## PCl5 example in lecture

Zaynab Hashm 2I
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### PCl5 example in lecture

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?

Alex Tchekanov Dis 2k
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Joined: Sat Aug 24, 2019 12:16 am

### Re: PCl5 example in lecture

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.
Example
K= [N2][H2]^3/[NH3]^2

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

MinuChoi
Posts: 100
Joined: Wed Sep 18, 2019 12:15 am

### Re: PCl5 example in lecture

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)