OXIDIZING AND REDUCING AGENTS

SUBJECT

CHEMISTRY

TERM :

FIRST TERM

WEEK :

WEEK 4

CLASS :

SS 2

TOPIC :

OXIDIZING AND REDUCING AGENTS

PREVIOUS KNOWLEDGE : 

The pupils have previous knowledge of

TYPES OF REACTION

that was taught as a topic in the previous lesson

 

BEHAVIOURAL OBJECTIVES :

At the end of the lesson, the learners will be able to

  • Define oxidation and reduction in terms of electron transfer.
  • Determine the oxidation state of P in each of the following structure  POCl3            b.    PH3
  • Balance the following redox equation: Cr2O72- +  SO2               Cr3+  +  SO42- in acidic medium

 

INSTRUCTIONAL  MATERIALS :

  • Wall charts
  • Pictures
  • Related Online Video
  • Flash Cards

 

 

METHODS OF TEACHING :

  • Class Discussion
  • Group Discussion
  • Asking Questions
  • Explanation
  • Role Modelling
  • Role Delegation

 

REFERENCE MATERIALS :

  • Scheme of Work
  • Online Information
  • Textbooks
  • New School Chemistry for Senior Secondary Schools by O.Y Ababio
  • New System Chemistry for Senior Secondary Schools by T. Y. Toon et al
  • S.S.C.E  Past Questions and Answers on Chemistry 
  • U.T.M.E Past Questions and Answers on Chemistry
  • Workbooks

 

CONTENT :

WEEK FOUR

TOPIC:OXIDIZING AND REDUCING AGENTS

CONTENT

  • Identification of Oxidizing and Reducing agents
  • Balancing of Redox equation in Acidic and Alkaline medium

OXIDIZING AND REDUCING AGENTS

An oxidizing agent is defined as a substance which loses oxygen or electronegative element to another substance. Or an oxidizing agent is a substance which gains hydrogen from a substance. Or an oxidizing agent is a substance which gains electron from a substance. Consider the reaction below 

 C(s)    +     ZnO(s)    CO2(g)    +       Zn(s)

ZnO is the oxidizing agent because it loses oxygen to C.

A reducing agent is defined as substance, which removes and accepts oxygen from other substances. Or a reducing agent is defined as a substance, which removes and accepts electronegative element from another substance. Or a reducing agent is defined as a substance which loses and donates electron to another substance. From the reaction above, C is the reducing agent because it removes and accepts oxygen from ZnO. 

In an oxidation and reduction reaction, the oxidizing agent is the reduced species while the reducing agent is the oxidized species. 

 

NOTE: An oxidizing agent accepts electron, is reduced and its oxidation number decreases while a reducing agent donate electron, is oxidized and its oxidation number increases.

 

IDENTIFICATION OF OXIDIZING AND REDUCING AGENTS

TESTS FOR OXIDIZING AGENTS: The presence of an oxidizing agent can be detected using any of the following reagents. 

  1. Acidified potassium iodide, KI with starch 
  2. Sulphur (IV) oxides, SO2 with acidified Barium trioxonitrate (V) solution 
  3. Iron (II) Chloride solution (FeCl2)
  4. Hydrogen sulphide gas (H2S)

 

SUMMARY OF TEST 

S/NO TEST OBSERVATION INFERENCE
1O.A + FeCl2(aq)Green colour of Fe2+ solution turns to reddish- brown of Fe3+O.A is present 
2O.A + H2S(g)Formation of yellow deposits of sulphurO.A is present 
3a.

  b.

O.A + acidified KI 

Red- brown solution + starch 

Reddish – brown coloration produced. Iodine is liberated. 

Reddish- brown turns dark blue. The iodine reacts with the starch  

O.A is present 
4O.A + SO2(g) +                  dilute HNO3(aq) + Ba(NO3)2(aq)White precipitate of insoluble BaSO4 is formed O.A is present 

 

TEST FOR REDUCING AGENTS: 

Reducing agent is detected in the laboratory using any of the following reagents.

  1.     Acidified Potassium tetraoxomanganate (VII) 
  2.     Acidified Potassium heptaoxodichromate (VI) 

 

S/NTEST OBSERVATION INFERENCE
1R.A + acidified KMO4Purple solution of KMnO4 turns colorless on addition of R.A R.A is present 
2R.A + acidified K2Cr2O7Orange solution of K2Cr2O7 turns green solution addition of R.A R.A is present 

Common oxidizing agents are: concentrated HNO3, H2SO4, KMnO4, K2Cr2O7, O2, Cl2 etc. 

Common reducing agents are: concentrated HCl, pure metals, carbon, H2, SO2, H2S, etc. 

 

EVALUATION

  1.   Define Oxidizing agent and Reducing agent in terms of electron transfer
  2.   Describe one test each for identifying an Oxidizing agent and a Reducing agent

       

BALANCING OF REDOX EQUATIONS

Redox equations are balanced by first considering the two half equations involved in such reaction. Steps involved are 

  1. Identify the oxidizing and reducing agents and deduce expected products.
  2. Write the half equations for oxidation and reduction. Balance the atoms and charges for each equation.
  3. Make sure that the electrons loss in the oxidation half equation is balanced by the electrons gain in the reduction half equation.
  4. Combine the halves equations to eliminate the electrons and get the overall redox equation.

 

EXAMPLE 1: Write a balanced ionic equation for the redox reaction between potassium tetraoxomanganate(VII) and Iron (II)tetraoxosulphate(VI)in acidic medium.

 

SOLUTION:   

O.A            MnO4– 

R.A             Fe2+

OXIDATION HALF EQUATION 

         Fe2+              Fe3+    +    e

 

REDUCTION HALF EQUATION 

        MnO4     +      H+        Mn2+    + H2O

BALANCED HALF EQUATIONS

        5Fe2+          Fe3+      +    5e

        MnO4  +     8H++     5e    Mn2+    +     4H2O

COMBINED EQUATION 

  5Fe2+    +       MnO4     +8H+         +   5e                     Fe3++  5e +    Mn2+ +4H2O

The electrons on both sides of the equation cancel out and the overall equation is

5Fe2+     +       MnO4     + 8H+            Fe3+    +    Mn2+  +  4H2O

 

EXAMPLE 2: Write a balanced equation for the following reaction in basic medium

          Cr3+ +    BrO        CrO42-  +   Br

 

SOLUTION:

O.A            BrO– 

R.A             Cr3+

OXIDATION HALF EQUATION 

         Cr3+              CrO42-

Balancing of atoms:  Cr3+  + 8OH        CrO42-  +  4H2O

Balancing of charges: Cr3+  + 8OH        CrO42-  +  4H2O + 3e

   

REDUCTION HALF EQUATION 

BrO        Br

Balancing the atoms: BrO  +  H2O        Br  +  2OH

Balancing the charges: BrO  +  H2O  +  2e        Br  +  2OH

But electron lost in the oxidation half must equal electron gained in the reduction half equation.

Multiplying the oxidation half equation by 2 and the reduction half equation by 3 gives

 

BALANCED HALF EQUATIONS

2Cr3+  +16OH–          2CrO42-  +  8H2O + 6e

3BrO  +  3H2O  +  6e        3Br  +  6OH

 

COMBINED EQUATION 

2Cr3+  +  16OH–  3BrO  +  3H2O  +  6e        2CrO42-  +  8H2O + 6e    3Br  +  6OH

The electrons on both sides of the equation cancel out and the overall equation is

2Cr3+  +  16OH–    3BrO  +  3H2O        2CrO42-  +  8H2O + 3Br  +  6OH 

 

GENERAL EVALUATION/REVISION

  1. Determine the oxidation number of 

(a) Fe in  Fe2O3      (b) Cu in [Cu(NH3)4]2+

  1.   Name the following compounds
  1. H2CO3          (b) KMnO4

3.The compound Na2S is called ————–

4.The IUPAC name of NaHSO4 is————-

5.Balance the following redox equation: I  +  MnO4IO3  +  MnO2in basic medium

 

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by O. Y. Ababio, pages 193-196

 

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1    What is the value of x in the following equation?

    Cr2O72-+  14H+   +   xe-      2Cr3+    +    7H2 O

  1.   1 B.    6    C.    8    D.    12

2    In which of the following is the oxidation number of sulphur equal to -2?

  1. S8 B. H2S C. SO2D. SO3.2-

3    Which species undergoes reduction in the reaction represented by the equation below?

        H2S(g)    + 2FeCl3(aq)                   S(s) +2HCl(aq)  +  2FeCl2(aq)

  1. Fe3+ B. H2S C. Cl D. S

4    Cr2O72-  +    6Fe2++   14H+    2Cr 3+    +    6Fe3+   +    7H2O                                                                                 In the equation above, the oxidation number of chromium changes from 

  1. +7 to +3 B. +6 to +3 C. -6 to +3 d. -2 to +6

5    When SO2 is passed into a solution of acidified potassium heptaoxodichromate(VI) (K2Cr2O7), the solution turns A. green B. orange C. purple d. yellow

SECTION B

  1. Determine the oxidation state of P in each of the following structure  POCl3            b.    PH3
  2. Balance the following redox equation: Cr2O72- +  SO2               Cr3+  +  SO42- in acidic medium