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

Intermolecular forces significantly 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, indicating resistance to flow. Conversely, stronger intermolecular forces result in lower vapor pressure, as they create a balance between condensation and vaporization. Understanding these relationships is crucial for grasping the behavior of substances in different states of matter and their interactions.

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 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 the 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 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 take advantage of to cross over bodies of water.

Finally, we have viscosity. Viscosity may not be as 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 of a substance being very viscous. That means it moves very slowly. Thinking of something that's very viscous, honey. Honey doesn't move very quickly. It's a viscous substance. Water, on the other hand, moves pretty quickly, very easily, so water has a low viscosity.

Now, we're going to say here, the greater your viscosity, then the slower the movement. Okay. So it deals with time. Now, 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 up 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? So 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, we have C₂H₅OH. Here we have hydrogen connected to an oxygen, so this would be hydrogen bonding. For option B, we have calcium sulfide in water, so we have an ionic compound in a polar solvent. This would be ion-dipole. For the next one, it's hard to visualize, but if we were to draw this out, we'd have a carbon in the center, and it'd be connected to the 2 hydrogens and the 2 bromines. This would not be a perfect shape because all the surrounding elements are not the same. So, it will be a polar covalent compound and therefore dipole-dipole. And then finally, we have a hydrocarbon, something composed of only carbons and hydrogens. So this is nonpolar and therefore London dispersion. So earlier we said the strongest intermolecular force would translate to the highest melting point. So here the strongest intermolecular force is ion-dipole, which would mean that 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 and a particular physical property. So, we're going to say under 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. And realize that vapor pressure is the result of an equilibrium between condensation and vaporization. So the gases on top basically push down on the liquid, creating a vapor pressure. And we're going to say here the stronger the intermolecular force, the weaker or lower the vapor pressure will be for a substance. So 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 force. If we take a look here, we have silver perchlorate in methanol. This is an ionic compound within a substance that has hydrogen bonding, which means it's polar. Therefore, this would be ion-dipole interaction. Next, we have a non-metal by itself, Krypton, which is non-polar and would have London dispersion forces. The next substance is a hydrocarbon, also non-polar, so it too would have London dispersion forces. Finally, we have hydrogen with sulfur. If you were to draw this out, you'd have sulfur with lone pairs, indicating that if your central element has lone pairs, the molecule is not a perfect shape, and by default, it is polar. This would be categorized as a dipole-dipole interaction.

The highest vapor pressure will belong to the substance with the weakest intermolecular force, which would be London dispersion forces. Here we are comparing Krypton to CH4 (methane). To compare their strengths, we consider their overall mass. Krypton weighs approximately 83.80 grams, and if you added up the mass of one carbon and four hydrogens in methane, it would be about 16 grams. Since Krypton weighs more, its London dispersion forces are stronger. However, we are looking for the weakest force, as it correlates 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 significantly influence the boiling point of a substance. The boiling point is the temperature at which a liquid and its vapor are in equilibrium. Stronger intermolecular forces require more energy to break, leading to a higher boiling point. For example, water has strong hydrogen bonds, resulting in a higher boiling point compared to other molecules of similar size. Understanding these forces helps explain why different substances boil at different temperatures.

How do intermolecular forces influence melting point?

Intermolecular forces play a crucial role in determining the melting point of a substance. The melting point is the temperature at which a solid and a liquid are in equilibrium. Stronger intermolecular forces require more energy to overcome, resulting in a higher melting point. For instance, ionic compounds, which have strong electrostatic forces, generally have higher melting points compared to molecular compounds with weaker van der Waals forces. This relationship helps explain the varying melting points of different substances.

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 directly related to the strength of intermolecular forces. Stronger intermolecular forces, such as hydrogen bonds in water, result in higher surface tension. This is why water forms droplets and allows certain insects to walk on its surface. Surface tension is crucial in various applications, including the behavior of liquids in capillary action and the formation of bubbles.

How does viscosity relate to intermolecular forces?

Viscosity is the measure of a fluid's resistance to flow. It is directly influenced by the strength of intermolecular forces. Stronger intermolecular forces result in higher viscosity, meaning the fluid flows more slowly. For example, honey has strong intermolecular forces, making it highly viscous, while water has weaker forces and flows easily. Temperature also affects viscosity; heating a fluid can decrease its viscosity by weakening intermolecular forces.

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

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. It is inversely related to the strength of intermolecular forces. Stronger intermolecular forces result in lower vapor pressure because fewer molecules can escape into the vapor phase. For instance, water has a lower vapor pressure compared to ethanol due to its stronger hydrogen bonds. Understanding vapor pressure is essential in fields like chemistry and meteorology.