First slide

Kinetics of chemical reactions

Question

If 75% of a first order reaction was completed in 90 minutes, 60% of the same reaction would be completed in approximately (in minutes)

Moderate

Solution

Use direct formulae:
$\begin{array}{l} \frac{tx %}{ty %} = \frac{\log (\frac{100}{100 - x})}{\log (\frac{100}{100 - y})} \\ \frac{t_{75 %}}{t_{60 %}} = \frac{2 \log 2}{\log 2 .5} \\ \frac{90}{t_{60 %}} = \frac{2 \times 0 .3}{0 .4} \Rightarrow t_{60 %} = \frac{90 \times 4}{6} = 60 \min \end{array}$

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