Download Thermodynamics Problem Set - smhs

Chemistry I-Honors
Thermodynamics Problem Set
Solution Set
1. For each reaction listed, tell whether the reaction this endothermic or exothermic.
EXO
a.
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(liq) + 890.31 kJ
 CO2(g) + CaO(s)
ENDO b.
CaCO3(s) +
EXO
SO2(g) + O2(g)  2 SO3(g) + 197.8 kJ
c.
176 kJ
2. For each of the reactions above, what would you predict about the sign for S?
a) neg. (n = -2)
b) pos. (n = +1)
c) about zero (n = 0)
3. Based on the values for H from #1 and the sign for S in #2, which reactions would you predict to be
spontaneous at room conditions; ie. at 25oC and 1 atm.?
Both A & C would be predicted to be spontaneous based on the signs and magnitude
of enthalpy and entropy.
4. If the molar heat of formation for ethanol – C2H5OH – is –277.0 kJ/mole at 25oC, calculate the heat of
reaction (heat of combustion) for ethanol with oxygen, assuming that the products are carbon dioxide gas
and water in the liquid form.
C2H5OH
+
3 O2 
2 CO2(g) +
3 H2O(liq)
Hcomb =  [ 2(-393.51 kJ/mol) + 3(-285.83 kJ/mol) ] - [ -277.0 kJ/mol ]
= -1367.5 kJ
5. How many kilojoules of heat would be released from the reaction above if only 1.00 gram of ethanol
underwent combustion?
1.00 g EtOH ( 1 mol EtOH / 46.0 g/mol)( 1367.5 kJ / mol) = 29.7 kJ released
6. Given that the molar heat of combustion for propane gas is –1541 kJ, what is the heat of formation for
propane gas. Assume 25oC, and that the products are carbon dioxide gas and water in the liquid form.
C3H8
+
5 O2 
3 CO2(g) +
4 H2O(liq)
Hcomb = -1541 kJ =  [ 3(-393.51 kJ/mol) + 4(-285.83 kJ/mol) ] - [Hf propane ]
Hf propane = -782.85 kJ
-27. Calculate the enthalpy change (H) for the following reaction:
WO3(s)
+

3 H2(g)
W(s) + 3 H2O(liq)
given the following information:
2 W(s) + 3 O2(g) 
O2(g) +
2 H2(g) 
H = -1685.4 kJ
H = -477.84 kJ
2 WO3(s)
2 H2O(liq)
2 WO3(s)  2 W(s) + 3 O2(g)
H = +1685.4 kJ
3 O2(g) +
6 H2(g) 
6 H2O(liq)
H = -1433.52 kJ
---------------------------------------------------------------------------------
8.
C
2 WO3(s)
+
6 H2(g) 
WO3(s)
+
3 H2(g)

2 W(s) + 6 H2O(liq)H = +251.88 kJ
W(s)
+
3 H2O(liq) )H = +125.94 kJ
For which of the following substances would Hf = 0 at standard conditions?
A) bromine vapor;
B) solid neon;
C) solid iodine;
D) CO(g);
E) C2H6(g)
9. If H = -240.0 kJ for the following reaction: A + B  C
What is the H for the following reaction:
3 C  3 A + 3 B ?
+ 720.0 kJ
10. If the H of fusion of ice is 6.01 kJ/mole, what is the G for melting one mole of ice at 0.0oC?
Since the temperature is at the freezing point of water, G = 0.
11. If the molar heat of fusion for ammonia is +5.65 kJ/mole, and the entropy increase for melting solid
ammonia is +28.9 joules/mole.K, what is the melting point for ammonia?
G = 0 = H - TS then
H = TS
T = H / S
T = 5.65 kJ / 0.0289 kJ = 195.5 K = -77.5oC
12. A bomb calorimeter is used to determine the specific heat of a metal. A 75.00-gram sample of the metal
is heated to a temperature of 93.0oC, then quickly dropped into 125.0 grams of cold water (initial
temperature is 10.0oC). If the final temperature of the water-metal mixture is 22.0oC, what is the
specific heat of the metal (in joules/g.oC)
Cmetal = mCT / mT
Cmetal = 1.18 J / g-oC
= [(125.0 g)(4.184 J/g-oC)(12.0oC)] / [(75.00 g)(71.0oC)]