A two-step reaction has the following activation energies and enthalpy changes: •…
CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics
Problem
A two-step reaction has the following activation energies and enthalpy changes:
• Step 1: Eₐ = 120 kJ, ΔH₁ = –10 kJ
• Step 2: Eₐ = 45 kJ, ΔH₂ = –30 kJ
A catalyst lowers the activation energy of only the first step to 60 kJ and does not affect any other energy values. Which statement is most consistent with the modified energy diagram and the resulting kinetics?
(a) Both peaks are lowered equally, so the second step now controls the rate because its rate constant increases the most.
(b) The first peak is lowered, causing the overall rate to increase and the overall ΔH to become more negative.
(c) Only the first peak is lowered, so the overall rate increases while the overall ΔH remains unchanged.
(d) Only the first peak is lowered, so the overall rate decreases because the second step now controls the rate while the overall ΔH remains unchanged.