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PRACTICE QUIZ (THERMODYNAMICS)
1. a. (2 pts.) Delta Grxn for: 2SO3(g) ----> 2SO2(g) + O2(g)
is 139.6 kJ. If delta Gf for SO2(g) is -300.1 kJ/mole,
calculate delta Gf for SO3(g).
b. (1 pt.) Is the formation of SO3 gas from SO2 gas and O2 gas
spontaneous at 25oC and 1 atm pressure? ___________________
c. (1 pt.) What is the maximum work that could be performed by
either of the above reactions?
2. (3 pts.) Give the expected sign on delta S for the following processes:
Process Sign on delta S
a. CaSO4(s) ----> CaO(s) + SO3(g) _______________
b. Ag+(aq) + Cl-(aq) ----> AgCl(s) _______________
c. I2(g) ----> I2(s) _______________
3. (2 pts.) Circle T (true) or F (false):
a. T F Endothermic reactions which become more
ordered are never spontaneous.
b. T F A system at equilibrium cannot perform any work.
4. (2 pts.) For the reaction
5Fe3+ + Mn2+ + 4H2O ----> MnO4- + 8H+ + 5Fe2+ Kc = 1.0 x 10-62
Given delta G = -2.3 RT log Kc, what is the sign of delta G for this reaction?
______
5. (2 pts.) 1.0 mole of SO2(g) and 1.0 mole of O2(g) are
mixed together in a 1.0 L flask. As SO2 and O2 react
to form SO3 and proceed toward equilibrium, how does
the value of the free energy change?
6. (3 pts.) The process benzene(s) ----> benzene(l) is spontaneous
at temperatures above 5oC and is spontaneous in the opposite
direction at temperatures below 5 oC.
a. Explain in terms of delta H, delta S, and delta G why the direction of
the spontaneous process depends on the temperature.
b. What does the temperature 5oC represent for benzene? ________
c. What is delta G for the above process at 5oC?___________________
7. (4 pts.) For the reaction, 3NO2(g) + H2O(l) ----> 2HNO3(l) + NO(g),
the following information is given at 25oC and 1 atm.
Grxn = + 8.4 kJ
Hf (kJ/mole):
+ 34.0 for NO2(g)
-286.9 for H2O(l)
-173.9 for HNO3(l)
+ 90.7 for NO(g)
Calculate delta S for the reaction (delta Srxn).
KEY
1. a. (2 pts.) Delta Grxn for: 2SO3(g) ----> 2SO2(g) + O2(g)
is 139.6 kJ. If delta Gf for SO2(g) is -300.1 kJ/mole,
calculate delta Gf for SO3(g).
delta Grxn = delta Gf(products) - delta Gf(reactants)
= [2(-300.0) - 2(delta Gf SO3)] = 139.6 kJ
(139.6 + 600.2)kJ = -2 delta Gf (SO3)
delta Gf (SO3) = -369.kJ/mole
b. (1 pt.) Is the formation of SO3 gas from SO2 gas and O2 gas
spontaneous at 25oC and 1 atm pressure?
yes, delta Grxn = -139.6 kJ (for the reverse reaction)
c. (1 pt.) What is the maximum work that could be performed
by either of the above reactions?
From reaction in b, delta Grxn = -Wmax
This is the work performed by the spontaneous reaction
Maximum work is +139.6 kJ
2. (3 pts.) Give the expected sign on delta S for the following processes:
Process
a. CaSO4(s) ----> CaO(s) + SO3(g) (delta S "+")
b. Ag+(aq) + Cl-(aq) ----> AgCl(s) (delta S "-")
c. I2(g) ----> I2(s) (delta S "-")
3. (2 pts.) Circle T (true) or F (false):
a. (T) F Endothermic reactions which become
more ordered are never spontaneous.
b. (T) F A system at equilibrium cannot perform any work.
4. (2 pts.) For the reaction
5Fe3+ + Mn2+ + 4H2O ----> MnO4- + 8H+ + 5Fe2+ Kc = 1.0 x 10-62
Given delta G = -2.3 RT log Kc, what is the sign of delta G for this reaction? +
5. (2 pts.) 1.0 mole of SO2(g) and 1.0 mole of O2(g) are mixed
together in a 1.0 L flask. As SO2 and O2 react to form
SO3 and proceed toward equilibrium, how does the value
of the free energy change?
The reaction going toward equilibrium is spontaneous.
Therefore, delta G is negative for the reaction,
meaning delta G(products) is less than delta G(reactants) or
free energy must be released. The free energy decreases
as the system goes to equilibrium
6. (3 pts.) The process benzene(s) ----> benzene(l) is spontaneous at
temperatures above 5oC and is spontaneous in the opposite
direction at temperatures below 5oC.
a. Explain in terms of delta H, delta S, and delta G why the direction of the
spontaneous process depends on the temperature.
When a solid melts, it absorbs heat (delta H is positive) and
delta S is also positive (more disorder);
therefore, delta G depends on temperature
(delta G = delta H - T delta S).
At hi temperature, the reaction is spontaneous as
written (delta G is negative since T delta S > delta H).
At low temperature the reaction is non-spontaneous as written.
The reverse reaction is spontaneous.
The liquid freezes spontaneously below 5oC because delta H > T delta S.
b. What does the temperature 5oC represent for benzene? (m.p. or f.p.)
c. What is delta G for the above process at 5o C? (delta G = 0 at equilibrium)
7. (4 pts.) For the reaction, 3NO2(g) + H2O(l) ----> 2HNO3(l) + NO(g),
the following information is given at 25oC and 1 atm.
delta Grxn = + 8.4 kJ
Hf (kJ/mole):
+ 34.0 for NO2(g)
-286.9 for H2O(l)
-173.9 for HNO3
+ 90.7 for NO(g).
Calculate delta S for the reaction (delta Srxn).
delta Grxn = delta Hrxn - T delta Srxn
Calculate delta Hrxn ; since you know delta Grxn , calculate delta Srxn
delta Hrxn = delta Hf(products) = delta Hf(reactants)
= [2(-173.9)+ 1(+90.7)]-[3(+34.0)+ 1(-286.9)]
= [-257.1]-[-184.9]
delta Hrxn = -72.2 kJ
delta Srxn = delta Hrxn - delta Grxn/T
= -72.2-(+8.4)/298
= -80.6/298 = -0.27 kJ/K (System goes to more order!!)
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