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### Quiz 3 Prep #7, setting up ICE box

Posted: Mon Dec 01, 2014 4:07 pm
Problem: For the following reaction, if the initial concentration of PCl3 (g) is 1.0M and x is the equilibrium concentration of P4 (g), what is the expression for the equilibrium constant? PCl3 <--> P4 + Cl2

I balanced the equation... 4PCl3 <--> P4 + 6Cl2

In setting up my ICE box, I realize that the initial for P4 is 0, and that you would add "x" to get your equilibrium value...I still don't understand what to add/subtract for the other components of the reaction? Would I add 6x for Cl2 and subtract 4x from PCl3 because of the stoichiometric coefficients?

Thank you!

### Re: Quiz 3 Prep #7, setting up ICE box

Posted: Mon Dec 01, 2014 4:41 pm
4PCl3 <---> P4 + 6Cl2
I: 1.0M 0 0
C: -4x +x +6x
E:1.0-4x +x +6x

the universal rule about ICE Box is that the ratios of "change" are the same as in the formula. Thus, the ratios: PCl3:P4:Cl2=4:1:6, are seen in the formula and the amount of change is the same.

### Re: Quiz 3 Prep #7, setting up ICE box

Posted: Mon Dec 01, 2014 8:02 pm
The expression for the equilibrium constant would thus be:

(x)(6x)6 = Kp
(1-4x)4

Correct?

### Re: Quiz 3 Prep #7, setting up ICE box

Posted: Tue Dec 02, 2014 1:37 am
That is what I got.

### Re: Quiz 3 Prep #7, setting up ICE box

Posted: Tue Dec 02, 2014 4:23 pm
Is it Kp or Kc?

### Re: Quiz 3 Prep #7, setting up ICE box

Posted: Tue Dec 02, 2014 4:28 pm
Kc I believe (talks about equilibrium concentration instead of equilibrium partial pressures)