to the charge of
the polyatomic ion
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Oxidation Numbers
Example What is the oxidation number of each
element in the following
compound. SrCl2 Sr is group 2 so it
always has an oxidation no. of 2 The sum of all
the charges of a compound is zero so the 2 Cl
must be -2. Therefore each of the Cl has an Ox.
Number of -1
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Oxidation Numbers
Example What is the oxidation number of each
element in the following
compound. PbCrO4 First you need to
recognize that CrO4 is a polyatomic with a
charge of -2 Since Oxygen is always -2, Cr must
be 6 (why?) Since CrO4 has a charge of -2, Pb
must be 2
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Oxidation Numbers
Problem What is the oxidation number of each
element in NH4NO3
In this compound there are 2 polyatomics (NH4,
NO3-) NH4 H 1 (Always) N - 3 why? NO3- O -
2 (Always) N 5 why?
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Find the oxidation s for each element in the
compound H2S2O7 H is always 1 when bonded to
a non-metal O is always -2 (except in H2O2 and
OF2) To find Sulfur, the sum of charges must
equal 0 2 (1) 2X 7 (-2) 0 2
2X -14 0 2X - 12 0 2X
12 X 6 Sulfur 6
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Redox
Were now going to discuss a topic that involves
something called Oxidation Reduction - These
reactions are commonly known as REDOX
REACTIONS
SO what is oxidation and what is
reduction? Originally oxidation referred to any
reaction in which Oxygen was one of the
reactants. Ex 4Fe 3O2 ? 2Fe2O3 2Cu
O2 ? 2CuO CH4 2O2 ? CO2 2H2O
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Redox
Opposite of Oxidation is Reduction . Originally,
this referred to any reaction in which Oxygen was
removed from a reactant. Ex 2Fe2O3 3C ?
4Fe 3CO2 CuO H2 ? Cu H2O
Today, oxidation Reduction still refer to these
type of reactions but both have taken on a much
broader meaning Recall that Oxygen is a very
electronegative element - EN 3.5 on a scale of
0 - 4 - This means it very strongly draws
electrons to itself (how many electrons does
it want ?) Right 2e- O 2e- ? O-2
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Redox
Since Oxygen wants electrons, substances that
react with oxygen Have to lose electrons -
Cu O ? Cu2O-2
From this comes the broader definition of
Oxidation/Reduction
Oxidation is the process by which substances lose
1 or more e- Reduction is the process by which
substances gain 1 or more e- HOW CAN WE REMEMBER
THIS? - Remember the phrase OIL RIG Oxidation
Is Lose of electrons / Reduction Is Gain of
electrons
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Redox
NOTE Oxidation and reduction ALWAYS occur
together Electrons cant just be lost they need
to be accepted by some Other substance. Metals
are typically oxidized (lose electrons) Na
? Na 1e- Non-metals are typically
reduced (gain electrons) Cl 1e- ? Cl-
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Half-cell reactions
The Equation that show how an element either
gains or loses Electrons is called a HALF CELL
reaction For example what is oxidized and what
is reduced in the following equation and what are
the 2 half cell reactions? Fe S ? FeS
sulfur is reduced (why?) S0 2e- ? S-2 This
is known as the REDUCTION half cell iron is
oxidized (why?) Fe0 ? Fe2 2e- This is known as
the OXIDATION half cell
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Half cell rxns
Oxidation half cell Rxns lose electrons (they
shows up on the product side) 2 F- ? F2 (g)
2 e - Mn2 (s) ? Mn7 5 e Reduction
half cell Rxns gain electrons (they show up on
the reactant side) F2(g) 2 e - ? 2 F- Mn7
5 e - ? Mn2 (s)
Oxidation Oxidation
Reduction Reduction
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Reducing Agent

• When something is Oxidized
• Its called the Reducing Agent
• When something is oxidized it
• Loses electrons (OIL)
• Something that accepts these
• Electrons is reduced (RIG)
• Therefore what is oxidized
• Facilitates the reduction of
• Something else

So Its the Reducing AGENT!
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Reducing agent
Lets look at an example of a reducing agent at
work
Fe O2 ? Fe2O3 What is the Reducing Agent?
And what is Reduced?
The first question to ask is which is WHATS
OXIDIZED? Iron is losing electrons Fe0 ?
Fe3 so Fe is oxidized. But Fe has to give
its electrons to something else. What is
it? Oxygen. So O2 is reduced. Therefore, Fe is
the Reducing Agent
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Oxidizing Agent
We said that when something is Oxidized its the
Reducing Agent. So When something is Reduced
Its called the Oxidizing Agent. We know that
when something GAINS electrons its
reduced Those electrons must come from another
Atom or Ion. The atom or ion that lost those
electrons is therefore oxidized. For example
2H2 O2 ? H20 (what is the Oxidizing
agent?) Oxidation half cell H0 ? H1 1e-
Reduction half cell O0 2e- ? O-2 Since
Oxygen is reduced its the agent that
oxidizes Hydrogen, so its the Oxidizing agent.
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• Problem for the following Reaction
• C(s) Cl2(g) ? CCl4 (l)
• A) What is reduced?
• B) What gets oxidized?
• C) What is the reducing agent?
• D) What is the oxidizing agent?
• Problem for the following Reaction
• 2 MgO ? 2 Mg O2
• A) What is reduced?
• B) What gets oxidized?
• C) What is the reducing agent?
• What is the oxidizing agent?
• Write the Oxidation and Reduction Half cell
  reactions.

Cl0 C0 C0 Cl0 Mg2 O-2 O-2 Mg2