Question

Which of the following reactions is a fast reaction at laboratory temperature?
A.Reaction between $KMn{O_4}$ and oxalic acid
B.Reaction between $KMn{O_4}$ and mohr’s salt
C.Hydrolysis of ethyl acetate
D.Thermal decomposition of ${N_2}{O_5}$

Answer 1

Hint: A reaction in which there is less rearrangement involved will be a fast reaction at laboratory temperature.

Complete step by step answer:
In order to determine which reaction is a fast reaction at room temperature, let’s find the balanced equations for each reaction.
In option A –
Reduction half reaction –
$ \Rightarrow 2KMn{O_4} + 3{H_2}S{O_4} \to {K_2}S{O_4} + 2MnS{O_4} + 3{H_2}O + 5[O]$
Oxidation half reaction –
$ \Rightarrow 5{(COOH)_2} + 5[O] \to 5{H_2}O + 10C{O_2} \uparrow $
Overall reaction –
$ \Rightarrow 2KMn{O_4} + 3{H_2}S{O_4} + 5{(COOH)_2} \to {K_2}S{O_4} + 2MnS{O_4} + 8{H_2}O + 10C{O_2} \uparrow $
In option B –
Reduction half reaction –
$ \Rightarrow 2KMn{O_4} + 3{H_2}S{O_4} \to {K_2}S{O_4} + 2MnS{O_4} + 3{H_2}O + 5[O]$
Oxidation half reaction –
\[ \Rightarrow [2FeS{O_4}{(N{H_4})_2}S{O_4}.6{H_2}O + {H_2}S{O_4}{\text{ + 5[O]}} \to F{e_2}{(S{O_4})_3}{\text{ + 2(N}}{{\text{H}}_4}{{\text{)}}_2}S{O_4}{\text{ + 13}}{{\text{H}}_2}O] \times 5\]
Overall reaction –
\[ \Rightarrow 2KMn{O_4} + 10FeS{O_4}{(N{H_4})_2}S{O_4}.6{H_2}O + 8{H_2}S{O_4} \to {K_2}S{O_4} + 2MnS{O_4} + 5F{e_2}{(S{O_4})_3}{\text{ + 10(N}}{{\text{H}}_4}{{\text{)}}_2}S{O_4}{\text{ + 68}}{{\text{H}}_2}O\]
In option C –
$C{H_3}COOC{H_2}C{H_3} + {H_2}O \rightleftharpoons C{H_3}COOH + C{H_3}C{H_2}OH$
In option D –
$ \Rightarrow 2{N_2}{O_5} \to 4N{O_2} + {O_2}$
In all the reactions there are some rearrangements that occur. When we compare the amount of rearrangements, the rearrangements that occur in case of the reaction between Mohr’s salt and potassium permanganate are the least.

Therefore, the reaction in option B occurs at a fast rate at laboratory temperature.

Note:
All these reactions involve a lot of rearrangement. The amount of rearrangement is inversely proportional to the speed of the reaction.
Whenever speed of a reaction is to be determined, we need to understand the molecular rearrangements in order to solve such a question.