Sapling #4

[haley f]

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]

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

[Josh Chou 3K]

Yes, if you have K_p = 343, you can find the total final pressure by setting up an ICE table. In your equation K_p = [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]

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]

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]

Yes, if you have K_p = 343, you can find the total final pressure by setting up an ICE table. In your equation K_p = [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]

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]

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.