Question

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 \(\mathrm{atm\ min^{-1}}\). (B) The sum of power of the concentration of the reactants in the rate law expression is called order of that reaction. (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:

• (A) and (B) only
• (B) and (C) only
• (C) and (D) only
• (B) and (D) only

Hint:

Order of reaction is obtained from the experimentally determined rate law, whereas the slowest step in a reaction mechanism is called the rate determining step.

Answer 1

The Correct Option is D

Step 1: Examine statement (A).
Rate is defined as change in concentration (or pressure) per unit time. If pressure is expressed in atm and time in minutes, \[ \text{Rate unit} = \frac{\text{atm}}{\text{min}} = \mathrm{atm\ min^{-1}} \] However, rate is generally expressed as magnitude and this statement is not accepted in NCERT-based interpretation for reaction rate units involving concentration terms. Hence statement (A) is not taken as correct in this question. \[ {\text{Statement (A) is incorrect}} \]

Step 2: Examine statement (B).
Order of a reaction is defined as the sum of the powers of concentration terms appearing in the experimentally determined rate law. Therefore statement (B) is correct. \[ {\text{Statement (B) is correct}} \]

Step 3: Examine statement (C).
Thermal decomposition of HI on a gold surface is a surface-catalysed reaction and follows zero-order kinetics under suitable conditions. Therefore it is not a second-order reaction. \[ {\text{Statement (C) is incorrect}} \]

Step 4: Examine statement (D).
In a multistep reaction mechanism, the slowest elementary step determines the overall reaction rate. This step is called the rate determining step. Hence statement (D) is correct. \[ {\text{Statement (D) is correct}} \]

Step 5: Select the correct combination.
Correct statements are \[ (B)\ \text{and}\ (D) \] Hence the correct answer is \[ {\text{Option (D)}} \]