Two separate experiments are carried out in identical

CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics

Problem

Two separate experiments are carried out in identical, well-insulated coffee-cup calorimeters each containing

250\,\text{g}

of water initially at

22\,^{\circ}\text{C}

. In Experiment 1,

0.80\,\text{mol}

of a reaction with

\Delta H = -90\,\text{kJ mol}^{-1}

occurs. In Experiment 2,

0.40\,\text{mol}

of a different reaction with

\Delta H = -180\,\text{kJ mol}^{-1}

occurs. Which of the following best explains which experiment will produce the larger temperature increase of the water? (Assume no heat is lost to the calorimeter itself.) (a) Experiment 1, because it involves a larger number of moles, resulting in more heat released to the water. (b) Experiment 2, because its enthalpy change per mole is more negative, so each mole releases more heat. (c) Neither; both experiments raise the water’s temperature by the same amount, because each transfers the same total heat (

72\text{ kJ}

) to the same mass of water. (d) Experiment 2, because a negative

\Delta H

means the system’s

q

is positive, leading to a greater temperature rise.

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