Exam 2 – ANSWER KEY TOTAL POINTS = 98
General Chemistry Spring 2007
Circle the best answer: correct choice in red, each 3 points

1. Entropy is:
a) a measure of spontaneity in a system
b) a measure of the possible arrangements of a system
c) the sum of heat and work
d) all of the above

2. Enthalpy is:
a) a measure of spontaneity in a system
b) a measure of the possible arrangements of a system
c) the sum of heat and work
d) none of the above

3. A reaction will be spontaneous when:
a) deltaS(sys) is positive
b) deltaS(surr) is positive
c) either (a) or (b)
d) none of the above


4.
In the reaction:
Ba(OH)2 8H2O (s) + 2 NH4NO3 (s) à Ba(NO3)2 (aq)  +  2 NH3 (g)  +  10 H2O(l) the reaction entropy is expected to:


a) increase
b) decrease
c) remain unchanged
d) there is too little info to tell


5. If deltaG = 0 for a system then
a) deltaS(total) > 0
b) deltaS(total) < 0
c) the system is at equilibrium
d) the temperature of the system is 0 K.

6. Which of the below reactions corresponds to the definition of C-H bond enthalpy?
a) C + H --> C-H
b) 1/4 [CH4 --> C (g) + 4H (g)]
e) 1/4 [C(graphite) + 2 H2 (g) --> CH4]
f) 1/4 [CH4 --> C(graphite) + 2 H2 (g) ]

7. Which statement below is false?
a) The ideal gas law shows the mathematical relationship between pressure, volume, temperature and moles of gas.
b) The pressure of a gas in a closed container will increase if the temperature is increased.
c) The volume of 1.0 mole of a gas at 1 bar pressure and 25 deg C will depend on how many atoms are in each molecule of gas.
d) The pressure of a gas in a closed container will increase if the container volume is decreased.

8. A metal spoon sitting on a hot stove has gotten very hot. If it is carefully picked up and placed into a glass of ice water, which of the following happens?
a) The entropy of the water increases.
b) The metal atoms in the spoon move more slowly.
c) deltaH(spoon) = - deltaH(water+glass)
d) All of the above will happen.

9. Lactose—C12H22O11—is the “sugar” in milk.  When lactose is oxidized by oxygen it produces carbon dioxide, water and heat.   Lots of heat:  DH = -5130 kJ/mol lactose.
How much heat is produced per mole of oxygen consumed? (5 points)

C12H22O11 + 12 O6 --> 11 H2O + 12 CO6
-5130 kJ/mol lactose x 1 mol lactose/ 12 mol O2 = - 427.5 kJ/mol O2

10. Hydrazine N2H4 is a rocket fuel. The reaction below shows how hydrazine reacts with the oxidant hydrogen peroxide. (All molecules are gas phase.) (5 points)
Without doing any calculations, explain why this reaction is so highly exothermic.

The bond enthalpy for a triple N-N bond is one of the very largest values. Thus, there is much more heat released on formation of a N-N triple bond than the heat required to break 4 N-H bonds and a O-O bond. There is in addition, heat produced by the formation of additional O-H bonds.

11. Explain why an air-filled balloon shrinks when placed into liquid nitrogen. (5 points)

Again using the relationship PV=nRT, only T has increased when the balloon is submerged into liquid N2 at –200 deg C. There is no change in pressure—atmospheric pressure on the balloon remains the same— and there is no change n the number of moles of gas. So V (of balloon) decreases.

12. The reaction PCl3(g) + Cl2(g) ‡ PCl5(s) is spontaneous at room temperature.
Predict whether the reaction is exothermic or endothermic and explain your prediction. (5 points)
The reaction of two gases to form a solid will certainly have decreasing entropy, a deltaS < 0. The reaction is stated to be spontaneous, so that deltaG must be < 0. Since delta G = deltaH – TdeltaS, the only way for deltaG to be negative is if deltaH is very negative to compensate for the –TdeltaS term that will be positive.

13. Many cultures worship the sun as a god.
Propose a thermodynamic justification for these spiritual practices. (8 points)
These cultures are humans who live by respiration, a process that produces energy (free energy of respiration is negative and spontaneous) to keep them alive through the oxidation of carbohydrates, like glucose. The carbohydrates originate from plants, whch make the carbohydrates through the process of photosynthesis. Photosynthesis is not a spontaneous process (free energy is positive) but is made possible through the energy in the form of light from the sun.
The sun is responsible for all life on earth.

14. Minerals exposed to the atmosphere can change over time. For example, the mineral magnetite, Fe3O4 in contact with oxygen in the air will form the mineral hematite, Fe2O3. (12 ponts)
a) Calculate the free energy change for the reaction of hematite formation, Fe2O3, from magnetite, Fe3O4. You may assume that the only species involved in the reaction are those shown below:

Fe3O4 and O2 form Fe2O3
First balance the equation!
2Fe3O4 + 1/2 O2 --> 3Fe2O3
delta G(rxn) = delta G(products) - delta G(rgts) = 3 delta G (Fe2O3) – 2 delta G (Fe3O4) – 1/2 delta G (O2) =
= 3 (-742.2) – 2(-1015.4) – 1/2 (O) = -195.8 kJ (for formation of 3 mole Fe2O3 )

If the reaction is balanced as:
4Fe3O4 + O2 ‡ 6Fe2O3
delta G(rxn) = delta G(products) - delta G(rgts) = 6 delta G (Fe2O3) – 4 delta G (Fe3O4) – delta G (O2) =
= 6(-742.2) – 4(-1015.4) – 1/2 (O) = -391.6 kJ (for formation of 6 mole Fe2O3

b) Is this reaction predicted by thermodynamics to spontaneously occur?
Yes it is spontaneous because delta G(rxn) <0.

15. The standard free energy of formation for N2O4 is +97 kJ/mol and indicates that
N2O4 is thermodynamically unstable towards decomposition to the elements. (12 points)

a) Explain what “standard free energy of formation “ means.Standard free energy of formation is the free energy to form one mole of a molecule from the elements in their most stable state at 25 deg C and 1 bar pressure.

b) What reaction is indicated by the phrase “decomposition to the elements”?
N2O4 --> N2 + 2O2

c) Why is decomposition of N2O4 likely to happen?

The decomposition, the reverse reaction, will have deltaG = -97 kJ/mol and will be spontaneous.

16. Using the thermodynamic values provided for ethanol, calculate its boiling point, that is, the Temperature (in deg C) where:          C2H5OH (l)  --> C2H5OH (g)    (10 points)

At the boiling point of ethanol, C2H5OH (l) is in equilibrium with C2H5OH, (g).
C2H5OH (l) --> C2H5OH (g)
Therefore for
delta G = deltaH – TdeltaS,
deltaG = 0.
And delta H = Tdelta S
Find deltaH and deltaS values for
C2H5OH (l) and C2H5OH (g).
delta H (
C2H5OH, (g)) - delta H (C2H5OH, (l )) = T delta S (C2H5OH, (g)) - delta S (C2H5OH, (l))
-235.1 – (-277.69) = T (282.7 – 160.7)

42.59 kJ/mol = T(0.122 kJ/K mol)
T = 349.1 K = 76.1 deg C.

17. A metal spoon sitting on a hot stove has gotten very hot. Actually, it is REALLY hot at 200 deg F. If the spoon is carefully picked up and placed into a half-full glass of room temperature water, what will the temperature of the water be when deltaG(spoon) = 0? (12 points)


Additional information:
- a half full glass contains 250 mL;
- the spoon is silver and weighs 100 g (it’s a big spoon)
- 1 deg C= 5/9 (deg F - 32 deg)
- the heat capacity of water = 4.18 J/g deg C.
- the heat capacity of silver = 0.22 J/g deg C
- room temperature is 25 deg C

delta H (spoon) = - deltaH (water+glass)
deltaH = mass x heat capacity x change temperature
so:
delta H (spoon) = - deltaH (water+glass)
100g x 0.22 J/ g deg x (T(final) – 93 deg C) = -(250 g x 4.18 J/ g deg x (T(final) – 25 deg C)
T(final) = 26.4 deg C.