Energy and Phase Changes

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Alexandra Carpenter 1G
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Joined: Sat Jul 22, 2017 3:00 am

Energy and Phase Changes

Postby Alexandra Carpenter 1G » Wed Jan 10, 2018 2:56 pm

In lecture today, we went over the heating curve of water. Why does the vaporization transition (40.7 kJ/mol) require much more energy than the melting transition (6.01 kJ/mol)?

Justin Lau 1D
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Joined: Sat Jul 22, 2017 3:00 am

Re: Energy and Phase Changes

Postby Justin Lau 1D » Wed Jan 10, 2018 2:59 pm

The hydrogen bonds that in water are probably what requires such a large enthalpy of vaporization. Going from ice to liquid water only requires a breaking of the lattice structure. However, liquid To gas needs energy to break the hydrogen bonds entirely and separate the molecules, as the liquid form still has molecules in close contact.

Tatiana Hage 2E
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Re: Energy and Phase Changes

Postby Tatiana Hage 2E » Wed Jan 10, 2018 3:16 pm

Melting only requires enough energy for the molecules to escape from their sites in the solid, but for vaporization, you need enough energy to be able to break most of the intermolecular attractions.

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Re: Energy and Phase Changes

Postby OliviaShearin2E » Wed Jan 10, 2018 3:21 pm

I agree with Justin and Tatiana. I found this description on Quora to summarize:
"In ice, water molecules are very closely packed together; the intermolecular forces of attraction are particularly strong in solids, which means that the molecules have little to no freedom to move. When you heat ice at 0C to get water at 0C, you're essentially weakening these intermolecular forces and giving the water molecules freedom of movement. Intermolecular forces still exist, but the water molecules are no longer stuck in a rigid structure. When going from liquid to vapor, you must provide sufficient heat so that the kinetic energy of the water molecules overcomes the intermolecular forces altogether, leaving you with molecules that no longer interact with each other."

Kaelie Blanes-Ronda 2L
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Re: Energy and Phase Changes

Postby Kaelie Blanes-Ronda 2L » Wed Jan 10, 2018 3:45 pm

Heat=Energy so the heat being applied is being used to break the bonds between water molecules instead of to heat up the water. The Max temperature (boiling point) of water is 100 degrees so after the liquid water reaches this point the added heat is used as energy to break the bonds. Ice is less dense than liquid water, which is a unique property of water. Since the water molecules are more tightly compacted between each other in the liquid state rather than the solid state more energy is required to break these short/strong bonds.

William Satyadi 2A
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Joined: Sat Jul 22, 2017 3:00 am

Re: Energy and Phase Changes

Postby William Satyadi 2A » Wed Jan 10, 2018 5:00 pm

Agreed, it might help to look at what molecules in solid, liquid, and gas phases look like. The main difference between solid and liquids is that molecules in liquids are still close but can move around past each other, while molecules in solids are close, compact, and cannot move around much. The difference between liquids and gases, however, is much greater than between liquids and solids. While molecules in liquids and solids are close, molecules in gases are way more spread out and fly around with greater velocities. In order to reach the gas phase, more energy would be required to go from liquid to gas than solid to liquid.

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