Explain classification of chemical reactions ?
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Classification of chemical reaction

A. Classification based on phases involved

1) Homogeneous Reaction :

Homogeneous reaction is one in which the only one phase I.e all the reacting materials, products and catalyst are in same phase.


Oxidation of nitrogen oxide to nitrogen dioxides with air, is a gas phase reaction.

$$ \mathrm{NO}+\frac{1}{2} \mathrm{O}_{2} \rightarrow \mathrm{NO}_{2} $$

2) Heterogeneous reaction

A Heterogeneous reaction is one which involves presence of more than one phase i.e in heterogeneous reaction, at least one of the reactants, catalyst or products is present in phase different from the remaining components of the reacting system.


Oxidation of sulfur dioxide to aulfur trioxide using vanadium pentaoxide catalyst. is a heterogeneous reaction as $\mathrm{SO}_{2} ~,\mathrm{O}_{2}$ and $\mathrm{SO}_{3}$ are gaseous while $\mathrm{V}_{2} \mathrm{O}_{5}$ is a solid material.

$$ \mathrm{SO}_{2}+\frac{1}{2} \mathrm{O}_{2} \rightarrow \mathrm{SO}_{3} $$

B. Classification based on catalyst property

1) Catalytic reaction

Catalytic reactions are those reactions which involve the use of catalyst to enhance the rate of a reaction speed of a reaction.


$$ \mathrm{C}_{2} \mathrm{H}_{4}+\mathrm{H}_{2} \underset{\text { Heat }}{\stackrel{\mathrm{Ni}}{\longrightarrow}} \mathrm{{C}_{2}H_6} $$

Hydrogenation of ethylene is a catalytic reaction which makes use of nickel catalyst.

2) Non-catalytic reactions

Non-catalytic reactions are those reactions which does not involve use of catalyst.

Oxidation of $\mathrm{NO}$ to $\mathrm{NO}_{2}$ is a non-catalytic reaction: $$ \mathrm{NO}+\frac{1}{2} \mathrm{O}_{2} \rightarrow \mathrm{NO}_{2} $$

C. Classification based on the molecularity of a reaction

i.e based upon the number of molecules that take part in the reaction (in rate determining step) as : unimolecular, bimolecular and termolecular reactions.


1) Decomposition of cyclobutane is a unimolecular reaction.

$$\text{cyclobutane}\rightarrow \text{ethylene}$$

2) Decomposition of hydrogen iodide is a bimolecular reaction which involves collision of two molecules.

$$ 2 \mathrm{HI} \rightarrow \mathrm{I}_{2}+\mathrm{H}_{2} $$

3) Oxidation of $\mathrm{NO}$ to $\mathrm{NO}_{2}$ is a trimolecular/ termolecular reaction which involves collision of three molecules.

$$ 2 \mathrm{NO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2} $$

D. Classification based on heat effect

as they give off or absorb heat to or from the surroundings as exothermic and endothermic reactions.

1) Exothermic reaction is the one in which heat is evolved

The reaction between $\mathrm {C O}$ and $\mathrm H_2$ wo produce methanol in exothermic reaction

$$\mathrm {CO_2+2H_2 \stackrel{Cu}\rightarrow CH_3OH+\text{heat}}$$

2) Endothermic reaction is the one in which heat in absorbed

Dehydration af ethyl alcohol to produce ethylene in a endothermic reaction.

$$ \mathrm {C_2H_5OH \stackrel{Al_2O_3}\rightarrow C_2H_4+H_2O -\text{Heat}}$$

E. Classification based on the order of a reaction

Such as first order reaction, second order reaction, third order reaction, etc

1) First order reaction of which overall order of reaction ie one (ie sum of orders with respect to participants in a reaction is unity) in called as first order reaction.

Decomposition of nitrogen pentaoxide is a first order reaction.

$$ \mathrm{N}_{2} \mathrm{O}_{5} \rightarrow \mathrm{NO}_{2}+\frac{3}{2} \mathrm{O}_{2} $$

2) Second order reaction at which the sum of orders with respect to reactants participated in reaction is two is called as second order reaction.

Saponification of ester is a second order reaction.

3) Third order reaction at which the sum of orders with respect to reactants participated in reaction is three is called as third order reaction.

$$ 2 \mathrm{NO}+\mathrm{H}_{2} \longrightarrow \mathrm{N}_{2} \mathrm{Q}+\mathrm{H}_{2} \mathrm{O} $$

F. Classification based on reaction direction

Such as reversible and irreversible reactions based upon whether they proceed in one or both the directions.

1) Reversible reaction are those in which the forward and reverse reactions take place simultaneously.

Example Esterification reaction is a reversible reaction.

$$\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}+\mathrm{CH}_{3} \mathrm{COOH} \stackrel{\mathrm{H}^{+}} \leftrightarrow \mathrm{CH}_{3} \mathrm{COOC_2H}_{5} +\mathrm{H}_{2} \mathrm{O}$$

2) Irreversible renction are those which can proceed only in one direction.

Example: Nitration of benzene is a irreversible reaction.

$$\mathrm {C_6H_6+HNO_3 \stackrel{H^+} \rightarrow C_6H_5NO_2 +H_2O}$$

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