Understanding Equilibrium Direction for PCl5 Reaction

[bobsmith76]

Homework Statement

In what direction will the reaction go if the volume is increased.

PCl₅⇔ PCl₃ + Cl₂

The Attempt at a Solution

I would think using the ideal gas equation PV = nRT that since V goes up P must go down. In order for P to go down you need less molecules. There are less molecules on the left side of the equation, so the direction should be to the left but the book says to the right.

 

[miniradman]

The volume of what is increased?

 

[miniradman]

sorry, I don't know what I was saying in my last post (for some reason the editing screen wouldn't load)

I like your logic with the whole PV =nRT, however the conclusion you've come to I disagree with (and so does your textbook :P).

You cannot simply "have less molecules". This is a closed system, so nothing gets in or gets out, the it's constant.

Take opening a soft drink can as an analogy.

Carbon dioxide(aq) <=> Carbon Dioxide(l)

if you opened this a can of soft drink, which way do you think the position of the equilibrium will be in favour of?

 

[bobsmith76]

But on the right side of the equation there are twice as many molecules as on the left side. The number of atoms is the same on both sides, but no the same amount of molecules.

 

[miniradman]

yes that may be, but the volume and the pressure has been changed. It will tell you which one is will drive the equilibrium... in favour of the products, or the reactants...

Have you learned anything about entropy?

 

[bobsmith76]

I have learned about entropy but I don't see how it applies in this case.

 

[miniradman]

hmmm... i was going to try and make connection, but I am having second thoughts.

when pressure is increased, density is also increased right?

which one is more dense out of the two?

 

[Borek]

Think in terms of LeChatelier's principle.

When the volume goes up, pressure goes down, system tries to increase the pressure.

 

[bobsmith76]

Yes, I know that pressure goes down. That's not helping me find out whether or not more products are being consumed or produced. I would think that because the pressure is going down the number of molecules would also go down which would shift the equation to the left since there are less molecules on the left.

 

[Borek]

You have read only a small part of my post and you are still not seeing the bigger picture.

What does the LeChatelier's principle state?

 

[bobsmith76]

Here's L's principle

If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

I'm still in the dark. The volume increases so the pressure decreases to restore equilibrium. I would think if pressure decreases than more reactants would form for reasons I've already stated.