Question

# What is the order of reaction? Write unit of rate constant $\text{K}$ for the zero order, first order and second order reaction.

Hint: Order of reaction involves the number of concentration terms present in the slowest step of the reaction. Rate constant is represented by $\text{K}$ is a proportionality constant that relates the molar concentration of reactants and the rate of a chemical reaction. Mathematically, represented as $\text{r = K}{{\left[ \text{reactants} \right]}^{\text{n}}}$.

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 ${{\text{M}}^{\text{-1}}}{{\text{L}}^{\text{1}}}{{\text{T}}^{-1}}\text{ or }\dfrac{\text{litres}}{\text{moles}\times \text{seconds}}\text{ or }\dfrac{1}{\text{molarity}\times \text{seconds}}$.