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Rabu, 19 Juni 2013

lesson plan redox

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.
reaction
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
  • The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an oxidation number of 0.
  • The oxidation number of monatomic ions is equal to the charge on the ion. The oxidation number of sodium in the Na+ ion is +1, for example, and the oxidation number of chlorine in the Cl- ion is -1.
  • The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Hydrogen is therefore in the +1 oxidation state in CH4, NH3, H2O, and HCl.
  • The oxidation number of hydrogen is -1 when it is combined with a metal. Hydrogen is therefore in the -1 oxidation state in LiH, NaH, CaH2, and LiAlH4.
  • The metals in Group IA form compounds (such as Li3N and Na2S) in which the metal atom is in the +1 oxidation state.
  • The elements in Group IIA form compounds (such as Mg3N2 and CaCO3) in which the metal atom is in the +2 oxidation state.
  • Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O2, O3, H2O2, and the O22- ion.
  • The nonmetals in Group VIIA often form compounds (such as AlF3, HCl, and ZnBr2) in which the nonmetal is in the -1 oxidation state.
  • The sum of the oxidation numbers of the atoms in a molecule is equal to the charge on the molecule.
  • The most electronegative element in a compound has a negative oxidation number.
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)
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.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/graphics/19_2.gif
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.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/graphics/19_3_1.gif
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/graphics/19_3_2.gif

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 groups 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.http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f7.gif
4.      Balance the redox equation by number state :
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f8.gif
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f11.gif
·         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
  1. Syllabus






Table Specifications Sheet Assessment
Indicator
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-)

+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-)








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-)


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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)
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.

Example:
reaction
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 :
reaction
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
  • The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an oxidation number of 0.
  • The oxidation number of monatomic ions is equal to the charge on the ion. The oxidation number of sodium in the Na+ ion is +1, for example, and the oxidation number of chlorine in the Cl- ion is -1.
  • The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Hydrogen is therefore in the +1 oxidation state in CH4, NH3, H2O, and HCl.
  • The oxidation number of hydrogen is -1 when it is combined with a metal. Hydrogen is therefore in the -1 oxidation state in LiH, NaH, CaH2, and LiAlH4.
  • The metals in Group IA form compounds (such as Li3N and Na2S) in which the metal atom is in the +1 oxidation state.
  • The elements in Group IIA form compounds (such as Mg3N2 and CaCO3) in which the metal atom is in the +2 oxidation state.
  • Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O2, O3, H2O2, and the O22- ion.
  • The nonmetals in Group VIIA often form compounds (such as AlF3, HCl, and ZnBr2) in which the nonmetal is in the -1 oxidation state.
  • The sum of the oxidation numbers of the atoms in a molecule is equal to the charge on the molecule.
  • The most electronegative element in a compound has a negative oxidation number.
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
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.
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f1.gif
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.
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f2.gif
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: http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f3.gif
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:
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f4.gif
Step 5: Balance the oxidation half-reaction: in this case, it is already balanced atomically and electronically:
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f5.gif
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 eon the left and only 1 e- on the right
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f6.gif
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:
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f7.gif
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: http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f8.gif
Step 2: Assign oxidation numbers to the atoms that are undergoing a change in oxidation state: http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f9.gif
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:
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f10.gif
Step 4: All we need to do is to adjust the coefficient of the water molecules. This gives a balanced equation!
http://www.physchem.co.za/OB11-mat/graphics/oxidation2_f11.gif
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.



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