Question

The half-life periods of decomposition of $\mathrm{NH_3}$ for different initial pressures are given below :

The order of the reaction is :

• A. zero order
• B. first order
• C. 0·5
• D. second order

Hint:

Half-life stays the same as pressure changes → that only happens for first order.

Answer 1

The Correct Option is B

Step 1: Watch the half-life, not the pressure.
Each reaction order leaves its own fingerprint on how the half-life changes when you change the starting amount. So the smart move is to scan the table and ask, does the half-life shift as the starting pressure shifts.

Step 2: Read the table values.
The starting pressure falls dramatically across the rows, yet the half-life stays pinned at the same value (80 min) the whole way down. In other words the half-life does not care about the starting pressure at all.

Step 3: Identify the matching order.
A half-life that is completely independent of the starting concentration is the signature of a first-order reaction, because there
\[ t_{1/2} = \frac{0.693}{k} \]
depends only on the rate constant and never on the starting amount.

Step 4: Conclusion.
Constant half-life means first order.
\[ \boxed{\text{Option (B): first order}} \]