Rank the following in order of increasing vapor pressure at 298 k: CH3CH2OH, CH3OCH3, CH3CH2CH3.
To rank the substances in order of increasing vapor pressure at 298 K, we can use the following reasoning:
Vapor pressure is a measure of the tendency of a substance to evaporate or escape into the gas phase. It depends on the intermolecular forces between the molecules of the substance. Generally, weaker intermolecular forces result in higher vapor pressure.
Let's analyze the intermolecular forces present in each substance:
1. CH3CH2OH (ethanol): It forms hydrogen bonds due to the presence of an -OH group. Hydrogen bonding is a strong intermolecular force that requires more energy to break, resulting in lower vapor pressure.
2. CH3OCH3 (dimethyl ether): It does not have a hydrogen atom directly bonded to an oxygen atom, so it cannot form hydrogen bonds. Instead, it experiences weaker dipole-dipole interactions called London dispersion forces. These are weaker than hydrogen bonds, resulting in a higher vapor pressure compared to ethanol.
3. CH3CH2CH3 (propane): As a hydrocarbon, it only experiences London dispersion forces. Since it does not have any polar groups, it has the weakest intermolecular forces of the three substances and, therefore, the highest vapor pressure.
Based on this analysis, the ranking of the substances in increasing order of vapor pressure at 298 K is as follows:
CH3CH2OH (ethanol) < CH3OCH3 (dimethyl ether) < CH3CH2CH3 (propane)
To rank these compounds in increasing order of vapor pressure at 298 K, we need to consider the intermolecular forces present in each compound. Vapor pressure is determined by the tendency of molecules to escape from the liquid and enter the gas phase, which is influenced by the strength of intermolecular forces.
Generally, weaker intermolecular forces result in higher vapor pressures. There are three common types of intermolecular forces: London dispersion forces, dipole-dipole forces, and hydrogen bonding.
Let's consider each compound in turn and determine the intermolecular forces present:
1. CH3CH2OH (ethyl alcohol): This compound has hydrogen bonding, as the hydrogen of the hydroxyl (-OH) group can form a hydrogen bond with an electron-donating atom, such as oxygen or nitrogen. Hydrogen bonding is a relatively strong intermolecular force.
2. CH3OCH3 (dimethyl ether): This compound has dipole-dipole forces. The oxygen atom is more electronegative than carbon, resulting in a partial negative charge on the oxygen and partial positive charges on the carbon atoms. Dipole-dipole forces are weaker than hydrogen bonding but stronger than London dispersion forces.
3. CH3CH2CH3 (propane): This compound only has London dispersion forces. These forces arise from temporary fluctuations of electron density that cause temporary dipoles. London dispersion forces are the weakest type of intermolecular force.
Based on the above analysis, we can conclude that:
CH3CH2CH3 (propane) < CH3OCH3 (dimethyl ether) < CH3CH2OH (ethyl alcohol)
Therefore, the ranking in increasing order of vapor pressure at 298 K is:
1. CH3CH2CH3 (propane)
2. CH3OCH3 (dimethyl ether)
3. CH3CH2OH (ethyl alcohol)