7th Edition HW Problem 5I.29

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Faith Fredlund 1H
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Joined: Fri Sep 28, 2018 12:18 am

7th Edition HW Problem 5I.29

Postby Faith Fredlund 1H » Mon Jan 14, 2019 5:04 pm

At 25°C, K=3.2X10^(-34) for the reaction 2HCl(g) --> <-- H2(g) + Cl2(g). If a reaction vessel of volume 1.0L is filled with HCl at 0.22 bar, what are the equilibrium partial pressures of HCl, H2, Cl2?

I understand how to fill out the ICE table, but am unsure of how to complete the calculation. The solutions manual says to just assume that x<<0.22 and to use 0.22 for the partial pressure of the reactant (HCl), but I'm not sure why. If someone could explain why it is okay to do this and when you know when it is okay to assume information like this, that would be amazing.

David Effio 1H
Posts: 38
Joined: Wed Feb 14, 2018 3:01 am

Re: 7th Edition HW Problem 5I.29

Postby David Effio 1H » Mon Jan 14, 2019 5:37 pm

I believe you would use 0.22 as the Ppressure of HCl because it was present at the start. As it is the only reactant present, the pressure present in the container has to come from the HCl, as there is nothing else within the container.

(Side note: You would assume nothing else is present in the container as the problem has not specified that there is any other reactant or product at the start of the reaction).

You should expect x to be less than 0.22 because in a reversible reaction like this one, all the reactant will not CHANGE into all the product. Therefore, part of the partial pressure from the reactant will remain, while some of the pressure in the container will come from the partial pressures of the products.

I hope that makes sense!

Camille Marangi 2E
Posts: 60
Joined: Fri Sep 28, 2018 12:26 am

Re: 7th Edition HW Problem 5I.29

Postby Camille Marangi 2E » Mon Jan 14, 2019 5:44 pm

I assume you're asking when it is ok to assume that 0.22-x can be simplified to just 0.22. Lavelle said it was fine to make that assumption as long as the equilibrium constant is at least 10^-3 smaller. So if there are 3 degrees of separation, then your initial value is at least 1000 times larger than the Kc value and it's safe to make the decision to discount the x. So is the case with this problem as K=3.2x10^-34 and the initial value is 0.22-- clearly the initial is way larger than the equilibrium constant and you can disregard the "-x".

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