Abstract
Heats of mixing have been measured at 25�,
35�, and 45� for mixtures of ethanol, propan-1-ol, butan-1-ol, hexan-1-ol, and
octan-1-ol with n-hexane and for methanol with n-hexane at 45�.
These heats of mixing were found to
increase with an increase in temperature and to decrease with an increase in
the size of the alcohol molecule. They showed anomalies for the systems
containing methanol and ethanol. An explanation of these anomalies has been
given.
The contribution of the hydroxyl groups to
the enthalpy of any mixture of a n-alcohol with a n-alkane is defined as the
enthalpy change on replacing the alcohol by its homomorphic alkane. The
experimental results given here are consistent with the postulate that the
contribution of the hydroxyl groups (per mole of alcohol) to the enthalpy
depends only on the ratio of the number of hydroxyl groups to that of
hydrocarbon units in the mixture. This postulate, if generally true, can be
used for the prediction of the heats of mixing of any n-alcohol with a n-alkane
and of the enthalpy difference between any n-alcohol and its liquid homomorphic
alkane.
Other data support the conclusion that the
postulate holds for all concentrations of alcohol, but show that the analogous
postulate applied to free energies holds only for infinitely dilute solutions
of alcohols. The interaction of the hydroxyl groups with the hydrocarbon units
contribute -7.8 and -4.5 kJ/mole of alcohol to the enthalpies and free energies
respectively of these infinitely dilute solutions.
The required values of the heats of mixing
of the lower alkanes with n-hexane were estimated from the enthalpies of the
alkanes using the principle of congruence; these were found to be appreciable
and negative. They are in sign agreement with values extrapolated, from
measured data at a higher temperature, using a principle of corresponding
states for chain molecules.
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