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Dr. C. Rogers --- Chem 205 GENERAL CHEMISTRY I --- SAMPLE TEST QUESTIONS
THERMODYNAMICS (Ch.6 Thermochemistry & Ch.19 Spontaneity)
# 1. A sample of an ideal gas with a volume of 6320 L at 0°C and 1 atm pressure is compressed into a cylinder, where it exerts a pressure of 136 atm ( » 2000 psi). The amount of work that can be done when this gas expands (e.g., if the cylinder is knocked over and the valve breaks off) is equivalent to the destructive force of around 100 g ( » ¼ lb) of an explosive such as dynamite. Calculate the work performed (in Joules) when this gas expands out of the cylinder at 0°C (constant temperature) against an opposing pressure of 1.0 atm.
[Hint #1: I wonder what volume this cylinder must have....]
[Hint #2: Make sure compressed-gas cylinders are always anchored to a secure heavy object...like a wall!]
# 2. Barium peroxide (BaO2) can be used as a solid fuel for propelling rockets. When it is added to hydrochloric acid, the following reaction takes place:
BaO2(s) + 2 HCl(g) ® BaCl2(s) + H2O2(l)
(a) Using the data provided, calculate DHo for the reaction.
(b) Calculate the number of grams of BaO2 that must react to produce 500 kcal of heat
(1 kcal = 4.184 kJ).
SUBSTANCE / DHof (kJ/mol) / SUBSTANCE / DHof (kJ/mol) / SUBSTANCE / DHof (kJ/mol)SO3(g) / -296.6 / BaO2(s) / -629.1 / BaCl2(s) / -859.2
CCl4(l) / -139.6 / HCl(g) / -92.4 / H2O2(l) / -187.3
# 3. Consider this situation: you drink 350mL of diet soda that is at a temperature of 5°C.
(a) How much energy will your body expend to raise the temperature of the liquid to body temperature (37°C)? Assume that the density and specific heat of diet soda is the same as water (even though this is probably not exactly true…).
(b) Compare the value in part (a) with the caloric content of the beverage. The label says that it has a caloric content of 1 Cal. (this is 1 dietary Calorie, which = 1 kcal = 4.184 kJ). What is the net energy change in your body resulting from drinking this beverage?
# 4. Peanuts and peanut oil are organic materials and will burn if ignited in air; thus, peanuts could be used as a fuel source. How many burning peanuts would it take to provide the energy to boil a cup of water (250 mL)? To solve this problem, we must make a number of potentially nutty assumptions (ha ha ha…sorry…):
· average mass of one peanut: 0.73 g
· peanut composition (mass%): 49% peanut oil
29% starch
remainder = non-combustible
· peanut oil = palmitic acid, C16H32O2; DHof = -124.8 kJ/mol
· starch = long chain of C6H10O6 units; each unit has DHof = -734 kJ/mol
# 5. The Romans used lime (calcium oxide, CaO) to produce a very strong mortar in stone structures. The CaO was mixed with water to give Ca(OH)2, which reacted slowly with CO2 from the air to give CaCO3 (limestone):
Ca(OH)2(s) + CO2(g) à CaCO3(s) + H2O(g)
(a) Calculate the standard enthalpy change for this reaction.
(b) What quantity of heat is evolved or absorbed if 1.00 kg of Ca(OH)2 reacts with a stoichiometric amount of CO2?
(c) Estimate the magnitude and sign of the entropy change for this reaction using logic. Compare this to the value of DSo calculated for this reaction using thermodynamic data.
(d) Calculate the Gibbs free energy change for this reaction at 35°C (a warm summer day in Rome). Would this reaction be correctly described as being driven by entropic contributions or by enthalpic contributions?
# 6. Wet limestone is used to scrub (remove) SO2 gas from the exhaust gases of power plants. One possible scrubbing reaction gives hydrated calcium sulfite:
CaCO3(s) + SO2(g) + ½ H2O(l) ® CaSO3 · ½ H2O(s) + CO2(g)
Another possible reaction gives hydrated calcium sulfate:
CaCO3(s) + SO2(g) + ½ H2O(l) + ½ O2(g) ® CaSO4 · ½ H2O(s) + CO2(g)
Which is the more product-favoured reaction? Use the following data and data from the textbook.
CaSO3 · ½ H2O(s) / CaSO4 · ½ H2O(s)DHof (kJ/mol) / -1311.7 / -1574.65
So (J/mol·K) / 121.3 / 134.8
# 7. Engineers who study the design of nuclear power plants have to worry about high-temperature reactions, since it is possible for water (used in power plants as a coolant) to decompose. Under what conditions would this reaction occur spontaneously? You will need to refer to thermodynamic data from your textbook.
Reaction: 2 H2O(g) ® 2 H2(g) + O2(g)
# 8. The formation of diamond from graphite is a process of considerable industrial importance.
(a) Calculate DSouniv, DHo, and DGo for this process.
(b) The calculations will suggest that this process is impossible. However, the synthesis of diamonds by this reaction (at high pressure and high temperature) is a commercial process. How can you rationalize this apparent contradiction?