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 Concept:
The order of a reaction is an experimental quantity representing the sum of the powers of the concentration terms in the rate law.

Step 2: Formula Application:
For a second-order reaction, Rate $= k[A]^2$ or Rate $= k[A][B]$.

Step 3: Explanation:
- (a) Decomposition of $H_2O_2$ is 1st order. - (b) Formation of $HI$ from $H_2$ and $I_2$ is a classic 2nd order reaction. - (c) Decomposition of acetaldehyde is $1.5$ order. - (d) Reaction between $NO$ and $H_2$ is 3rd order ($2^{nd}$ order in $NO$ and $1^{st}$ order in $H_2$).

Step 4: Final Answer:
The second-order reaction is $H_2(g) + I_2(g) \rightarrow 2HI(g)$.