A first order reaction has a half-life of 693 sec. What will be its r…

The correct answer is Option (3) → 0.001 sec⁻¹

For a first order reaction, the half-life is constant and related to the rate constant by:

    \( \displaystyle t_{1/2}=\frac{0.693}{k}\quad\Rightarrow\quad k=\frac{0.693}{t_{1/2}} \)

    \( \displaystyle k=\frac{0.693}{693\ \mathrm{s}}=1\times10^{-3}\ \mathrm{s^{-1}} \)

So the rate constant is 0.001 s⁻¹.