Help!! I don't understand why equilibrium constant doesn't change with volume?

[Padfoot]

---

Members don't see this ad.

Okay, so I'm doing question 32 on Passage V in the Equilibrium section for TBR Gen chem.

Here's the reaction: 1HCl(g) + 1CO(g) <--> 1HCOCl(g)
The question asks how the partial pressures of the gases will be affected and if the equilibrium changes if the volume increases at constant temperature.

Here's the equilibrium formula: PHCOCl/(PHCl* PCO)
It says that the ratio of PHCl to PHCOCl will increase, since the reaction will move left to compensate for the greater volume, which I understand. But I don't understand how equilibrium constant stays the same, since PHCl and PCO both increase, while PHCOCl decreases?

I get the equilibrium constant stays the same no matter what except for temperature changes, but I don't understand it on a mathematical level.
If someone can explain this to me it would be much appreciated!

 

[SuperSneaky24]

So at constant temperature, as volume increases, pressure decreases according to:
PV/T = PV/T

So when pressure decreases, equilibrium favors the side with more particles, as there is now more room for molecules to be free. Therefore, increasing the volume of the reaction ultimately causes the equilibrium to shift to the left side. I think you're thinking that both HCl, CO, and HCOCl decrease/increase at the same |rates|, but in actuality it is much more complicated than that. The equilibrium constant is never affected by concentration because when one side is low, the other side increases to immediately compensate. Now in this example, the equilibrium constant stays the same because the concentration of HCl and CO increases at the same total rate HCOCl decreases.

Edit: whoops, I meant partial pressures, not concentration. But they work basically the same way

 

[Padfoot]

So at constant temperature, as volume increases, pressure decreases according to:
PV/T = PV/T

So when pressure decreases, equilibrium favors the side with more particles, as there is now more room for molecules to be free. Therefore, increasing the volume of the reaction ultimately causes the equilibrium to shift to the left side. I think you're thinking that both HCl, CO, and HCOCl decrease/increase at the same |rates|, but in actuality it is much more complicated than that. The equilibrium constant is never affected by concentration because when one side is low, the other side increases to immediately compensate. Now in this example, the equilibrium constant stays the same because the concentration of HCl and CO increases at the same total rate HCOCl decreases.

Edit: whoops, I meant partial pressures, not concentration. But they work basically the same way

I'm confused because isn't the equilibrium constant the partial pressure of HCOCl divided by the partial pressures of HCl and CO? So if the HCOCl decreases, that means the numerator of Keq gets smaller. And as HCl and CO concentrations increase, the denominator is now larger. It said that the equilibrium constant is PHCOCl/(PHCL x PCO), so that's why I feel like it should get smaller.

 

[1bls2g]

The Q changes but Keq does not. Keq only changes with temperature.