Electrochemistry
Oxidation - Reduction
Table of Contents
Section 1 Introduction
Section 2 Oxidation Numbers
Section 3 Predicting Spontaneity of a
Redox Reaction
Questions to think About
What element must be present for carbon
to burn?
 What element must be present in order for
rusting to occur?
 Can you think of a name or a term for
turning FeS or Al2O3 into the element Fe
or the element Al? Are you adding or
subtracting something during this process?

What happens when?
Solid copper is put in AgNO3?
2. Write an equation.
3. What is happening to the solution?
4. What is happening to the copper?
Watch the demo and think about what
you see?
1.
The Demo
Describe what happened to the
solution.
 Describe what happened to the
copper.
 Can you make an equation to show the
ions that are still in solution and the
metal forming on the copper wire?

The equation
2AgNO3 (aq) + Cu(a) 
CuNO3(aq)
2Ag(s) +
Write the net ionic equation for this
reaction.
Where is the silver when it is a reactant?
Where is the silver when it is a product?
Where is the copper when it is a reactant?
Where is the copper when it is a product?
What happened to the nitrate?
Net ionic equation:
2Ag+ (aq)+ Cu(a)  2Ag(s) + Cu2+(aq)
 How many protons and electrons
there in Ag+ (aq)?
 How many protons and electrons
there in Ag(s)?
 How many protons and electrons
there in Cu(s)?
 How many protons and electrons
there in Cu2+?
are
are
are
are
What do you notice about the
electrons in the reactants and the
products?
 What is electricity?
 How does electricity come into your
house?
 How does electricity come from a
battery?

Section 1 -Introduction
Electrochemistry – the branch of
chemistry which is concerned with
the conversion of chemical energy
into electrical energy and vice
versa.(Hebden, p. 189)
Oxidation – reduction reactions
– Two kinds of reactions
– Always occur together
– Involve the loss or gain of electrons
Oxidation
A reaction that involves the loss of
electrons
Example
Complete equation, net ionic equation,
oxidation equation
2AgNO3 (aq) + Cu(a) 2Ag(s) + CuNO3(aq)
2Ag+ (aq)+ Cu(a)  2Ag(s) + Cu2+(aq)
Cu(a)  Cu2+(aq)+ 2e-
Reduction
A reaction that involves the gain of
electrons
Reduction Example
Example: Complete equation, net ionic
equation, reduction equation
2AgNO3 (aq) + Cu(a) 2Ag(s) + CuNO3(aq)
2Ag+ (aq)+ Cu(a)  2Ag(s) + Cu2+(aq)
2Ag+ (aq)+ 2e-  2Ag(s)
Memory Aid
LEO the lion says GER
LEO = Loss of Electrons is Oxidation
GER = Gain of Electrons is Reduction
(Hebden, p. 190)
Oxidizing Agent
The species that causes the oxidation
 The species that gets reduced
 Look for the species whose oxidation
number (looks like a charge) is
reduced

Example
2Ag+(aq)+ Cu(a)  2Ag(s) + Cu2+(aq)
2Ag+(aq) is the oxidizing agent – it
changes from 2Ag+(aq) to 2Ag(s)
Ag+(aq) to Ag(s) – changes from +1 to 0
or no number, changes from an ion to
a metal
Reducing Agent
The species that causes the
reduction
 The species that gets oxidized
 Look for the species whose oxidation
number (looks like a charge) is
increased.

Example
2Ag+(aq)+ Cu(a)  2Ag(s) + Cu2+(aq)
Cu(a) is the reducing agent – it changes
from Cu(a) to Cu2+(aq)
Cu(a) to Cu2+(aq)– changes from 0 (no
number) to 2+ - changes from a metal
to an ion
Redox Reactions

Any time an atom or an ion changes
its charge during a reaction a redox
reaction has occurred.
Recognizing Oxidation
Reactions
Oxidized species become more
positive because they lose electrons
 They have fewer electrons as a
product than they had as a reactant
 Protons > electrons as a product
 Cu(a)  Cu2+(aq)+ 2e See examples of oxidation reactions
(half-reactions) page 191 in Hebden

Recognizing Reduction
Reactions
Reduced species become more
negative because they gain electrons
 They have more electrons as a
product than as a reactant
 Protons < electrons as product
 Ag+ (aq)+ e-  2Ag(s)
 See examples of reduction reactions
(half-reactions) page 191 in Hebden

In a redox reaction: you
should be able to
1.
2.
3.
4.
Identify the oxidizing agent
Identify the reducing agent
Identify the species being oxidized
Identify the species being reduced
5.
6.
7.
8.
Write complete balanced equations
Write net ionic equations
Write reactions (half-reactions) for
oxidation
Write reactions (half-reactions) for
reduction
Practice



Go to the following websites to
review and practice
World of Chemistry: The Home Page of Ralph
Logan at
http://members.aol.com/logan20/agents.html
http://hyperphysics.phyastr.gsu.edu/hbase/chemical/redoxcon.html#c1
Section 2 – Oxidation
Numbers
An arbitrary system
 Used by chemists so they can
describe what happens in Redox
reactions
 It is the charge an ion would have if
the species containing the atom were
made up of ions (Hebden, p. 193)

Assumptions
All group 1 elements (alkali metals)
have oxidation numbers of 1+
 All group 2 elements (alkaline earth
metals) have oxidation numbers of 2+
 Oxygen is USUALLY 2 Hydrogen is USUALLY 1+ - not when
in a metallic hydride NaH, or BaH2

All group 17 (halogens) have oxidation
numbers of 1- but there are many
exceptions
 NEUTRAL ATOMS – ATOMS IN
THEIR ELEMENTAL FORM ARE
ZERO

Rule for calculating
Oxidation Numbers (ON’s)
THE SUM OF THE POSITIVE
OXIDATION NUMBERS
AND THE NEGATIVE
OXIDATION NUMBERS
MUST EQUAL THE
OVERALL CHARGE ON THE
SPECIES
Examples
See page 625 in Heath
 See page 626 in Hebden
 Go to this website to for a tutorial and to
practice assigning Oxidation Numbers
Assigning Oxidizing Numbers
Instructor: Dr. Angela G. King
Programming & Design: Yue-Ling Wong
Chemistry Department, Wake Forest
University
http://www.wfu.edu/~ylwong/redox/assignoxid-num/

Section 3- Predicting the
Spontaneity of a Redox
Reaction
Use the Standard Reduction
Potentials Table
 IT IS VERY SIMILAR TO THE
TABLE OF RELATIVE STRENGTHS
OF ACIDS

Table of
Standard Reduction
Potentials
Metals are on the bottom half on the
right
 EXCEPT FOR FOR THIS CLUSTER

–
–
–
–
Cu
Ag
Hg
Au
Halogens are in the top half
 Oxyanions – anions that have oxygen
are in the top half
 Some metals like Fe, Sn, Cr, Hg, Cu
have more than one oxidation number
so they are on both sides of the table
– find copper on the table – see how
many times you can find it and make
sure you notice the side of the table

Fe, Sn, Cr, Hg, Cu
Some metals like Fe, Sn, Cr, Hg, Cu
have more than one oxidation number
 they are on both sides of the table
 Find copper on the table –How many
times do you see Cu+, Cu2+, Cu(s)?
 What sides of the table are they on
each time? Is it always the same?

H2O 2
Look for H2O2 on the table.
 Where is it on the left side – top or
bottom? How much is its potential?
 Where is it on the right side – top or
bottom? How much is its potential?

Reaction Tendencies
Species on the upper left – tend to go
forward
 Species on the lower right – tend to
go in reverse

Writing Half-Reactions
Isolated half reactions – write with a
double arrow
 Half reactions made to undergo
reduction or oxidation use a single
arrow


On the table – oxidation reactions can
be thought of as written backwards
(reverse reactions)

When you write an equation you must
turn the oxidation reaction around
Example
For the reaction of Cu2+ and Zn
Cu2+ + 2e- Cu (reduction reaction)
Zn  Zn2+ + 2e-(oxidation reaction)
Cu2+ + 2e-  Cu
Zn  Zn2+ + 2e
Cu2+ + Zn  Cu + Zn2+
Oxidation or Reduction?


If two half-cells (or half reactions)
are joined, the higher half reaction
on the Table will undergo reduction
and the lower one will undergo
oxidation (Hebden, p. 196)
In a redox half-reactions there are two
species that can reduce and two species
that can oxidize.
Find the potential reducing species
that is highest on the table, that is
the one that will make the reduction
half-reaction
 Find the potential oxidizing species
that is lowest on the table, that is
the one that will make the oxidizing
half-reaction

Predicting Spontaneity

1.
2.
Given two potential reactions
Find the reactants on the table
Check to see which side the
reactants are on
Spontaneous Reactions

If one reactant is on the left and one
is on the right sides of the table, a
reaction will be spontaneous if the
one on the left, the oxidizing agent, is
higher on the table (above) than the
one on the right, the reducing agent.
Non Spontaneous Reactions
1.
2.
If both reactants are on the same
side the reaction will not occur
If one reactant is on the left and
one is on the right the reaction will
not occur if the reactant on the
left, the oxidizing agent, is lower on
the table the reactant on the right,
the reducing agent
+
H
H+ is like any other reactant
 A substance like SO42- will only
reduce, take part in a reaction, in an
acidic environment so H+ must be
present

H+ is a necessary component in many
reactions
 2 H++ 2e-  H2 is the only one that
has only 2e