PCl5 example in lecture

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Zaynab Hashm 2I
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PCl5 example in lecture

Postby Zaynab Hashm 2I » Fri Jan 10, 2020 3:26 pm

During today's lecture, a student asked what would we change X to if PCl3 had a stoichiometric coefficient of 2 instead of 1, I think Professor Lavelle said that it would be 2X. Is that right? (I wanted to check if my notes were correct!)

Also, when would it be 2X^2 for PCl3? (he mentioned it, but I didn't quite catch that.)

Can someone also elaborate on the logic behind those changes and how we must modify X for different examples?

Thanks in advance.

Alex Tchekanov Dis 2k
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Re: PCl5 example in lecture

Postby Alex Tchekanov Dis 2k » Fri Jan 10, 2020 3:31 pm

When you are doing the ice tables, the number in front of the x is equal to the stoichiometric coefficient.
Here's an example with different coefficients:
N2 + 3H2 = 2NH3
x 3x 2x

And now when you are writing the equilibrium constant, K, you have to use those stoichiometric coefficients to raise to the power.
K= [N2][H2]^3/[NH3]^2

Then finally, you plug in the values you found earlier (with the x's) into the K expression

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Re: PCl5 example in lecture

Postby MinuChoi » Fri Jan 10, 2020 3:40 pm

The 2X change is correct.
Remember, the Xs represent how much product is produced when reactant is consumed, and this is based on the stoichiometric coefficients.
The 2x^2 is probably referring to the K constant.
From his example, we used the ICE chart's equilibrium values to determine constant K = (X^2)/(3 - X).
If, in that reaction, PCl3's coefficient changed from 1 to 2, its equilibrium value from the ICE chart would now be 2X.
The K constant would instead be (X*2X)/(3 - X) = (2X^2)/(3 - X)

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