How do intermolecular forces determine the vapor pressure of a liquid?
A. A nonpolar compound has a low vapor pressure because of London dispersion forces.
B. A compound that can form hydrogen bonds has a low vapor pressure.
C. A polar compound has a high vapor pressure because of London dispersion forces.
D. A compound that has strong covalent bonds has a low vapor pressure.
Water forms H bonds and has a low vapor pressure. How does that fit with B?
It's B
Intermolecular forces play a crucial role in determining the vapor pressure of a liquid. Among the given options, the correct statement is:
A. A nonpolar compound has a low vapor pressure because of London dispersion forces.
London dispersion forces occur between all molecules, regardless of their polarity. However, their magnitude depends on the size and shape of the molecules involved. In nonpolar compounds, such as hydrocarbons, the only intermolecular force present is the London dispersion force. These forces are relatively weak compared to other intermolecular forces and therefore, nonpolar compounds tend to have lower vapor pressures.
To determine the vapor pressure of a liquid, we need to consider the intermolecular forces present in the substance.
The vapor pressure of a liquid is the pressure exerted by its vapor when in equilibrium with the liquid phase at a given temperature. Intermolecular forces play a crucial role in determining the vapor pressure because they affect the tendency of liquid particles to escape from the surface and form a vapor.
A. A nonpolar compound has a low vapor pressure because of London dispersion forces.
Nonpolar compounds, such as noble gases and hydrocarbons, have only London dispersion forces (also known as Van der Waals forces) as their intermolecular forces. London dispersion forces arise due to temporary fluctuations in the electron cloud, resulting in temporary dipoles. These forces are relatively weak compared to other intermolecular forces.
Since nonpolar compounds have weak intermolecular forces, it is easier for their molecules to overcome these forces and escape into the vapor phase. Hence, nonpolar compounds tend to have relatively higher vapor pressures compared to compounds with stronger intermolecular forces.
Therefore, option A is incorrect because nonpolar compounds have higher vapor pressures due to their weaker intermolecular forces.
B. A compound that can form hydrogen bonds has a low vapor pressure.
Hydrogen bonding is a strong type of intermolecular force that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. These bonds are much stronger than London dispersion forces.
When a compound can form hydrogen bonds, the intermolecular forces holding its molecules together are stronger than in nonpolar compounds. As a result, more energy is required to break these bonds and convert the liquid into a vapor.
Therefore, option B is incorrect because compounds that can form hydrogen bonds tend to have higher vapor pressures due to their stronger intermolecular forces.
C. A polar compound has a high vapor pressure because of London dispersion forces.
Polar compounds have permanent dipoles due to the unequal sharing of electrons between atoms. These dipoles can induce partial charges in neighboring molecules, leading to London dispersion forces in addition to other intermolecular forces like dipole-dipole interactions.
The presence of polar intermolecular forces affects the vapor pressure of polar compounds. While these forces are generally stronger than London dispersion forces in polar compounds, they still contribute to vapor pressure.
However, it is important to note that the strength of the intermolecular forces and the associated vapor pressure depend on factors such as molecular size, shape, and temperature. Generally, polar compounds tend to have higher vapor pressures than nonpolar compounds at comparable conditions.
Therefore, option C is incorrect because polar compounds usually have higher vapor pressures, but this is not solely due to London dispersion forces present in them.
D. A compound that has strong covalent bonds has a low vapor pressure.
The strength of covalent bonds within a molecule is not directly related to the vapor pressure of a substance. The vapor pressure primarily depends on intermolecular forces between molecules rather than intramolecular forces within a molecule.
Intramolecular forces like covalent bonds hold the atoms within a molecule together, while intermolecular forces determine the interaction between different molecules.
Therefore, option D is incorrect because the strength of covalent bonds within a compound does not directly affect its vapor pressure.
In conclusion, the correct answer is none of the given options. The vapor pressure of a liquid is determined by the nature and strength of its intermolecular forces, with nonpolar compounds generally having higher vapor pressures and polar compounds having relatively lower vapor pressures. However, the presence of hydrogen bonding can increase the vapor pressure of a compound.