GATE 2026 CY – Question 56

Correct answer: (B) Zero order kinetics is followed at sufficiently high pressure of the reacting molecules; (C) First order kinetics is followed at very low pressure of the reacting molecules; (D) Rate of the reaction is proportional to the fraction of surface covered by the reacting molecules

Explanation

For a unimolecular gaseous reaction on a catalyst surface the **Langmuir isotherm** gives the fraction of the surface covered:
$$\theta=\frac{KP}{1+KP},$$
and the rate is proportional to the covered fraction, $r=k\theta$.

• **A. It follows the Freundlich isotherm.** False. The simple surface-reaction model is based on the Langmuir isotherm.
• **B. Zero order at sufficiently high pressure.** True. For $KP\gg1$, $\theta\to1$, so the rate is constant ($r\approx k$), independent of pressure. ✓
• **C. First order at very low pressure.** True. For $KP\ll1$, $\theta\approx KP$, so $r\approx kKP$, proportional to pressure. ✓
• **D. The rate is proportional to the fraction of surface covered.** True: $r=k\theta$. ✓

Answer **B, C and D**.