Intermolecular Forces and Physical Properties - Video Tutorials & Practice Problems

Intermolecular Forces and Physical Properties Concept 1

in this video, we're gonna take a look at the direct relationship between the inter molecular forces and physical properties. Now recall that physical properties are measurable and observed. The senses that describe the states of matter. We're going to say the inter molecular forces themselves are just the attractive forces that exists between molecules and influence these physical properties. Now, when we say direct relationships realize that under direct relationships, we're gonna say the stronger the inter molecular force than the greater the physical property. And the four physical properties that we will observe are boiling point melting point. Uh 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 vaporizes into a gas and the gas can condense down into a liquid. So boiling point is just the equilibrium between these two states. And we're gonna say, the stronger the inter molecular force than the greater the boiling point. Next we have melting point melting point is the temperature or 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 inter molecular force than 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 body into 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 are an insects take advantage of in order to cross over bodies of water and then finally we have viscosity. Viscosity may not be as known as the other three. We're gonna 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 gonna say here, the greater viscosity than the slower the movement. Okay, 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 gonna say just like the other three. Before it, the stronger the inter molecular 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 inter molecular forces. So highest melting point here would be the strongest inter molecular force. So if we take a look at these options, we have C two H 50 H. Here we have hydrogen connected to an oxygen. So this would be hydrogen bonding. Okay, 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 to see. But if we draw this out and we have a carbon in the center and be connected to the two hydrogen is and the two bro means this would not be a perfect shape because all the surrounding elements are not the same. So it would be a polar covalin compound and therefore dipole dipole. And then finally we have a hydrocarbon, something composed of only carbon and hydrogen. So this is non polar and therefore London dispersion. So earlier, we said, strongest inter molecular force would translate to the highest melting point. So here the strongest inter molecular 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 inter molecular forces in a particular physical property. So we're gonna stay here under indirect relationships, the stronger the inter molecular force than the weaker the physical property, the physical property in question is a vapor pressure. Now, vapor pressure is just the pressure exerted by a gas at the surface of a liquid and realize that baby pressure is the result of an equilibrium between compensation and vaporization. So the gases on top basically pushed down on the liquid, creating a vapor pressure and we're going to stay here. The stronger the inter molecular 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 inter molecular force translates to a lower or weaker vapor pressure.

Intermolecular Forces and Physical Properties Example 2

choose the substance with the highest vapor pressure. So highest vapor pressure translates to the weakest inter molecular force. So we take a look here, we have silver per chlorate 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, right? So it's a nonmetal by itself. So it's non polar. So it's going to be London dispersion forces for the next one. We have a hydrocarbon which is non polar as well. So it would be also London dispersion forces. And then finally we have hydrogen with sulfur. We're gonna say here hydrogen would solve for if you to draw this out, you have solved for one 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 covalin compound. So this is dipole dipole, highest vapor pressure belong to the weakest inter molecular force, which would be London dispersion. So here we're comparing krypton to CH four, which is methane. Remember to compare their streets, we look at their overall mass. Krypton weighs approximately 83. g. And if you added up to one carbon in the four hydrogen of methane it be about g 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 cream put 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