energy in gases

子路

2010-04-05 11:51 pm

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I am concerned about Q only

The solution said Q is the latent heat of vaporization
I agree with this.

At first I though it was
"change in internal energy of the content(water actually) inside the syringe"
Since the temperature does change, the change in internal energy must be the rise in potential energy

but it contradicts with
"the potential energy of a gas is zero"

So what actually is the latent heat of vaporization/fusion?
when water vaporizes, the molecules are farther apart
there should be a gain in potential energy, right?
This gain is the latent heat of vaporization?
(you MAY neglect the work done against external pressure)

how about the latent heat of fusion?
what is it in fact?

更新1:

why there is a gain in potential energy? because the forces between gas molecules are attractive? why not repulsive?

回答 (1)

天同

2010-04-06 1:00 am

✔ 最佳答案

It is just the same for fusion. The energy supplied to melt a substance is mainly used in increasing the potential energy of molecules when changing from the solid to liquid state.

In applying the First Law of Thermodynamics, Q = dU + W
Q is the heat supplied, dU is the change in internal energy, and W is the work done by the substance.

Since there is no change of temperature on melting, dU represents the increase in potential energy of all molecules. W is the wrok done against atmospheric pressure when the substance changes its volume (expand or contract) on melting. In the case of melting, the change in volume is not as large as that in vapourization. W is thus small as compared with dU, and is generally negligible.

Therefore, Q = dU on melting, and Q equals to the latent heat of fusion if a unit mass of the melted substance is involved.

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