Ch 15 Volume and K Questions

[Zellmer, Robert]

I got the following question, which I've received in previous semesters in some form.

**********
"I am currently reviewing some chapter 15 concepts and am a little confused pertaining
to when the volume matters in an equilibrium problem. I know that volume does not
affect the K constant, but I remember for some problems done in this chapter, if the
question was given and included a volume of 2L, we would have to divide each value
by 2L to get the 1L value.

I may just be completely misinterpreting something, but for example I am on question
15.33 of the EOC questions and the volume is said to be 5L. Why are the pressure values
given not divided by 5L to obtain the 1L value?

I guess in conclusion, when should we worry about the volume of a vessel, if ever? I do
know shifting in a reaction is affected by volume and pressure.

**********

The value of K doesn't depend on the volume (although the molarities and pressures used
in K do depend on the volume of the container).  So, when is volume important?

If a problem gives you moles and a volume and asks you to solve for Kc or you're given
Kc and asked to find the conc. of one of the substances in the expression for Kc you need
to convert the moles to molarities.  You need to use molarities in the expression for Kc.

For a Kp you should be using partial pressures in atm.  If you're given masses or moles
of substances, temperature and the container volume you need to convert to pressure (in
atm) before substituting into the express for Kp.

Therefore, when we do ICE table problems we convert to molarities or atmospheres and
use those in the ICE table.  If you don't use molarities in Kc or atm in Kp you aren't going
to get the correct answers for the conc. or pressures at equilibrium.

I know this can get a little confusing because in Ch 16 & 17 we will need to use moles for
neutralization reactions when a strong acid or base is added to a buffer system or when
doing a titration.  A neutralization rxn involving a strong acid or base goes to completion and
we're dealing with a limiting-reactant stoichiometry problem.  These are done in moles using
a BCA table (or as I call them, ICC table, Initial-Change-Completion).  These tables look like
ICE tables but are done in moles.

For EOC exercise 15.33 it gives a rxn and partial pressures in atm and asks you to determine
Kp at 500.0 K in part (a).  In part (b) it gives a volume of 5.0 L and asks for Kc.  The volume
is a "red herring", it's not needed to answer the question.  You should remember the
relationship between Kp and Kc,

            Kp = Kc (RT)^delta(n)

There is no volume in this eqn. so you don't need it.  It's one of those questions that really
gets at how confident you are with the material.  You need to quickly recognize you don't need
the volume to answer the question.  This is like being given the density in a conc. conversion
problem when you don't need it.

Dr. Zellmer
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