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Let us first discuss the order of reaction; order of reaction is defined as the sum of all the exponents of the concentration terms in the rate expression. Characteristics of order for a chemical reaction are:

- Order of reaction does not depend on the stoichiometric coefficients of each species in the balanced reaction.

- Order of a chemical reaction is expressed in reactant concentrations.

To find order of a reaction, the power-law form of the rate equation is generally used. Rate law is given by $\text{r = k}{{\left[ \text{A} \right]}^{\text{x}}}{{\left[ \text{B} \right]}^{\text{y}}}$.

Let us find the unit of rate constant ‘K’, the general formula to find it is ${{\text{M}}^{\text{1-n}}}{{\text{L}}^{\text{n-1}}}{{\text{T}}^{-1}}$; where $\text{M}$ are the moles of reactants, $\text{L}$ is the volume expressed in litres, $\text{n}$ is the order of reaction and$\text{T}$is the time.

- For zero order reaction, n is equal to 0; the units of rate constant will be ${{\text{M}}^{\text{1-0}}}{{\text{L}}^{\text{0-1}}}{{\text{T}}^{-1}}$ is equal to ${{\text{M}}^{\text{1}}}{{\text{L}}^{\text{-1}}}{{\text{T}}^{-1}}\text{ or }\dfrac{\text{moles}}{\text{litre}\times \text{seconds}}\text{ or }\dfrac{\text{molarity}}{\text{seconds}}$.

- For first order reaction, n is equal to 1; the units of rate constant will be ${{\text{M}}^{\text{1-1}}}{{\text{L}}^{\text{1-1}}}{{\text{T}}^{-1}}$ is equal to ${{\text{M}}^{0}}{{\text{L}}^{0}}{{\text{T}}^{-1}}\text{ or }\dfrac{1}{\text{seconds}}\text{ or second}{{\text{s}}^{-1}}$.

- For second order reaction, n is equal to 2; the units of rate constant will be ${{\text{M}}^{\text{1-2}}}{{\text{L}}^{\text{2-1}}}{{\text{T}}^{-1}}$ is equal to ${{\text{M}}^{\text{-1}}}{{\text{L}}^{\text{1}}}{{\text{T}}^{-1}}\text{ or }\dfrac{\text{litres}}{\text{moles}\times \text{seconds}}\text{ or }\dfrac{1}{\text{molarity}\times \text{seconds}}$.

MOLECULARITY OF REACTION | ORDER OF A REACTION |

It is the number of molecules taking part in a reaction. | In rate expression, the sum of powers in concentration terms; is called order. |

It cannot be zero, fractional and negative. | It can be zero, integer, fractional and can even be negative. |

It is independent of pressure and temperature. | It depends on temperature and pressure. |

It is a theoretical concept. | It is derived from rate expression. |

Assigned to every elementary step. | Assigned for overall reaction. |