GOB Chemistry

Learn the toughest concepts covered in your GOB - General, Organic, and Biological Chemistry class with step-by-step video tutorials and practice problems.

Gases, Liquids and Solids

Intermolecular Forces and Physical Properties

Intermolecular forces influence physical properties.  We will look at direct and indirect relationships.

Intermolecular Forces and Physical Properties

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concept

Intermolecular Forces and Physical Properties Concept 1

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recall the physical properties are measurable and observed for the senses that describe these states of matter and that the inter molecular forces are the attractive forces that exists between molecules and influence these physical properties. Now, here we're gonna talk about direct relationships, we're gonna say under direct relationships the stronger the inter molecular force than the greater or stronger the physical property. So here we can establish a direct relationship with boiling point, abbreviated BP melting point, abbreviated Mp surface tension and viscosity. Remember boiling point. This is where the temperature between a liquid and gas are in equilibrium. So you could go from liquid to gas or condense back down to liquid. We're gonna say the connection between them is where boiling point resigns melting point, which is M. P. This is the temperature where a solid and liquid are in equilibrium. So here we have our solid and we could melt to give us our liquid. We can go backwards and freeze to make our solid. It's here in the middle between the two. That melting point can exist Now, surface tension, surface tension is just the measure of attractive forces on a liquid surface. So sometimes you might see um insects being able to walk on water that is a direct result of the hydrogen bonding that exists within water molecule. They form such a tight knit packed surface on top. It acts like a false floor and allows insects to be able to basically take advantage of this phenomenon and allow them to walk on water. So here there are cohesive or strong, attractive forces at the top. And this allows the bucks to walk on the water. Now, finally, we have what's called viscosity. Now, viscosity is a resistance to flow. We're going to say a constant temperature. We're going to say the viscosity is the resistance to flow for substance. So I'm pouring in a substance within a journey time. It we're going to say here that if you have a very viscous material, so the higher the viscosity that equals a slower speed, slower the movement. But here's the thing, we can help speed up the movement of a substance by affecting its temperature if we increase the temperature of a substance that will loosen up the molecules within it and help it move faster. And as a consequence are viscosity would decrease. So increasing temperature lowers viscosity. Viscosity is the resistance to flow, something that is viscous moves very slowly.
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Intermolecular Forces and Physical Properties Example 1

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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.
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Intermolecular Forces and Physical Properties Concept 2

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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.
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Intermolecular Forces and Physical Properties Example 2

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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.
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Problem

Which of the following will have the lowest boiling point?

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Problem

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

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Problem

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

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