Page 160 - Physical chemistry understanding our chemical world

INDIRECT MEASUREMENT OF ENTHALPY 127

Energy is needed to break the ionic bonds in the solid salt and energy is liberated
forming hydration complexes like VI. We also break some of the natural hydrogen
bonds in the water. The overall change in enthalpy is termed the enthalpy of solu-
tion, H O . Typical values are −207 kJ mol −1 for nitric acid; 34 kJ mol −1 for
(solution)
potassium nitrate and −65.5kJ mol −1 for silver chloride.
One of the most sensitive ways of determining a value of H O is to measure
(solution)
the temperature T at which a salt dissolves completely as a function of its solubility
s. A plot of ln s (as y) against 1 ÷ T (as x) is usually linear. We obtain a value
of H O by multiplying the gradient of the graph by −R, where R is the gas
(solution)
constant (as described in Chapter 5, p. 210).

How does a camper’s ‘emergency heat stick’ work?

Enthalpies of Complexation

A camper is in great danger of exposure if alone on the moor or in ‘Exposure’ is a condi-
the desert when night falls and the weather becomes very cold. If tion of being exposed
a camper has no additional heating, and knows that exposure is not to the elements, lead-
far off, then he can employ an ‘emergency heat stick’. The stick ing to hypothermia,
is long and thin. One of its ends contains a vial of water and, at and can lead to death.
the other, a salt such as anhydrous copper sulphate, CuSO 4 .Both
compartments are housed within a thin-walled glass tube, itself encased in plastic.
Bending the stick breaks the glass, allowing the water to come into contact with the
copper sulphate and effect the following hydration reaction:

CuSO 4(s) + 5H 2 O (l) −−→ CuSO 4 · 5H 2 O (s) (3.36)

The reaction in Equation (3.36) is highly exothermic and releases 134 kJ mol −1 of
energy. The camper is kept warm by this heat. The reaction in Equation (3.36)
involves complexation. In this example, we could also call it ‘hydration’ or ‘adding
water of crystallization’. We will call the energy released the ‘energy of complexation’
H (complexation) .
Heat is liberated when adding water to anhydrous copper sulphate because a new
crystal lattice forms in response to strong, new bonds forming between the water and
Cu 2+ and SO 2− ions. As corroborative evidence of a change in the crystal structure,
4
note how ‘anhydrous’ copper sulphate is off-white but the pentahydrate is blue.