Types of reactions: oxidation and reduction

Chemical reaction can be classified under the following:

1. Double decomposition
2. Combination
3. Decomposition
4. Displacement
5. Oxidation and reaction
6. Catalytic reactions
7. Reversible reactions
8. Thermal dissociation

Combination

Combination is the process whereby two substances combine together to form a single bond. Combination reactions can take place in organic and inorganic reactions. E.g. Fe_s + S_s > FeS_s

Thermal dissociation

Thermal takes place when heat is applied. It breaks down a substance into two or more simpler molecules or atoms. Thermal dissociation is reversible.

Reversible reactions

The process whereby the product of a reaction can react to form the original reactants exceeds is known as reversible reaction.e.g.

A+B > C+D normal reaction

C+D< A+B reverse reaction

Catalytic reaction

Catalytic reaction has to do with the use of a particular substance to alter the rates of chemical reactions. Note, this substance remains the same chemically and quantitatively.

There are two types of catalyzed reactions which are Homogeneous and heterogeneous catalysts.

Double decomposition

Here the reactants decompose to form new substances through the process of exchanging radicals. For example BC+DE > BE+DC

Displacement

Displacement has to do with one element replacing another in a compound, for example C+BD > B+CD

Redox reaction

The term oxidation was originally thought of in simple terms as reactions in which oxygen is added to a substance, while reduction was described as the removal of oxygen from an oxygen containing compound. The meanings of the terms have gradually been broadened and modified:

Note: reactions involving oxidation and reaction are known as redox reactions.

Definitions of oxidation and reduction

1. Addition of oxygen: at first, oxidation is simply a reaction in which oxygen is combined with another substance, while, reduction is that in which oxygen is removed from a substance.

2. Removal of hydrogen: the definition of oxidation is extended to mean the removal of hydrogen, while that of reduction means the addition of hydrogen.

3. Addition of electronegative elements: here oxidation further extended to include the addition of electronegative elements and the removal of electropositive elements, while reduction include the removal of electronegative elements and addition of electropositive elements.

4. Transfer of electron: the current definition of both oxidation and reduction look at the two concepts in terms of electron transfer. Oxidation is a process involving loss of electron(s), while reduction is a process involving gain of electron(s).

Oxidation number

Oxidation number of an element is the electrical charge it appears to have both in the free and combined states, as determined by a set of rules. These rules are:

1. All elements in the uncombined state have an oxidation number of zero.e.g. k⁰, Na⁰, Cl⁰₂, etc. This zero, however, is often omitted.

2. The oxidation number of a simple ion has the same size and sign as the charge of the ion.e.g Zn²⁺ and  Cl^– are +2 and -1 respectively. The signs always come before the number.

3. The sum of all the oxidation numbers of the elements in a compound is zero.e.g NaCl

(Oxidation number of Na) + (oxidation number of Cl) = (oxidation number of NaCl)= 0

(+1) +(-1) = (+1) + (-1) = 0

4. The oxidation number of hydrogen is taken to be +1 (except in hydrides where it is -1), while that of oxygen is -2 (except in peroxide where it is -1)

Note : this rule allows us to calculate the unknown oxidation number of any element.

Oxidizing and reducing agents

An oxidizing agent is a substance that is reduced in a chemical reaction, thereby, bringing about oxidation, while a reducing agent is oxidized thereby, bringing about reduction.e.g

CuO + CO > Cu + CO₂

Some common oxidizing and reducing agents

Balancing redox Equations

1.the half cell reaction method:

a.identify the oxidizing agent and the reducing agent and deduce what the product should be .

b.write the half Equation for each of oxidation and reduction. Then, balance the atoms and charges for each half Equation.

Questions

1.The oxidation number of sulphur in Fe₂(SO₄)₃ is? (Wassce 2014)

A. +2

B. +3

C. +4

D. +6

Ans: D

Solution: Fe₂(SO₄)₃

2Fe³⁺ + 3(S + 40²⁺) = 0

2×3+3(S-8)=0

6+3S-24=0

3S-18=0

3S=18

S=+18/3 =+6

2. What is the function of manganese (IV) oxide on the reaction represented by the following equation? (Wassce 2014)

MnO₂ + 4HCl > MnCl₂ + 2H₂O + Cl₂

A. Catalyst

B. Dehydrating agent

C. Oxidizing agent

D. Reducing agent

Ans: C

3. Reduction is the process of? (Wassce 2014)

A. Loss of electron 

B. Loss of hydrogen

C. Loss of oxygen

D. Addition of Electronegative elements

Ans: C

4. Consider the reaction represented by the following equation: K₂Cr₂O₇ + HCl > KCl + CrCl₃ + H₂O + Cl₂

(I) explain briefly why this reaction is redox

(II) write balanced half equation for the reaction

(III) write the overall balanced reaction equation.

Ans:(I) the reaction is redox because chromium was reduced from oxidation state of +6 to +3 by HCl

(II) Cr₂O₇^2- + 14H⁺ + 9e^– > Cr²⁺ + 7H₂O

(III) K₂Cr₂O₇ + 14HCl > 2KCl + CrCl₃ + 7H₂O + 3Cl₂

5. Which of the reactions below is NOT an example of a redox reaction? (Jamb 1989)

i. Fe + 2Ag⁺ > Fe²⁺ + 2Ag|

ii. 2H₂S + SO₂ > 2H₂O + 3S|

iii. N₂ + O₂>< 2NO

iv. CaCO₃ >< CaO + CO₂

A. I, II and III

B. II and III

C. III and IV

D. IV only

Ans: D

6. Gun powder is made from charcoal, sulphur and potassium trioxonitrate (v). The salt in the mixture performs the function of? (Jamb 1989)

A. An oxidant

B. A reductant

C. A solvent

D. A catalyst

Ans: A

7. Sulphur (IV) oxide bleaches by? (Jamb 2004)

A. Hydration

B. Reduction

C. Adsorption

D. Oxidation

Ans: B

Solution: SO₂ bleaches by reduction, it produces H₂SO₃ with water which bleaches the dye.

SO_2(g) + H₂O_(l) > H₂SO_3(aq)

H₂SO_3(aq) + Dye coloured > H₂SO_3(aq) + (Dye – [O]) colorless

                                                                           |

                                                                      Oxygen

8. The efficiency of a solid catalyst to increase reaction rates depends on its?

(Jamb 2006)

A. Surface area

B. Solubility

C. Concentration

D. Quantity

Ans: A