Question

Which of the following is an example of second order reaction?

• A. $2H_2O_2(g) \rightarrow 2H_2O(l) + O_2(g)$
• B. $H_2(g) + I_2(g) \rightarrow 2HI(g)$
• C. $CH_3CHO(g) \rightarrow CH_4(g) + CO(g)$
• D. $2NO(g) + 2H_2(g) \rightarrow N_2(g) + 2H_2O(g)$

Hint:

While you cannot always guess the rate law from the balanced equation, $H_2 + I_2 \rightarrow 2HI$ is one of the rare elementary reactions where the order exactly matches its molecularity (bimolecular).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This is a factual question testing the knowledge of experimentally determined orders for common chemical reactions.

Step 2: Detailed Explanation:
The order of a reaction cannot be definitively deduced just by looking at the stoichiometric coefficients; it must be determined experimentally. Let's review the known kinetics for these classic examples:
- (a) Decomposition of hydrogen peroxide ($2H_2O_2 \rightarrow 2H_2O + O_2$) is a classic example of a first-order reaction.
- (b) The reaction between hydrogen and iodine vapor ($H_2 + I_2 \rightarrow 2HI$) is a fundamental textbook example of a second-order reaction. The rate law is $\text{Rate} = k[H_2][I_2]$.
- (c) The thermal decomposition of acetaldehyde ($CH_3CHO \rightarrow CH_4 + CO$) is a complex chain reaction that experimentally shows a fractional order of 1.5.
- (d) The reduction of nitric oxide ($2NO + 2H_2 \rightarrow N_2 + 2H_2O$) is a third-order reaction. The rate law is $\text{Rate} = k[NO]^2[H_2]$.

Step 3: Final Answer:
The formation of hydrogen iodide is a second-order reaction, matching option (b).