compression vs pressure

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cristiancampana 2H
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compression vs pressure

Postby cristiancampana 2H » Sun Dec 10, 2017 12:05 am

what is the relationship between pressure and compression? i believed that changing the pressure did not affect the overall Kc but i have a problem where it says that compression doubles the molarity.

Mia Navarro 1D
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Re: compression vs pressure

Postby Mia Navarro 1D » Sun Dec 10, 2017 12:12 am

Compression is the decrease of volume within the system, and pressure is the force upon the moles of reactants and products within the system. Their relationship is inverse. Pressure and compression both only change the molarities and the side of the reaction that the system favors, but they will not eventually change the Kc of the reaction.

Phan Tran 1K
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Re: compression vs pressure

Postby Phan Tran 1K » Thu Jan 10, 2019 5:36 pm

Wait, if compression changes which side of the equation is favored, would that eventually change the Kc too, since Kc tells you which side of the reaction is favored? I'm not really sure, can someone clarify?

Matthew Tran 1H
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Re: compression vs pressure

Postby Matthew Tran 1H » Sat Jan 12, 2019 1:00 am

If the system is compressed (decreasing the volume), then the concentrations or partial pressures of the reactants and products increase, giving you a value of Q that is different from K. Comparing the value of Q to K allows us to determine which side of the reaction is favored. This is the method that Dr. Lavelle emphasized during lecture, but you can also use the shortcut that if the volume is decreased, the reaction will favor the side that has less moles of gas, and vice versa. Kc only depends on the temperature, and the whole idea of Le Chatelier's principle is that the reaction at equilibrium will respond to mitigate any stress on it. When stress is applied to the system, the reaction is no longer at equilibrium, so the reaction that will return the system to equilibrium will be favored.

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Re: compression vs pressure

Postby JacobHershenhouse3G » Sat Jan 12, 2019 1:59 am

Yes! Kc will change if you alter volume (compression), but be careful because Kc does not change when you change pressure in the case that, instead of changing the volume to affect pressure, you change pressure by adding or removing inert gas from the system ({C=[num of moles]/[Volume]} concentrations stay the same this way)! So, if the concentrations can be changed then Kc may be different therefore a new equilibrium with new equilibrium concentrations of reacting molecules is reached after some time.

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Re: compression vs pressure

Postby Chem_Mod » Sat Jan 12, 2019 11:41 am

The relationship between pressure and volume is described by Boyle's Law which is PV=a constant. This shows us that pressure and volume have an inverse relationship. This means increasing the volume leads to a decrease in pressure while decreasing the volume leads to an increase in pressure. This is only true for gasses though. Imagine a brick sitting in a vessel, changing the volume of the container would not change the pressure in any way.

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Re: compression vs pressure

Postby MichaelMoreno2G » Sat Jan 12, 2019 12:13 pm

To the comment above me, you said the inverse relationship between volume and pressure only applies to gases. How would it apply to liquids, solids or aqueous solutions?

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