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SP8. Miscibility

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  • SP8. Why do some things mix while others don't?

    So far, we've been thinking about how one molecule could interact with another molecule of the same compound to form a liquid or a solid. What about two different compounds? It turns out that whether two different compounds interact with each other depends on the same kinds of interactions we have looked at so far.

    Two liquids that do not really mix well together, such as oil and water, are described as immiscible. Two liquids that appear to mix completely together are said to be miscible. Water and ethanol are one example of a pair of miscible liquids, because you can take any amount of ethanol and mix it with any amount of water and you will always end up with a clear, colorless liquid just like the ones you started with.

    Problem SP8.1.

    What kind of intermolecular attraction allows water and ethanol to mix together?

    Problem SP8.2.

    The following pairs of liquids are miscible. Identify the predominant type of intermolecular attraction between each pair.

    1. 2-propanone and ethyl ethanoate (ethyl acetate)
    2. pentane and octane
    3. dichloromethane and 2-butanone
    4. methanol and ethanoic acid

    Problem SP8.3.

    Explain, in terms of intermolecular forces, why water and octane are not miscible.

    Problem SP8.4.

    Explain, in terms of intermolecular forces, why ethanenitrile and hexane are not miscible.

    Sometimes, the attractions between molecules are a little more complicated. Two molecules may have different interactions on their own, but when placed together still manage to interact with each other. For example, dichloromethane and hexane mix together pretty well.

    How can that be? Dichloromethane has dipole interactions. Hexane has nothing more than London interactions. Ethanenitrile and hexane didn't mix for that very reason. But the problem wasn't whether those two molecules could interact with each other; they could. The problem was that ethanenitrile would not give up its dipole-dipole interactions for the small amount it could gain by mixing with hexane.

    In this case, the dipoles between dichloromethane are much smaller; they aren't held back from the hexane molecules as strongly. On the other hand, the interaction between the the hexane and dichloromethane is actually amplified a little bit. Whereas hexane molecules rely solely on weak, transient London interactions to cling to each other, dichloromethane has a permanent dipole. That permanent dipole is able to enhance the transient dipole in hexane.

    This is called a dipole / induced dipole interaction. The slight tug dichloromethane exerts on hexane's electrons actually helps the two different molecules interact more strongly.

    Problem SP8.5.

    Describe the intermolecular interaction between each of the following pairs.

    a) 2-propanone; benzene b) 2-propanol; dichloromethane c) ethanol; ethanoic acid d) octanol; methylbenzene

    Solubility is conceptually similar to miscibility. If you can mix a solid with a liquid and the solid particle gets smaller and smaller until it disappears, it must have dissolved in the liquid. At that point it may look like you are left only with the original liquid, but really the individual molecules of the solid are just distributed throughout the liquid instead of all sitting together on the bottom. You can see this when blue kool-aid powder dissolves in water to give blue kool-aid. The blue substance has not disappeared; it has become intimately mixed with the water. Just as you cannot see an individual "chunk" of water, you cannot see an individual "chunk" of the original solid anymore -- it has separated into individual units (molecules) that are too small to be seen.