Question

Hydrogenation of ethene represented by, 
\[ \text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_6\text{(g)} \] 
is an example of: 
 

• A. Second order reaction.
• B. First order reaction.
• C. Zero-order reaction.
• D. Third Order reaction.

Hint:

Standard NCERT reaction order examples:
- First order: Hydrogenation of ethene, thermal decomposition of \(\text{N}_2\text{O}_5\), radioactive disintegrations.
- Zero order: Decomposition of \(\text{NH}_3\) on hot Pt, decomposition of \(\text{HI}\) on gold at high pressure.

Answer 1

The Correct Option is B

Concept:
The order of a reaction is an experimentally determined quantity reflecting the sum of the powers to which the reactant concentrations are raised in the empirical rate law.
It cannot be deduced solely from the stoichiometric coefficients of the balanced chemical equation.

Step 1: Chemical Equation and Experimental Conditions:
The catalytic hydrogenation of ethene is represented by:
\[ \text{C}_2\text{H}_4(\text{g}) + \text{H}_2(\text{g}) \xrightarrow{\text{catalyst}} \text{C}_2\text{H}_6(\text{g}) \] This reaction takes place heterogeneously on the surface of finely divided transition metal catalysts such as platinum, palladium, or Raney nickel.

Step 2: Experimental Rate Law Formulation:
Under typical laboratory and industrial hydrogenation conditions, molecular hydrogen gas is present in large excess, or the catalyst surface is predominantly covered with adsorbed hydrogen atoms.
Consequently, the rate of the reaction depends only on the concentration of ethene in the gas phase.
Experimental kinetic investigations show that the rate law is:
\[ \text{Rate} = k [\text{C}_2\text{H}_4]^1 \] Because the exponent of the concentration of ethene is \(1\), the reaction follows first-order kinetics.
This is explicitly highlighted as a standard example of a first-order reaction in the NCERT Chemical Kinetics curriculum alongside radioactive decay processes.

Step 3: Verification of Other Orders:
Although two molecules appear in the stoichiometry, the reaction does not exhibit second-order kinetics because hydrogen is maintained in excess.
Zero-order behavior typically applies to decompositions on fully saturated surfaces at high pressure (such as \(\text{NH}_3\) on Pt), which is not the case here.
Final Answer:
The hydrogenation of ethene is an example of a first order reaction, corresponding to option (B).