Phase changes

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Phase changes

Postby KaitlynWatkins2D » Sun Jan 10, 2016 11:11 pm

Why are phase changes (vaporization, melting, sublimation) endothermic? The definition for endothermic in the course reader says that endothermic reactions cause the surroundings to cool so how does this happen during phase changes in which the temperature stays the same?

Hannah Markovic 3C
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Re: Phase changes

Postby Hannah Markovic 3C » Mon Jan 11, 2016 12:13 am

Not all phase changes are endothermic - just the ones you mentioned (vaporization, melting, and sublimation) are endothermic. The other three phase changes that go in the opposite direction (condensation, freezing, and deposition) are exothermic but not mentioned in the course reader because in calculating these values you just take the negative of what you get for the phase change in the opposite direction.

For those three you mentioned (liguid -> gas, solid -> liquid, solid -> gas), energy has to be put into the substance in order to create more space between the molecules and more it to the higher energy state. The surroundings cool because energy from the surroundings goes into the material to expand the spaces between molecules. Because the energy expands space between molecules rather than changing how the molecules are vibrating, the temperature of the material stays constant during a phase change.

For condensation, freezing, and deposition the opposite is true - the surroundings are heated in an exothermic process as energy that was maintaining the distance between molecules now decreases and the molecules get closer together. Again, the vibrations of the molecules do not change, so the temperature stays constant.

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Re: Phase changes

Postby Eduardogonzalez1G » Mon Jan 11, 2016 1:24 am

In your definition of endothermic and exothermic, it may be difficult to identify whether a reaction is endothermic or exothermic because you look at in in the perspective of whether the reaction cools or heats its surroundings. Look at it this way; if a reaction requires heat for it to be performed, then its endothermic( ex: melting an ice cube requires it to be heated or put in a warm enough temperature for it to melt). For exothermic, its the opposite. So if melting an ice cube is endothermic, then freezing it would be exothermic. Also, you may want to look at the enthalpies. If its negative, its exo. If its positive, its endo. Vaporization requires heat to heat particles and make steam, therefore its endothermic. The opposite which is condensation is exothermic. Same for sublimation, its requires energy to heat to change from a solid to a gas, so its endothermic. Its counterpart,deposition, is therefore exothermic. Basically the ones that are going from a low state(like a solid) to a solid state(like a gas) are endothermic. and the ones going from a high state to a low state are considered exothermic
this is how a exothermic reaction looks like. Its just the opposite for an endothermic reaction.
endo.png (25.19 KiB) Viewed 424 times

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