Ammonium nitrate,

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Ammonium nitrate,

\mathrm{NH_4NO_3}

, dissolves endothermically in water according to the process

\mathrm{NH_4NO_3(s)\;\longrightarrow\; NH_4^{+}(aq)+NO_3^{-}(aq)}

with an enthalpy change of

\Delta H=+25.7\,\text{kJ\,mol}^{-1}

for the dissolution of one mole of solid. A laboratory calorimeter shows that, during one trial, the solution absorbed

154\,\text{kJ}

of thermal energy from the surroundings. (a) Describe, in terms of energy flow and sign convention, what the positive value of

\Delta H

signifies for this process. (b) Without performing a formal calculation, explain qualitatively how the numerical value of

154\,\text{kJ}

could be used to estimate the amount of

\mathrm{NH_4NO_3}

that dissolved. In your explanation, refer to the relationship between the heat absorbed and the tabulated enthalpy change. (c) If a second trial absorbed exactly twice as much heat as the first, explain how the amount of

\mathrm{NH_4NO_3}

required would compare to the first trial and why.

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