By Sunil Bhardwaj

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The number of molecules (or atoms) whose concentration (or pressure) determines the rate of reaction is called the order of the reaction. $$ 2NH_3(g) \overset { W }{ \longrightarrow } { N }_{ 2 }(g) + 3{ H }_{ 2 } (g) \qquad \text{ (Zero order reaction) } $$ $$ N_2O_2(g) \longrightarrow N_2O_4(g) + \frac12{ O }_{ 2 }(g) \qquad \text{ (First Order reaction) } $$ $$ 3KClO \longrightarrow KClO_3+2KCl \qquad \text{ (Second Order reaction) } $$ The order of reaction may not be always a whole number, It can be fraction also. $$CH_3CHO \overset { 450C }{ \longrightarrow } CH_4+CO $$ $$ \text{ Rate of Reaction } \propto { \left[ CH_3CHO \right] }^{ 1.5 } $$ $$ \therefore \text{ Order of Reaction } = 1.5 $$