Lesson
Plan: Reduction-Oxidation
Education
Unit : Senior High School
School
Name : State Senior High School 4
Subject : Chemistry
Class / Semester : XII / Odd Semester
Learning Materials : Reduction-Oxidation
Allocation of Time : 2x 45 minutes
I. Competence Standards : 2. To apply the concept of
oxidation-reduction reactions and electrochemistry in technology and everyday
life.
II. Basic Competence : 2.1 To Apply the concept
of oxidation-reduction reactions in electrochemical systems involving
electrical energy and its usefulness in preventing corrosion and also for the
industry.
III. Indicator
A.
Cognitive
1. Differentiate the concept of
oxidation and reduction in terms of the incorporation of oxygen release,
discharge and electron acceptance, as well as the increase and decrease in oxidation
state.
2. Determine the oxidizing and reducing
agents in redox reactions.
3. Balance the redox equation by half
reaction (ion-electron).
4. Balance the redox equation by changes
in the oxidation state.
B. Affective
Character
skills :
Show an appreciation of scientific
attitudes such as . honest, responsibility,curiousity, passionate about learning and open Mindness learning about
reduction-oxidation reaction.
IV. Learning
Objectives:
A. Cognitive
a.
Given
an question worksheet and also handsout about reduction-oxidation reaction,
students can differentiate the concept of oxidation and reduction in terms of
the incorporation of oxygen release, discharge and electron acceptance, as well
as the increase and decrease in oxidation state correctly based on LP 1:
Product in accordance with the answer key.
b.
Given
an question worksheet and handsout about reduction-oxidation reaction, student
can determine the oxidizing and reducing agents in redox reactions correctly
based on LP 1: Product in accordance with the answer key
c.
Given
an question worksheet and handsout about reduction-oxidation reaction, student
can balance the redox equation by half reaction (ion-electron) correctly based
on LP 1: Product in accordance with the answer key
d.
Given
an question worksheet and handsout about reduction-oxidation reaction, student
can balance the redox equation by changes in the oxidation state correctly based
on LP 1: Product in accordance with the answer key
B. Affective
Character
skills :
Show an
appreciation of scientific attitudes such as . honest, responsibility,
curiousity, passionate about learning
and open mindness in learning about reduction-oxidation reaction.
V.
Learning Materials
By the turn of the 20th
century, it seemed that all oxidation reactions had one thing in common 
oxidation always
seemed to involve the loss of electrons. Chemists therefore developed a model
for these reactions that focused on the transfer of electrons. Magnesium metal,
for example, was thought to lose electrons to form Mg2+ ions
when it reacted with oxygen. By convention, the element or compound that gained
these electrons was said to undergo reduction. In this case, O2 molecules
were said to be reduced to form O2- ions.
oxidation always
seemed to involve the loss of electrons. Chemists therefore developed a model
for these reactions that focused on the transfer of electrons. Magnesium metal,
for example, was thought to lose electrons to form Mg2+ ions
when it reacted with oxygen. By convention, the element or compound that gained
these electrons was said to undergo reduction. In this case, O2 molecules
were said to be reduced to form O2- ions.
Chemists therefore
developed the concept of oxidation number to extend the idea
of oxidation and reduction to reactions in which electrons are not really
gained or lost. The most powerful model of oxidation-reduction reactions is
based on the following definitions.
Oxidation involves an increase in the
oxidation number of an atom.
Reduction occurs when the oxidation number
of an atom decreases.
Note that it doesn't
matter whether the compound actually contains ions. The oxidation number is the
charge an atom would have if the compound was ionic. The concept of oxidation
number is nothing more than a bookkeeping system used to keep track of
electrons in chemical reactions. This system is based on a series of rules,
summarized in the table below.
|
Rules for Assigning Oxidation
Numbers
|
|
Any set of rules, no
matter how good, will only get you so far. You then have to rely on a
combination of common sense and prior knowledge. Questions to keep in mind
while assigning oxidation numbers include the following: Are there any
recognizable ions hidden in the molecule? Does the
oxidation number make sense in terms of the known electron configuration of the
atom?
Chemical reactions are
often divided into two categories: oxidation-reduction or metathesis
reactions. Metathesis reactions include acid-base reactions
that involve the transfer of an H+ ion from a Brnsted acid to a
Brnsted base.
|
CH3CO2H(aq)
|
+
|
OH-(aq)
|
 |
CH3CO2-(aq)
|
+
|
H2O(l)
|
|
Brnsted
acid
|
Brnsted
base
|
Brnsted
base
|
Brnsted
acid
|
They can also involve
the sharing of a pair of electrons by an electron-pair donor (Lewis base) and
an electron-pair acceptor (Lewis acid).
|
Co3+(aq)
|
+
|
6 NO2-(aq)
|
|
Co(NO2)63-(aq)
|
|
|
Lewis acid
|
Lewis base
|
Oxidation-reduction
reactions or redox reactions can involve the
transfer of one or more electrons.
Cu(s) + 2 Ag+(aq) Cu2+(aq)
+ 2 Ag(s)
Cu2+(aq)
+ 2 Ag(s)
They can also occur by the transfer of
oxygen, hydrogen, or halogen atoms.
|
CO2(g)
+ H2(g)
CO(g)
+ H2O(g) |
|
SF4(g)
+ F2(g)
 SF6(g) |
Fortunately, there is an
almost foolproof method of distinguishing between metathesis and redox
reactions. Reactions in which none of the atoms undergoes a change in oxidation
number are metathesis reactions. There is no change in the oxidation
number of any atom in either of the metathesis reactions, for example.
The word metathesis literally
means "interchange" or "transposition," and it is used to
describe changes that occur in the order of letters or sounds in a word as a
language develops. Metathesis occurred, for example, when the Old English
word brid became bird. In chemistry, metathesis is
used to describe reactions that interchange atoms or groups of atoms between
molecules.
When at least one atom
undergoes a change in oxidation state, the reaction is an oxidation-reduction
reaction. Each of the reactions in the figure below is therefore an example of
an oxidation-reduction reaction.
VI. Model
and Learning Method:
Model Learning: Cooperative Learning Model
Learning Method: Problem Based Learning, Discussion
VII.
Learning Tools
·
Whiteboard
·
Stationery
·
Power point contains
about reduction-oxidation
·
LCD
VIII.
Learning
Resources
1.
Erlangga Michael Purba Chemistry
textbooks for class XII odd semester
2.
Redox Handsout
IX.
Learning Activities
|
No
|
Teaching-Learning Activities
|
Allocation of Time
|
|
A.
|
Introduction:
· Teacher
starts the lesson by greeting, prayer, checking the attendance and cleanliness of
classroom
·
Teacher ask students,
“what will happen if we put iron in outdoor for weeks?” and “ what will
happen if we put sliced apple in the open air?” to grow up the curiousity of
the students. Teacher guides student to answer correctly.
|
10 minutes
|
|
2.
|
Core activities
Exploration:
·
Teacher ask few questions related reduction and oxidation reaction
·
Students
are guided to
communicate and contribute their opinions and the others listen
respectly the idea posed and also tell them to be honest if they still not fully understood
·
Teacher guide the student to make groups
·
The representative of
each group are guided to take their
handsout
·
Teacher
ask each students to discuss the learning material about redox reaction in
handsout
Elaboration:
·
Teacher ask students
to discuss the questions from the handsout and answer it.
·
Teacher ask students
to answer the questions in evaluation sheet
·
Teacher facilities
each group :
a.
to collaborate and discuss the question and how to answer it
b. to Show an
appreciation of scientific attitudes such as . honest,
responsibility,curiousity, passionate about
learning and open mindness in discussion and learning about
reduction-oxidation reaction.
c. to Show an appreciation of scientific
attitudes such as asking , contribute an idea or opinion, communicate and be
a good listener in discussion and also learning process.
·
Each representative
of group are asked to change their answers.
·
Teacher asks each
group to presentation their answer.
Confirmation:
·
Teachers
give the response for the group’s answers,
and re-explain or correct the answers
·
Provide
positive feedback and reinforcement in the form of oral, written, gesture, or
gift for all
group’s answer
·
Provide
motivation to some groups who get bad scores.
|
10 minutes
10 minutes
35 minutes
15 minutes
|
|
3.
|
The final activity (Closing):
·
Students
guided by the teacher to make the conclusion.
1. Oxidation involves an increase in the oxidation
number of an atom.Reduction occurs when the oxidation number of an atom
decreases. oxidation always seemed to involve the
loss of electrons.The element or compound
that gained these electrons was said to undergo reduction.
2.
An oxidizing agent is a chemical substance, which in
a chemical reaction has a high tendency to get reduced while causing the
other reactant to get oxidized. An reducing agent is a chemical substance,
which in a chemical reaction has a high tendency to get oxidized while
causing the other reactant to get reduced.
3.
Balance the redox equation by half reaction :
Let us take the reaction between potassium permanganate,KMnO4,
and iron(II) sulphate,FeSO4, in which the permanganate
ion, MnO4-, is reduced to manganese(II)
cations, Mn2+, while the iron(II) cations, Fe2+,
are oxidised to iron(III) cations, Fe3+. The reaction
takes place in acid solution.
4.
Balance the redox equation by number state :
 
·
Teacher
gives students structure task to answer the questions in
Erlangga Michael Purba Chemistry book 3A page 46 exercises no 1-10 as homework and ask the student if they can
collect it before next meeting.
·
Teacher gives
students unstructured task reading course about volta cell from Erlangga Chemistry
Book 3A page 52- 86 for next meeting
|
10 minutes
|
X.
Assessment
1. Cognitive evaluation-key
(LP-1)
2. Table
Specifications Sheet Assessment
- Syllabus
Table Specifications Sheet
Assessment
|
LP and the Grain Problem
|
Key and the Grain Problem LP
|
|
|
Product:
a.
Given
an question worksheet and also handsout about reduction-oxidation reaction,
students can differentiate the concept of oxidation and reduction in terms of
the incorporation of oxygen release, discharge and electron acceptance, as
well as the increase and decrease in oxidation state correctly based on LP 1:
Product in accordance with the answer key.
b. Given an question worksheet and
handsout about reduction-oxidation reaction, student can determine the
oxidizing and reducing agents in redox reactions correctly based on LP 1:
Product in accordance with the answer key
c. Given an question worksheet and
handsout about reduction-oxidation reaction, student can balance the redox
equation by half reaction (ion-electron) correctly based on LP 1: Product in
accordance with the answer key
d. Given an question worksheet and
handsout about reduction-oxidation reaction, student can balance the redox
equation by changes in the oxidation state correctly based on LP 1: Product
in accordance with the answer key
|
LP 1: Product
Item 1
LP 1: Product
Item 2
LP 1: Product
Item 3
LP 1: Product
Item 4
|
LP 1: Product
Item 1
LP 1: Product
Item 2
LP 1: Product
Item 3
LP 1: Product
Item 4
|
COGNITIVE
EVALUATION LP 1: PRODUCT
EVALUATION QUESTIONS :
1. Explain the concept of oxidation and
reduction and in terms of the incorporation of oxygen release, discharge and
electron acceptance, as well as the increase and decrease in oxidation state!
2.
Some oxidation-reduction reactions
around us are :
Respiration : C6H12O6 (aq)
+ 6O2(g) à 6CO2 (g) + 6H2O(l)
+energy
Corrosion : 4Fe(s) + 3O2(g)
+ nH2O(l) à 2Fe2O3.nH2O(s)
Photosynthesis : 6CO2(g) +
6H2O(l) + sunlight à C6H12O6(aq)
+ 6CO2(g)
Identify the reactant oxidized, the
reactant reduced, reductor and oxidator from the following reactions of
respiration, corrosion and photosynthesis.
3.
Balance each redox reaction using ion-electron method.
a.
Cr2O72- (aq) + Fe2+(aq) à Fe3+ (aq)+Cr3+(aq) (Acid)
b.
Cr2O72- + Hg à Hg2++Cr3+ (Base)
c.
C2O42-(aq)
+ MnO4-(aq) à CO2 (g) + MnO2(s)
(Base)
4.
Balance each
redox reaction using oxidation numbers
state.
a.
KClO3
+ C12H22O11 à KCl + H2O + CO2
b.
H2C2O4
+ K2MnO4 à
CO2 + K2O + Mn2O3 + H2O
c.
Mn(NO3)
2 + NaBiO3 + HNO3 à HMnO4 + Bi(NO3) 3
+ NaNO3 + H2O
KEY COGNITIVE EVALUATION LP 1: PRODUCT
1.
Reduction reaction used
to describe reaction in which substances loss oxygen
Oxidation involves an increase in the
oxidation number of an atom.
Reduction occurs when the oxidation number
of an atom decreases.
Compound
or elements that loss of electrons is Oxidation reaction
Compound
or elements that gain of electrons
is Reduction reaction
2. Respiration : C6H12O6
(aq) + 6 O2 (g) à
6CO2 (g) + 6H2O(l) +energy
Oxidizing agent : O2
Reducing agent : C
Corrosion : 4Fe(s) + 3O2(g)
+ nH2O(l) à 2Fe2O3.nH2O(s)
Oxidizing agent : O2
Reducing agent : Fe
Photosynthesis : 6CO2(g) +
6H2O(l) + sunlight à C6H12O6(aq)
+ 6CO2(g)
Oxidizing agent : C
Reducing agent : O2
3.
·
Cr2O72- +
Fe2+ à Fe3+
+ Cr3+
a. Cr2O72- à
Cr3+
Fe2+ à Fe3+
b. Cr2O72- à2
Cr3+
Fe2+ à Fe3+
c. Cr2O72- à2
Cr3+ + 7H2O
Fe2+ à Fe3+
d. Cr2O72-
+14H+ à2
Cr3+ + 7H2O
Fe2+ à Fe3+
e. Cr2O72-
+14H+ +6e à
2Cr3+ + 7H2O
Fe2+ à Fe3+ + e
f. Cr2O72-
+14H+ +6e à
2Cr3+ + 7H2O
(Fe2+ à Fe3+ + e) x6
g. Cr2O72-
+14H+ +6e à
2Cr3+ + 7H2O
(Fe2+
à Fe3+ + e) x6
Cr2O72-
+14H+ +6 Fe2+
à
2Cr3+ + 6Fe3+ + 7H2O
·
Cr2O72- +
Hg à Hg2+
+ Cr3+
a. Cr2O72- à
Cr3+
Hg à
Hg2+
b. Cr2O72- à2
Cr3+
Hg à
Hg2+
c. Cr2O72- + 7H2O à2
Cr3+
Hg à
Hg2+
d. Cr2O72-
+ 7H2O
à
2Cr3+ +14OH- +6e
Hg à
Hg2+
e. Cr2O72-
+ 7H2O
à
2Cr3+ +14OH- +6e
Hg à
Hg2+
f. Cr2O72-
+14OH- à
2Cr3+ + 7H2O +6e
(Hg à Hg2+ +2e)x3
g. Cr2O72-
+ 7H2O
à
2Cr3+ +14OH- +6e
(Hg
à
Hg2+ +2e)x3
7H2O +
Cr2O72-
+ 3Hg ------->
3Hg2+ + 14OH- + 2Cr3+
·
C2O42-(aq)
+ MnO4-(aq) à CO2 (g) + MnO2(s)
(Base)
a. C2O42-
à
CO2
MnO4- à
MnO2
b. C2O42-
à
2CO2
MnO4- à
MnO2
c. C2O42-
à
2CO2
MnO4- à
MnO2 +2H2O
d. C2O42-
à
2CO2
MnO4-+4H+ à
MnO2 +2H2O
e. C2O42-
à
2CO2+2e
MnO4-+4H+ +3eà
MnO2 +2H2O
f. (C2O42-
à
2CO2+2e)x3
(MnO4-+4H+ +3eà
MnO2 +2H2O)x2
g. (C2O42-
à
2CO2+2e)x3
(MnO4-+4H+
+3eà
MnO2 +2H2O)x2
3
C2O42- +
2MnO4-+8H+à 6CO2+
2MnO2 +4H2O
4. +1 +5 -2(3) 0 +1(22) -2 (11) +1-1
+(2) -2 +4 -2 (2)
8KClO3 + C12H22O11 à 8KCl + 11H2O + 12CO2
8(6e-) -4(8e-)
8KClO3 + C12H22O11 à 8KCl + 11H2O + 12CO2
8(6e-) -4(8e-)
+1 +3
-2 +1 +6
-2 +4
-2 +1 -2 +3 -2
+1-2
3H2C2O4 + 2K2MnO4 à 6CO2 + 2K2O + Mn2O3 + 3H2O
3(-2e-) 2(+3e-)
3H2C2O4 + 2K2MnO4 à 6CO2 + 2K2O + Mn2O3 + 3H2O
3(-2e-) 2(+3e-)
+2 +5 -2
+1+5 -2
+1+5-2 +1+7
-2 +3+5-2
+1+5-2 +1-2
2Mn(NO3) 2+5NaBiO3+16HNO3 à 2HMnO4 +5Bi(NO3) 3+5NaNO3+7H2O
2(+5e-) 5(-2e-)
2Mn(NO3) 2+5NaBiO3+16HNO3 à 2HMnO4 +5Bi(NO3) 3+5NaNO3+7H2O
2(+5e-) 5(-2e-)
Guidelines Scoring
|
No
|
Answer
|
Score
|
|
1.
2.
3.
4.
|
Reduction reaction used to describe reaction
in which substances loss oxygen.
Oxidation involves an increase in the
oxidation number of an atom.Reduction occurs when the oxidation number
of an atom decreases.
Compound
or elements that loss of electrons is Oxidation reaction
Compound
or elements that gain of electrons
is Reduction reaction
Respiration : C6H12O6
(aq) + 6 O2 (g) à
6CO2 (g) + 6H2O(l) +energy
Oxidizing agent :
O2
Reducing agent :
C
Corrosion : 4Fe(s) + 3O2(g) + nH2O(l)
à
2Fe2O3.nH2O(s)
Oxidizing agent :
O2
Reducing agent :
Fe
Photosynthesis
: 6CO2(g) + 6H2O(l)
+ sunlight à
C6H12O6(aq) + 6CO2(g)
Oxidizing agent : C
Reducing agent :
O2
·
Cr2O72- +
Fe2+ à Fe3+
+ Cr3+
·
Cr2O72- à Cr3+
Fe2+ à Fe3+
·
Cr2O72- à2 Cr3+
Fe2+ à Fe3+
·
Cr2O72- à2 Cr3+ + 7H2O
Fe2+ à Fe3+
·
Cr2O72- +14H+ à2 Cr3+ + 7H2O
Fe2+ à Fe3+
·
Cr2O72- +14H+
+6e à 2Cr3+ + 7H2O
Fe2+
à Fe3+ + e
·
Cr2O72- +14H+
+6e à 2Cr3+ + 7H2O
(Fe2+
à Fe3+ + e) x6
·
Cr2O72- +14H+
+6e à 2Cr3+ + 7H2O
(Fe2+
à Fe3+ + e) x6
Cr2O72-
+14H+ +6 Fe2+
à 2Cr3+ +
6Fe3+ + 7H2O
·
Cr2O72- +
Hg à Hg2+
+ Cr3+
·
Cr2O72- à Cr3+
Hg
à Hg2+
·
Cr2O72- à2 Cr3+
Hg
à Hg2+
·
Cr2O72- + 7H2O à2 Cr3+
Hg
à Hg2+
·
Cr2O72- + 7H2O à 2Cr3+ +14OH- +6e
Hg
à Hg2+
·
Cr2O72- + 7H2O à 2Cr3+ +14OH- +6e
Hg
à Hg2+
·
Cr2O72- +14OH- à 2Cr3+ + 7H2O +6e
(Hg
à Hg2+ +2e)x3
·
Cr2O72- + 7H2O à 2Cr3+ +14OH- +6e
(Hg à Hg2+ +2e)x3
7H2O +
Cr2O72-
+ 3Hg ------->
3Hg2+ + 14OH- +
2Cr3+
·
C2O42-(aq)
+ MnO4-(aq) à
CO2 (g) + MnO2(s)
(Base)
o C2O42-
à
CO2
MnO4- à
MnO2
o C2O42-
à
2CO2
MnO4- à
MnO2
o C2O42-
à
2CO2
MnO4- à
MnO2 +2H2O
o C2O42-
à
2CO2
MnO4-+4H+ à
MnO2 +2H2O
o C2O42-
à
2CO2+2e
MnO4-+4H+ +3eà
MnO2 +2H2O
o (C2O42-
à
2CO2+2e)x3
(MnO4-+4H+ +3eà
MnO2 +2H2O)x2
o (C2O42-
à
2CO2+2e)x3
(MnO4-+4H+
+3eà
MnO2 +2H2O)x2
C2O42-
+ 2MnO4-+8H+à
6CO2+ 2MnO2 +4H2O
+1 +5
-2(3) 0 +1(22) -2 (11)
+1-1
+(2) -2 +4 -2 (2)
8KClO3 + C12H22O11 à 8KCl + 11H2O + 12CO2 8(6e-) -4(8e-)
+1 +3
-2 +1 +6
-2 +4
-2 +1 -2 +3 -2
+1-2
3H2C2O4 + 2K2MnO4 à 6CO2 + 2K2O + Mn2O3 + 3H2O 3(-2e-) 2(+3e-)
+2 +5 -2
+1+5 -2
+1+5-2 +1+7
-2 +3+5-2
+1+5-2
+1-2
2Mn(NO3) 2+5NaBiO3+16HNO3 à 2HMnO4 +5Bi(NO3) 3+5NaNO3+7H2O 2(+5e-) 5(-2e-) |
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
4
4
4
|
|
|
Maximum Score
|
50
|
STUDENT’S HANDSOUT
Name :
_________________________
Class :
________________________
Group : ________________________
Competence Standards : 2. To apply the concept of oxidation-reduction
reactions and electrochemistry in technology and everyday life.
Basic Competence : 2.1 To Apply the concept of
oxidation-reduction reactions in electrochemical systems involving electrical
energy and its usefulness in preventing corrosion and also for the industry.
1.
Learning Material
1.1 The Process of Discovery: Oxidation and Reduction
Lavoisier proposed the
name oxygene (literally, "acid-former") for the
substance absorbed from air when a compound burns. He chose this name because
the products of the combustion of nonmetals such as phosphorus are acids when
they dissolve in water.
|
P4(s)
+ 5 O2(g)
P4O10(s) |
|
P4O10(s)
+ 6 H2O(l)
4 H3PO4(aq) |
Lavoisier's oxygen
theory of combustion was eventually accepted and chemists began to describe any
reaction between an element or compound and oxygen as oxidation.
The reaction between magnesium metal and oxygen, for example, involves the
oxidation of magnesium.
2 Mg(s) + O2(g) 2 MgO(s)
2 MgO(s)
By the turn of the 20th
century, it seemed that all oxidation reactions had one thing in common oxidation always seemed to involve the loss of electrons. Chemists
therefore developed a model for these reactions that focused on the transfer of
electrons. Magnesium metal, for example, was thought to lose electrons to form
Mg2+ ions when it reacted with oxygen. By convention, the element or compound that gained these
electrons was said to undergo reduction.
In this case, O2 molecules were said to be reduced to form O2- ions.
oxidation always seemed to involve the loss of electrons. Chemists
therefore developed a model for these reactions that focused on the transfer of
electrons. Magnesium metal, for example, was thought to lose electrons to form
Mg2+ ions when it reacted with oxygen. By convention, the element or compound that gained these
electrons was said to undergo reduction.
In this case, O2 molecules were said to be reduced to form O2- ions.
Example:
Chemists therefore
developed the concept of oxidation number to extend the idea
of oxidation and reduction to reactions in which electrons are not really
gained or lost. The most powerful model of oxidation-reduction reactions is
based on the following definitions.
Oxidation involves an increase in the
oxidation number of an atom.
Reduction occurs when the oxidation number
of an atom decreases.
Example :
According to this model,
CO2 is reduced when it reacts with hydrogen because the
oxidation number of the carbon decreases from +4 to +2. Hydrogen is oxidized in
this reaction because its oxidation number increases from 0 to +1. An oxidizing
agent is a chemical substance, which in a chemical reaction has a high tendency
to get reduced while causing the other reactant to get oxidized. An reducing
agent is a chemical substance, which in a chemical reaction has a high tendency
to get oxidized while causing the other reactant to get reduced.
Question :
Determine
if each of the following changes is oxidation, reduction or neither.
SO32- --------> SO42- : ______________
CaO --------> Ca :
______________
CrO42- --------> Cr2O72- : ______________
1.2 Assigning Oxidation Numbers
The key to identifying
oxidation-reduction reactions is recognizing when a chemical reaction leads to
a change in the oxidation number of one or more atoms. It is therefore a good
idea to take another look at the rules for assigning oxidation numbers. By
definition, the oxidation number of an atom is equal to the charge that would
be present on the atom if the compound was composed of ions. If we assume that
CH4 contains C4- and H+ ions,
for example, the oxidation numbers of the carbon and hydrogen atoms would be -4
and +1.
Note that it doesn't
matter whether the compound actually contains ions. The oxidation number is the
charge an atom would have if the compound was ionic. The concept of oxidation
number is nothing more than a bookkeeping system used to keep track of
electrons in chemical reactions. This system is based on a series of rules,
summarized in the table below.
|
Rules for Assigning Oxidation
Numbers
|
|
Any set of rules, no
matter how good, will only get you so far. You then have to rely on a
combination of common sense and prior knowledge.
Example :
+1 +6
-2 +1
-2 0 4 -2
+1-2+1 +3 -2
2K2Cr2O7 + 2H2O + 3S ® 3SO2 + 4KOH + 2Cr2O3
2K2Cr2O7 + 2H2O + 3S ® 3SO2 + 4KOH + 2Cr2O3
Question :
State the Oxidation Number of each of the elements
that is underlined.
a) NH3 :_________
b) H2SO4 :_________
c) Al(OH)3 :_________
1.3 Balancing Redox Reaction
1.3.1 The ion-electron method
The following will demonstrate how the ION-ELECTRON method can be used
to balance redox reactions. Let us take the reaction between potassium
permanganate,KMnO4, and iron(II) sulphate,FeSO4,
in which the permanganate ion, MnO4-, is
reduced to manganese(II) cations, Mn2+, while the
iron(II) cations, Fe2+, are oxidised to iron(III)
cations, Fe3+. The reaction takes place in acid
solution.
Step 1: We
first summarise the above chemical facts in a rough unbalanced reaction:
Note that we ignore the presence of both the potassium and sulphate ions, which do not undergo any change. They are called theSPECTATOR IONS.
Note that we ignore the presence of both the potassium and sulphate ions, which do not undergo any change. They are called theSPECTATOR IONS.
Step 2: We
now split this reaction into two half-reactions which separately describe the
reduction and oxidation.
In the next two steps, we will balance these half-reactions atomically and electronically.
In the next two steps, we will balance these half-reactions atomically and electronically.
Step 3: Balancing
the reduction half reaction:
The oxygen atoms of the permanganate ion and the hydrogen ions must combine to form water. Since there are 4 oxygen atoms, 4 water molecules must appear on the right- hand side of the equation. To produce 4 water molecules, 8 hydrogen atoms are required. These must be derived from 8 H+ ions on the left-hand side. This gives us the reaction on the right:
The oxygen atoms of the permanganate ion and the hydrogen ions must combine to form water. Since there are 4 oxygen atoms, 4 water molecules must appear on the right- hand side of the equation. To produce 4 water molecules, 8 hydrogen atoms are required. These must be derived from 8 H+ ions on the left-hand side. This gives us the reaction on the right:
Step 4: The
above does not balance electronically: on the left-hand side we have a net
charge of 6+, while on the right hand-side we have 2+. This means that we must
add 4 negative charges (4 electrons) on the left-hand side. This brings the
total number of electrons on the left to 5, which gives us a the reaction on
the right which is electronically balanced:
Step 5: Balance
the oxidation half-reaction: in this case, it is already balanced atomically
and electronically:
Step 6: The final reaction is the sum of the above two
half-reactions, bearing in mind that the final reaction must not only balance
atomically, but electronically as well. Just adding the two half-reactions
above will result in a reaction which will be balanced atomically, but there
will be 5 e- on the left and only 1 e- on the
right
Step 7: All
that needs to be done is to multiply the oxidation half-reaction by 5, and add
it to the balanced reduction half-reaction. After cleaning it up by removing
items which appear in equal amounts on both sides, we finally get:
Questions:
Balance
each equation using half-cell reactions.
1. S2O32- → SO42- (Acid)
2. NO3- → NO
(Base)
1.3.2 The
oxidation numbers method
Consider the reaction between copper(II) oxide, CuO,
and ammonia, NH3, which produces metallic copper,
nitrogen gas, and water.
Step
1: We first summarise the above chemical facts in a
rough unbalanced reaction:
Step 2: Assign
oxidation numbers to the atoms that are undergoing a change in oxidation state: 
Step 3: We
see that the Cu atoms undergo a decrease in
oxidation number from +2 to 0. At the same time, the N atoms increase their
oxidation number from -3 to 0. The decrease in oxidation number must match the
increase, so that the net change in oxidation numbers is zero. To achieve this,
we must adjust the coefficients in our rough equation. We note however that the
equation does not yet balance as the number of hydrogen and oxygen atoms differ
on the left and right sides of the equation:
Step 4: All
we need to do is to adjust the coefficient of the water molecules. This gives a
balanced equation!
Question:
Balance
each equation using oxidation numbers state methods.
1. Fe
+ V2O3 à Fe2O3
+ VO
2.
KMnO4 + NaCl + H2SO4 à Cl2 + K2SO4
+ MnSO4 + H2O + Na2SO4
EVALUATION QUESTIONS :
1. Explain the concept of oxidation and
reduction and in terms of the incorporation of oxygen release, discharge and
electron acceptance, as well as the increase and decrease in oxidation state!
2.
Some oxidation-reduction reactions
around us are :
Respiration : C6H12O6 (aq)
+ 6O2(g) à 6CO2 (g) + 6H2O(l)
+energy
Corrosion : 4Fe(s) + 3O2(g)
+ nH2O(l) à 2Fe2O3.nH2O(s)
Photosynthesis : 6CO2(g) +
6H2O(l) + sunlight à C6H12O6(aq)
+ 6CO2(g)
Identify the reactant oxidized, the
reactant reduced, reductor and oxidator from the following reactions of
respiration, corrosion and photosynthesis.
3.
Balance each redox reaction using ion-electron method.
a.
Cr2O72- (aq) + Fe2+(aq) à Fe3+ (aq)+Cr3+(aq) (Acid)
b.
Cr2O72- + Hg à Hg2++Cr3+ (Base)
c.
C2O42-(aq)
+ MnO4-(aq) à CO2 (g) + MnO2(s)
(Base)
4.
Balance each
redox reaction using oxidation numbers
state.
a.
KClO3
+ C12H22O11 à KCl + H2O + CO2
b.
H2C2O4
+ K2MnO4 à
CO2 + K2O + Mn2O3 + H2O
c.
Mn(NO3)
2 + NaBiO3 + HNO3 à HMnO4 + Bi(NO3) 3
+ NaNO3 + H2O
Affective Evaluation
Character Skill
|
No
|
Details of Task Performance
|
Arief
|
Irine W
|
Fani K
|
|
1
|
Honesty
|
|
|
|
|
2
|
Curiousity
|
|
|
|
|
3.
|
Responsible
|
|
|
|
|
4.
|
Passionate
about Learning
|
|
|
|
|
5.
|
Open
Mindness
|
|
|
|
|
|
Total score
|
|
|
|
BIBLIOGRAPHY
Anonymous. 2012. Redox Reaction. http://en.wikipedia.org/wiki/Redox_Reaction . diakses tanggal 05 February 2013
Arends L, Richard. 2001. Learning
to Teach : Cooperative Learning. New
York: McGraw-Hill
BSNP (Badan Standar Nasional Pendidikan). 2007. Model Silabus dan Rencana
Pelaksanaan Pembelajaran. Mata Pelajaran: Ilmu Pengetahuan Alam. SMP/MTS.
Digandakan oleh Kegiatan Penyelenggaraan
Sosialisasi/Diseminasi/Seminar/Workshop/Publikasi Direktorat Jenderal Manajemen
Pendidikan Dasar dan Menengah.
Justiana, Sandy.
2011. Chemistry for Senior High School.
Jakarta : Yudhistira
Purba,
Michael. 2000. Kimia Kelas XII semester 1. Jakarta: Erlangga
Wilbraham, Staley, Matta, and Waterman,
(2007). Chemistry (Teacher’s Edition).
Boston: Pearson Prentice Hall.
