Question 13.3: Using Intermolecular Forces to Predict Solution Formation Pr...

Using Intermolecular Forces to Predict Solution Formation

Predict whether or not a solution will form in each of the following mixtures and whether the solution is likely to be ideal: (a) ethyl alcohol, CH_3CH_2OH, and water; (b) the hydrocarbons hexane, CH_3{(CH_2)}_4CH_3, and octane, CH_3{(CH_2)}_6CH_3; (c) octanol, CH_3{(CH_2)}_6CH_2OH, and water.

Analyze
Keep in mind that ideal or nearly ideal solutions are not too common. They require the solvent and solute(s) to be quite similar in structure.

(a) If we think of water as \begin{matrix} H-OH \end{matrix} , ethyl alcohol is similar to water. (Just substitute the group \begin{matrix} CH_3CH_2- \end{matrix} for one of the H atoms in water.) Both molecules meet the requirements of hydrogen bonding as an important intermolecular force. The strengths of the hydrogen bonds between like molecules and between unlike molecules are likely to differ, however.

(b) In hexane, the carbon chain is six atoms long, and in octane it is eight. Both substances are virtually nonpolar, and intermolecular attractive forces (of the dispersion type) should be quite similar both in the pure liquids and in the solution.

(c) At first sight, this case may seem similar to (a), with the substitution of a hydrocarbon group for a H atom in \begin{matrix} H-OH \end{matrix}. Here, however, the carbon chain is eight members long. This long carbon chain is much more important than the terminal \begin{matrix} -OH \end{matrix} group in establishing the physical properties of octanol. Viewed from this perspective, octanol and water are quite dissimilar.