Page 159 - Physical chemistry understanding our chemical world

126 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

The cause of the cooling sensation is the unusually positive enthalpy of solution.
Most values of H O are positive, particularly for simple inorganic solutes.
(solution)
Pure IV is a solid at s.t.p. Dissolving IV in the mouth disrupts its molecular
structure, especially the breaking of the hydrogen bonds associated with the hydroxyl
group. These bonds break concurrently with new hydrogen bonds forming with the
water of the saliva. We require energy to break the existing bonds, and liberate energy
as new bonds form. Energetically, dissolving (−)-menthol is seen to be endothermic,
meaning we require energy. This energy comes from the mouth and, as we saw earlier,
the macroscopic manifestation of a lower microscopic energy is a lower temperature.
Our mouth feels cold.
The other substance sometimes added to foodstuffs to cause cooling of the mouth
is xylitol (V). It is added as a solid to some sweets, chewing gum, toothpastes and
mouth-wash solutions.

CH CH
CH 2 CH CH 2
OH OH
OH OH OH
(V)

Measuring values of H O
(solution)

It is quite difficult to measure an accurate enthalpy of solution H O with a
(solution)
calorimeter, but we can measure it indirectly. Consider the example of sodium chlo-
ride, NaCl. The ions in solid NaCl are held together in a tight array by strong ionic
bonds. While dissolving in water, the ionic bonds holding the constituent ions of
−
+
Na and Cl in place break, and new bonds form between the ions and molecules
of water to yield hydrated species. Most simple ions are surrounded with six water
+
molecules, like the [Na(H 2 O) 6 ] ion (VI). Exceptions include the proton with four
water molecules (see p. 235) and lanthanide ions with eight.
+
OH 2
H 2 O OH 2
Na
H 2 O OH 2
OH 2
(VI)
The positive charge
does not reside on
the central sodium Each hydration bond is partially ionic and partially covalent.
alone. Some charge Each oxygen atom (from the water molecules) donates a small
is distributed over the amount of charge to the central sodium; hence the ionicity. The
whole ion. orbitals also overlap to impart covalency to the bond.