Types of reactions: oxidation and reduction
Chemical reaction can be classified under the following:
- Double decomposition
- Oxidation and reaction
- Catalytic reactions
- Reversible reactions
- Thermal dissociation
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. Fes + Ss > FeSs
Thermal takes place when heat is applied. It breaks down a substance into two or more simpler molecules or atoms. Thermal dissociation is reversible.
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 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.
Here the reactants decompose to form new substances through the process of exchanging radicals. For example BC+DE > BE+DC
Displacement has to do with one element replacing another in a compound, for example C+BD > B+CD
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 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
|Oxidizing agents||Reducing agents|
3. Manganese(IV) oxide
4. Hydrogen peroxide
6. Acidified potassium tetraoxomanganate(VII)
7. Acidified potassium heptaoxodichromate(VI)
8. Concentrated trioxonitrate (V) acid
9. Hot concentrated tetraoxosulphate (VI) acid
10. Silver salt and any metal ion low in the activity series
Carbon (II) oxide
Sulphur (iv) oxide
sodium and any metal atom high in the reactivity series
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.
1.The oxidation number of sulphur in Fe2(SO4)3 is? (Wassce 2014)
2Fe3+ + 3(S + 402+) = 0
2. What is the function of manganese (IV) oxide on the reaction represented by the following equation? (Wassce 2014)
MnO2 + 4HCl > MnCl2 + 2H2O + Cl2
B. Dehydrating agent
C. Oxidizing agent
D. Reducing agent
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
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) Cr2O72- + 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
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
7. Sulphur (IV) oxide bleaches by? (Jamb 2004)
Solution: SO2 bleaches by reduction, it produces H2SO3 with water which bleaches the dye.
SO2(g) + H2O(l) > H2SO3(aq)
H2SO3(aq) + Dye coloured > H2SO3(aq) + (Dye – [O]) colorless
8. The efficiency of a solid catalyst to increase reaction rates depends on its?
A. Surface area