Pressures and Points

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asalest 2K
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Pressures and Points

Postby asalest 2K » Thu Nov 19, 2020 2:56 pm

How do we identify stronger melting and boiling points? Also what is vapor pressure and how can it be higher? for example, why is the vapor pressure of diethyl ether (C2H5OC2H5) is greater than that of water.

Melody Haratian 2J
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Re: Pressures and Points

Postby Melody Haratian 2J » Thu Nov 19, 2020 4:25 pm

The stronger the intermolecular forces between the molecules, the higher the melting and boiling points will be. Vapor pressure refers to the amount of vapor released when a liquid turns into a gas. The more vapor released, the higher the vapor pressure. Weaker intermolecular forces have a higher vapor pressure, since they are more likely to move into a gas phase. The vapor pressure of diethyl ether is greater than that of water since diethyl ether has weaker intermolecular forces than water. Water has hydrogen bonding forces, Dipole-Dipole, and LDF while diethyl ether has Dipole dipole and LDF forces. Since hydrogen bond forces are stronger than dipole dipole forces, water has stronger intermolecular forces and won’t exert as much pressure in its gas phase than diethyl ether will.

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Re: Pressures and Points

Postby Anthony_3C » Sun Nov 22, 2020 4:25 am

To further clarify Melody Haratian's points, I want to point out that vapor pressure is a definition for convenience of calculation and comparison. You could tell which substance has higher boiling point by its vapor pressure. The correct definition of vapor pressure is the pressure (N/m^2 = pascal(Pa)) of the gas phase of a substance at 25C in a enclosed container where the gas and liquid phase of the substance is in equilibrium.

To compare boiling and melting points, compare the atomic mass or vapor pressure, because larger atomic mass means more electrons, which means a larger London force(IMF) and thus a higher boiling and melting point and a lower vapor pressure means a stronger IMF and thus a higher boiling and melting point. To summarize, large atomic mass --> high bp, mp, and low vapor pressure --> high bp, mp. In addition, let me put vapor pressure in a visual context. Imagine an enclosed container. When the temperature is below boiling point for a substance, for example water, some moleculres of H2O spontaneously gets freed from the liquid phase on the outer surface and floats in the air. Even below boiling point, not all water molecules are stuck together as liquid. If the temperature is raised until 100C, at this point all the water molecules have enough kinetic energy to move freely in space and not be restricted by other water molecules. Specifically, the vapor pressure at 100C is 760 torr(another unit), which equals 1 atm, which means that the water molecules finally have enough energy to resist the downward force of the atmosphere and become gas. Thus, molecules with higher vapor pressure can resist atmospheric pressure at lower temperature because they reach this point faster. So higher vapor pressure means lower boiling point.

chinmayeec 2H
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Re: Pressures and Points

Postby chinmayeec 2H » Sun Nov 22, 2020 12:57 pm

When IMFs are stronger, the boiling points and melting points are higher. For example, the boiling point of methane CH4 is much lower than that of water because CH4 is a nonpolar molecule that only experiences LDFs while water can hydrogen bond. A higher vapor pressure indicates that the boiling point of the compound is lower because it is being turned to a gas at a lower temperature. As the amount of gas increases, the vapor pressure also increases. For example, CH4 would have a higher vapor pressure than water because it has weaker IMFs leading to a lower boiling point.

Giselle Granda 3F
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Re: Pressures and Points

Postby Giselle Granda 3F » Sun Nov 22, 2020 1:10 pm

The molecule with the higher boiling/melting point will always have the stronger intermolecular forces. The order of increasing strength of IMF are ion-ion> ion-dipole> H-bonding> dipole-dipole> London dispersion. You can identify the forces each molecule can have by drawing the lewis structures. Remember that ion-ion is between two ions, ion-dipole is between an ion and a polar molecule, H-bonding is in molecules with H bonded to N,O, or F, dipole-dipole is between two polar molecules, and LDF is in all nonpolar and polar molecules. When you draw the Lewis structures determine polarity of the molecules. If both are nonpolar, they only have LDF and you will determine higher boiling/melting points by which molecule is bigger in size. If one is nonpolar and the other is polar, the polar one will have the higher boiling/melting point because it will have more IMF. If both are polar, you will have to determine their IMF forces, and which ones are greater than the other. Hope this is helpful!

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