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: Understanding the Concept:
Order is the sum of exponents in the rate law. Molecularity is the number of reactant molecules in an elementary step.
Step 2: Key Formula or Approach:
Rate law: $\text{r} = \text{k}[\text{A}]^\text{x} \rightarrow \text{order} = \text{x}$.
Step 3: Detailed Explanation:
- From the given rate law $\text{r} = \text{k}[\text{C}_2\text{H}_5\text{I}]^1$, the power of concentration is 1. Thus, order $= 1$.
- Molecularity is defined for elementary reactions. This decomposition is a simple unimolecular process involving only one molecule of ethyl iodide. Hence, molecularity $= 1$.
Step 4: Final Answer:
Both order and molecularity are 1.