Arrhenius behavior: The rate constant of a chemical reaction will decrease under which change in operating conditions?

Introduction / Context:
Reaction rate constants capture intrinsic kinetics at given temperature and catalyst state. They are distinct from observed rates, which also depend on concentrations and transport phenomena. Knowing what affects k directly is essential for reactor troubleshooting and control.

Given Data / Assumptions:

• Arrhenius form: k = A * exp(-Ea / (RT)).
• Focus on changes that alter k itself, not just the observed rate r = k * f(concentrations).

Concept / Approach:
Temperature is the dominant variable in the Arrhenius expression. Lowering T reduces the exponential factor, thus decreasing k. Other listed variables (pressure for ideal gas at constant T, concentration, time, or surface area) may change the rate r, but do not change the intrinsic rate constant k defined at the specified T.

Step-by-Step Solution:
Start from k = A * exp(-Ea/(RT)).Reduce T → the exponent becomes more negative → k decreases.Therefore, a decrease in temperature directly decreases the rate constant.

Verification / Alternative check:
Arrhenius plots (ln k vs. 1/T) show linear behavior; moving to higher 1/T (lower T) reduces ln k and hence k.

Why Other Options Are Wrong:

• Pressure/concentration/time/catalyst area: these change the observed reaction rate or equilibrium but do not redefine k at that temperature.

Common Pitfalls:
Conflating rate constant with rate; lower concentration reduces rate but not the value of k.

Final Answer:
Decrease in temperature