Problem Set #10 (18:170 CAPA) - 9 Marks
These "hints" should ONLY be consulted after you have given serious effort to
answering these questions with your own efforts. Remember, it’s more important
to be able to identify the method of solving a particular problem than in just
getting a correct answer; after all, there aren’t any "hints" on an exam paper.
Question #1: (2003 mid-term)
These statements will confirm that you do indeed understand the theory behind the
concept of equilibrium. If not, read over your notes and Chapter 15 a bit more carefully.
Question #2-#3:
Some simple math, just to get practiced up for the calculations that lie ahead.
a)
draw an Initial/Change/Equilibrium (ICE) table
b)
fill in the Initial quantities of each reagent and product.
c)
fill in the Equilibrium quantity of the given material.
d)
calculate the other quantities and fill in all nine boxes of the ICE table.
Questions #4-#5:
Some more simple math, albeit with a different example.
a)
draw an Initial/Change/Equilibrium (ICE) table
b)
fill in the Initial quantities of each reagent and product.
c)
fill in the Equilibrium quantity of the given material.
d)
calculate the other quantities and fill in all nine boxes of the ICE table.
By now you should have developed an approach that will do you well in the problems
ahead.
Questions #6-#7: (2002, 2003 m-t, 2003 final, 2004 m-t)
Let's step the math up a notch, not too difficult, but requiring an equilibrium expression.
For the reaction:
x B(g)
—>
y A(g)
+ z C(g)
y
z
the equilibrium expression will be: Keq = [A] [C]
[B]x
Question #8: (2001, 2003 m-t, 2004 m-t)
Calculating concentrations and inserting them into an equilibrium expression in order to
determine a K:
Keq = _____[PCl5]_____
[PCl3] [Cl2]
If you can't get an answer on the first try, I'd suggest you go back and do Questions #1
through #7 for the necessary practice.
Questions #9-#10: (2003 f)
On the same difficulty level as #8, but now you've got to think in terms of an ICE table.
a)
draw an ICE table.
b)
put in the Initial quantity of A.
c)
what level of dissociation of A will give you equal quantities of A, B, and C?
You might want to try out a few amounts to notice the trend that develops.
a)
substitute your Equilibrium values into the equilibrium expression:
Keq = [B][C]
[A]
Questions #11-#12: (2001, 2002, 2003 m-t, 2004 m-t, 2004 f)
Duplicates. However, it’s good to revisit the theory before we get into the tough
questions. Check your notes again if you are having difficulty with the concepts.
Question #13:
The Kc for a reverse reaction is the inverse/reciprocal of the Kc for the forward reaction.
Question #14: (2002, 2003 m-t, 2004 f)
When the molar volumes of the reactants is different from the molar volumes of the
products:
Kp = Kc (RT)delta n
Question #15: (2004 f)
a)
calculate the molar concentrations of each reagent
b)
substitute into the equilibrium expression:
Keq = ____[NH3]2_____
[N] [H2]3
Questions #16:
a)
"tweek" the units of mass and molecules into terms of moles
b)
divide by the volume, and you've got concentration values that can be used in an
equilibrium expression:
Keq = [PCl3] [Cl2]___
[PCl5]
Question #17: (2003 f)
a)
determine delta”n” for the reaction
b)
convert % to mass, then to moles, then to atmospheres.
c)
substitute into the equilibrium expression:
Kp = p CO 2
p CO2
Question #18: (2003 m-t)
Since we already know that
Kp = Kc (RT)delta n
this should be a breeze.
Question #19:
Back to the theory, one more time. However, last year some students found that one of
their six statements was:
The variable "s4h" selected by choose() has not yet been defined.
Please note that this is code, not the actual question (which is unknown). Therefore, in
this situation you’ll have to answer the other five parts, and then guess either T or F for
this “statement”.
Question #20: (2003 m-t, 2004 m-t)
a)
calculate Kc or Kp for each of the five situations.
b)
delta(n) for this equilibrium is zero, therefore Kc = Kp.
c)
if the Kc / Kp value is less than 0.090 then the equilibrium shifts RIGHT; if the
value is less than 0.090 then equilibrium shifts LEFT; if the value is 0.090 then
there is no change.
Question #21-#22: (2003 m-t, 2004 f)
Remember that "quadratic equation"? Well, you get to use it here, several times over.
These questions are probably too tough for a first year course, but a nice challenge.
Any takers?
Question #23-#24: (2004 f)
Method 1:
a)
construct an ICE table for the equilibrium concentrations of each species
b)
substitute these values into an equilibrium expression.
Ensure that the HF numerator of the equilibrium expression is shown as
(2x)E+02.
Method 2:
a)
take the square root of all parts of the expression. This will avoid using the
quadratic equation method of solution and give only one possible solution.
You will prefer Method 2.
Question #25:
Construct an ICE table for the equilibrium concentrations of each species, with the
additional H2 added.
However, this time you will have to use the quadratic equation method of solving.
Questions #26 - #28: (2004 m-t)
These may be optional questions, but they are the exact equilibrium that you analyse in
Lab Procedure 170-07, Colourimetric Determination of Kc.
Questions #29: (2001, 2002, 2004 m-t)
A great review of your comprehension of the theory for this, and previous, parts of the
course.
Question #30:
Combining equilibrium constants. There is no worked example in Chang.
However, those who are using Brown, LeMay, & Bursten can try Review Sample
Exercise 15.4 (9th ed.).
Questions #31-#32: (2001, 2002, 2004 m-t)
Take it slow and careful, so as not to make silly mistakes since some of the possible
answers are tricky.
Suggestion:
a)
write down the equilibrium expression for each of the four parts of each question
b)
see which one of the possible solutions actually match your efforts.
Hint: What happens to an equilibrium expression when one of the species is in its
liquid or solid form?
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