Boiling Points
Boiling Point Determination
Similar to the melting point, the boiling point is a physical property. If the sample is a pure compound, then the boiling…
Liquids are held together by weak interactions between their molecules. To transition to the gas phase, a molecule must gain enough energy from heat or other sources to overcome these interactions.
Let's consider a closed container of a pure liquid at room temperature. Some of the liquid phase molecules at the surface gain enough energy to evaporate, and some of that vapor loses energy and condenses back to liquid. When the rates of evaporation and condensation are the same, the liquid and vapor reach equilibrium. At this point, the space above the liquid is saturated with vapor, and there is no net change in the amount of liquid and vapor in the container. The pressure exerted by the vapor is called the vapor pressure.
Now, consider an open container of liquid. Here the vapor can escape, keeping the area above the liquid from being saturated. Thus, evaporation could continue until there is no liquid left. As the temperature of a liquid increases, so does the evaporation rate, and therefore, so does the vapor pressure. If enough heat is added, molecules deeper in the liquid start vaporizing, which we see as bubbles of vapor forming.
This is called boiling, and it starts when the vapor pressure of the compound is equal to the atmospheric pressure. The temperature at which this occurs is the boiling point. Since each pure substance has different types and strengths of intermolecular interactions, different substances have different boiling points.
Now, let's take a look at how intermolecular interactions impact the boiling point. Many intermolecular interactions involve areas with uneven electron density called dipoles. All molecules have brief fluctuations in their electron distribution, so even nonpolar molecules can temporarily have dipoles. In response, opposing dipoles form in nearby molecules.
The attractive forces between temporary and induced dipoles are called London dispersion forces, and they are the dominant interactions between nonpolar molecules like hydrocarbons. London dispersion forces are stronger between large molecules with many electrons because those molecules hold their electrons weakly, making it easier for neighboring molecules to affect their electron distribution.
For example, a high molecular weight unbranched alkane has a higher boiling point than a lower molecular weight unbranched alkane. Additionally, an unbranched alkane has a higher boiling point than a branched alkane with a comparable molecular weight due to its higher surface area.
Molecules with polar bonds have permanent dipoles, enabling regular dipole-dipole interactions between them. Dipole-dipole interactions are stronger than London dispersion forces, so it usually takes more thermal energy to boil polar liquids than nonpolar liquids. For example, the boiling point of one chloropentane, a polar molecule, is 108 °C. Hexane, which is similar in surface area but lacks a dipole, has a lower boiling point.
Molecules with both a hydrogen covalently bound to a more electronegative atom and an electron-withdrawing atom with a lone pair of electrons are capable of hydrogen bonding. Hydrogen bonds are stronger than dipole-dipole interactions and London dispersion forces, so it takes even more thermal energy to overcome them. For example, propane and ethanol have similar molecular weights and surface areas. But the boiling point of propane is much lower than ethanol's because ethanol molecules can form hydrogen bonds with each other, while propane cannot.
In this lab, you will use the capillary method to determine the boiling point of two organic solvents.
Liquids are held together by weak interactions between their molecules. To transition to the gas phase, a molecule must gain enough energy from heat or other sources to overcome these interactions.
Let's consider a closed container of a pure liquid at room temperature. Some of the liquid phase molecules at the surface gain enough energy to evaporate, and some of that vapor loses energy and condenses back to liquid. When the rates of evaporation and condensation are the same, the liquid and vapor reach equilibrium. At this point, the space above the liquid is saturated with vapor, and there is no net change in the amount of liquid and vapor in the container. The pressure exerted by the vapor is called the vapor pressure.
Now, consider an open container of liquid. Here the vapor can escape, keeping the area above the liquid from being saturated. Thus, evaporation could continue until there is no liquid left. As the temperature of a liquid increases, so does the evaporation rate, and therefore, so does the vapor pressure. If enough heat is added, molecules deeper in the liquid start vaporizing, which we see as bubbles of vapor forming.
This is called boiling, and it starts when the vapor pressure of the compound is equal to the atmospheric pressure. The temperature at which this occurs is the boiling point. Since each pure substance has different types and strengths of intermolecular interactions, different substances have different boiling points.
Now, let's take a look at how intermolecular interactions impact the boiling point. Many intermolecular interactions involve areas with uneven electron density called dipoles. All molecules have brief fluctuations in their electron distribution, so even nonpolar molecules can temporarily have dipoles. In response, opposing dipoles form in nearby molecules.
The attractive forces between temporary and induced dipoles are called London dispersion forces, and they are the dominant interactions between nonpolar molecules like hydrocarbons. London dispersion forces are stronger between large molecules with many electrons because those molecules hold their electrons weakly, making it easier for neighboring molecules to affect their electron distribution.
For example, a high molecular weight unbranched alkane has a higher boiling point than a lower molecular weight unbranched alkane. Additionally, an unbranched alkane has a higher boiling point than a branched alkane with a comparable molecular weight due to its higher surface area.
Molecules with polar bonds have permanent dipoles, enabling regular dipole-dipole interactions between them. Dipole-dipole interactions are stronger than London dispersion forces, so it usually takes more thermal energy to boil polar liquids than nonpolar liquids. For example, the boiling point of one chloropentane, a polar molecule, is 108 °C. Hexane, which is similar in surface area but lacks a dipole, has a lower boiling point.
Molecules with both a hydrogen covalently bound to a more electronegative atom and an electron-withdrawing atom with a lone pair of electrons are capable of hydrogen bonding. Hydrogen bonds are stronger than dipole-dipole interactions and London dispersion forces, so it takes even more thermal energy to overcome them. For example, propane and ethanol have similar molecular weights and surface areas. But the boiling point of propane is much lower than ethanol's because ethanol molecules can form hydrogen bonds with each other, while propane cannot.
In this lab, you will use the capillary method to determine the boiling point of two organic solvents.
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Q1: What is the relationship between vapor pressure and boiling point?
Boiling occurs when the vapor pressure of a liquid equals atmospheric pressure. As temperature increases, more molecules gain energy to escape the liquid phase, raising vapor pressure. When vapor pressure matches the surrounding atmospheric pressure, bubbles form throughout the liquid, marking the boiling point. This temperature varies by compound and atmospheric conditions.
Q2: How do intermolecular forces affect a compound's boiling point?
Stronger intermolecular forces require more thermal energy to overcome, resulting in higher boiling points. Hydrogen bonding is the strongest, followed by dipole-dipole interactions, then London dispersion forces. For example, ethanol boils at 78°C due to hydrogen bonding, while propane boils at -42°C because it lacks this interaction despite similar molecular weight.
Q3: Why do larger nonpolar molecules have higher boiling points than smaller ones?
Larger molecules with more electrons experience stronger London dispersion forces because their electrons are held more loosely, making them easier for neighboring molecules to polarize. High molecular weight unbranched alkanes have higher boiling points than lower molecular weight alkanes. Additionally, unbranched structures have greater surface area, enabling stronger intermolecular attractions.
Q4: What happens during evaporation and condensation in a closed container?
In a sealed container, liquid molecules at the surface gain energy and evaporate into the gas phase, while vapor molecules lose energy and condense back to liquid. When these rates become equal, the system reaches equilibrium, and the space above the liquid becomes saturated with vapor. The pressure exerted by this vapor is called vapor pressure, and no net change in liquid or gas amount occurs.
Q5: How does atmospheric pressure influence the boiling point of a liquid?
Boiling occurs when vapor pressure equals atmospheric pressure. At higher elevations where atmospheric pressure is lower, liquids boil at lower temperatures because less heat is needed to raise vapor pressure to match the reduced atmospheric pressure. Conversely, at sea level with higher atmospheric pressure, more heat is required to reach the boiling point.
Q6: What is the capillary method and how does it determine boiling point?
The capillary method uses an inverted empty glass capillary tube inserted into a pure liquid. As the liquid heats, vapor pressure increases, forcing air out and creating bubbles. When cooled, liquid enters the tube once vapor pressure equals atmospheric pressure inside it. The temperature at which liquid enters the capillary marks the boiling point of the compound.
Q7: Why does a polar molecule like chloropentane have a higher boiling point than nonpolar hexane?
Chloropentane has permanent dipoles enabling dipole-dipole interactions, which are stronger than the London dispersion forces in nonpolar hexane. Although both molecules have similar surface area, the stronger intermolecular forces in chloropentane require more thermal energy to overcome, resulting in a boiling point of 108°C compared to hexane's lower boiling point.