Choose the correct statements
(A) For a gaseous reaction, if the concentration of gas is expressed in terms of their partial pressure ‘atm’ and time in ‘minutes’ then unit of rate are atm−1 min.
(B) The sum of power of the concentration of the reactants in the rate law expression is called order of that reaction.
(C) The thermal decomposition of HI on gold surface is an example of second order reaction.
(D) The overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
The rate of a reaction is the change in concentration (or in partial pressure, for gases) per unit time. For a rate law Rate = k[A]x[B]y, the order is x + y, and it is found by experiment. A complex reaction takes place in several steps, and the rate of the whole reaction is set by the slowest step.
(A) For a gaseous reaction, if the concentration of gas is expressed in terms of their partial pressure ‘atm’ and time in ‘minutes’ then unit of rate are atm−1 min. — Incorrect. Rate is a change in pressure divided by time, so its unit is atm min−1, not atm−1 min:
\( \displaystyle \text{Rate}=\frac{\Delta p}{\Delta t}\ \Rightarrow\ \frac{\mathrm{atm}}{\mathrm{min}}=\mathrm{atm\,min^{-1}} \)
(B) The sum of power of the concentration of the reactants in the rate law expression is called order of that reaction. — Correct. This is the definition of the order of a reaction. For Rate = k[A]x[B]y, the order is x + y.
(C) The thermal decomposition of HI on gold surface is an example of second order reaction. — Incorrect. The gold surface gets saturated with HI molecules, so a change in [HI] does not change the amount of HI on the surface. The rate does not depend on [HI], and the reaction is of zero order:
\( \displaystyle \ce{2HI(g)} \xrightarrow{\text{Au surface}} \ce{H2(g) + I2(g)} \)
\( \displaystyle \text{Rate}=k[\ce{HI}]^0=k \)
(D) The overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step. — Correct. The products cannot form faster than the slowest step allows, so this step decides the rate of the whole reaction. It is called the rate determining step.
NCERT (2026-27 edition):
Rate of a Chemical Reaction – “… it is clear that units of rate are concentration time–1. For example, if concentration is in mol L–1 and time is in seconds then the units will be mol L-1s–1. However, in gaseous reactions, when the concentration of gases is expressed in terms of their partial pressures, then the units of the rate equation will be atm s–1.”
Order of a Reaction – “… Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction.”
Zero Order Reactions – “… At high pressure, the metal surface gets saturated with gas molecules. So, a further change in reaction conditions is unable to alter the amount of ammonia on the surface of the catalyst making rate of the reaction independent of its concentration. The thermal decomposition of HI on gold surface is another example of zero order reaction.”
Molecularity of a Reaction – “… Similarly, the overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step.”