Define order of a reaction

Explanation

The order of a reactions with respect to a reactant is the power (exponent) of the concentration of the reactant to which the rate of the reactions is directly proportional. The overall order of the .reaction is the sum of the exponents of all the reactants in the rate expression.
For example: For the reaction `aA + bB rarr` Products
Let rate law expression is,
Rate = `k[A]^(alpha)[B]^(beta)`
Then order of reaction w.r.t. reactants A and B are `alpha` and `beta` respectively and overall order of reaction `alpha+beta`.
For an nth order reaction
Rate = `"k[conc.]"^(n)`
where k in rate constant of the reaction
`therefore k=("Rate")/("[Conc.]"^(n))`
If concentration is expressed in mol `L^(-1)`
Units of k=`("mol L"^(-1)s^(-1))/(("mol L"^(-1))^(n))`
`therefore` Units of k for the 1st order reaction (i.e.,n=1)
`=mol^(1-1)L^(-1)s^(-1)=s^(-1)`
Units of k for a 2nd order reaction (i.e.,n=2)=`mol^(1-2)L^(2-1)s^(-1)=mol^(-1)Ls^(-1)`