Order of Reaction - Practice Questions & MCQ

Order of reaction is defined as the number of reactants which determine the rate of reaction. In other words, it is the sum of exponents raised on active masses of reactants in a rate law equation.
Let us consider this reaction
Rate law equation for the reaction can be given as:

               $\begin{aligned} & \mathrm{R} \propto[\mathrm{A}]^p[\mathrm{~B}]^q \\ & \mathrm{R}=\mathrm{K}[\mathrm{A}]^{\mathrm{P}}[\mathrm{B}]^{\mathrm{q}}\end{aligned}$
Order of reaction $=\mathrm{p}+\mathrm{q}$
Rate constant and order for various reacting species is an experimental finding and cannot be predicted directly from the stoichiometry of the balanced reaction.

1. It is an experimental value.

2. It may be zero, negative or in fraction.

3. The order of reaction depends upon temperature, pressure and concentration etc.

4. It determines rate of reaction.

5. Order of reaction is determined by the slowest step of the reaction.

6. High order reactions are rare due to less chance of effective collisions between molecules.

7. Anything in excess is not counted in order of the reaction. Example, in hydrolysis of ester and sugar water is in excess so it is neglected for order.

• In a complex reactions, there are many steps. Among those only one step is the rate-determining step as it is a slow reaction.
• The order of the reaction can be found out using the stoichiometric coefficients of the rate-determining step or the slowest step.
• In a simple reaction, the reaction completes in one single step and that is the rate-determining step. For simple reactions, Gulberg and Waage law of mass action is valid.
  For example:
  $\mathrm{H}_2+\mathrm{I}_2 \rightarrow 2 \mathrm{HI}$
  Since it is a simple reaction, thus the rate of reaction is given as:
  Rate $=\mathrm{K}\left[\mathrm{H}_2\right]^1\left[\mathrm{I}_2\right]^1$

Consider the reaction:

$2 \mathrm{~A}+\mathrm{B} \rightarrow \mathrm{P}$

The rate law for this reaction is given as follows:

Rate $=\mathrm{K}[\mathrm{a}]^{\mathrm{x}}[\mathrm{B}]^{\mathrm{y}}$

Now, x and y are the coefficients that are determined experimentally.

K = Rate constant
x,y = Order of reaction with respect to A and B.