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Solutions — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistrySolutions2 Marks
Question
For a reaction
$2\text{H}_2\text{O}_2\xrightarrow[\text{alkaline medium}]{\text{I}^-}2\text{H}_2\text{O}+\text{O}_2$
the proposed mechanism is as given below:
  1. $\text{H}_2\text{O}_2+\text{I}^{-}\xrightarrow{\ \ \ \ \ \ \ }\text{H}_2\text{O}+\text{IO}^{-}(\text{slow})$
  2. $\text{H}_2\text{O}_2+\text{IO}^{-}\xrightarrow{\ \ \ \ \ \ \ }\text{H}_2\text{O}+\text{I}^{-}+\text{O}_2(\text{fast})$
  1. Write rate law for the reaction.
  2. Write the overall order of reaction.
  3. Out of steps (1) and (2), which one is rate determining step?
✓

Answer

$2\text{H}_2\text{O}_2\xrightarrow[\text{alkaline medium}]{\text{I}^-}2\text{H}_2\text{O}+\text{O}_2$In some cases, the rate of reaction depends not only on the reactant but may also depend on the substance present as a catalyst. This is also seen in the above reaction. The rate of reaction depends on the slowest step in case of a complex reaction.
  1. $\text{H}_2\text{O}_{2} +\text{I}^- \xrightarrow{\ \ \text{Slow}\ \ }\text{H}_{2}\text{O} + \text{IO}^-$
  2. $\text{H}_2\text{O}_{2} +\text{IO}^- \xrightarrow{\ \ \text{Fast}\ \ }\text{H}_{2}\text{O} + \text{I}^-+\text{O}_2$
  1. Rate law:
$\text{Rate}=-\frac{1}{2}\frac{\text{d}\big[\text{H}_2\text{O}_2\big]}{\text{dt}}=\text{k}\big[\text{H}_2\text{O}_2\big]\big[\text{I}^-\big]$
  1. The overall order of reaction:
$\text{Rate}=\text{k}\big[\text{H}_2\text{O}_2\big]\big[\text{I}^-\big]$
$\text{Order}=1+1=2$
  1. Slowest step is the rate determining step. Hence, step 1 is rate determining step.

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