CHE 303; version 9/21/10
Hot Cocktails or Cold?
The heat of mixing of ethanol and water
Water is the most important molecule to life, and its unique properties affect virtually all
of the processes that go on in organisms and the environment.
One of the most basic chemical
operations occuring throughout the planet is having things dissolve in water.
solvation properties, stemming from its unusual combination of partial charges and a very open
"holey" condensed phase structure, create a driving force for much of the chemistry we see
There are various reasons why one substance dissolves in another.
Entropy is a major
driving force, since a mixture has a more random structure than pure components, but other
forces such as neutralization of charge and changes in volume also play a role.
thermodynamic language, if the free energy goes down upon mixing, a solution will be formed.
Since the free energy is a linear combination of entropy and enthalpy [
], if the
entropy change is large and positive, a solution can form for either positive or negative
enthalpies of solution.
Rather surprisingly, NaCl, which we think of as one of the most soluble
of substances, has a
enthalpy of solution and thus cools a solution when it dissolves.
[remember that an enthalpy is
if heat is given off in a reaction].
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The sign and magnitude of
depends upon the intricate details of the interactions
between the solute and solvent molecules in both their pure and mixed states.
If the interactions
in the pure substances are stronger than those in the solution (the case for NaCl), the mixing can
be driven by entropy, even though the reaction is endothermic. In water solutions, hydrogen
bonds are important, so molecules such as acetone have a strong propensity to go into solution
because strong hydrogen bonds are formed.
Normal hydrocarbons, however, do not form
hydrogen bonds and therefore do not dissolve ("oil and water don't mix").
Since alcohols are
polar on one end and oil-like on the other end, they can go either way.
One finds that long-chain
alcohols are not soluble, and that methanol has a fairly negative
The question then
arises--When you add water to your shot of rum, does it warm up or cool down? [the desirability
a particular outcome might vary whether you are on a Caribbean beach or in a ski lodge]
hydrocarbon part of ethanol fits neatly into a hole in the water structure, heat may be given off--
otherwise the solution might cool down.
Our object here is to determine and plot the enthalpy change
constant] over the complete range of possible compositions: