Unit of Rate Constant

The unit of the rate constant is proportionality constant where the rate of reaction is directly correlated to the concentration of the reactant. In first-order reactions, the reaction rate is directly proportional to the reactant concentration and the units of first-order rate constants are 1/sec. In bimolecular reactions with two reactants, the second-order rate constants have units of 1/M*sec. Second-order reactions can be made to appear as first-order reactions, such reactions are called pseudo-first-order since adding one reactant in excess will make the reaction first order with respect to the other reactant. There are also zero-order reactions in which the reaction is independent of the reactant concentrations where the units of the rate constant are mol/L*sec.

     Characteristics of the rate constant

1. The rate constant of a particular reaction is constant at a particular temperature.
2. It is independent of the initial concentration of reactants at a particular reaction.
3. The rate of reaction depends on the rate constant.
4. The larger the value of the rate constant faster will be the rate of reaction and vice versa.
5. The value of ‘K’ is different for a different reaction.
6. The value of ‘x’ increases with an increase in temperature

Lets us consider the following chemical reaction

A → Products

Its rate law can be given as

r= K[A]x

Where r= rate of chemical reaction

K= rate of constant

X= order of reaction w.r.t. A

The unit for a different order of reactions are:

1. For zero-order reaction, x = 0
   we have,
   r= K[A]x
   or, r= K[A]0
   or, r= k
   taking the unit only
   molLit-1sec-1= k
   Unit of the rate constant for a zero-order reaction is mole lit-1sec-1
2. For first-order reaction r= 1
   we have
   r= K[A]x
   or, r= K[A]1
   or r= K[A]
   unit of the rate constant for the first-order reaction is sec-1
3. For second-order reaction r= 2
   we have
   r= K[A]x
   or, r= k[A]2
   
   unit of the rate constant for 2nd order reaction is mole-1lit-1sec-1