CHM 2046C Module Ten-Chapter 16 Sample Test
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Module Ten: Chemical Equilibria Chapter 16 |
Possible |
Actual |
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A.
Equilibrium Constant Derivation from Reaction Rates Lecture |
10 |
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B.
Writing Equilibrium Constant Expressions Section 16.2 p760-1 |
10 |
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C.
Meaning of the Equilibrium
Constant: K; Section 16.2 p765-7 |
10 |
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D.
Meaning of the Equilibrium
Quotient: Q; Section 16.2 p767-9 |
10 |
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E.
Determination of Equilibrium Constants from Lab Data Section 16.3-16.4 |
10 |
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10 |
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G.
Determination of Equilibrium Conc from Kc Problems Section 16.4-16.5 |
10 |
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10 |
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10 |
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J.
Key Terms - Chapter 16 & More about Equations & Eq Coefficients 16.5 |
10 |
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25 |
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Module Ten Total: |
125 |
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Part A: Equilibrium Constant Derivation 10 points
Derive the equilibrium constant expression from the rate expressions of the following reactions:
a A + b
B ç è c C + d D
McMurry: Read Chapter 13 Section 13.1, 13.2 and this derivation is
shown in Section 13.11 p 528-529. Look at worked example 13.15. Look at
Questions 13.90-13.95 on page 538 for good multiple choice questions for the
exam.
For the equilbrium reaction above, lets assume the forward reaction
and the reverse reaction are elementary reactions. Then we can write a rate
expression for each:
Rateforward = kforward [A]x[B]y
And
Ratereverse = kreverse [C]z[D]w
Now what about the
subscripts: x,y,z,w?
Since this is a dynamic equilibrium, the
forward reaction may be considered an elementary step in the mechanism of the
reactions and we may use the reactions stoichiometry:
therefore
x=a, y=b
Rateforward = kforward [A]x[B]y which is =
to kforward [A]a[B]b
And the stoichiometric
coefficients become the rate order of each reactant.
Likewise, for the reverse reaction the stoichiometric coefficients become the rate order of each
product:
so that z=c, w-d
or
Ratereverse = kreverse [C]z[D]w which is =
to kreverse [C]c[D]d
When dynamic equilibrium is achieved:
The
Rateforward = Ratereverse
Therefore: kforward [A]a[B]b = kreverse [C]c[D]d
Divide both sides by kreverse and [A]a[B]b
And a constant divided
by a constant is a constant:
kforward /kreverse = Kc or
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[C]c [D]d |
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Kc = --------------- |
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[A]a [B]b |
Part B: Equilibrium Constant Expressions Answers 10 points
McMurray: Chapter 13 Section 13.2. Study the worked example
13.1 p 501, Worked example 13.3 p 503. Try Problems 13.1 and
13.3 p 503. Look at Worked example 13.7 in Section 13.4 p 507 and try
Problem 13.7 on page 508. At the end of the chapter are many additional
problems 15.40-15.61 pages 535-536.
Write equilibrium
constant expressions, Kc, for the following reactions they represent:
a. PCl3 (g) + Cl2 (g) ç è PCl5 (g)
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[PC5]1 |
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Kc = --------------------- |
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[PCl3]1
[Cl2]1 |
b. 2 NOCl (g) ç è 2 NO (g) + Cl2 (g)
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[NO]2
[Cl2]1 |
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Kc = ------------------- |
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[NOCl]2 |
c. 4 HCl (g) + O2 (g) ç è 2 H2O (g) +
2 Cl2 (g)
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[H2O]2
[Cl2]2 |
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Kc = --------------------- |
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[HCl]4
[O2]1 |
d. CS2
(g) + H2 (g) ç è CH4
(g) + H2S (g)
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[CH4]1 [H2S]1 |
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Kc = --------------------- |
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[CS2]1
[H2]1 |
e. CaCO3 (s) ç è CaO (s) + CO2 (g)
Kc = [CO2]1
Note: the concentrations of any solid reactants or products are omitted
from the equilibrium constant expressions as long as some of each solid is
present.
f. NH3 (g) + H2O (l) ç è NH4 1+(aq) + OH1- (aq)
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[NH4 1+]1 [OH1-]1 |
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Kc = ----------------------- |
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[NH3]1 |
Note: There are special considerations for reactions
occurring in aqueous solutions (or in any solvent). Because the water
concentration is very high in the above equilibrium, the concentration of the
water is unchanged by the reaction. As for solids, the
molar concentration of water (55.5M) (or any other liquid reactant or product)
is omitted from the equilibrium constant expression.