Question

Under what condition the order of the reaction, $2\text{HI}\xrightarrow{\Delta,\text{catalyst}}\text{H}_2(g)+\text{I}_2(g)$, is zero

• A. At high temperature
• B. At high partial pressure of HI
• C. At low partial pressure of HI
• D. At high partial pressure of \( \text{H}_2 \)
• E. At high partial pressure of \( \text{I}_2 \)

Hint:

For any heterogeneous catalysis, if the surface is completely covered by the reactant, the reaction order drops to zero.

Answer 1

The Correct Option is B

Concept: Surface-catalyzed reactions, such as the decomposition of HI on gold or platinum, follow the Langmuir adsorption isotherm principles. The rate depends on the surface coverage of the catalyst.

Step 1: Surface Saturation.
At low pressures, the rate of reaction is proportional to the concentration of the reactant because many active sites on the catalyst surface are empty.

Step 2: Effect of High Pressure.
When the partial pressure of HI is very high, the reactant molecules occupy almost every available active site on the catalyst surface. At this point, the surface is "saturated."

Step 3: Zero-Order Kinetics.
Since all active sites are filled, adding more HI (increasing pressure) cannot increase the rate of reaction further. The rate becomes constant and independent of the concentration of HI. This behavior characterizes a zero-order reaction.