Sapling #4

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haley f
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Sapling #4

Postby haley f » Sun Jan 10, 2021 10:53 pm

At a certain temperature, the given reaction has an equilibrium constant of Kp=343 .

PCl3(g)+Cl2(g)<->PCl5(g)

PCl5 is placed in a sealed container at an initial pressure of 0.0620 bar. What is the total pressure at equilibrium?

What is the equation used for pressure? Would it be PCl5/ (PCl3 x PCl2)?

Ryan Hoang 1D
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Re: Sapling #4

Postby Ryan Hoang 1D » Sun Jan 10, 2021 10:55 pm

Yep, that equation looks good since Kc= reactants/products (and any coefficients are exponents).

Josh Chou 3K
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Re: Sapling #4

Postby Josh Chou 3K » Mon Jan 11, 2021 12:19 pm

Yes, if you have Kp = 343, you can find the total final pressure by setting up an ICE table. In your equation Kp = [PCl5] / ('[PCl3][Cl2]), your PCl5 term would be (0.062 - x), and PCl3 and Cl2 would both be +x. You'd then have to do some rearranging to get a quadratic equation 343x2 + x - 0.062 = 0, and then you'd plug that into a quadratic formula program to get the x value. You'd then find the partial pressure of PCl5, PCl3, and Cl2 individually and then find their sum to get the total pressure in bar.

David Chibukhchian 2G
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Re: Sapling #4

Postby David Chibukhchian 2G » Mon Jan 11, 2021 1:19 pm

Yes, that's the equation you should use. I think it's important to remember that the Kc and Kp expressions are basically very similar, we just have to know when to distinguish between the two based on whether the given compositions are concentrations or pressures. In addition, it's helpful to know that we can convert between partial pressures and concentrations by using the Ideal Gas Law, PV=nRT. Rearranging it can help us understand the relationship between Kc and Kp. That's how I like to think of it, I hope that helps!

abby hyman
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Re: Sapling #4

Postby abby hyman » Mon Jan 11, 2021 1:25 pm

Yes that would be the equation. It would be helpful for this question to set up the table to determine the values in terms of x and then can use the quadratic formula to solve for x

haley f
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Re: Sapling #4

Postby haley f » Thu Jan 14, 2021 6:59 pm

Josh Chou 3K wrote:Yes, if you have Kp = 343, you can find the total final pressure by setting up an ICE table. In your equation Kp = [PCl5] / ('[PCl3][Cl2]), your PCl5 term would be (0.062 - x), and PCl3 and Cl2 would both be +x. You'd then have to do some rearranging to get a quadratic equation 343x2 + x - 0.062 = 0, and then you'd plug that into a quadratic formula program to get the x value. You'd then find the partial pressure of PCl5, PCl3, and Cl2 individually and then find their sum to get the total pressure in bar.


This is super helpful, thank you! When I plug in the quadratic I keep getting an error that says my answer includes unreal numbers, is this correct: x = (-1+/-sqrt(1^2-4(343)(.062)))/2(343)?

Queena Chu 3E
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Re: Sapling #4

Postby Queena Chu 3E » Fri Jan 15, 2021 1:11 am

Yep, that's the equation! I would recommend setting up an ICE table to get an equation in terms of x so that you can solve for x after (using quadratic formula).

Mackenzie Stockton 2H
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Re: Sapling #4

Postby Mackenzie Stockton 2H » Fri Jan 15, 2021 11:31 am

Yes, that is correct!

Now, you set the Kp expression you provided equal to the given K value. Also, set up an ice table with initial, change, and equilibrium concentrations to attain your quadratic, solve for x, and then you can find the equilibrium pressures for all species.


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