Chemistry 1A
Spring 2008 Exam 2 Key
Chapters 5, 6, and 7 (part)
You might find the following useful.
0.008314 kJ
ΔH° = ΔE° + ( Δn)RT
R =
K • mol
0.00418 kJ ⎤
q = - ⎡⎢Ccal +
mw ⎥ ΔT
K • mol
⎣
⎦
ΔH°rxn = Σ ΔHf° (products) − Σ ΔHf° (reactants)
Heats of Formation for Some Common Inorganic Substances
Substance
State
ΔHf°kJ/mol
Substance
State
ΔHf°
kJ/mol
AgCl
s
−127.04
H2O2
l
−187.6
Al2O3
s
−1669.79
HI
g
25.94
HBr
g
−36.23
MgO
s
−601.8
C(diamond)
s
1.90
MgCO3
s
−1112.9
CO
g
−110.5
Mg(OH)2
s
−924.54
CO2
g
−393.5
NH3
g
−46.25
CaC2
s
−59.8
NO
g
90.37
CaO
s
−635.55
NO2
g
33.85
CaCO3
s
−1206.88
N2O4
g
9.66
Ca(OH)2
aq
−1002.82
N2O
g
81.56
CuO
s
−155.23
O3
g
142.2
FeO
Fe2O3
Fe3O4
HCl
HF
H2O
H2O
s
s
s
g
g
g
l
−272
−824.2
−1118
−92.31
−268.61
−241.83
−285.8
S8(rhombic)
S8(monoclinic)
SO2
SO3
H2S
ZnO
s
s
g
g
g
s
0
0.30
−296.06
−395.18
−20.15
−347.98
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Heats of Formation for some Common Organic Substances
Substance
Formula
State
ΔHf°
kJ/mol
Acetic acid
HC2H3O2
l
−484.21
Acetaldehyde
CH3CHO
g
−246.81
Acetone
CH3COCH3
l
−246.81
Acetylene
C2H2
g
226.6
Benzene
C6H6
l
49.04
Ethane
C2H6
g
−84.68
Ethylene
C2H4
g
52.3
Formic acid
HCO2H
l
−409.20
Glucose
C6H12O6
s
−1274.45
Methane
CH4
g
−74.85
Methanol
CH3OH
l
−238.66
Sucrose
C12H22O11
s
−2221.70
Substance
battery acid
benzene, C6H6
blood (whole)
bromine, Br2
chocolate
copper, Cu
chromium(III) oxide, Cr2O3
ethanol, C2H5OH(l)
gasoline
gold, Au
Iron, Fe
magnesium, Mg
nitrobenzene, C6H5NO2
oleic acid
phosphorus oxychloride, POCl3
sulfuric acid (concentrated)
water at 20 °C
Density in g/mL
1.29
0.879
1.05
3.12
1.75
8.94
5.2
0.7893
0.7025
19.0
7.87
1.74
1.20
0.895
1.675
1.84
0.997
2
For each of the following, write the word, words, letter, letters, or number in each blank
that best completes each sentence. (2 points per blank)
1. Hydrated means bound to one or more water molecules.
2. A substance that ionizes or dissociates incompletely in an aqueous solution is a(n) weak
electrolyte.
3. An acid that can donate two hydrogen ions per molecule in a reaction is a(n) diprotic acid.
4. A(n) weak base is a substance that produces fewer hydroxide ions in water solution than
particles of the substance added.
5. Miscible means able to be mixed in any proportion, that is, infinitely soluble.
6. A(n) Brønsted-Lowry acid is a substance that donates protons, H+, in a Brønsted-Lowry
acid-base reaction.
7. Any chemical change in which at least one element gains electrons, either completely or
partially is reduced.
8. A(n) reducing agent is a substance that loses electrons, making it possible for another
substance to gain electrons and be reduced.
9. Potential energy is a retrievable, stored form of energy an object possesses by virtue of its
position or state.
10. A(n) endergonic (endogonic) change is a change that absorbs energy.
11. Specific heat capacity is the heat necessary to increase the temperature of one gram of pure
substance by one kelvin (or one degree Celsius).
12. If the pH of a solution is 3.5, it is acidic (acidic, basic, or neutral).
13. Of the two forms of radiant energy, infrared and microwaves, infrared has the highest
energy for its photons, and microwaves have the longest wavelength.
14. Write a complete, balanced equation for which the ΔH is the heat of formation of
phosphoric acid, H3PO4(l). (4 points)
¼ P4(s) + 3/2 H2(g) + 2 O2(g) → H3PO4(l)
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15. Write the complete, complete ionic, and net ionic equations for the precipitation reaction
that occurs when water solutions of lithium phosphate and iron(II) nitrate are combined.
(7 points)
2Li3PO4(aq) + 3Fe(NO3)2(aq) →
6LiNO3(aq) + Fe3(PO4)2(s)
6Li+(aq) + 2PO43−(aq) + 3Fe2+ (aq) + 6NO3−(aq)
→ 6Li+(aq) + 6NO3−(aq) + Fe3(PO4)2(s)
2PO43−(aq) + 3Fe2+ (aq) → Fe3(PO4)2(s)
16. Write the complete, complete ionic, and net ionic equations for the acid-base reaction that
occurs when solid chromium(III) hydroxide and aqueous acetic acid are combined.
(7 points)
Cr(OH)3(s) + 3HC2H3O2(aq) →
Cr(C2H3O2)3(aq) + 3H2O(l)
Cr(OH)3(s) + 3HC2H3O2(aq) →
Cr3+(aq) + 3C2H3O2−(aq) + 3H2O(l)
Cr(OH)3(s) + 3HC2H3O2(aq) →
Cr3+(aq) + 3C2H3O2−(aq) + 3H2O(l)
17. Complete the following table. (1/2 point each box)
Substance
Strong, weak, or nonelectrolyte?
Strong acid, weak acid, strong base,
weak base, or neutral?
H2SO3
NaH2PO4
NaC2H3O2
CH3OH
HCl(aq)
NaNO3
NH3
Weak electrolyte
Strong electrolyte
Strong electrolyte
Nonelectrolyte
Strong electrolyte
Strong electrolyte
Weak electrolyte
Weak acid
Weak acid
Weak base
Neutral
Strong acid
Neutral
Weak base
18. Write the oxidation number for each atom in the following equation. Put your answers
above each symbol in the equation. Identify which substance is oxidized, which is reduced,
the oxidizing agent, and the reducing agent. (8 points)
+6 –2
2−
–1
−
+1
+3
+
3+
0
+1 –2
Cr2O7 (aq) + 6Cl (aq) + 14H (aq) → 2Cr (aq) + 3Cl2(g) + 7H2O(l)
Cl in Cl– is oxidized
Cr in Cr2O72− is reduced
Cl– is the reducing agent
Cr2O72− is the oxidizing agent.
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19. Use quantum numbers to show why the 3d sublevel has five orbitals. (6 points)
The quantum numbers are 3,2 (for n,l) for the 3d sublevel. When l=2, ml can be 2, 1, 0,
−1, -2. Therefore, there are five sets of three quantum numbers that include 3,2.
3,2,2 3,2,1 3,2,0 3,2,-1 3,2,-2
Each set of three quantum numbers describes an orbital, so five sets of three quantum
numbers leads to five orbitals.
For the following numerical problems, be sure to show your work and put a box around
your answer. Remember that there will be part credit, so even if you cannot complete a
problem, be sure to set up as much of the problem as possible.
20. When 12.7 mL of a 0.726 M NaOH solution is added from a burette to 25.00 mL of a
sulfuric acid solution that contains phenolphthalein, the solution changes from
colorless to red.
a. What is the titrant for this process? (2 points) NaOH is the titrant.
b. What is the molarity of the sulfuric acid? (6 points)
? mol H 2 SO 4
12.7 mL NaOH soln ⎛ 0.726 mol NaOH ⎞ ⎛ 1 mol H 2 SO4 ⎞ ⎛ 103 mL ⎞
=
⎜
⎟
⎜
⎟
L H2 SO4 soln 25.00 mL H2 SO4 soln ⎝ 103 mL NaOH soln ⎠ ⎜⎝ 2 mol NaOH ⎟⎠ ⎝ 1 L ⎠
= 0.184 M H2SO4
21. Ethanol is the alcohol in alcoholic beverages. The first step in the breakdown of
ethanol in the liver is its conversion to acetaldehyde, CH3CHO(g). The acetaldehyde
is then converted, in a series of steps, into carbon dioxide and water, providing
energy for our bodies in a process that is similar to combustion.
a. What is the heat of combustion of acetaldehyde? (6 points)
CH3CHO(g) + 5/2O2(g)
→ 2CO2(g) + 2H2O(l)
ΔH°rxn = Σ ΔHf° (products) − Σ ΔHf° (reactants)
ΔH°rxn = 2[ΔHf° CO2(g)] + 2[ΔHf° H2O(l)] − ΔHf° CH3CHO(g)
= 2(−393.5 kJ) + 2 (−285.8 kJ) − (−246.81kJ) = −1111.8 kJ
b. How much heat is released in the complete combustion of acetaldehyde produced
from 15 mL of pure ethanol, C2H5OH(l)? (One mole of ethanol forms one mole of
acetaldehyde.) (6 points)
⎛ 0.7893 g C2 H5 OH ⎞ ⎛ 1 mol C2 H5 OH ⎞ ⎛ 1 mol CH3 CHO ⎞⎛ −1111.8 kJ ⎞
? kJ = 15 mL C2 H5 OH ⎜
⎟⎜
⎟⎜
⎟⎜
⎟
⎝ 1 mL C2 H5 OH ⎠ ⎝ 46.069 g C2 H5 OH ⎠ ⎝ 1 mol C2 H5 OH ⎠⎝ 1 mol CH3 CHO ⎠
= −2.9 × 102 kJ
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Answer the following in short answer form. (6 points each)
22. Explain why it is useful to have two sets of definitions for acids and bases (the Arrhenius
definitions and the Brønsted-Lowry Definitions).
Positive Aspects of Arrhenius Definitions
•
All isolated substances can be classified as acids (generate H3O+ in water), bases
(generate OH− in water), or neither.
•
Allows predictions, including (1) whether substances will react with a base or acid,
(2) whether the pH of a solution of the substance will be less than 7 or greater than
7, and (3) whether a solution of the substance will be sour.
Negative Aspects of Arrhenius Definitions
•
Does not include similar reactions (H+ transfer reactions) as acid-base reactions.
Positive Aspects of Brønsted-Lowry Definitions
•
Includes similar reactions (H+ transfer reactions) as acid-base reactions.
Negative Aspects of Brønsted-Lowry Definitions
•
Cannot classify isolated substances as acids (generate H3O+ in water), bases
(generate OH− in water), or neither. The same substance can sometimes be an acid
and sometimes a base.
•
Does not allow predictions of (1) whether substances will react with a base or acid,
(2) whether the pH of a solution of the substance will be less than 7 or greater than
7, and (3) whether a solution of the substance will be sour.
23. Explain why some chemical reactions release heat to their surroundings.
Reactants → products
Weaker bonds → stronger bonds
Less stable → more stable
Higher PE → Lower PE + energy released
PE converted to KE
Increased KE means increased temperature
Temperature inside container > temperature outside container
Heat transferred to surroundings
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