Chemistry Question

[FishyTheFish]

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Consider the Haber Process occurring in a rigid container:

N2 + 3H2 <--> 2NH3

Decreasing the volume would increase the pressure and because there are 4 moles of gas on the left side and only 2 on the right side, the products would be favored, right? Well mathematically, no.

Consider Q , the reaction quotient, for this reaction: Q = [NH3]²/[H2]³[N2].
We can rewrite the concentrations as moles/L (so [NH3]² becomes (moles NH3)²/L² where L is the volume of the container)

Doing this for all the concentrations,Q = L² * (moles NH3)²/[(moles H2)³*(moles N2)]

By this expression for Q, if we were to decrease the volume of the container, L² would decrease and thus Q would decrease, meaning there would be less product and more reactant.

So why is this and which one is actually correct?

 

[milski]

Your first description is right - you will be getting more product.

You cannot say that Q increases because none of the moles are constants - Hmoles^3* Nmoles can is increasing and NH3moles is decreasing. You cannot evaluate the whole expression without having some sort of dependency between the four variables.

 

[MedPR]

Consider the Haber Process occurring in a rigid container:

N2 + 3H2 <--> 2NH3

Decreasing the volume would increase the pressure and because there are 4 moles of gas on the left side and only 2 on the right side, the products would be favored, right? Well mathematically, no.

Consider Q , the reaction quotient, for this reaction: Q = [NH3]²/[H2]³[N2].
We can rewrite the concentrations as moles/L (so [NH3]² becomes (moles NH3)²/L² where L is the volume of the container)

Doing this for all the concentrations,Q = L² * (moles NH3)²/[(moles H2)³*(moles N2)]

By this expression for Q, if we were to decrease the volume of the container, L² would decrease and thus Q would decrease, meaning there would be less product and more reactant.

So why is this and which one is actually correct?

More pressure = favor side with less mol of gas. Rules are rules. If you want to redefine them, go get a phd in chemistry and forget about med school. Otherwise, just use the quick shortcuts for MCAT and don't look back 👍

 

[chemtopper]

This is applicable when Q= K(equilibrium constant)
if you decrease volume this will try to decrease Q and hence the equilibrium will shift towards the product to make it again equal to K