re: HSC Chemistry Marathon Archive
Oh, ok cool.
Explain the differences and similarities in their solvent behaviour in terms of their molecular structures. Include a diagram in your answer. 4 MARKS
Diagram: Draw a molecule of water, clearly showing its bent structure and that it has a polar oxygen (slightly negative), and two polar hydrogens (slightly positive). For sophistication, suppose NaCl is dissolved in the water, then draw Na+ ions attracted to the polar oxygen, and Cl- ions attracted to the polar hydrogen to show dissolution.
Then, draw a molecule of ethanol, clearly showing the nature of the hydroxyl group as polar. Draw it like -O-H to show the slightly negative oxygen and the slightly positive hydrogen. Also, state that the hydrocarbon chain is neutral. For sophistication, suppose the ethanol was dissolved into the water. Then, draw the -OH interacting with the polar O in water. Suppose the ethanol dissolved an alkane. Draw the alkane beneath it (or wherever, depending on orientation of diagram) and show dispersion forces between molecules (zigzag lines I think are ok here).
--------
(My answer would be shrunk in an exam due to writing space)
Although water is typically classified as the 'universal solvent', comparison of their molecular structure shows how water and ethanol can dissolve similar substances, whilst ethanol can dissolve others. As a rule of thumb, like substances dissolve like substances. In this situation, water, having its bent molecular structure results in formation of permanent dipoles, whereby the oxygen is slightly negative and the hydrogens are slightly positive. These will be involved with dipole-dipole interactions and possibly hydrogen bonding. These permanent dipoles can interact with other polar molecules, or charged species, as shown in the diagram with the interaction of NaCl.
In a similar way, ethanol, containing the polar hydroxyl group can dissolve many polar substances in a similar manner. It also allows ethanol to interact with water, as clearly the slightly positive H can interact with the slightly negative O in water. The difference is, however, present in the fact that ethanol also has the carbon chain structure typical to alkanes and alkenes. When ethanol meets alkanes or alkenes, temporary dipoles can be induced due to the identical molecular structures of the substances, which allow forming of dispersion forces. These dispersion forces mean that the resulting difference is that ethanol can also dissolve many hydrocarbon such as 1-hexene.
___________
The actual correct terminology is that it is miscible. Can someone explain what miscible means?
