Intermolecular Forces and Physical Properties - Online Tutor, Practice Problems & Exam Prep

Intermolecular forces directly influence physical properties such as boiling point, melting point, surface tension, and viscosity. Stronger intermolecular forces lead to higher boiling and melting points, increased surface tension, and greater viscosity, which is the resistance to flow. Conversely, vapor pressure is indirectly related; stronger intermolecular forces result in lower vapor pressure. Understanding these relationships is crucial for grasping the behavior of substances in different states and conditions.

Intermolecular Forces and Physical Properties Concept 1

In this video, we're going to take a look at the direct relationship between the intermolecular forces and physical properties. Now recall that physical properties are measurable and observed through the senses that describe the states of matter. We're going to say the intermolecular forces themselves are just the attractive forces that exist between molecules and influence these physical properties. Now when we say direct relationships, realize that under direct relationships, we're going to say the stronger the intermolecular force, then the greater the physical property. And the 4 physical properties that we will observe are boiling point, melting point, surface tension, and viscosity.

For the first one, we deal with boiling point. Now boiling point is just the temperature where a liquid and a gas are in equilibrium with each other. The liquid can get vaporized into a gas, and the gas can condense down into a liquid. So boiling point is just the equilibrium between these two states. We're going to say the stronger the intermolecular force, then the greater the boiling point.

Next, we have the melting point. Melting point is the temperature where a solid and a liquid are in equilibrium. Going from a solid to a liquid involves melting or fusion. Going from a liquid to a solid is freezing. The greater the intermolecular force, then the higher the melting point.

Next, we have surface tension. Surface tension is just a measure of attractive forces on a liquid surface. So, just imagine that below the surface itself, we have water molecules. These water molecules are hydrogen bonding to one another. This causes a strong attraction between them. So much in fact that there's a cohesive force that results on the top of the water. This kind of creates a false floor for certain insects to be able to walk across the water. Okay. So this is the phenomenon that insects or insects take advantage of in order to cross over bodies of water.

And then finally, we have viscosity. Viscosity may not be as well-known as the other 3. We're going to say at constant temperature, viscosity is just the resistance to flow for a substance. So you might have heard a substance being very viscous. That means it moves very slowly. Thinking of something very viscous, honey. Honey doesn't move very quickly. It's a viscous substance. Water, on the other hand, it moves pretty quickly, very easily, so water has low viscosity. Now, we're going to say here the greater your viscosity, the slower the movement. Okay. So it deals with time.

How could I reduce the viscosity of a substance? I can introduce heat. So let's say that I heat up some of this honey. If I heat it enough, I can help make it flow a lot faster. So we can say here that if I increase my temperature, I could help to decrease my viscosity. And we're going to say just like the other 3 before it, the stronger the intermolecular force, the greater the viscosity.

Intermolecular Forces and Physical Properties Example 1

Which of the following compounds would have the highest melting point? Remember, we're establishing a direct relationship between physical properties and the intermolecular forces. The highest melting point corresponds to the strongest intermolecular force. So, if we take a look at these options:

• For C2H5OH, we have hydrogen connected to an oxygen, which would be hydrogen bonding.
• For calcium sulfide in water, we have an ionic compound in a polar solvent, which would be ion-dipole.
• For the next option, it's hard to just see, but if we were to draw this out, we'd have a carbon in the center connected to two hydrogens and two bromines. This would not be a perfect shape because all the surrounding elements are not the same, making it a polar covalent compound and therefore, dipole-dipole.
• Finally, we have a hydrocarbon, something composed of only carbons and hydrogens. This is non-polar and therefore exhibits London dispersion forces.

Since the strongest intermolecular force translates to the highest melting point, the strongest intermolecular force here is ion-dipole. Therefore, option B is the correct answer.

Intermolecular Forces and Physical Properties Concept 2

Now it's time to relate the indirect relationships that we have between intermolecular forces in a particular physical property. So we're going to say in these indirect relationships, the stronger the intermolecular force, then the weaker the physical property. The physical property in question is vapor pressure. Now, vapor pressure is just the pressure exerted by a gas at the surface of a liquid. Realize that vapor pressure is the result of an equilibrium between condensation and vaporization. The gases on top basically push down on the liquid, creating vapor pressure. We're going to say here the stronger the intermolecular force, the weaker the vapor pressure, or the lower the vapor pressure will be for a substance. They are basically indirectly related or opposites of one another; one is high, the other one is low. A stronger intermolecular force translates to a lower or weaker vapor pressure.

Intermolecular Forces and Physical Properties Example 2

Choose the substance with the highest vapor pressure. The highest vapor pressure translates to the weakest intermolecular forces. If we take a look here, we have silver perchlorate in methanol. This is an ionic compound within a substance that has hydrogen bonding, meaning it's polar. So this interaction would be ion-dipole.

Next, we have a nonmetal by itself: Krypton. Being a nonmetal by itself, it's nonpolar, resulting in London dispersion forces. For another substance, we have a hydrocarbon, which is nonpolar as well, so it would also be subjected to London dispersion forces.

Finally, we have hydrogen with sulfur. If you were to draw this out, you'd see sulfur with lone pairs, and remember, if your central element has lone pairs, it is not a perfect shape, and by default, it is polar. So we're dealing with a polar covalent compound, which means the intermolecular force here is dipole-dipole.

The highest vapor pressure will belong to the weakest intermolecular force, which would be London dispersion. Here, we're comparing Krypton to CH₄ (methane). To compare their strengths, we look at their overall mass. Krypton weighs approximately 83.80 grams. If you add up the one carbon and the four hydrogens of methane, it would be about 16 grams. Since Krypton weighs more, it's stronger in terms of London dispersion forces. But remember, we're looking for the weakest one. The weakest one would translate to the highest vapor pressure. Since methane weighs less, it is weaker and, therefore, would have the highest vapor pressure. Thus, the answer would be methane (option C).

Which of the following will have the lowest boiling point?

• A. CH₃–O–CH₃
• B. KI
• C. C₆H₅OH
• D. C₆H₁₄
• E. C₆₀

Which molecules would most likely cause a liquid to have the lowest viscosity?

• A. Large, polar molecules
• B. Small, nonpolar molecules
• C. Small, polar molecules
• D. Large, nonpolar molecules

Which of the following should have the highest surface tension at a given temperature?

• A. CH₄
• B. CF₄
• C. CCl₄
• D. CBr₄
• E. CI₄

Here’s what students ask on this topic:

What are intermolecular forces and how do they affect boiling point?

Intermolecular forces are the attractive forces that exist between molecules. They play a crucial role in determining the physical properties of substances. Boiling point is the temperature at which a liquid turns into a gas. The stronger the intermolecular forces, the higher the boiling point. This is because more energy is required to overcome these forces and convert the liquid into a gas. For example, water has strong hydrogen bonds, leading to a relatively high boiling point compared to other molecules of similar size.

How do intermolecular forces influence melting point?

Melting point is the temperature at which a solid turns into a liquid. Intermolecular forces significantly influence the melting point of a substance. Stronger intermolecular forces result in a higher melting point because more energy is needed to break these forces and transition the solid into a liquid state. For instance, ionic compounds typically have high melting points due to the strong electrostatic forces between ions.

What is surface tension and how is it related to intermolecular forces?

Surface tension is a measure of the cohesive forces acting on the surface of a liquid. It is caused by the attraction between molecules at the surface of the liquid. Stronger intermolecular forces result in higher surface tension. For example, water has high surface tension due to hydrogen bonding between water molecules, allowing certain insects to walk on water without sinking.

How does viscosity relate to intermolecular forces?

Viscosity is the resistance of a liquid to flow. It is directly related to the strength of intermolecular forces. Stronger intermolecular forces result in higher viscosity, meaning the liquid flows more slowly. For example, honey has a high viscosity due to strong intermolecular forces, whereas water has a low viscosity and flows easily. Heating a liquid can decrease its viscosity by weakening the intermolecular forces.

What is vapor pressure and how do intermolecular forces affect it?

Vapor pressure is the pressure exerted by a gas in equilibrium with its liquid at a given temperature. It is inversely related to the strength of intermolecular forces. Stronger intermolecular forces result in lower vapor pressure because fewer molecules can escape the liquid phase to become gas. For example, water has a lower vapor pressure compared to ethanol due to its stronger hydrogen bonds.