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) For the heterogeneous reaction

	2MnO2(s) <---> 2MnO(s) + O2(g)
the equilibrium constant expression for Kc is which of the following ?

1. K = 1/[O2]
2. K = k[MnO2]
3. K = ([MnO][O2])/[MnO2]
4. K = [MnO2]/([MnO][O2])
5. K = [O2]

2) Equilibria involving species in more than one phase are called

1. Heterogeneous equilibria
2. Homogeneous equilibria
3. Reversible reaction
4. Chemical equilibria

3) When a reaction reaches equilibrium

1. all reaction stops

2. the rate of the reverse reaction is zero

3. the rate of forward reaction is zero

4. the forward and reverse reaction rates are equal

5. the rate of the forward reaction and the reate of the reverse reaction 
are equal to zero

Hello again -

1)  The equilibrium expression is written with the concentration of
products over reactants as:

 
       [C]^x[D]^y        where [A][B][C][D] are concentrations of
A,B,C,D
k = --------------       and x,y,a,b are the stoichiometric number of
that
       [A]^a[B]^b        compound in the reaction (ie the number of
molecules)

so in this case: 2MnO2(s) <---> 2MnO(s) + O2(g)

     [MnO]^2 * O2    in this case the partial pressure of O2 is used
instead
k = --------------   of a concentration as O2 is a gas
     [MnO2]^2

this answer is closest to your answer 3 - k = [MnO2]^2 * O2 / [MnO2]^2


2) equilibria involving species in multiple forms are "heterogenous
equilbria", "heterogenous" meaning from different sources.

answer - 1

3) When a reaction reaches equilibrium, it doesn't mean that the
reaction has stopped, merely that the forward and reverse rates are
the same.  This is given by answer 4.


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

synarchy

reference:

A nice site with a tutorial on chemical equilibria:
http://www.learnchem.net/tutorials/cequil1.shtml

Another good site:
http://www.sfu.ca/person/lower/Chem1Text/equilibrium/

A simpler site that doesn't address all of the issues, but is good
background:
http://www.biology.ucsc.edu/classes/bio3/TUT/CHEM_TUT/equil.html

search
chemical equilibrium tutorial

---

Clarification of Answer by synarchy-ga on 15 Oct 2003 20:47 PDT

It is likely that one person has locked the question while they try to
answer it - it is possible for a researcher to continue to "re-lock" a
question and thus prolong the amount of time that they have to answer
it.  I will watch for it to come unlocked and will try to answer it if
possible.

Thank you again ! You have helped us so much today, and I am forever
grateful. Now I am just waiting for help with that last large set of
questions. Do you think that something happened, because I see that
its locked for more than 3 hours, or is it perhaps that different
people tried to answer the questions. Do you think it would be wise
for me to break them all up ?

As I am not an answerer forr google, but do have an advanced degree in
chemistry and have taught the subject, I would suggest looking for a
tutor at a local college, either an advanced undergraduate student,
but preferably a graduate student. Expect to pay about $20-30 per
hour, but I think she would learn alot more than having the answers.