In the rate equation
Rate = k [A]x [B]y
x and y indicate how sensitive the rate is to the change in concentration of A and B. Sum of these exponents, i.e., x + y gives the overall order of a reaction whereas x and y represent the order with respect to the reactants A and B respectively.
Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. Order of a reaction can be 0, 1, 2, 3 and even a fraction. A zero-order reaction means that the rate of reaction is independent of the concentration of reactants.
Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.
What is the overall order of a reaction which has the following rate expression?
Rate = k[A]5/2[B]−1
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(\frac{3}{2}\)
The passage states that the overall order of a reaction is the sum of the powers of the concentration terms in the rate law (x + y for Rate = k[A]x[B]y). A negative power is added with its sign.
Order with respect to A: 5/2
Order with respect to B: −1
\( \displaystyle \text{Overall order}=\frac{5}{2}+(-1)=\frac{3}{2} \)
A fractional order is possible, since order is found from experiment, not from the balanced equation.