Oxidizing and Reducing Agents

 

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In today’s Chemistry class, We will be learning about Oxidizing and Reducing Agents. We hope you enjoy the class!

 

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 an 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 a substance, which removes and accepts oxygen from other substances. Or a reducing agent is defined as a substance, which removes and accepts the electronegative element from another substance. Or a reducing agent is defined as a substance which loses and donates an 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 an electron, is reduced and its oxidation number decreases while a reducing agent donate electron, is oxidized and its oxidation number increases.

 

redox reactions chemistry classnotesng

 

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
1 O.A + FeCl2(aq) Green colour of Fe2+ solution turns to reddish-brown of Fe3+ O.A is present

 

2 O.A + H2S(g) Formation of yellow deposits of sulphur O.A is present
3a.

 

b.

O.A + acidified KI

 

Red- brown solution + starch

Reddish-brown colouration produced. Iodine is liberated.

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

O.A is present
4 O.A + SO2(g) +                  dilute HNO3(aq) + Ba(NO3)2(aq) A 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/N TEST OBSERVATION INFERENCE
1 R.A + acidified KMO4 The purple solution of KMnO4 turns colourless on the addition of R.A R.A is present
2 R.A + acidified K2Cr2O7 The orange 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 loss of the electron 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

(a) H2CO3 (b) KMnO4

  1. The compound Na2S is called ————–
  2. The IUPAC name of NaHSO4 is————-
  3. Balance the following redox equation:

I +  MnO4–                    IO3  +  MnO2 in 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

A. 1   B.    6   C.    8  D.    12

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

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

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

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

A. 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

A. +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
  2. POCl3          b.           PH3
  3. Balance the following redox equation: Cr2O72- +  SO2                          Cr3+  +  SO42- in acidic medium

 

 

We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

In our next class, we will be learning about Ionic Theory. We are very much eager to meet you there.

 

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