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Redox Reaction


Redox Reaction

• one or more elements change oxidation number

•
•
•
•

all single displacement, and combustion,
some synthesis and decomposition
always have both oxidation and reduction
split reaction into oxidation half-reaction and a
reduction half-reaction
aka electron transfer reactions
half-reactions include electrons
oxidizing agent is reactant molecule that causes oxidation
contains element reduced
reducing agent is reactant molecule that causes reduction
contains the element oxidized
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Oxidation & Reduction
• oxidation is the process that occurs when
oxidation number of an element increases
element loses electrons
compound adds oxygen

compound loses hydrogen
half-reaction has electrons as products

• reduction is the process that occurs when
oxidation number of an element decreases
element gains electrons
compound loses oxygen
compound gains hydrogen
half-reactions have electrons as reactants
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Rules for Assigning Oxidation States
• rules are in order of priority
1. free elements have an oxidation state = 0
 Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)

2. monatomic ions have an oxidation state equal
to their charge
 Na = +1 and Cl = -1 in NaCl

3. (a) the sum of the oxidation states of all the
atoms in a compound is 0
 Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0
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Rules for Assigning Oxidation States
3. (b) the sum of the oxidation states of all the atoms in
a polyatomic ion equals the charge on the ion

 N = +5 and O = -2 in NO3–, (+5) + 3(-2) = -1

4. (a) Group I metals have an oxidation state of +1 in all
their compounds
 Na = +1 in NaCl

4. (b) Group II metals have an oxidation state of +2 in
all their compounds
 Mg = +2 in MgCl2
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Rules for Assigning Oxidation States
5. in their compounds, nonmetals have oxidation
states according to the table below
 nonmetals higher on the table take priority
Nonmetal

Oxidation State

Example

F

-1

CF4

H

+1

CH4

O

-2

CO2

Group 7A

-1

CCl4

Group 6A

-2

CS2

Group 5A

-3

NH3
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Oxidation and Reduction
• oxidation occurs when an atom’s oxidation state
increases during a reaction
• reduction occurs when an atom’s oxidation state
decreases during a reaction

CH4 + 2 O2 → CO2 + 2 H2O
-4 +1

0

+4 –2

+1 -2

oxidation
reduction
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Oxidation–Reduction
• oxidation and reduction must occur simultaneously
 if an atom loses electrons another atom must take them

• the reactant that reduces an element in another reactant
is called the reducing agent
 the reducing agent contains the element that is oxidized

• the reactant that oxidizes an element in another reactant
is called the oxidizing agent

 the oxidizing agent contains the element that is reduced

2 Na(s) + Cl2(g) → 2 Na+Cl–(s)
Na is oxidized, Cl is reduced
Na is the reducing agent, Cl2 is the oxidizing agent
Tro, Chemistry: A Molecular Approach

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Identify the Oxidizing and Reducing Agents
in Each of the Following
3 H2S + 2 NO3– + 2 H+ 3S + 2 NO + 4 H2O

MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O

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Identify the Oxidizing and Reducing Agents
in Each of the Following
red ag

ox ag

+1 -2

+5 -2

3 H2S + 2 NO3– + 2 H+ 3S + 2 NO + 4 H2O

+1

0

+2 -2

+1 -2

oxidation
reduction

ox ag

red ag

+4 -2

+1 -1

MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O
+2 -1

0

+1 -2

oxidation
reduction
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