# JEE Main & Advanced Chemistry Chemical Kinetics Law Of Mass Action And Rate Constant

Law Of Mass Action And Rate Constant

Category : JEE Main & Advanced

The rate at which a substance reacts is directly proportional to its active mass and the rate at which a reaction proceeds is proportional to the product of the active masses of the reacting substances.

• For a reaction, $aA+bB\to$ product

Rate $=\left( \frac{dx}{dt} \right)\propto {{[A]}^{a}}{{[B]}^{b}}$; $\left( \frac{dx}{dt} \right)=k{{[A]}^{a}}{{[B]}^{b}}$

Where k is called rate constant or velocity constant.

When $[A]=[B]=1\ mol/litre$, then $\frac{dx}{dt}=k$

Thus, rate constant k is also called specific reaction rate.

• The value of rate constant depends on, nature of reactant, temperature and catalyst. It is independent of concentration of the reactants.
• Unit of rate constant $={{\left[ \frac{\text{litre}}{\text{mol}} \right]}^{n-1}}\times {{\sec }^{-1}}$$\ ={{\left[ \frac{\text{mol}}{\text{litre}} \right]}^{1-n}}\times {{\sec }^{-1}}$

Where $n=$ order of reaction.

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