Explain why pentane has a significantly higher vapor pressure than 1-butanol?
Pentane doesn't have hydrogen bonding as does 1-butanol.
Pentane has a significantly higher vapor pressure than 1-butanol due to differences in their intermolecular forces.
Vapor pressure is a measure of a substance's tendency to evaporate and enter the gas phase. It depends on the strength of the forces holding the molecules together in the liquid phase. In general, the higher the vapor pressure, the more volatile a substance is.
Pentane is an alkane with the molecular formula C5H12, consisting of only carbon and hydrogen atoms. It is a nonpolar molecule, meaning it lacks a permanent dipole moment. The intermolecular forces in pentane are London dispersion forces, which are weak temporary attractions between the electron clouds of adjacent molecules.
On the other hand, 1-butanol is an alcohol with the molecular formula C4H9OH. It contains both polar and nonpolar moieties. The –OH group in 1-butanol introduces polarity to the molecule, resulting in stronger intermolecular forces. In addition to London dispersion forces, 1-butanol also experiences hydrogen bonding between the –OH group of one molecule and the oxygen atom of another molecule.
Hydrogen bonding is a strong intermolecular force that arises when hydrogen atom bonded to a highly electronegative atom (in this case, oxygen) interacts with another electronegative atom. It leads to more significant attractions between molecules in 1-butanol compared to pentane, resulting in a higher boiling point, stronger intermolecular forces, and lower vapor pressure. Hydrogen bonding enhances the forces of attraction and makes it more difficult for the individual molecules to escape into the gas phase.
To quantitatively compare the vapor pressures of pentane and 1-butanol, we can examine their boiling points. The boiling point of pentane is approximately 36°C, while 1-butanol has a boiling point around 118°C. The higher boiling point of 1-butanol indicates stronger intermolecular forces and a lower vapor pressure compared to pentane.
Therefore, the difference in vapor pressure between pentane and 1-butanol can be attributed to the contrast in their intermolecular forces. Pentane's nonpolar nature and weaker intermolecular forces (London dispersion forces) allow more molecules to escape into the gas phase, resulting in a higher vapor pressure. In contrast, 1-butanol's polar nature and stronger intermolecular forces (London dispersion forces as well as hydrogen bonding) restrict the escape of molecules, resulting in a lower vapor pressure.