Question

A consecutive reaction,$[A\xrightarrow{{{k_1}}}B\xrightarrow{{{k_2}}}C]$, is characterised by:
A. maxima in the concentration of A
B. maxima in the concentration of B
C. maxima in the concentration of C
D. highly exothermic

Answer 1

Hint: In a consecutive reaction, the initial material for one reaction is the product of the previous chemical reaction. During consecutive reaction, the intermediate is produced by reactants and the intermediate formed reacts in single or multiple steps to produce a product. The examples of consecutive reaction include polymerisation and chlorination of hydrocarbons.

Complete step by step answer:
The given below is a consecutive reaction.
$[A\xrightarrow{{{k_1}}}B\xrightarrow{{{k_2}}}C]$
In the above reaction, the initial reactant A reacts to form an intermediate B, which again reacts to produce C which is a product. An intermediate is a molecule which has short life, high energy and very high reactivity and it is formed from reactants and then converted to the final product of the reaction. Reaction intermediaries are produced in one step during a reaction and then consumed in the next step. Therefore, intermediates generally do not appear in the overall chemical equation for that reaction.
The reaction of formation of intermediate B from A and then conversion of intermediate B to C both are irreversible reactions.
The $[{K_1}]$ and $[{K_2}]$ denotes the rate constants. The rate constant for conversion of A to intermediate B is $[{K_1}]$. The rate constant for the conversion of intermediate B to product C is $[{K_2}]$.
Here, at the start of the reaction, only reactant A was present. As the reaction proceeds further, reactant A starts converting into an intermediate B. Therefore, the concentration of reactant A starts decreasing and that of B increases. Simultaneously, as the concentration of intermediate B increases, the conversion of intermediate B to the product C also increases. Therefore, the concentration of intermediate B will be maximum in the mentioned consecutive reaction.
Therefore, a consecutive reaction, $[A\xrightarrow{{{k_1}}}B\xrightarrow{{{k_2}}}C]$, is characterised by maxima in the concentration of B.

So, the correct answer is Option B.

Additional information:
The example of consecutive reaction is formation of iodine and hydrogen chloride by the reaction between hydrogen gas and iodine chloride.
$[{H_2} + 2ICl \to {I_2} + 2HCl] $
This is an overall chemical equation for the said chemical reaction.
But, the reaction is actually a 2 step reaction with the formation of an intermediate.
Step 1: This is a slow step of the reaction. The intermediate, $[HI]$ is formed during this step.
$[{H_2} + ICl \to HI + HCl]$
Step 2: This is a fast step of the reaction, in which the final products are produced.
$[HI + ICl \to {I_2} + HCl]$
In the first step, $[HI]$ is formed as an intermediate but it is consumed in the next step as soon as it is formed to form the final product.

Note: In a chemical kinetics, for a consecutive chemical reaction, the slowest step of the reaction is taken into consideration for determining the rate of that chemical reaction.
In a consecutive chemical reaction, intermediates are produced and consumed immediately to produce the final product. The formation of intermediate is generally a slow reaction but consumption of intermediate to form a product is a fast reaction. The slow step is considered as the rate determining step for that reaction.