Question

What is rate constant?
(A) Equality constant
(B) Shows relation between rate of reaction and pressure
(C) Proportionality constant between rate of reaction and concentration
(D) None of the above

Answer 1

Hint: Rate constant as the name says is a constant that depends upon the temperature and reactant but does not depend upon the concentrations of the reactant. So, in the rate law expression rate constant has a fixed value.

Complete step by step answer:
Rate constant is a constant that is in the rate law expression. The rate of a reaction is defined as the rate of change of the concentration with the change in time. This means that our rate of reaction depends upon the concentrations of the reactant which is given by the rate law expression which is derived below. Suppose we have a reactants (A) and (B) which are going to product (P) as shown, then the rate law is written as:
 $ A + B \to P $
 $
  Rate \propto {[A]^a} \\
  Rate \propto {[B]^b} \\
  $
Where $ [A] $ is the concentration of reactant A and $ [B] $ is the concentration of the reactant B, and the powers $ a,b $ represents the order of the reaction with respect to their respective reactants and $ a + b $ gives us the complete order of the reaction. Now combining these we get:
 $ Rate \propto {[A]^a}{[B]^b} $
Now if we remove this proportionality sign then we have to introduce a new constant $ k $ which is known as the rate constant and our final rate law expression will be:
 $ Rate = k{[A]^a}{[B]^b} $
Now the units of this rate constant are dependent on the order of the reaction, so for different order reactions the units of the rate constant $ k $ are different.

Note:
The order of the reaction can be zero or even fractional so the units of the rate constant can also be fractional. Also for many questions we can directly look at the units of the rate constant and tell the order of the reaction.