A chemist determines that the direct conversion of solid to solid releases
CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics
Problem
A chemist determines that the direct conversion of solid A to solid C releases 200kJmol−1. In a separate experiment, A first converts to an intermediate solid B, releasing 120kJmol−1, and B then converts to C, releasing 80kJmol−1. Which of the following best accounts for the fact that both experimental routes yield the same total enthalpy change?
(a) Because enthalpy is a state function only when the process occurs at constant pressure, and both experiments were run at 1atm, the totals must match.
(b) The measured heat released in the direct route is underestimated owing to calorimetric heat loss, and the extra step in the two-step pathway exactly compensates for this loss.
(c) The overall enthalpy change depends only on the initial and final states, so alternative reaction pathways sum to the same value.
(d) Any intermediate such as B has zero enthalpy of formation by definition, so it contributes nothing to the net heat released.