Assertion (A): For a gaseous reaction at constant temperature, the rate can be expressed as the rate of change in partial pressure of a reactant or a product, in units such as atm s−1.
Reason (R): At constant temperature, the concentration of a gaseous species is directly proportional to its partial pressure.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
The units of rate are concentration time−1, for example mol L−1 s−1. For gases the concentration can also be followed through the partial pressure.
Assertion (A): For a gaseous reaction at constant temperature, the rate can be expressed as the rate of change in partial pressure of a reactant or a product, in units such as atm s−1. — True. When the concentration of gases is expressed as partial pressures, the rate has the units atm s−1.
Reason (R): At constant temperature, the concentration of a gaseous species is directly proportional to its partial pressure. — True. At a fixed temperature, a higher partial pressure means more moles of the gas in each litre.
Why R explains A?
Since partial pressure rises and falls in step with concentration, the change in partial pressure per unit time follows the change in concentration per unit time.
So R is the reason the rate of a gaseous reaction can be written in pressure units.
NCERT (2026-27 edition):
3.1 Rate of a Chemical Reaction – “…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.”
“For a gaseous reaction at constant temperature, concentration is directly proportional to the partial pressure of a species and hence, rate can also be expressed as rate of change in partial pressure of the reactant or the product.”