Question

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

Answer 1

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 $\text{r = K}{{\left[ \text{reactants} \right]}^{\text{n}}}$.

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

Note: Do not get confused between molecularity and order of reaction, because sometimes the terms seem to be similar but they are different. Let us see the differences between the two:

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.