Homework Help: Chem II

Posted by Ken on Monday, June 16, 2008 at 9:48pm.

The equilibrium constant, Kp for the reaction PCl5 <---> PCl3 + Cl2 is 1.05 at 250 degrees C. The reaction is started with PCl5, PCL3 and Cl2 at 0.177, 0.223, and 0.111 atm at 250 degrees C. When the reaction comes to equilibrium, the pressure of _________

PCl3 must have decreased
Cl2 must have decreased
PCl5 must have decreased
PCl5 must have increased

I am not sure how to do this problem. I have tried subing 1.05 into the Kp formula and leaving one of the pressures out, but the answers that I would get when plugged back into the formula would not give 1.05 as an answer

Chem II - DrBob222, Monday, June 16, 2008 at 11:25pm
This is about reaction quotient. Here is a site to give you some background.
http://elchem.kaist.ac.kr/vt/chem-ed/courses/equil/intro/reactquo.htm

Write the Kp expression.
Kp = pPCl3 x pCl2/pPCl5 = 1.05
Of course, substituting the values you won't get 1.05 but substitute anyway. Like this.
Reaction quotient = Qp = (0.223)(0.111)/0.177 = about 0.14 if I punched my calculator correctly. So look at the value of Qp. It is LESS THAN Kp (which is 1.05). That means the numerator (products) is too small and the denominator (reactants) is too large. So you know the reaction will go to the right as it reaches equilibrium. That means PCl5 will decrease, PCl3 will increase, Cl2 will increase. Now you only need to pick out the correct answer from your choices.

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