Question

Define order of a reaction.

Hint:

Remember: \[ \text{Rate}=k[A]^m[B]^n \] Then, \[ \text{Order}=m+n \] and \[ \text{Molecularity}=a+b \] Do not confuse order with molecularity. Order is determined experimentally, whereas molecularity is based on the reaction mechanism.

Answer 1

Concept: The speed of a chemical reaction is expressed in terms of its rate. Experimental studies show that the rate of a reaction depends on the concentration of reactants. This dependence is represented mathematically by the rate law or rate equation. For a general reaction \[ aA+bB \rightarrow \text{Products} \] the rate law may be written as \[ \text{Rate}=k[A]^m[B]^n \] where \[ k=\text{rate constant} \] \[ [A]\text{ and }[B]=\text{molar concentrations of reactants} \] \[ m\text{ and }n=\text{orders with respect to }A\text{ and }B \] respectively. The values of \(m\) and \(n\) are determined experimentally and need not be equal to the stoichiometric coefficients of the balanced chemical equation.

Step 1: Understanding the meaning of order. The exponents of the concentration terms in the rate equation indicate the extent to which the rate depends upon the concentration of the corresponding reactants. For example, in the rate law \[ \text{Rate}=k[A]^2[B] \] the reaction is:

• Second order with respect to \(A\)

• First order with respect to \(B\)

Step 2: Calculating the overall order of reaction. The overall order of the reaction is obtained by adding all the exponents appearing in the rate law. Thus, \[ \text{Order of reaction}=m+n \] For the above example, \[ \text{Rate}=k[A]^2[B] \] \[ \text{Order}=2+1=3 \] Hence, it is a third-order reaction.

Step 3: Important characteristics of order of reaction.

• Order is determined experimentally.

• It is obtained from the rate law and not from the balanced chemical equation.

• It may be zero, fractional, integral or even negative in some special cases.

• It provides valuable information about the reaction mechanism.

Step 4: Writing the formal definition. Therefore, the order of a reaction is defined as the sum of the powers of the molar concentration terms of reactants in the experimentally determined rate equation.

Final Definition: \[ \boxed{ \text{Order of a reaction is the sum of the powers of the concentration terms of the reactants in the rate law.} } \] If \[ \text{Rate}=k[A]^m[B]^n \] then \[ \boxed{\text{Order}=m+n} \]