If gases cool while they expand, doesn't that conflict with Endothermicity?

[MedicalMan14]

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If the process is endothermic that means it gains thermos; right? Why in this case does a gas cool while it expands? What is the difference?

Also in a Princeton Review Explanation I am told; the vaporization of a liquid to a gas is an endothermic process. Therefore when liquid water evaporates, the remaining liquid is chilled.

I am confused; is it the gas that is cooled or its surroundings or both?

 

[SN2ed]

Thread moved. These types of questions are better suited for the Study Q&A forum.

 

[docelh]

Entropy.

I love short answers.

 

[badmintondr]

If the process is endothermic that means it gains thermos; right? Yes that means it needs the proper amount of energy to proceed with the reaction.

Why in this case does a gas cool while it expands? What is the difference? I believe the gas expansion you're referring to is the endothermic vaporization of a liquid, which is not the same as the expansion of a gas in terms of volume from gas --> gas. If I'm right in assuming you are talking about this, then these are actually 2 completely seperate reactions as one concerns a phase change while the other only results in a change in volume. Regarding the expansion of gas in terms of volume, the application of this can be seen in the blowing of hot air from your mouth. Air in your mouth is blown through your small mouth opening and as a result, it "feels" cold. But what is happening is that the gas is just expanding and thus resulting in a "cold" temperature.

I am confused; is it the gas that is cooled or its surroundings or both?
If your talking about liquid water evaporating then the gas would have to have a higher temperature than the liquid right? That should make sense. Also the surroundings, might be a little vague to answer your question. If your referring to something like evaporation of water form the skin, then assuming the skin to the surrounding then yes, the surrounding is cooled in the sense that heat is being drawn from it as the water molecules are shifting into the gas phase.

Also, this might be better posted in the Q/A forum. (More people there are chronic checkers...like myself...can never read too much on question/answers lol) For future notes!

 

[ACSheldor]

If the environment is isolated and no heat is coming in then the gas will cool as it expands because it uses some of its own internal energy for the expansion.

Endothermic just means the process requires heat from somewhere. When you boil water you add heat from the stove to the water. So water is usually absorbing heat from the environment, it doesn't spontaneously evaporate. So when you add heat from an external source like a stove then the heat goes directly into increasing the waters kinetic energy.

 

[Danlee07]

Endothermic means surrounding to system

In order for gas to convert to liquid, you need to put energy into it...

Gas expands when it cools since if you think about it, the higher the temp, the faster the particles move within their confined space (if they have one). That's gonna eventually make the intramolecular forces to change states into liquid. Expansion has to do with energy from Joule-Thomson effect. It's an inverted relationship.

 

[Enoko]

To make something boil you need to put heat into it. Another way to say this is a liquid draws heat from around it to boil. It draws heat from the hotplate or bunsen burner or whatever.

When a gas expands it gets cold for the exact same reason. A gas gets cold when it expands because it draws heat from around it to expand.