Question

Which of the following statements is not correct about the order of a reaction?
A. The order of a reaction can be a fractional number.
B. The order of a reaction is experimentally determined quantity.
C. The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in a balanced chemical equation of a reaction.
D. The order of a reaction is the sum of the powers of the molar concentration of the reactants in the rate law expression.

Answer 1

Order of a reaction is always determined the rate law or the rate equation. It is the sum of the coefficients of the powers of the reactants expressed in rate law. The concentrations of the reactants can be fractional or natural numbers and so can the order. It is important as it tells us about the mechanism of the reaction.

Complete step by step answer:
- The rate law can be shown as
$rate=k{{\left[ A \right]}^{x}}{{\left[ B \right]}^{y}}$

where A and B are reactants of the equation and k is a constant parameter. The sum of x and y gives the overall order of the reaction.
-If the rate of the equation does not depend on the reactant concentration;i.e; it is constant irrespective of the reactant concentration, then the equation is zero order.
-If the rate depends on one reactant, it is a first order reaction and if it depends on two reactants or square of 1 reactant, then it is a second order reaction.
- The equation for first order reaction is $\ln \left[ A \right]=-kt+\ln \left[ {{A}_{0}} \right]$
The rate law for second order reaction is $\dfrac{1}{\left[ A \right]}=\dfrac{1}{{{\left[ A \right]}_{\circ }}}+kt$
The rate law for nth order reaction is $\frac{1}{{{\left[ A \right]}^{n-1}}}=\dfrac{1}{{{\left[ A \right]}_{\circ }}^{n-1}}+\left( n-1 \right)kt$

-There are certain reactions such that the order of those reactions comes out to be fractional. It usually indicates that the reaction is occurring in chains and not in single or 2 steps. It also occurs for the reactions which are very complex in nature. Pyrolysis of compounds like acetaldehyde have the order in fraction which is 1.5.
-These reactions follow the processes of chain initiation, chain propagation and chain termination which makes the powers of the concentrations occurring in the rate law to be fractional. This happens because the reactants are consumed in some step and are formed in some other step resulting in fractional powers.

-Order of a reaction is very crucial as it tells us the method in which the reaction will occur and the products formed by such method. Same reactants will give different products if they have different orders as they will occur in completely different forms.
-So, observing the options we see that the first option tells us that order can be fractional which is correct as the order of a reaction can be zero, any natural positive number as well as a fraction value. Order is calculated experimentally only and we cannot find the order just by observing the reaction. So option B is also correct.
-The fourth option gives us the actual definition of the order of a reaction and so it is also correct. Thus option C is incorrect as order can vary sometimes from the stoichiometric summations due to some internal factors.
So, the correct answer is “Option C”.

Note: The order can also be found by knowing the time duration in which the reaction is being completed. The duration in which half of the reaction gets completed is called half-life of the reaction and is denoted by ${{t}_{1/2}}$.