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Chemical Kinetics
First Order Reaction Kinetics
1 question from 1 paper, each with its answer.
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- 1
- paper
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- years asked, 2017–2026
- 1 of 10
- of Chemical Kinetics
- 1.25%
Concepts tested
- Half-Life and Reaction Completion
All Questions
2025 1 question
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If the half-life ($t_{1/2}$) for a first order reaction is $1\text{ minute}$, then the time required for 99.9% completion of the reaction is closest to:
For a first order reaction, $t_{99.9\%} \approx 10 \times t_{1/2} = 10 \times 1\text{ min} = 10\text{ minutes}$.
For a first order reaction: $k = \frac{0.693}{t_{1/2}} = \frac{0.693}{1\text{ min}} = 0.693\text{ min}^{-1}$. Time for 99.9% completion: $t = \frac{2.303}{k} \log\left(\frac{100}{100 - 99.9}\right) = \frac{2.303}{0.693} \log\left(\frac{100}{0.1}\right) = \frac{2.303}{0.693} \log(10^3) = \frac{2.303 \times 3}{0.693} = \frac{6.909}{0.693} \approx 10\text{ minutes}$.