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Chapter 13: Problem 78

An important test for the purity of an organic compound is to measure its melting point. Usually, if the compound is not pure, it begins to melt at a lower temperature than the pure compound.(a) Why is this the case, rather than the melting point being higher in some cases and lower in others?(b) Are there any conditions under which the melting point of the impure compound is higher than that of the pure compound? Explain.

Short Answer

Expert verified
Impure organic compounds typically melt at a lower temperature because impurities disrupt regular intermolecular forces. Rarely, if impurities interact with the compound in a way that strengthens these forces, the melting point might be higher.

Step by step solution

01

Answering Question A

Impurities in an organic compound disrupt the uniformity of the forces between the molecules of that compound. A pure organic compound melts at a specific temperature because that's when the intermolecular forces of the compound are overcome and the solid starts to turn into liquid. When there are impurities, these may disrupt the forces between the molecules, causing the compound to start melting at a lower temperature because the intermolecular forces between the molecules are not as strong as they would be in a pure compound.
02

Answering Question B

In general, impurities lower the melting point of a compound as they disrupt the regular structure of the compound. An exception to this general rule might occur if the impurity interacts with the compound in a way that actually strengthens the intermolecular forces. However, such situations are rare with organic compounds. In fact, it is more common for the opposite to occur with the melting point being lowered due to impurity.

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