Reduction – Oxidation Reactions “REDOX” - Weebly

Reduction – Oxid
“RED

• Is a chemical reaction in which e
• Must have both reduction and

reaction to occur

▫ REDUCTION – a process in wh
▫ OXIDATION – a process in whi

▫ How can you remember this?

“LEO the lion says
LEO = Losing Electrons
GER = Gaining Electrons

Other memory devices:
OIL RIG (Oxidation Is Losing electrons, Reduct
REGOLE (Reduction Electron Gain Oxidation L

dation Reactions
DOX”

electrons are transferred
oxidation happening for the

hich electrons are gained by an entity
ich electrons are lost by an entity

GER”
= Oxidation
s = Reduction

tion Is Gaining electrons)
Loss of Electron)

Reduction – Oxid
“RED

• Examples of Redox Reaction

¾Synthesis, decomposition, co
cellular respiration, photosy
(NOT double displacemen

dation Reactions
DOX”

ns:

ombustion, single displacement,
ynthesis,
nt)

An Introduction to

• Imagine that a reaction is a combination of

▫ A half reaction represents what is happeni
▫ Another half-reaction is required to compl

• Example: When metal is placed into hydro
the zinc slowly disappears.
Zn(s) + 2HCl(aq) Æ

• What happens to the zinc? To the HCl(aq)?

Zn(s) Æ Zn

2 H+(aq) +

Notice that both of these half-reactions are bala
element on both sides) and by ch

▫ A half reaction is a balanced chemical equa
electrons by a substance

o Redox

f two parts called half-reactions.

ing to one reactant, it tells one part of the story.
lete the description of the reaction.

ochloric acid solution, gas bubbles form as

Æ ZnCl2(aq) + H2(g
Look at the half-reactions.

n 2+ (aq) + 2 e-
+ 2 e- Æ H2 (g)

anced by mass (same number of atoms/ions of each
harge (same total charge on both sides)

ation that represents either a loss or gain of

An Introduction to

• Imagine that a reaction is a combination of

▫ A half reaction represents what is happenin
▫ Another half-reaction is required to compl

• Example: When metal is placed into hydroc
the zinc slowly disappears.
Zn(s) + 2HCl(aq) Æ

• What happens to the zinc? To the HCl(aq)?

Zn(s) Æ Zn
REDUCTION - entity gains electrons 2 H+(aq) +

▫ Where is oxidation occurring?? (LEO)

▫ Where is reduction occurring?? (GER)

o Redox

f two parts called half-reactions.

ng to one reactant, it tells one part of the story.
lete the description of the reaction.

chloric acid solution, gas bubbles form as
Æ ZnCl2(aq) + H2(g

Look at the half-reactions.

n 2+ (aq) + 2 e- OXIDATION - entity loses electrons
2 e- Æ H2 (g)

)

)

An Introduction to

• Example: When a piece of copper is placed
beaker of silver nitrate, th

Cu(s) + AgNO3(aq) Æ Cu(NO3)2(aq) + A

• Write the balanced half-reaction equations

▫ To show that the number of electrons gain
half-equations, it may be necessary to mu
coefficient to balance the electrons. I.e. Ag

Cu(s) Æ Cu
2 [Ag+(aq)

• Where is Oxidation occurring?

• Where is Reduction occurring?

o Redox

d into a
he following changes occur.

Ag(s)

s:

ned equals the number of electrons lost in two
ultiply one or both half-reaction equations by a
g half reaction must be multiplied by 2

u 2+ (aq) + 2 e- OXIDATION
) + e- Æ Ag (s)] REDUCTION

An Introduction to

OXIDATION Cu(s) Æ Cu
REDUCTION 2 [Ag+(aq)

• Now add the half-reactions and cancel the t
equation to obtain the net-ionic equation

2 Ag+(aq) + 2 e- + Cu(s) Æ

2 Ag+(aq)+ Cu(s)

▫ Silver ions are reduced to silver metal by
copper metal is oxidized to copper(II) ion

o Redox

u 2+ (aq) + 2 e-
+ e- Æ Ag (s)]

terms that appear on both sides of the
n

Æ 2 Ag(s) + Cu 2+ (aq) + 2 e-
Æ 2 Ag(s) + Cu 2+ (aq)

reaction with copper metal. Simultaneously,
ns by reaction with silver ions.

An Introduction to

▫ Silver ions are reduced to silver metal by
copper metal is oxidized to copper(II) ion

o Redox

y reaction with copper metal. Simultaneously,
ns by reaction with silver ions.

An Introduction to

▫ There are two methods for developin
1) ½ reaction method we just le

OR

2) Using the net-ionic equation me

Cu(s) + 2AgNO3(aq) Æ Cu(NO3)2(aq) + 2Ag

Cu(s) + 2Ag + + 2 NO3- (aq) Æ Cu2+(aq)+
(aq)

2 Ag+(aq)+ Cu(s) Æ 2 Ag(s) + Cu 2+ (aq)

o Redox

ng net ionic equations:
earned

ethod from Chem 20
g(s) (dissociate high sNolOubNili-tIyOaNndICionic compounds)
+ 2NO3-(aq) + 2Ag(s) (canTceOl TspAeLctIaOtoNrIiCons)
) (DoNwEe Tge-tIOthNe IsCame net ionic reaction?? YES!)

Summary “Electron

• A redox reaction is a chemical r
transferred between entities

• The total number of electrons gain
total number of electrons lost in th

• Reduction is a process in which

• Oxidation is a process in which e

• Both reduction and oxidation are
reaction equations.

n Transfer Theory”

reaction in which electrons are

ned in the reduction equals the
he oxidation
electrons are gained by an entity
electrons are lost by an entity
represented by balanced half-

REDOX React

Reduction

• Historically, the formation of a metal
from its “ore” (or oxide)

▫ I.e. nickel(II) oxide is reduced by
hydrogen gas to nickel metal

NiO(s) + H2(g) Æ Ni(s) + H2O(l)

Ni +2 Æ Nio

• A gain of electrons occurs (so the entity
becomes more negative)

• Electrons are shown as the reactant in
the half-reaction

tions…. so far

Oxidation

• Historically, reactions with oxygen

▫ I.e. iron reacts with oxygen to
produce iron(III) oxide

4 Fe(s) + O2(g) Æ Fe2O3(s)

Fe 0 Æ Fe+3

• A loss of electrons occurs (so the entity
becomes more positive)

• Electrons are shown as the product in
the half-reaction

Oxidation States

• An oxidation state is defined as the
would have if the electron pairs in a co
most electronegative atom.

• An oxidation number is a positive o
oxidation state of the atom in a compo

• Example: In water, which is the most e

▫ Oxygen, so we act as if the oxygen owns

apparent net electric charge an atom
ovalent bond belonged entirely to the

or negative number corresponding to the
ound. (These are NOT charges!)

electronegative atom, H or O?

both electrons in the electron pair.

Each oxygen atom has 8 p+ and 8 e-.
But if the oxygen atom gets to count the
two hydrogen electrons (red dots) in the
two shared pairs, as its own, then it has
8 p+ but 10 e-, leaving an apparent net

charge of -2

Each hydrogen atom has 1 p+, but with
no additional electron (since oxygen has
already counted it), that leaves hydrogen

with an apparent net charge of +1

Oxidation States

Tip:
• The sum of the oxidation numbers for
• The sum of the oxidation numbers for

** This method only works if there is only one

r a neutral compound = 0
r a polyatomic ion = ion charge

e unknown after referring to the above table

Oxidation States

• Example: What is the oxidation num

▫ After referring to Table 1, we assign a
▫ So now we have some simple ma
▫ Since a methane molecule is electrical

one carbon atom and the four hydroge

x + 4(+1)

x So the oxidation number of car
x How do we write this?

mber of carbon in methane CH4?

an oxidation number of +1 to hydrogen
ath…
lly neutral, then the oxidation number of the
en atoms 4(+1) must equal zero.

) =0

rbon is = -4

Oxidation States

• Example: What is the oxidation numbe
MnO4- ?

▫ After referring to Table 1, we assign an

▫ Since a permanganate ion has a charge o
manganese atom and the four oxygen ato

x + 4(-

x+ -8

x So the oxidation number of mang

• Example: What is the oxidation numbe

▫ We know the oxidation numbers of both

2(+1) +
2+

So the oxida

er of manganese in a permanganate ion,

oxidation number of -2 to oxygen
of 1-, then the oxidation number of the one
oms 4(-2) must equal -1.
-2) = -1
8 = -1

ganese is = -7

er of sulfur in sodium sulfate?

h Na and O, and solve algebraically

+ x + 4(-2) = 0
+ x + -8 = 0

ation number of sulfur is +6

Redox in Living Organ

• The ability of carbon to take on diffe
oxidation states is essential to life on
Photosynthesis involves a series of re
reactions in which the oxidation num
carbon changes from +4 in carbon di
an average of 0 in sugars such as glu

• The smell of a skunk is caused by a th
compound (R-SH). To deodorize a p
sprayed by a skunk, you need to conv
smelly thiol to an odourless compoun
Hydrogen peroxide in a basic solutio
an oxidizing agent to change the thio
disulfide compound (RS-SR), which
odourless.

nisms

erent
n Earth.
eduction
mber of
ioxide to
ucose.

hiol
pet
vert the
nd.
on acts as
ol to a
is

Determining Oxidation

1. Assign common oxidation numbers
2. The total of the oxidation numbers o

value of the net electric charge of th

a) The sum of the oxidation numbers for a
b) The sum of the oxidation numbers for a

3. Any unknown oxidation number is deter
known oxidation numbers and the net ch

n Numbers Summary

s (Table 1 on page 658)
of atoms in a molecule or ion equals the
he molecule or ion.

a compound is zero.
a polyatomic ion equals the charge of the ion.

rmined algebraically from the sum of the
harge on the entity.

Oxidation Numbers an

• Although the concept of oxidation states
on assigned charges, it is self-consistent a

▫ Chemists believe that if the oxidation
chemical reaction, then an electron tra

▫ Based on oxidation numbers,

x If the oxidation numbers do not chan

x An increase in the oxidation numbe

x A decrease in the oxidation number

nd Redox

is somewhat arbitrary, because it is based
and allows predictions of electron transfer.
number of an atom or ion changes during a
ansfer (oxidation-reduction reaction) occurs.

nge = no transfer of e-’s = not a redox rxn
er = oxidation
r = reduction

Oxidation Numbers an

• Example: Identify the oxidation and reducti
hydrochloric acid.

▫ First write the chemical equation (as it is not p

▫ Determine all of the oxidation numbers

▫ Now look for the oxidation number of an atom
label the change as oxidation. There must al
decreases. Label this change as reduction.

nd Redox

ion in the reaction of zinc metal with

provided)

m/ion that increases as a result of the reaction and
lso be an atom/ion whose oxidation number
.

Oxidation Numbers an

• Example: When natural gas burns in a furn
oxidation and reduction in this reaction.

▫ First write the chemical equation (as it is not p

▫ Determine all of the oxidation numbers

▫ Now look for the oxidation number of an atom
label the change as oxidation. There must al
decreases. Label this change as reduction.

nd Redox

nace, carbon dioxide and water form. Identify

provided)

m/ion that increases as a result of the reaction and
lso be an atom/ion whose oxidation number
.

Balancing Redox Equations u

1. Assign oxidation numbers a
oxidation numbers change

2. Using the change in oxidation
electrons transferred per ato

3. Using the chemical formulas, d
electrons transferred per rea
do this)

4. Calculate the simplest whole nu
reactants that will balance the
transferred. Balance the reacta

5. Balance the O atoms using H2O
using H+(aq)

using Oxidation Numbers

and identify the atoms/ions whose

n numbers, write the number of
om.
determine the number of
actant. (Use formula subscripts to

umber coefficients for the
e total number of electrons
ants and products.
O(l), and then balance the H atoms

Balancing Redox Equations u

Example: When hydrogen sulfide is burned in the pre
water vapour. Use oxidation numbers to balance this

1.Assign oxidation numbers to all atoms/ions and loo
▫ Notice that a sulfur atom is oxidized from -2
This is a change of 6 meaning 6 e- have been t

▫ An oxygen atom is reduced from 0 to -2. This
of 2 or 2e- transferred.

▫ Because the substances are molecules,
not atoms, we need to specify the change
in the number of e-’s per molecule

2.The next step is to determine the simplest whole nu
transferred for each reactant. The numbers become t

1.The coefficients for the products can be obtained by
changed and then any other atoms.

using Oxidation Numbers #1

esence of oxygen, it is converted to sulfur dioxide and

s equation. H2S(g) + O2(g) Æ SO2(g) + H2O(g)

ok for the numbers that change. Highlight these.

to +4.
transferred.

s is a change

umbers that will balance the number of electrons
the coefficients of the reactants

y balancing the atoms whose oxidation numbers have

Balancing Redox Equations us

Example: Chlorate ions and iodine react in an acidic s

Balance the equation for this reactions. ClO3-(aq)

1.Assign oxidation numbers to all atoms/ions and look for
Remember to record the change in the number of electron

1.The next step is to determine the simplest whole numbers
each reactant. The numbers become the coefficients of the
obtained by balancing the atoms whose oxidation number

2.Although Cl and I atoms are balanced, oxygen is not. Add

3.Add H+(aq) to balance the hydrogen. The redox equation s
checking the total numbers of each atom/ion on each side
balanced.

sing Oxidation Numbers #2

solution to produce chloride ions and iodate ions.
) + I2(aq) Æ Cl-(aq) + IO3-(aq)
the numbers that change. Highlight these.
ns per atom and per molecule or polyatomic ion.

s that will balance the number of electrons transferred for
e reactants. The coefficients for the products can be
rs have changed and then any other atoms.

d H2O(l) molecules to balance the O atoms.

should now be completely balanced. Check your work by
e and checking the total electric charge, which should also be

Balancing Redox Equations us

Example: Methanol reacts with permanganate ions in a
shown below. Balance the equation for this reaction.
•Assign oxidation numbers to all atoms/ions and look for t

•Remember to record the change in the number of electron

•Determine the simplest whole numbers that will balance t
numbers become the coefficients of the reactants. The coe
atoms whose oxidation numbers have changed and then a

•Add H2O(l) to balance the oxygen, add H+(aq) to balance the

sing Oxidation Numbers #3

a basic solution. The main reactants and products are
the numbers that change. Highlight these.
ns per atom and per molecule or polyatomic ion.
the number of electrons transferred for each reactant. The
efficients for the products can be obtained by balancing the
any other atoms.

e hydrogen.

Balancing Redox Equations us

Example: Household bleach contains sodium hypochlo
(react with themselves) to produce chloride ions and c
the disproportionation.

For disproportionation reactions, start with
side and follow the usual procedure for bala

sing Oxidation Numbers #4

orite. Some of the hypochlorite ions disproportionate
chlorate ions. Write the balanced redox equation for

h two identical entities on the reactant
ancing equations.

Balancing Redox Equations us

• Example: A person suspected of being intoxicate
person’s breath reacts with an acidic dichromate
and aqueous chromium(III) ions. Balance the eq

• Remember to balance the C and Cr first, then ad
stop because this is an acidic solution

sing Oxidation Numbers #5

ed blows into this device and the alcohol in the
e ion solution to produce acetic acid (ethanoic acid)
quation for this reaction.

dd H2O(l) to balance O, H+(aq) to balance H and then