Download 1. Consider the thermochemistry of C

Chemistry 131
Name _________________________
Problem Solving Section. Please place your answers in the spaces provided. Show your
work!
1. Consider the thermochemistry of CaCl2 dissolving in water, as it is commonly available for
purchase as a de-icer in winter. [25 points]
CaCl2 (s) 
Ca2+ (aq) +
2 Cl- (aq)
-795.0 kJ/mol
-542.96 kJ/mol
-167.46 kJ/mol
a) Determine the enthalpy of this reaction from the enthalpies of formation.
!Hrxn =
=
!n!H (products) - !n!H (reactants)
[(1 mol Ca )( -542.96 kJ/mol) + (2 mol Cl )( -167.46 kJ/mol)]
f
f
2+
-
-
[(1 mol CaCl )(-795.0 kJ/mol)]
2
= -82.9 kJ/mol
b) Determine the enthalpy change if 3.25 kg of CaCl2 is used to de-ice a sidewalk.
1000 g
mol
x
= 29.3 mol CaCl2
1 kg
110.986 g
-82.9 kJ
29.3 mol x
= -2430 kJ
mol
3.25 kg x
c) If the same 3.25 kg of CaCl2 is added to 100.0 mL of water at 4.00°C, what is the final
temperature of the water? The density of water at this temperature is 1.00 g/mL.
This is a heat transfer problem, where q1 = - q2. We use the heat calculated in part b for q1,
and the heat of water will be q2. The mass of the water is found from the volume and density.
-2340 kJ x
'!
*
1000 J
1.00 g $ !
J $
&& (Tf - 4.00°C),
= - )# 100.0 mL x
& ## 4.184
)("
,+
1 kJ
ml % "
g°C%
Tf = 5.60 x 103 °C
It’s a good thing that heat is actually lost to the surroundings!
Exam 2
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Chemistry 131
Name _________________________
2. Consider the following reaction between ammonia and carbon dioxide,
2 NH3 (aq) + CO2 (g)  (NH2)2CO (aq) + H2O
0.750g of CO2 are added to 85.0 ml of a 0.225M solution of NH3. (Assume the addition of
the CO2 does not alter the volume of the solution). [21 points]
Molecular weights(g/mol): NH3 17.031 CO2 44.009 (NH2)2CO 60.056 H2O 18.015
a) How many grams of (NH2)2CO are produced by this reaction?
This is a limiting reagent problem. Moles of CO2 can be calculated directly from the grams,
and moles of NH3 can be calculated from the molarity and the volume. For all exams, NH3 was
the limiting reagent.
CO2 : 0.750 g CO2 x
NH3 :
mol
1 mol (NH2 )2 CO
x
= 0.0170 mol (NH2 )2 CO
44.009 g
1 mol CO2
0.225 mol
1 mol (NH2 )2 CO
x 0.0850 L x
= 9.56 x 10-3 mol (NH 2 )2 CO
L
2 mol NH3
NH3 is the limiting reagent
9.56 x 10-3 mol (NH 2 )2 CO x
60.056 g
= 0.574 g (NH2 )2 CO
mol
b) What is the molar concentration of (NH2)2CO at the end of the reaction? You may assume
that the amount of water produced is negligible.
To calculate this, take the number of moles of (NH2)2Co produced from part a and divide this
by the volume, in L.
For all exams, the concentration is 0.112 M
c) How many grams of NH3 and CO2 remain at the end of the reaction?
By definition, the limiting reagent is completely used up, so no NH3 remains at the end of the
reaction. To determine the amount of CO2 remaining, subtract the moles of (NH2)2CO
produced by NH3 from the amount produced by CO2, and then convert back to grams of CO2.
0.0170 mol - 9.56 x 10-3 mol = 7.44 x 10-3 mol excess (NH2 )2 CO
7.44 x 10-3 mol x
Exam 2
1 mol CO
44.009 g
x
= 0.327 g CO2 in excess
1 mol (NH2 )2 CO
mol
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Chemistry 131
Name _________________________
Predicting Reactions. Predict the products of the reaction described below. Please give the
complete molecular equation and net ionic equation to each reaction. [8 points each; 24 points
total]
1. The reaction of calcium metal and aqueous hydrobromic acid.
Molecular Equation:
Ca (s) + 2 HBr (aq)  CaBr2 (aq) + H2 (g)
Net Ionic Equation:
Ca (s) + 2 H+ (aq)  Ca2+ (aq) + H2 (g)
The reaction of barium metal and aqueous hydrobromic acid.
Molecular Equation:
Ba (s) + 2 HBr (aq)  BaBr2 (aq) + H2 (g)
Net Ionic Equation:
Ba (s) + 2 H+ (aq)  Ba2+ (aq) + H2 (g)
2. The reaction between lead (II) phosphate and sodium sulfate.
Molecular Equation:
Pb3(PO4)2 (s) + 3 Na2SO4 (aq)  3 PbSO4 (s) + 2 Na3PO4 (aq)
Net Ionic Equation:
Pb3(PO4)2 (s) + 3 SO42- (aq)  3 PbSO4 (s) + 2 PO43- (aq)
3. The reaction between aqueous nitric acid and potassium sulfite.
Molecular Equation:
2 HNO3 (aq) + K2SO3 (aq)  2 KNO3 (aq) + H2O (l) + SO2 (g)
Net Ionic Equation:
2 H+ (aq) + SO32- (aq)  H2O (l) + SO2 (g)
Exam 2
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Chemistry 131
Name _________________________
Multiple choice section. For each problem, circle the best choice for the solution. [3 points
each; 30 points total] The three exams have the same questions, but the answers are in
different order.
D
1. The heat of formation of propene gas, C3H6, is, by definition, the enthalpy change
for the reaction:
a) 3/2 C2 (g) + 3 H2 (g)  C3H6 (g)
b) 3 C (g) + 6 H (g)  C3H6 (g)
c) C3H8 (g)  C3H6 (g) + H2 (g)
d) 3 C (s) + 3 H2 (g)  C3H6 (g)
e) C3H4 (g) + H2 (g)  C3H6 (g)
D
2. How many moles of OH- are in 27.5 mL of 0.250 M calcium hydroxide?
a) 13.8 mol
b) 0.0688 mol
c) 9.09 mol
d) 0.0138 mol
e) 0.110 mol
E
3. When solution of barium chloride and sodium sulfate are mixed, the spectator ions
in the resulting solutions are:
a) only Ba2+.
b) only SO42-.
c) only Na+.
d) only Cl-.
e) both Na+ and Cl-.
B
B
Exam 2
4. A precipitate will be formed when a solution of aqueous calcium hydroxide is added
to an aqueous solution of:
a) sodium bromide
b) ammonium carbonate
c) barium nitrate
d) potassium iodide
e) lithium acetate
5. What is the change in enthalpy when 1 mol (28.0 g) of carbon monoxide is oxidized
to carbon dioxide?
2 CO (g) + O2 (g)  2 CO2 (g)
∆H = -566 kJ
a) –566 kJ
b) –566/2 kJ
c) 566/2 kJ
d) 566 kJ
e) 566 x 2 kJ
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Chemistry 131
Name _________________________
E
6. The net ionic equation for the reaction of nitric acid with lithium hydroxide is:
a) HNO3 (aq) + LiOH (aq)  LiNO3 (aq) + H2O (l).
b) HNO3 (aq) + LiOH (aq)  Li+ (aq) + NO3- (aq) + H2O (l).
c) HNO2 (aq) + Li+ (aq) + OH- (aq)  Li+ (aq) + NO2- (aq) + H2O (l).
d) H+ (aq) + NO2- (aq) + Li+ (aq) + OH- (aq)  Li+ (aq) + NO2- (aq) + H2O (l).
e) H+ (aq) + OH- (aq)  H2O (l).
C
7. This question is concerned with the heat change when ammonia is formed from its
elements:
N2(g) + 3H2(g)  2NH3(g)
∆H = –92 kJ
Therefore, 92 kJ is the quantity of heat that is
a) lost to the surroundings when 1 mol of hydrogen is consumed.
b) lost to the surroundings when 1 mol of ammonia is formed.
c) lost to the surroundings when 2 mol of ammonia are formed.
d) gained from the surroundings when 1 mol of ammonia is formed.
e) gained from the surroundings when 2 mol of ammonia are formed.
B
A
D
Exam 2
8. The enthalpy change for the thermochemical equation
2HF(g)  H2(g) + F2(g)
is 542.2 kJ. The enthalpy of formation of HF(g) in kilojoules per mole must be
a) –542.2.
b) –271.1.
c) –1084.
d) 271.1.
e) 542.2
9. All the following compounds are insoluble in water EXCEPT
a) Fe(NO3)2.
b) CuCO3.
c) BaSO4.
d) CaCO3.
e) CuS.
10. Calculate the standard enthalpy of formation of carbon disulfide from its elements
given that
C (s) + O2 (g)  CO2 (g)
∆H = –393.5 kJ
SO2 (g)  S (s) + O2 (g)
∆H = 296.8 kJ
CS2 (l) + 3 O2 (g)  CO2 (g) + 2 SO2(g)
∆H = –1076.8 kJ
a) 386.5 kJ
b) –1173.5 kJ
c) –1767.1 kJ
d) 89.7 kJ
e) 1276.9 kJ
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