SUBJECT: CHEMISTRY

CLASS:� SS 2

DATE:

TERM: 1st TERM

REFERENCE MATERIALS

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

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WEEK FOUR

TOPIC: OXIDIZING AND REDUCING AGENTS

CONTENT

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Identification of Oxidizing and Reducing agents

Balancing of Redox equation in Acidic and Alkaline medium

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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.�

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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.

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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.�

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

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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 coloration 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)	White precipitate of insoluble BaSO4 is formed�	O.A is present�

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TEST FOR REDUCING AGENTS:� Reducing agent is detected in the laboratory using any of the following reagents.

� � Acidified Potassium tetraoxomanganate (VII)�
� � Acidified Potassium heptaoxodichromate (VI)�

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S/N	TEST�	OBSERVATION�	INFERENCE
1	R.A + acidified KMO4	Purple solution of KMnO4 turns colorless on addition of R.A�	R.A is present�
2	R.A + acidified K2Cr2O7	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.�

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EVALUATION

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

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BALANCING OF REDOX EQUATIONS

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

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

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

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SOLUTION:��

O.A�� MnO4-�

R.A �� Fe2+

OXIDATION HALF EQUATION�

��Fe2+�� Fe3+� � +� � e-

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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

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

�� Cr3+ +� � BrO-�� CrO42-� + � Br-

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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-

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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

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BALANCED HALF EQUATIONS

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

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

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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-�

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GENERAL EVALUATION/REVISION

Determine the oxidation number of�

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

� Name the following compounds

H2CO3� � � � � (b) KMnO4

3.The compound Na2S is called --------------

4.The IUPAC name of NaHSO4 is-------------

5.Balance the following redox equation: I-� +� MnO4-IO3-� +� MnO2in basic medium

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READING ASSIGNMENT

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

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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 B.� � 6�� C.� � 8�� D.� � 12

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

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)

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�

+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

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

Lesson Notes By Weeks and Term - Senior Secondary School 2