Name
CHEM 115 Exam #4
PRACTICE
Circle the correct answers. (2.5 points for each question)
Complete the following questions/calculations. (Show all of your work.)
(12.5 points)
1. Use Hess’ law and the following data
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)
CH4 (g) + CO2(g) → 2 CO (g) + 2 H2 (g)
CH4 (g) + H2O (g) → CO (g) + 3 H2 (g)
to determine ΔH for the reaction
4 CH4 (g) + 2 O2 (g) → 4 CO (g) + 8 H2 (g)
ΔH = -890 kJ
ΔH = +247 kJ
ΔH = +250 kJ
ΔH = ??? kJ
(12.5 points)
2. Reaction of solid white phosphorus (P4) with chlorine gas (Cl2) yields phosphorus trichloride as
the sole product at 100 % yield. (a) Write a balanced reaction.
(b) When 1.00 g of solid white phosphorus (P4) is reacted with excess chlorine gas, the reaction
gives off 9.27 kJ of heat. What is the heat of reaction per mole of PCl3 formed?
(12.5 points)
3. A bomb calorimetry experiment is performed with isobutane [(CH3)3 CH molar mass = 58.12
g/mole] as the combustable substance. The data obtained are:
mass of isobutane burned: 0.9932 g
heat capacity of calorimeter: 4.947 kJ/ºC
final calorimeter temperature: 32.74ºC
initial calorimeter temperature: 22.88ºC
a. What is qv, in kJ/g, for the combustion of isobutane?
b. What is qv, in kJ/mole, for the combustion of isobutane?
c. How many grams of isobutane would need to be burned to produce 1,000 kJ of heat?
(10 points)
4. Calculate the amount of energy required to (a) heat 20.0 grams of water from 50.0ºC to 100.0ºC
(the normal boiling point) AND also (b) vaporize ONLY HALF of the water at 100ºC? Make
sure the sign of the energy is correct!!!
[for water ΔHvap = 40.6 kJ/mole, specific heat of liquid water = 4.184 J/gºC]
2
(6 points)
5. Draw Lewis structures for water (H2O) and n-hexane (CH3CH2CH2CH2CH2CH3).
Discuss why water is a (highly) polar molecule while hexane is non-polar.
(6 points)
6.
Discuss why ethanol (CH3CH2OH) has a lower boiling point than water (H2O) even though
ethanol is a larger molecule (with larger dispersion forces) AND is ALSO capable of H-bonding.
(12.5 points, 2.5 points each)
7. a. the critical temperature is (in ºC)
b. the normal boiling point pressure (in atm)
c. the temperature at the triple point is (in ºC)
d. which letter in the diagram corresponds to the region of gas
e. which has the higher density the solid or liquid phase
Pressure (atm) [not to scale]
16.5
C
B
1.00
1
2
3
4
0.22
A
-13
85
Temperature (°C) [not to scale]
444
3
(4 points each, 16 points total)
8. a. Circle the choice with the lower surface tension: C8H18 vs. CH3(CH2)6OH
EXPLAIN your choice.
b. Circle the choice with the lower boiling point: CO vs. N2
EXPLAIN your choice.
c. Select compound with the highest melting point: NaOH vs. CH3OH vs. MgO
EXPLAIN your choice.
d. Circle the substance which has the highest solubility in water: chloroform (CHCl3)
vs. benzoic acid (C6H5COOH) vs. propylene glycol (CH3CHOHCH2OH)
EXPLAIN your choice.
(14 points)
9. a. A sample of metal with a mass of 1.50 x 102 g absorbs 99.0 J of heat, and the
temperature of the metal increases 1.03ºC. What is the specific heat of the metal in J/gºC
a. 1.56
b. 0.641
c. 1.47
d. 0.680
e. none of the above
b. Calculate the amount of energy required to (1) melt 18.0 grams of water at 0.0 ºC, (2)
heat ALL of that water to 100.0 ºC (the normal boiling point), and (3) vaporize HALF of
that water at 100 C. Make sure the sign of the energy is correct!!!
[for water: ΔHfus = 6.01 kJ/mole, ΔHvap = 40.0 kJ/mole , Cp = 4.184 J/gºC]
4