In solid catalysed reactions the diffusional effects are more likely to affect the overall rate of reaction for

The correct answer is:

A. Fast reactions in catalyst of small pore diameter

✅ Explanation:
In solid-catalyzed reactions, the overall rate can be affected by diffusional resistance, especially internal diffusion within catalyst pores.

🔍 Diffusional effects become significant when:
The reaction is fast:

Reactants are consumed rapidly at active sites.

To sustain the reaction, reactants must diffuse quickly to the internal surface.

The pore diameter is small:

This increases resistance to internal diffusion (pore diffusion).

Reactants and products move more slowly, creating a concentration gradient and lowering the effective rate.

📌 Therefore:
If the reaction is fast and pore diameter is small, the system becomes diffusion-limited, i.e., the rate of mass transfer can't keep up with the rate of reaction.

This leads to diffusional effects controlling the overall rate.

❌ Why other options are incorrect:
B. Large pores reduce diffusion resistance.

C. Slow reactions don't demand fast diffusion, so diffusion isn't rate-limiting.

D. Both slow reaction and large pores mean minimal diffusion impact.

✅ Final Answer: A. Fast reactions in catalyst of small pore diameter

C

answer is B