Download Form-5-Oxidation-and-Reduction-Note

CHAPTER 2: OXIDATION AND REDUCTION
Name: _______________________ (
2.1
)
Class: _________
REDOX REACTIONS
1. Many processes that occur around us are redox reactions such as combustion, rusting,
photosynthesis, respiration and decomposition.
2. Redox reactions are chemical reactions involving oxidation and reduction occurring
simultaneously.
3. Redox reactions can be defined based on:
(A) ____________________________________________________________________
(B) ____________________________________________________________________
(C) ____________________________________________________________________
(D) ____________________________________________________________________
(A)
1.
Oxidation and reduction based on loss or gain of oxygen
Oxidation is a gain of oxygen, therefore, when substance gains oxygen, it is said to be
oxidized.
2. Reduction is a loss of oxygen, therefore, when substance loses its oxygen, it is said to be
reduced.
3. Oxidizing agent: substance that caused oxidation in another substance but itself is being
reduced.
4. Reducing agent: substance that causes reduction in another substance but itself is being
oxidized.
For example:
In a reaction between aluminium and copper(II) oxide, copper(II) oxide is reduced to copper,
aluminium is oxidized to aluminium oxide.
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Example 1
When hydrogen gas is passed over hot copper(II) oxide powder, copper and water are
produced.
H2(g) + CuO(s)  Cu(s) + H2O(l)
 Hydrogen is ___________to water because it___________ oxygen.
 Copper(II) oxide is ___________ to copper because it ________oxygen.
 Reducing agent: ______________________
 Oxidizing agent: ______________________
(B)
Oxidation and reduction based on loss or gain of hydrogen
1.
Not all redox reactions involve oxygen.
2.
Oxidation is a loss of hydrogen, therefore, when substance loses its hydrogen, it is said
to be oxidized.
3.
Reduction is a gain of hydrogen, therefore, when substance gains hydrogen, it is said to
be reduced.
Example 2
Hydrogen sulphide, H2S reacts with chlorine gas, Cl2 to produce sulphur and hydrogen
chloride.
Lose hydrogen
H2S(g) + Cl2(g)  S(s) + 2HCl(g)
Gain hydrogen
 Hydrogen sulphide is ___________to sulphur because it ___________hydrogen.
 Chlorine is ________ to hydrogen chloride because it ___________hydrogen.
 Reducing agent: ___________________________________
 Oxidizing agent: ___________________________________
 _____________ colour precipitate of sulphur is formed and ______________ gas is
released.
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(C)
Oxidation and reduction based on transfer of electrons
1.
Many redox reactions do not involve oxygen or hydrogen. These reactions can be
explained based on the transfer of electrons.
2.
Oxidation is a loss of electrons and reduction is a gain of electrons.
3.
Thus, the electron acceptor acts as the oxidizing agent and the electron donor acts as
the reducing agent.
For example:
When sodium is burnt in chlorine gas, sodium
chloride is formed.
2Na(s) + Cl2(g)  2NaCl (s)
1. Sodium is __________to sodium ion because it ___________an electron.
Half equation: _________________________________
2. Sodium act as a _________agent because it ________chlorine to chloride ion but itself is
being __________to sodium ion.
3. Chlorine is ___________because it __________electron.
Half equation: __________________________________
4. Chlorine acts as an __________agent because it __________sodium to sodium ion but
itself is being _______________to chloride ion.
Example 5
When magnesium is added into copper(II) sulphate solution, magnesium displaces copper
from the solution to form copper and magnesium sulphate solution.
Mg(s) + CuSO4(aq)  MgSO4(aq) + Cu(s)
(a) Write half equations to represent the oxidation and reduction.
Oxidation: ______________________________________________________
Reduction: ______________________________________________________
(b) State the oxidizing agent and reducing agent.
Oxidising agent: ____________________ Reducing agent: _____________________
(c) Write the ionic equation to represent the redox reaction.
______________________________________________________________________
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Example 6
The chemical equation for the combustion of magnesium in air is as follow:
2Mg(s) + O2(g)  2MgO(s)
(a) Is magnesium oxidized or reduced? ___________________________________________
(b) Explain your answer in term of transfer of electrons.
________________________________________________________________________
________________________________________________________________________
(D)
Oxidation and reduction based on changes in oxidation number
1. Complete transfer of electrons occurs during redox reactions involving ions. However,
there are redox reactions involving molecules that do not involve complete transfer of
electrons.
2. Oxidation number (oxidation state) of an element is the charge that the atom of the
element would have if complete transfer of electrons takes place.
Rules for assigning oxidation number
Rule
Condition
1
The oxidation number of atoms
and molecules of elements is
zero.
2
3
Example
Atom/molecule
Hydrogen gas, H2
Helium, He
Bromine gas, Br2
Sodium, Na
Oxidation number
0
0
0
0
Ion
Na+
Cu2+
ClO2-
Oxidation number
+1
+2
-1
-2
The oxidation number of a
monoatomic ion is equal to its
charge.
In compounds, the more
electronegative elements are Fluorine, oxygen, nitrogen, chlorine, bromine, iodine
given a negative oxidation
Increasing electronegativity
number.
HCl
+1
4
Cl2O7
-1
+7
-2
4
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The oxidation number for
hydrogen in most of its
compound is +1 except in metal
hydride.
H2 O
+1
The oxidation number for
oxygen in most of its compound
is -2 except in peroxide and
when combines with a more
electronegative element such as
fluorine.
MgH2
+2
-2
PbO
+2
F2O
-2
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The sum of the oxidation
number of all the elements in the
formula of a compound must be
zero.
+2
-1
H2O2
+1
6
-1
CO2
-1
CaCO3
+4
-2
Sum of oxidation number
+2
+4
-2
Sum of oxidation number
= (+4) +2(-2)
=0
=(+2) + (+4) + 3(-2)
=0
The sum of the oxidation
number of all the elements in the
SO42formula of a polyatomic ion
+6
-2
must be equal to the charge of
Sum
of
oxidation
number
the ion.
= (+6) +4(-2)
= -2
Example 9
Determine the oxidation number of nitrogen in N2O5.
N2O5
The oxidation number of nitrogen in N2O5 is ______.
Example 10
Determine the oxidation number of chromium in K2Cr2O7.
K2Cr2O7
The oxidation number of chromium in K2Cr2O7 is ______.
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MnO4+7
-2
Sum of oxidation number
=(+7) + 4(-2)
= -1
Oxidation number and nomenclature of compounds
1. Many elements, especially transition metals, exhibit more than one oxidation number in
their compounds.
2. For example, oxidation numbers are included in names of ionic compounds in the IUPAC
nomenclature.
Formula
FeO
Fe2O3
CuCl
CuCl2
PbO
PbO2
IUPAC name
Iron(II) oxide
Iron(III) oxide
Copper(I) chloride
Copper(II) chloride
Lead(II) oxide
Lead(IV) oxide
Oxidation number of transition metal
+2
+3
+1
+2
+2
+4
3. For elements that have only one oxidation number such as those in Group 1, 2, and 13,
their oxidation number are not included in their name. i.e potassium oxide, K2O
4. Oxidation numbers are also included in the systematic naming of anions containing
metals that have more than one oxidation number.
Formula
KMnO4
K2CrO4
K2Cr2O7
Systematic name
Potassium manganate(VII)
Potassium chromate(VI)
Potassium dichromate(VI)
Redox reaction based on changes in oxidation number
1. Oxidation occurs when there is an increase in oxidation number.
2. Reduction occurs when there is a decrease in oxidation number.
For example:
When chlorine gas is bubbled into sodium
bromide solution, bromide ions, Br- are
displaced to form bromine molecules, Br2,
A reddish-brown solution is formed.
How do we know which substance has been
oxidized?
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Example 11:
Reaction of potassium bromide solution with chlorine gas
2KBr (aq) + Cl2 (g)  2KCl (aq) + Br2 (aq)
a. Oxidation number of bromide ion change from ___ to ___ (_________ in oxidation
number) ______________ reaction.
b. Potassium bromide solution act as ___________agent because it ____________chlorine to
chloride ion but itself being________________ to bromine.
c. Oxidation number of chlorine change from ____ to ______ (________ in oxidation
number)  __________reaction.
d. Chlorine acts as ______________ agent because it___________ bromide ion to bromine
but itself is being ___________to chloride ion.
Example 12
Identify the oxidizing agents (OA) and reducing agents (RA) in the following reactions.
Explain your answer in terms of oxidation number.
(a) SnCl2 + 2FeCl3  SnCl4 + 2FeCl2
(b) Zn + Pb(NO3)2  Zn(NO3)2 + Pb
(c) Br2 + 2KI  2KBr + I2
Answer:
Oxidizing agent
( decrease in oxidation number )
Reducing agent
( increase in oxidation number )
a
b
c
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Example 13
Mg(s) + O2(g)  MgO(s)
a. Oxidation number of magnesium ______________from ____ to ____.
Magnesium undergoes ________________ reaction.
b. Oxidation number of oxygen ____________from_____ to _____.
Oxygen undergoes ________________ reaction.
c. Oxidising agent : ______________ and reducing agent: ______________
Example 14
CuSO4 (aq) + Zn (s)  Cu(s) + ZnSO4 (aq)
a. Zn acts as ____________agent. It ____________Cu2+ to Cu. Zn itself is being
_____________to Zn2+.
b. CuSO4 acts as ____________agent. It __________Zn to Zn2+ but itself is being
_________________to Cu.
2.2
IDENTIFYING REDOX REACTIONS
1.
There can be more than one way of identifying a particular reaction as a redox
reaction.
For example:
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Are all reactions redox reaction?
Not all chemical reactions are redox reactions. Neutralisation and precipitation are some
examples of non-redox reactions.
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2.3
OXIDISING AGENTS AND REDUCING AGENTS
1.
An oxidizing agent is a substance that causes another substance to be oxidized.
2.
A reducing agent is a substance that causes another substance to be reduced.
Some examples of oxidizing agents and reducing agents
Oxidising agent
Reducing agent
Bromine, Br2
Reactive metals,i.e: Zn, Mg, Al
Chlorine, Cl2
Sulphur dioxide, SO2
Concentrated sulphuric acid, H2SO4
Hydrogen sulphide, H2S
Concentrated nitric acid, HNO3
Potassium iodide, KI
Hydrogen peroxide
Carbon, C
Acidified potassium manganate(VII)
Hydrogen, H2
solution, KMnO4
Acidified potassium dichromate(VI) solution,
Sodium sulphite solution, Na2SO3
K2Cr2O7
Identifying the oxidizing and reducing agents in a reaction
Example: Reaction between chorine and hydrogen sulphide
Chlorine + hydrogen sulphide
hydrogen chloride + sulphur
In this reaction, hydrogen sulphide is the reducing agent while chlorine is the
oxidizing agent.
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Testing for an oxidizing agent
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