Two separate beakers each contain 100 mL of water at 25 °C

CHEMISTRY · Single correct · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties

Problem

Two separate beakers each contain 100 mL of water at 25 °C. After 5.0 g of CaCl₂(s) is added to the first beaker, the temperature rises to 32 °C. After 5.0 g of NH₄NO₃(s) is added to the second beaker, the temperature falls to 19 °C. Which statement best explains the opposite temperature changes? (a) The lattice energy of

CaCl_{2}

is greater than its hydration enthalpy, so the excess energy is released as heat, whereas the lattice energy of

NH_{4}NO_{3}

is smaller than its hydration enthalpy, so energy must be absorbed for it to dissolve. (b) Because 5.0\,g of

CaCl_{2}

represents more moles than 5.0\,g of

NH_{4}NO_{3}

, the dissolution of

CaCl_{2}

produces a larger temperature change, while the smaller number of moles of

NH_{4}NO_{3}

causes the temperature to fall. (c) The hydration of

Ca^{2+}

and

Cl^{-}

ions releases more energy than is required to separate the

CaCl_{2}

lattice, whereas the hydration of

NH_{4}^{+}

and

NO_{3}^{-}

ions releases less energy than is required to separate the

NH_{4}NO_{3}

lattice. (d) Both dissolutions are exothermic, but water evaporates from the

NH_{4}NO_{3}

beaker and that evaporation absorbs heat, leading to the observed cooling.

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