chemistry class problem

[abbot]

could you help me with this, we're learning this in ap chem, and i
don't get it

given A(g) + B(g) <----> C(g) + D(g) consider the following
situations,
i) You have 1.3 M A and 0.8 M B initially.
ii) You have 1.3 M A, and 0.8 M B, and 0.2 M C, initially
iii) You have 2.0 M A and 0.8 M B, initially.
Order the preceeding situations in terms of increasing equilibrium
concentration of D. Explain your order. then give he order in terms
of increasing equilibrium concentration of B and explain.

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Posted at:
www.GroupSrv.com
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[Bob]

On 6 Nov 2004 16:30:20 -0600, jla...@hotmail-dot-com.no-spam.invalid
(abbot) wrote:

>could you help me with this, we're learning this in ap chem, and i
>don't get it

Have you read the section on Le Chatelier’s Principle? If not, go read
it, and do some of the (perhaps simpler) problems from the book.

For example, assuming you have an equilibrium mixture of all four
chemicals (for your rxn below), what would happen if you add some more
A?

Now look at the cases below. I'm inclined to suggest that you start by
comparing i and iii.

If you still have trouble, post a more specific question, so we can
focus on where you are stuck.

bob

[Mark Tarka]

jla...@hotmail-dot-com.no-spam.invalid (abbot) wrote in message news:<418d507c$1...@Usenet.com>...

> could you help me with this, we're learning this in ap chem, and i
> don't get it

Yep.

>
>
> given A(g) + B(g) <----> C(g) + D(g) consider the following
> situations,
> i) You have 1.3 M A and 0.8 M B initially.
> ii) You have 1.3 M A, and 0.8 M B, and 0.2 M C, initially
> iii) You have 2.0 M A and 0.8 M B, initially.
> Order the preceeding situations in terms of increasing equilibrium
> concentration of D. Explain your order. then give he order in terms
> of increasing equilibrium concentration of B and explain.
>
> *-----------------------*
> Posted at:
> www.GroupSrv.com
> *-----------------------*

Try asking one of the other students in the class.
I doubt if anyone here could understand this
problem, let alone give the correct answer -- they're
all pretty lame when it comes to fundamental
undergraduate chemistry concepts. If anyone other than
Bob is stupid enough to post an answer, don't believe
it -- they're lying (practicing for job interviews
with the government).

Mark (Try the library, get a different text book, read it :-)

[Abast44]

From the information you've given me, it looks like you're just supposed to
apply Le Chatlier's principle to each case, not grind out numerical answers.
Obviously, it would have been nice to have values of K...but that would have
rendered the problem completely trivial. As you know, for a system at
equilibrium, adding more reactant will cause the conversion to product to
increase and vice versa until said equilibrium is reestablished.

For the first case (eq. conc. of D), the order should be II,I,III. You'll get
the most amount from rxn. III as the 2 moles of A (and the lack of any products
to start with) will drive the reaction strongly to the right. Rxn I is just
like the last case, but won't go to the right as stongly, due to decreased
amount (well, technically concentration...but you know that M = mol/L) of A. II
will form the least amount of D, as now you have some product initially, so
equilibrium will have to be established with the initial 0.2 moles of C, which
will drive down the total amount of D that is formed. The second case is just
like the first, but reversed (III,I,II). Just apply similiar reasoning as I did
for that one and you'll have your explanation.

Also, I just found a webpage that has a java applet that'll let you play with
conditions for a reaction. Maybe it'll help you get more of a grasp on chemical
equilibrium.
http://www.chm.davidson.edu/java/lechatelier/lechatelier.html

Hope this helps!
-Shane