To make a long story short, my youngest daughter decided to endure
through 4 semesters of chemistry, and now is paying up for it. She is
a talented little girl, but science is not her thing and it hurts for
me to see her study so hard but understand so little. It runs in the
family. I wanted to help myself, but me and my wife are no scientists
at all, plus it was so long ago. Tutoring is very very weak at where
she goes, and we know few friends around here who are capable of
helping, most of which already are looking at us the wrong way for
asking so much. I decided to open my pocket and let you, Chemistry
folks, help us out. I secretly took the next set of questions from my
daughters assignment section, and here they are. I understand that
many (or most) of you would have the problem to giving direct help
like this, "learning and understanding the material is the most
important", right ? Wrong. Only but a few of you would understand the
pain I feel when I see my daughter quietly cry in her room over her
low assignment grades after hours and hours of studying daily.
 
Below I copied down some of the questions and choice answers that came
along. I tried my best to copy down everything number to number, word
for word. Its a little bit difficult to transfer everything to text,
but I tried. Please feel free to clarify if you did not understand a
part.
 
I am asking for a list of answers to the problems, in numerical +
content format, as such :
 
Question 1: Answer 3. The number of moles is 4.0. 
Question 2: Answer 5. The reaction is reversed. 
 
Time is an issue, but I would like the answers to be correct. I do not
want to check my daughters answers and tell her that one of her
correct answers is incorrect simply because what was given to me is a
wrong answer. Once again I apologize for such rude question, but I am
desperate, and I ran out of all other options. I am also considering a
big tip for those who'll work hard on these, do all (or most) of the
questions, provide the correct answers and hopefully as soon as
possible.
 
Thank you once more. You are my last hope. 

1) Explain why equilibrium constants are dimensionless.


1. The statement is not true. Equilibrium constants have units that
involve some multiple of atmospheres of moles per liter.

2. They are not really dimensionles but we must treat them as such in
order to be able to take ln(k) in the expression Delta G (deg) = -RT
ln(K)

3. Every concentration or pressure that enters into Kc or Kp is really
divided by the corresponding concentration or pressure of the
substance in its standard state.

4. They are dimensionless because the pressures or concentrations we
put in are all for the substances in their standard states.

5. They are dimensionless because concentrations and pressures have no
units.

2) A 1.000 liter vessel is filled with 1.000 mole of N2, 2.000 moles
of H2, and 3.000 moles of NH3. When the reaction

	N2(g) + 3H2(g) <--> 2NH3(g)

comes to equilibrium, it is observed that the concentration of NH3 is
2.67 moles/liter. What is the numerical value of the equilibrium
constant, Kc ?

<Numerical Answer>

Hello 

1)  This statement is untrue - equilibrium constants have units that
involve multiples of atmospheres and/or moles per liter.

The equilibrium constant is defined as the ratio between either the
partial pressure or concentration of reactants and the the partial
pressure or concentration of products.

such as:

       A
k = -------   where A is the partial pressure of the product A and
      B * C   B and C are the partial pressures of the reactants B and
C

similarly, in solution

    [C][D]
k = ------- where [C] and [D] are the concentrations of the products
and
    [A][B]  [A] and [B] are the concentrations of the reactants

note, the equilibrium constant can be dimensionless (as in the last
example) in cases where the units of the products exactly equal the
units of the reactants.  In the first example this is not the case,
and the equilibrium constant would have units   1/atmosphere.


2) Looking at the stoiciometry of the equation (the number of
reactants to products), we can see that one mole of nitrogen plus
three moles of hydrogen yields two moles of ammonia.  If we started
with 3.0 moles of ammonia and now have only 2.67 moles, then we
converted 0.33 moles of ammonia into nitrogen and hydrogen.  As the
ratio of ammonia to nitrogen is 2:1, this means that we produced 0.165
moles of nitrogen.  The ratio of ammonia to hydrogen is 2:3, so we
produced 0.495 moles of hydrogen.  Adding these values to our starting
values gives us:
    starting  equilibrium
N2 -  1.000      1.165
H2 -  2.000      2.495
NH3 - 3.000      2.67


The equilibrium constant is given by the equation (where ^ indicates
raised to the power)

      [NH3]^2            (2.67 moles/L)^2
k = -------------   =  --------------------------------- =  0.148
L^2/moles^2
    [N2] * [H2]^3       1.165 moles/L* (2.495 moles/L)^3


A good, simple reference for this type of question:
http://www.hyper-ad.com/tutoring/chemistry/chem_eqm.htm#The%20Equilibrium%20Constant

Please let me know if you wish further explanation/clarification.

synarchy

search:
equilibrium chemistry

Thank you, you're a great help!