A solid ionic compound dissolves in water according to the overall process XY(s)…

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

Problem

A solid ionic compound

XY(s)

dissolves in water according to the overall process

XY(s) \rightarrow X^+(aq) + Y^-(aq)

The dissolution can be viewed as the sum of two conceptual steps whose enthalpy changes are given below. • Lattice disruption:

XY(s) \rightarrow X^+(g) + Y^-(g)

\Delta H_1 = +760\,\text{kJ\,mol}^{-1}

• Hydration of the gaseous ions:

X^+(g) + Y^-(g) \rightarrow X^+(aq) + Y^-(aq)

\Delta H_2 = -800\,\text{kJ\,mol}^{-1}

(a) Predict the sign of

\Delta H^\circ

for the overall dissolution of

XY(s)

. Support your answer using the data provided. (b) A student dissolves a small amount of

XY(s)

in a coffee-cup calorimeter containing water at

25\,^\circ\text{C}

. Will the temperature of the solution increase, decrease, or remain the same? Explain. (c) Rank

|\Delta H_1|

and

|\Delta H_2|

from greater to smaller magnitude and discuss how that ranking governs the temperature change you predicted in part (b).