Question

The system described by the reaction
CO(g)+Cl2(g)?COCl2(g)
is at equilibrium at a given temperature when PCO= 0.30atm , PCl2= 0.12atm , and PCOCl2= 0.58atm . An additional pressure of Cl2(g)= 0.41atm is added. Find the pressure of CO when the system returns to equilibrium.

Answer 1

his is a standard RICE (reaction - initial - change - equilibrium) problem.

First of all, write down the definition of the equilibrium constant Kp. in this case, you are not told the equilibrium constant, but you are told that you are at equilibrium under certain conditions, before the additional material is added, so you can substitute in pressures and find Kp

Now consider the situation after the additional Cl2 is added. There will be a change, a certain partial pressure (call it Y) of Cl2 will react, and every time you lose one Cl2 you also lose one CO and make one COCl2. A new equilibrium will be established. You can write down the new equilibrium situation in terms of Y, and solve for Y.

Easier to do it than to describe it:

R: CO + Cl2 <=> COCl2
I: 0.30 0.12 ..... 0.58 (this is the situation after the extra Cl2 is added)
C: -Y -Y ..... + Y (these are the relative changes. Negative for a decrease. Ratios, in this case just 1:1:1, from the balanced equation)
E: 0.30 - Y ... 0.12 - Y .... 0.58 + Y

Kp = pCOCl2/(pCO x pCl2) (definition)
= (0.58 + Y)/[(0.30 - Y)(0.12 - Y)]

Insert the value of Kp and solve for Y.