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Post by buckleymanor1 on Jun 8, 2011 16:17:25 GMT 1
bucklymanor, I'll let STA give you the definmitive answer, but on reading you post I think I can see your "error". The molecules are the gas. Between the gas molecules there is nothing. However, there is an interesting point that you allude to and that is the space that is occupied by the molecules and why molecules that are larger do not need more space at equal levels of energy than those that are smaller? For example, let's take two identical containers each containig a different gas. We increase the pressure to an infinite level by forcing more gas into each container. Temperature and volume are the same. I assume the answer is that the larger molecules travel slower and so "occupy" the same space as the smaller, faster moving molecules. P P If larger molecules occupy the same space as smaller ones then why does a balloon inflated with hydrogen deflate quicker than one filled with air if tied and left.
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Post by speakertoanimals on Jun 8, 2011 17:24:22 GMT 1
Occupy the same space, and size of the molecule are DIFFERENT concepts. Balloon deflating have to do with whether or not the molecules can get through the rubber, which depends on the size of the molecule, not average spacing between molecules.
I suggest 'occupy the same space' is a bad phrase, designed to confuse, and we stick to 'average spacing between molecules', which is clear and unambiguous.
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Post by robinpike on Jun 9, 2011 9:19:05 GMT 1
STA, thanks for your replies. I think I've got it now:
In a mixture of different gases, such as oxygen and hydrogen at a certain temperature, the oxygen molecules on average will have a slower speed than the hydrogen molecules in relation to their different masses. And this leads to the consequence that we see in Avogadros' principle.
The way I understand how they are able to keep their different speeds is by simplifying the problem and only consider the "head-on" and "rear-end" collisions. Starting with the oxygen and hydrogen molecules at their respective different speeds and looking at those two types of collision (and taking the collisions as being perfect inelastic collisions).
Two hydrogen molecules colliding head-on (both having the same speed), will change direction but remain at that speed. This will be true for two oxygen molecules colliding head-on with equal speeds as well.
So what happens when a hydrogen molecule and an oxygen molecule collide head-on? The hydrogen molecule (with less momentum than the oxygen molecule) will bounce backwards and gain extra momentum and therefore gain a faster speed, whereas the oxygen molecule will continue on but will lose the same amount of momentum and therefore have a slower speed.
And conversely, when a hydrogen molecule collides with an oxygen molecule by catching up with it, the oxygen molecule will gain momentum and therefore end up with a faster speed, whereas the hydrogen molecule will lose the same amount of momentum and therefore lose some speed.
So overall, the two types of molecules will have a spread of speeds from their average speed, but in general they will retain their different speeds that equates to that temperature?
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Post by speakertoanimals on Jun 9, 2011 11:51:29 GMT 1
Yep, that's right. Although what equates to temperature is kinetic energy (hence different speeds for different masses of molecule).
Then, of course, we have the collisions of molecules with the sides of the box, which keep the temperature of the gas the same as the walls, but that is another story!
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