Chemistry 122
Chapter 18-19 PRE-TEST: Chemical Equilibrium, Including Acid-Base
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I.
Multiple Choice
1.
Equilibrium is theoretically defined as
a. a balance between forward and reverse processes. b. constant changes in observable properties.
c. constant microscopic properties of a closed system. d. two unrelated reactions occurring simultaneously.
2.
A 1.0 mol amount of solid calcium hydroxide is added to water to make 1.0 L of solution. At 25°C, the reaction
at equilibrium is 8.0%.
Ca(OH)2(s)X Ca2+(aq) + 2 OH-(aq)
The correct interpretation of this evidence is
a. the products are favored. b. the concentration of hydroxide ions is 0.08 mol/L.
c. the concentration of calcium hydroxide is 0.92 mol/L. d. the concentration of calcium ions is 0.16 mol/L.
3.
A benzoic acid solution was prepared as a 0.50 M solution. At equilibrium, 0.48 mol of benzoic acid was
present in the solution. Based on this evidence, the % reaction for lactic acid is
a. 96 b. 48 c. 4 d. 2
4.
The formation of acid rain from sulfur dioxide emissions can be demonstrated in a lab, as shown by the
following equilibrium expression:
SO2(g) + H2O(g) X H2SO3(aq) + energy
Which of the following conditions would help prevent the forming of sulfurous acid?
a. adding sulfur dioxide to the system. b. increasing the humidity in the system.
c. decreasing the temperature of the system. d. decreasing the pressure in the system.
5.
In a closed system, the equilibrium expression shows 3 moles of gaseous reactants and 2 moles of gaseous
products, and the forward reaction is endothermic. What will happen if the system is heated?
a. the equilibrium will shift to the left b. it will be favourable for the reactants
c. the pressure will increase. d. the concentration of products will increase
6.
2 NO2(g) X N2O4(g)
brown
colorless
If the equilibrium mixture is heated, the color becomes a darker brown. The correct interpretation is that the
forward reaction is
a. exothermic and the equilibrium has been shifted to the left.
b. exothermic and the equilibrium has been shifted to the right.
c. endothermic and the equilibrium has been shifted to the right.
d. endothermic and the equilibrium has been shifted to the left.
7.
Calculate the value of the equilibrium constant using the following information.
Consider the following information:
2 SO2(g) + O2(g) X 2 SO3(g)
0.33 M
0.17 M
0.27 M
a. 0.25 b. 2.0 c. 3.9 d. 4.8
8.
Which of the following best shows the equilibrium of a mixture of water and ice at 0°C?
a. H2O(s) X H2O(l) X H2O(g)
b. H 2O(s) X H 2(g) + O 2(g)
c. H 2O(s) X H 2O(l) + energy
d. H2O(s) + energy X H2O(l)
II. Problems.
1.
The molar enthalpy of formation of nitrogen dioxide is +33.9 kJ/mol, and under certain conditions is an 85%
reaction.
a. Write the balanced equilibrium thermochemical equation (including states of matter, equilibrium arrows,
correct amount of energy on the proper side, percent yield).
85%
N2(g) + 2 O2(g) + 67.8 kJ º 2 NO2(g)
b. The chart below indicates stresses applied to the system, and then new equilibrium established. Copy the
chart below and fill in the information properly (print “left” or “right” under direction of shift, and
“increase” or “decrease” under the concentration amounts of the reactants and product as they would be at
the new equilibrium in comparison to the original equilibrium:
Chemistry 122
Chapter 18-19 PRE-TEST: Chemical Equilibrium, Including Acid-Base
Stress
Page 2 of 3
Direction of Shift
[N2(g)]
[O2(g)]
[NO2(g)]
Temperature
right
dn
dn
up
dn
temperature decreased
left
up
up
dn
dn (net)
volume increased
left
up
up
dn
up
oxygen removed
left
up
dn (net)
dn
up
right
dn
dn
dn (net)
dn
pressure increased
nitrogen dioxide removed
2.
In the production of chloroform vapour (CHCl3(g)) from its elements, the product is favoured.
a. Write the chemical reaction equation using appropriate equilibrium arrows with percent reaction.
b. Use the equilibrium law to write the mathematical expression for the equilibrium constant.
Answer:
K=
[CHCl3(g)]2
2
[C(s)] [H2(g)] [Cl2(g)]3
c. If it is quantitatively determined to be an 80.0% reaction, and after the reaction has occurred and
equilibrium has been established, the amount of CHCl3(g) is 0.440 mol/L, calculate the value of the
equilibrium constant. [Answer: K = 6.48 x 104 M-4]
3.
At high temperatures, ammonia will decompose into its elements. Use the given equilibrium concentrations
below, measured at 1000°C, to calculate the value of the equilibrium constant. [Answer: K = 4.29 M2]
2 NH3(g)
0.102 mol/L
4.
X
N2(g) + 3 H2(g)
1.03 mol/L 1.63 mol/L
A small industrial company was having trouble with rust building up on the inside of their sealed iron vessels.
A keen chemistry connoisseur was trying to prevent, or at least, reduce the problem. He wrote the following
balanced thermochemical equilibrium equation:
4 Fe(s) + 3 O 2(g) X 2 Fe 2O 3(s) + 1644.4 kJ
Based on an understanding of equilibrium chemistry, what should the keen chemistry connoisseur recommend
to do with:
1) temperature, 2) pressure, and 3) the product as it forms (leave it alone or try to keep scraping it off?).
To prevent rust (i.e. avoid the formation of the products; shift left):
1. increase temperature
2. lower pressure
3. leave the product alone (don’t scrape it off)
5.
Do each of the following (percent ionization of weak acids is given in brackets):
A. Calculate the hydrogen ion concentration in a solution prepared by dissolving 2.7 g of barium hydroxide in
290 mL of water (assume 100% dissociation).
B. What mass of HCl is contained in 390 mL of hydrochloric acid solution that has a pOH of 11.6?
Chemistry 122
Chapter 18-19 PRE-TEST: Chemical Equilibrium, Including Acid-Base
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C. What is the hydrogen ion concentration of a 0.10 mol/L solution of HCN (hydrocyanic acid)? (0.0078%)
D. What mass of HF is in a 450 mL solution that has a pOH of 11.6? (7.8%)
E. What is the pH of a 0.56 mol/L phosphoric acid solution? (assume the triprotic acid ionizes 23%)
F.
46g of HF is added to water to make 309 mL of solution. What is the hydroxide ion concentration? (7.8 %)