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Thread: Why does temperature change with pressure?

  1. #1 Why does temperature change with pressure? 
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    According to Gay-Lussac's law, pressure and temperature are directly proportional. I understand why pressure changes with temperature but I don't understand why temperature changes with change in pressure.
    Same question applies for Charles' Law: Why does temperature change with volume?


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  3. #2  
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    The way I've always looked at it (and I suppose I could be wrong...but at least this is one way of making sense of it) is that a higher pressure forces molecules together and makes them interact more, and all then bumping around raises the energy, and thus the temperature. With less pressure, the molecules are allowed to be further apart and there's not as much activity, and less energy.

    As for Charles' Law, if you heat up a gas, it's naturally going to expand. The particles will fly outward as they gain energy. If we reverse it and increase volume, temperature will increase because the particles are able to gain this energy. In a small area they are unable to do so.

    Only problem is that these seem to counter each other... It's been a while since I've done the gas laws. Hopefully someone can provide a better answer. At least that's a way, if incorrect, of looking at it.


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    Charles law only works if the pressure is held constant, all though the theory tells us that increasing the volume at constant pressure will increase the temperature. it is difficult to show this process in real life situations. In fact the volume is causul of the temperature not the other way round.
    if you increase the volume by pulling on a moveable wall or some other method, you would decrease the internal pressure which is not allowed by charles law.
    therefore the only way to change the volume is to heat it up.
    everything is mathematical.
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  5. #4  
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    I can give you a good reason for why a decrease in pressure will give you a decrease in temperature, and I am about to presume the concept works just as well the other way around:

    Consider an endothermic reaction; the reaction happens but it cools down. This is in direct contradiction to what you assume to be common sense - surely things need energy to react? And that it precisely (?spelling) it. For a gas to expand, it must overcome an energy barrier. Every action requires energy input of some sort to occur. Because you are not providing an energy source to the mixture of gasses, you are forcing it to gain energy by stripping itself of heat. Hence the temperature drops, because there is less heat energy available to 'feel' or record, because it is all being transformed into energy to provide expansion of the gas.
    This is also true when you employ low presure distilation conditions in the lab. Lower the pressure, and set the temperature you want the mixture to be at, the thing doing the heating will HAVE TO WORK HARDER to get the temperature to your desired point, because the decreasing pressure and expansion of gasses removes this excess heat energy.

    There are other considerations to take into account here to, such as intermollecular repulsions and attractions between molecules which make our picture so much more complicated. As I am by no means a colloidal scientist I am not best to go into these effects; but it is best you acknowledge that any model or explaination will not be complete. For example, pushing mollecules together requires energy because you overcome repulsive forces from the nuclei, but also pulling molecules appart requires energy because of attractive Van Der Waals forces. All of these should be considered when making a thermodynamic assesment of why gasses expand and contract with fluctuations in temperature.

    I hope this is sufficient,
    Luka
    Chemistry is everything!
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    Information in this post wasn't factual. First of all, "ambient radiation" doesn't have a place here, and second, an "abundance of free electrons" does not equal heat, among other things.
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    Post content removed. Nothing to back up the information provided, and off topic.
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  8. #7  
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    Two of the most absurd explanations I have ever read!
    Not even worth Debate.
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  9. #8  
    Forum Freshman asxz's Avatar
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    [quote="Chemboy"]The way I've always looked at it (and I suppose I could be wrong...but at least this is one way of making sense of it) is that a higher pressure forces molecules together and makes them interact more, and all then bumping around raises the energy, and thus the temperature. With less pressure, the molecules are allowed to be further apart and there's not as much activity, and less energy.[quote]
    That's what I thought, but I'm only a high-school student, so I could be wrong!
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  10. #9  
    Forum Senior Booms's Avatar
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    it's very simple really, heat causes molecules to move faster, they collide with everything, this increases the pressure because pressure is simply a measurement of average collisions per unit time


    Increases pressure increases the collisions with increases the activity and speed of molecules, otherwise known as heat
    It's not how many questions you ask, but the answers you get - Booms

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