Answer:
First choice. The boiling points, surface tension, and viscosities all increase because the strength of dispersion forces increases.
Explanation:
Those properties, <em>boiling points, surface tension, and viscosities</em> are due to the attractive intermolecular forces of the substances.
<em>Ethanol, propanol, and n-butanol</em> (three alcohols) have these chemical formulae:
- <em>Ethatnol</em>: CH₃CH₂ OH
- <em>Propanol</em>: CH₃ CH₂ CH₂ OH
- <em>n-butanol</em>: CH₃CH₂ CH₂ CH₂ OH
So, the molecules of those substances are polar and exhibit all these types of intermolecular bonding forces:
- <u>Dipole-dipole interactions</u>: attractive forces between the positive end of one polar molecule and the negative end of another polar molecule;
- <u>London dispersion forces</u>: a temporary attractive force that results of temporary assymetry in the electron distribution around the nuclei. This occurs when the electrons in two adjacent atoms occupy positions that cause the atoms form temporary dipoles. These forces exist between any two molecules when they are very, very close to each other (almost touching).
- <u>Hydrogen bonding</u>: the strongest intermolecular force. It is a type of dipole-dipole attraction, which arises between an hydrogen atom covalently bonded to a very electronegative atom (in this case oxygen) and the very electronegative atom (oxygen) of a neighboor molecule.
What answer your question is that such <em>hydrogen bonding and dipole-dipole interactions</em> are similar for all the three alcohols, <em>ethanol, propanol, and n-butano</em>l, so they are not responsible for the difference in the mentioned properties, boiling points, surface tension, and viscosities. What is different is the <em>dispersion forces</em>: they increase as the size of the alcohols increase.
As the molecules contain more carbon atoms, lengthen, contain more electrons and, as consequence, the dispersion forces become greater (the magnitude of the temporary dipoles formed increase). This explains the validity of the first statement, that <em>the boiling points, surface tension, and viscosities, all increase because the strength fo the dispersion forces increase.</em>
