The Model: Dipole-Dipole Forces A polar compound is one in which there is a sepa
ID: 977582 • Letter: T
Question
The Model: Dipole-Dipole Forces A polar compound is one in which there is a separation of charge within a molecule of the substance. Neighboring molecules in a bulk sample of a polar compound will arrange themselves in order to maximize attractive forces and to minimize repulsive forces between the molecules. (Of course, in a liquid there is still enough thermal energy so that the molecules are constantly moving past each other. So this is a "time-averaged" picture we're considering.) The net attractive forces between polar molecules are called dipole-dipole forces. Dipole-dipole forces are typically only about 1% as strong as a covalent bond. If intermolecular forces are stronger between neighboring molecules in a bulk sample, will it require more or less heat energy to separate molecules from each other? (Circle your choice.) Consider the molecules on the right. One of the compounds has a boiling point of 37 degree C and the other has a boiling point of 60 degree C. Which one has each boiling point? Explain how you made your decision.Explanation / Answer
For 4a, if intermolecular forces are strong within molecules, it will require more energy to break those intermolecular forces and to separate them.
For 4b, as we can see, molecule on the left has both chlorines in one side of the molecule, and hence, will leave one side of the molecule "weak". For the other molecule, both chlorines are one in each carbon, keeping the molecule together and generating stronger attraction between molecules. So we can say that molecule on the right (1,1 dichloroethene) has a lower bp, while molecule on the left (z-1,2-dichloroethene) has a greater bp.