Question

Which are complex reaction
(i) Oxidation of ethane
(ii) Thermal decomposition of HI on gold surface
(iii) Saponification of methyl acetate
(iv) Nitration of phenol
(v) Decomposition of \( \text{NH}_3 \) on hot Pt surface

Hint:

Reactions such as complete combustion, or those that generate multiple intermediate side products like nitration, are always complex because they cannot happen via a single simultaneous molecular collision.

Answer 1

Step 1: Understanding the Concept:
In chemical kinetics, reactions that occur in a single step are called elementary reactions.
Reactions that occur in a sequence of elementary steps (a mechanism) to give the final products are called complex reactions.
Step 2: Key Formula or Approach:
The approach is to classify reactions based on their known mechanisms; multi-step mechanisms involving intermediates are complex.
Step 3: Detailed Explanation:
According to standard Chemical Kinetics theory, complex reactions involve multiple intermediate steps or yield mixed side products.
(i) Oxidation of ethane: It passes through a series of intermediate steps (forming alcohols, aldehydes, and acids) before final conversion to \( \text{CO}_2 \) and \( \text{H}_2\text{O} \).
This is a classic complex chain reaction.
(ii) Thermal decomposition of HI on a gold surface: This is a classic example of a zero-order reaction on a solid catalyst surface, often treated as elementary at high concentrations.
(iii) Saponification of methyl acetate: This is a bimolecular second-order elementary reaction in basic kinetics discussions.
(iv) Nitration of phenol: It occurs via electrophilic aromatic substitution, involving intermediate sigma complexes, and yields a mixture of ortho and para products.
This multi-step pathway makes it a typical complex reaction.
(v) Decomposition of \( \text{NH}_3 \) on hot Pt surface: This is another zero-order elementary-like catalytic surface reaction.
Therefore, based on standard textbook classifications, (i) and (iv) represent complex consecutive and parallel reaction mechanisms respectively.
Step 4: Final Answer:
(i) Oxidation of ethane and (iv) Nitration of phenol are complex reactions.