Read the passage given below and answer the questions based on it.
The rate of a reaction is concerned with decrease in concentration of reactants or increase in concentration of products per unit time. It can be expressed as instantaneous rate at a particular instant of time and average rate over a large interval of time. A number of factors such as temperature, concentration of reactants and catalyst affect the rate of a reaction. Mathematical representation of rate of a reaction is given by rate law. It has to be determined experimentally and cannot be predicted. Order of a reaction with respect to a reactant is the power of its concentration which appears in the rate law equation. Molecularity is defined only for an elementary reaction. Molecularity and order of an elementary reaction are the same.
Given below is a bimolecular elementary reaction.
2HI → H2 + I2
The order of the above reaction is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2
For an elementary reaction (one that takes place in a single step), the rate law follows directly from the equation, and the order equals the molecularity. The decomposition of HI involves the collision of two HI molecules (bimolecular):
\( \displaystyle \ce{2HI -> H2 + I2} \qquad \text{Rate} = k[\ce{HI}]^2 \)
So the order of the reaction is 2.