Question

Rate law for the reaction,
$\text{C}_2\text{H}_5\text{I}_{(g)} \longrightarrow \text{C}_2\text{H}_{4(g)} + \text{HI}_{(g)}$ is $\text{r} = \text{k} [\text{C}_2\text{H}_5\text{I}]$
What is the order and molecularity of this reaction?

• A. order and molecularity both are 1
• B. order is 1 and molecularity is 2
• C. order and molecularity both are 2
• D. order is 2 and molecularity is 1

Hint:

For elementary reactions, molecularity and order are usually identical.

Answer 1

The Correct Option is A

Step 1: Concept Order is the sum of powers of concentrations in the rate law. Molecularity is the number of reactant species involved in an elementary reaction.

Step 2: Meaning The rate law $r = k[C_2H_5I]^1$ shows the power is 1.

Step 3: Analysis The reaction involves the decomposition of a single molecule of ethyl iodide. In such elementary gas-phase decompositions, molecularity matches the number of reactant molecules.

Step 4: Conclusion Both the order (from rate law) and molecularity (from stoichiometry of elementary step) are 1. Final Answer: (A)