A student studies the gas-phase decomposition of X₂ at two temperatures
CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics
Problem
A student studies the gas-phase decomposition of X₂ at two temperatures. At 25 °C the initial rate is 0.020 mol L⁻¹ s⁻¹. The student estimates that the increase in average molecular speed from 25 °C to 35 °C should raise the collision frequency by only about 2 %. In a second trial at 35 °C, however, the measured initial rate is 0.040 mol L⁻¹ s⁻¹. Which statement best accounts for the observed rate doubling?
(a) Heating a closed vessel by 10 °C nearly doubles the gas pressure, so the effective concentration of X2 rises enough to double the rate.
(b) A common rule of thumb states that most reaction rates double for every 10 °C rise in temperature, so the result is expected without considering activation energy.
(c) A higher temperature greatly increases the fraction of collisions with sufficient energy to react, far beyond the small increase in collision frequency.
(d) Because activation energy drops roughly in half for each 1 °C increase, a 10 °C rise produces a very large increase in the rate constant, explaining the doubling.