Download CHM 122_practice problem 1

Chemistry 122
Practice problems – Exam 1
Dang
Question I:
Multiple choices: Choose one best answer for each of the following question below (2pts each)
2.
If Kc is the equilibrium constant for a forward reaction, 2 A⇌ B, what is Kc´ for the reaction 4 A⇌
2B?
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A) Kc
2
B) Kc
C) 2 Kc
D) (Kc)2
4.
Which statement is true for a reaction with Kc equal to 2.43 × 10-12?
A)
Increasing the temperature will not change the value of Kc.
B)
There are appreciable concentrations of both reactants and products.
C)
The reaction proceeds hardly at all towards completion.
D)
The reaction proceeds nearly all the way to completion.
5.
Shown below is a concentration vs. time plot for the reaction A ⇌ 2B. For this reaction the value of
the equilibrium constant is
A) Kc < 1.
B) Kc = 0.
C) Kc = 1.
D) Kc > 1.
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6.
The following pictures represent equal volumes of aqueous solutions of three acids HA (A = X, Y,
or Z); water molecules have been omitted for clarity
Arrange the acids in order of increasing value of Ka.
A)
Ka(HZ) < Ka(HX) < Ka(HY)
B)
Ka(HX) < Ka(HZ) < Ka(HY)
C)
Ka(HZ) < Ka(HY) < Ka(HX)
B)
Ka(HY) < Ka(HZ) < Ka(HX)
7..
A solution with a hydrogen ion concentration of 3.25 × 10-2 M is ________ and
has a hydroxide concentration of ________.
A) acidic, 3.08 × 10-12 M
B) acidic, 3.08 × 10-13 M
C) basic, 3.08 × 10-12 M
D) basic, 3.08 × 10-1
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Question II
A. Consider the following reaction
C(s) + 2 H2(g) ⇌ CH4(g)
ΔH° = –74.8 kJ
Does the amount of H2(g) increases, decreases or remain constant when equilibrium of reactants and
products is subjected to the following changes?
a. The temperature decreases; Also account for the change of Kc
increases
b. The volume increases; also account for the change of Qc
Increases
c. CH4 is added
Increases
d. The catalyst is added
No change
A. If you mixed an equal concentrations of reactants and products, would the equilibrium proceed to the
right or to the left? Explain your answer using the theory of conjugated acid base pair and their relative
strength. Use the data in table 14.1 and remember that the stronger base gets the proton
a. CH3CO2H(aq) + H2O(l)
CH3CO2-(aq) + H3O+(aq)
Favor the formation of a weaker acid
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Question III
A. For each of the reaction, identify the Lewis acid and the Lewis base. Explain your answer by showing a
complete Dot Lewis structure for the two reactants. (Extra Credits – 3pts: using arrow to show how
electrons are moved)
a. NH3 +
BF3 
Base
Acid
Draw Dots Lewis structure for each
B. Consider the following reaction:
At a certain temperature, Kc = 8.5 x 10-3. A reaction mixture at this temperature containing solid NH4NS
has [NH3] = 0.166 M and [H2S] = 0.166. Will more of the solid form or will some of the existing solid
decompose as equilibrium is reached? Calculation must be shown to support your answer.
More solid
C. The decomposition of ammonia is: 2 NH3(g) ⇌ N2(g) + 3 H2(g). If the partial pressure of ammonia
is 1.6 × 10-3 atm and the partial pressures of N2 and H2 are each 0.25 atm at equilibrium, what is the
value for Kc at 400°C for the forward reaction?
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Question IV
Acrylic acid (HC3H3O2) is used in the manufacture of paints and plastics. The pKa of acrylic acid is 4.25
a. Write an acid dissociation equation
b. Calculate the pH and the concentrations of all species (H3O+, C3H3O2-. HC3H3O2, and –OH) in
0.150 M acrylic acid
c. Calculate the percent dissociation in 0.0500M acrylic acid.
b. X = 0.0029
2.54, 2.9 x 10-3, 2.9 x 10-3, 0.147, 4.3 x 10-12
c. 3.4 %
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