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

Intermolecular forces play a crucial role in determining the physical properties of substances, such as boiling point, melting point, surface tension, and viscosity. Boiling point is the temperature at which a liquid and gas are at equilibrium, and a stronger intermolecular force leads to a higher boiling point. Melting point, the temperature where solids and liquids are in equilibrium, also increases with stronger intermolecular forces. Surface tension, which allows insects to walk on water, is a measure of the cohesive forces at a liquid's surface and is greater when intermolecular forces are strong. Viscosity, the resistance to flow, is higher in substances with strong intermolecular forces; however, it can be reduced by increasing temperature. In contrast, vapor pressure, the pressure exerted by a gas on a liquid's surface, is lower when intermolecular forces are strong, indicating an inverse relationship between the two. Strong intermolecular forces lead to lower vapor pressure due to the decreased tendency of molecules to escape into the gas phase.

Intermolecular Forces & 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 to the senses that describe these 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 going to say the stronger the intermolecular force then the greater the physical property.

And the four 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 an equilibrium with each other. The liquid can get vaporizing to what gas and the gas can condense down into a liquid. So boiling point is just the equilibrium between these two States and 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 that involves melting or fusion. Going from a liquid to a solid is freezing the greater the intermolecular force than the higher the melting point. Next we have surface tension. Surface tension is just the 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 is 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. OK, so this is the phenomenon that insects are insects take advantage of in order to crossover bodies of water. And then finally we have viscosity. Viscosity may not be as known as the other three. We're going to say at constant temperature. Viscosity is just the resistance, so flow for a substance. So you might have heard 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, it moves pretty quickly, very easily. So water has a low viscosity. Now we're going to say here the greater viscosity than the slower the movement, OK, so it deals with time. Now, how could I, 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 three before it, the stronger the intermolecular force, the greater the viscosity.

Intermolecular Forces & 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. So highest melting point here would be the strongest intermolecular force.

So if we take a look at these options we have C₂H₅O. Here we have hydrogen connected to an oxygen, so this would be hydrogen bonding. For B, we have calcium sulfide in water, so we have an ionic compound in a polar solvent. So this would be ion dipole.

For the next one, it's hard to see, but if we were to draw this out, we'd have a carbon in the center and be connected to the two hydrogens and the two 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 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 & 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 here 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 pushed down on the liquid creating a vapor pressure.

And we're going to say here the stronger the intermolecular force, the weaker the vapor pressure or lower the vapor pressure will be for substance. So they are basically indirectly related or opposites of one another. One is high, the other one is low. Stronger intermolecular force translates to a lower or weaker vapor pressure.

Intermolecular Forces & Physical Properties Example 2

Choose a substance with the highest vapor pressure. So highest vapor pressure translates to the weakest intermolecular force. So if we take a look here we have silver perchlorate in methanol. So this is an ionic compound within a substance that has hydrogen bonding. So that means it's polar. So this would be ion dipole.

Next we have is a nonmetal by itself, so we have Krypton, so it's a nonmetal by itself. So it's nonpolar. So it's going to be London dispersion forces. For the next one, we have a hydrocarbon which is nonpolar as well, so it would be also London dispersion forces.

And then finally we have hydrogen with sulfur. We're going to say here hydrogen with sulfur. If you were to draw this out, you'd have 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. So this is dipole dipole.

Highest vapor pressure will belong to the weakest intermolecular force, which would be London dispersion. So here we're comparing Krypton to chapter 4, which is methane. Remember to compare their strengths we look at their overall mass. Krypton weighs approximately 83.80g, and if you added up the one carbon in the four hydrogens of methane, it'd 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 1. The weakest one would translate to the highest framework pressure. Since methane weighs less, it is weaker and therefore would have the highest vapor pressure. So here the answer would be 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 physical properties are determined by the strength of intermolecular forces in solids and liquids?

Intermolecular forces play a crucial role in determining the physical properties of solids and liquids. These forces are the attractions that occur between molecules, and their strength can significantly influence several properties:

1. Melting and Boiling Points: Stronger intermolecular forces require more energy to overcome, leading to higher melting and boiling points. Substances with weak intermolecular forces, like London dispersion forces, tend to have lower melting and boiling points.
2. Vapor Pressure: Liquids with weaker intermolecular forces have higher vapor pressures because the molecules can escape into the gas phase more easily.
3. Viscosity: The resistance to flow in liquids, or viscosity, is higher when intermolecular forces are strong because the molecules have a greater attraction to each other, making it harder for them to move past one another.
4. Surface Tension: Strong intermolecular forces result in higher surface tension, which is the elastic tendency of a fluid surface that makes it acquire the least surface area possible.
5. Solubility: The solubility of substances in solvents is influenced by the compatibility of intermolecular forces between the solute and solvent molecules.
6. Capillary Action: This is the ability of a liquid to flow in narrow spaces without the assistance of external forces, and it is stronger in liquids with stronger intermolecular forces.

How do intermolecular forces affect physical properties?

Intermolecular forces (IMFs) are the forces that mediate interaction between molecules, including attraction or repulsion. These forces significantly influence the physical properties of substances.

For instance, boiling and melting points are directly related to the strength of IMFs. Substances with strong intermolecular forces, like hydrogen bonds, require more energy to change phases, resulting in higher boiling and melting points. Conversely, substances with weaker forces, such as London dispersion forces, have lower boiling and melting points because less energy is needed to overcome the intermolecular attractions.

Vapor pressure, which is the pressure exerted by a vapor in equilibrium with its liquid or solid phase, is inversely related to the strength of intermolecular forces. A substance with weak IMFs will have a higher vapor pressure at a given temperature compared to a substance with strong IMFs.

Viscosity, or the resistance of a liquid to flow, is also affected by intermolecular forces. Liquids with stronger IMFs will be more viscous because the molecules are more tightly held together and do not flow past each other as easily.

Surface tension, the elastic tendency of a fluid surface, is higher in liquids with strong intermolecular forces because the molecules at the surface are more strongly attracted to each other, creating a "film" on the surface that resists external force.

What happens to these physical properties as the strength of intermolecular forces increases?

As the strength of intermolecular forces increases, several physical properties of a substance are affected:

1. Melting Point and Boiling Point: Substances with stronger intermolecular forces require more energy to overcome these forces, leading to higher melting and boiling points. This is because more heat is needed to break the intermolecular bonds during the phase transitions from solid to liquid (melting) and from liquid to gas (boiling).
2. Viscosity: The viscosity, or thickness, of a liquid increases with stronger intermolecular forces. A higher viscosity means that the liquid flows less easily, as the molecules have a greater attraction to each other.
3. Surface Tension: Surface tension is the tendency of a liquid's surface to resist rupture when placed under tension or stress. Stronger intermolecular forces enhance surface tension, making the liquid more resistant to external forces and less likely to spread out.
4. Volatility: Volatility decreases with stronger intermolecular forces. A less volatile substance will evaporate more slowly because the molecules are more strongly attracted to each other and do not escape into the gas phase as readily.
5. Solubility: The solubility of solids and liquids can either increase or decrease with stronger intermolecular forces, depending on the types of forces involved and the solvent. Generally, like dissolves