"Quick Way" for predicting response to changes in volume/pressure

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Janet Nguy 2C
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Joined: Sat Aug 24, 2019 12:17 am

"Quick Way" for predicting response to changes in volume/pressure

Postby Janet Nguy 2C » Mon Jan 20, 2020 4:32 pm

Hi guys,

Dr. Lavelle summarized the 'quick way' for predicting the response of chemical equilibria to changes in volume as follows:

-if you decrease the volume and there's more moles of gas on the left, then the reaction shifts right.
-if you decrease the volume and there's more moles of gas on the right, then the reaction shifts left.

Can somebody explain the intuition behind this method? I don't understand how it corresponds to what we did in the long example.

Daniela Shatzki 2E
Posts: 53
Joined: Sat Aug 24, 2019 12:16 am

Re: "Quick Way" for predicting response to changes in volume/pressure

Postby Daniela Shatzki 2E » Mon Jan 20, 2020 4:48 pm

if you decrease the volume, that means you are adding more pressure to the system. Therefore, if there are more moles on the left of an equation, decreasing the volume will cause the reaction to go to the right because there are less moles of gas on that side/more space so the reaction will want to even out. The same for the vice versa.

Qiu Ya Wu 4I
Posts: 107
Joined: Fri Sep 20, 2019 12:17 am

Re: "Quick Way" for predicting response to changes in volume/pressure

Postby Qiu Ya Wu 4I » Mon Jan 20, 2020 4:57 pm

To add on, this system works this way due to Le Chatelier's principle since favoring the side of the reaction with less moles of gas when pressure is applied means that there will be less molecules ultimately bouncing off the walls of the container and creating additional pressure.

Brian Tangsombatvisit 1C
Posts: 119
Joined: Sat Aug 17, 2019 12:15 am

Re: "Quick Way" for predicting response to changes in volume/pressure

Postby Brian Tangsombatvisit 1C » Mon Jan 20, 2020 5:17 pm

The explicit way to determine the response to a change in volume or pressure is to calculate the new concentrations of the each reactant and product. For example, since concentration is represented by n/V, increasing V would decrease concentration. Similarly, since pressure and volume are inversely related, increasing pressure (by compression) would decrease volume, increasing the concentration of all reactants and products by a factor. If you plug in these new concentration values for the reaction quotient, you should be able to determine the proceeding direction of the reaction based on whether Q > K, Q < K, or Q = K.


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