Sapling WK 1, #4

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Melody Haratian 2J
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Sapling WK 1, #4

Postby Melody Haratian 2J » Sun Jan 17, 2021 9:37 am

Hi guys!
I’m having a lot of trouble getting started with the chemical equilibrium sapling HW, problem #4.
The problem is:
At a certain temperature, the given reaction has an equilibrium constant of Kp= 329.
PCl3 + Cl2 <—> PCl5

PCl5 is placed in a sealed container at an initial pressure of .0750 bar. What is the total pressure at equilibrium?

Can someone guide me through how to do this problem?
Thanks

Bethany Yang 2E
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Re: Sapling WK 1, #4

Postby Bethany Yang 2E » Sun Jan 17, 2021 9:54 am

You are going to want to start with the ICE table and use the initial bar pressure of PCl5 in the beginning, with the other two at 0. Then at equilibrium you would set the equation equal to the Kp that was given. You should be able to do the quadratic formula and solve for x and then plug it back in. For the total pressure you would just add all the equilibriums together.

Chance Herbert 3A
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Re: Sapling WK 1, #4

Postby Chance Herbert 3A » Sun Jan 17, 2021 10:18 am

The first thing you will want to do is create an ICE table. You are given the initial pressure of 0.0750 bar for PCl5. The initial pressure will be 0 for PCl3 and PCl2. The change in pressure will be +x for both PCl3 and PCl2 and -x for PCl5 since there is a 1:1 molar ratio for each of the reactants and products. This leaves you with Equilibrium pressure values of x for PCl3 and Cl2 and 0.0750 - x for PCl5.

You are provided the value of Kp which you can use to determine the value of x.
Kp=(PPCl5)/((PPCl3)(PCl2))
329=(0.0750-x)/(x^2)
329x^2+x-0.0750=0
use the quadratic formula to solve for the values of x and plug this into your equilibrium values. It is important to note that total pressure ate equilibrium is found by finding the sum of partial pressures of each gas at equilibrium. Hope this helps!

Megan Lu 3D
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Re: Sapling WK 1, #4

Postby Megan Lu 3D » Sun Jan 17, 2021 11:36 am

Hi! Like others have stated above, we want to start with an ICE table.


(I) The initial pressures of PCl3 and Cl2 will be 0, as the container only consists of PCl5. The initial pressure of PCl5 is given as 0.0750 bar.

(C) Because the system will favor the reverse reaction to reach equilibrium, PCl5 will decrease and PCl3 and Cl2 will increase. Thus, we can say that the change is +x for PCl3, +x for Cl2, and -x for PCl5.

(E) Putting this all together, we get the resulting equilibrium pressures of P(PCl3) = x, P(Cl2) = x, and P(PCl5) = 0.0750 - x.


We can then set the equilibrium constant equal to the expression P(PCl5) / P(PCl3) P(Cl2), where the equilibrium pressures in terms of x from the ICE box are used, and then solve for x. After solving for x, plug the value in to solve for the partial pressures at equilibrium, and then add these all up to get your final total pressure. Hope this made sense!


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