CH186
Thermodynamics Exam Questions
From Spring 2001 Semester
Useful constant: R = 8.314 J/K mol; Boltzman’s constant, k = 1.38 x 10-23 J/K.
1)a.For each pair below, indicate the substance which possesses the larger entropy.
- 1 mol Cu(s) at 25C, 1 atm vs. 1 mol Cu(s) at 105C, 1 atm
- 1 mol NaCl(aq) at 25C, 1 atm vs 1 mol NaCl(s) at 25C, 1 atm
- 1 mol C2H6(g) at 25C, 1 atm vs. 1 mol C2H2(g) at 25C, 1 atm
- 1 mol NH3(g) at 25C, 1 atm vs. 15 mol NH3(g) at 25C, 1 atm
- 0.1 mol H2O(l) 100C, 1 atm vs. 0.1 mol H2O(g) at 100C, 1 atm
b.Predict the sign of S for each of the following processes.
CO2(g) dissolves into water
MgCO3(s) decomposes into MgO(s) and CO2(g)
water vapor condenses at 25C, 1 atm
A leaf falls off of a tree and decomposes in the autumn
AgCl(s) precipitates when Ag+(aq) and Cl-(aq) are mixed
2)Calculate (a) the standard Gibb’s free energy change, ΔG, and (b) the equilibrium constant, K, for the following precipitation equilibrium:
3Ca2+(aq) + 2PO43-(aq) Ca3(PO4)2(s)
ΔGf = -553.04 -1018 -3899 kJ/mol
(c) In which direction will the reaction be spontaneous under standard state conditions?
3)Consider the following reaction
2Mg(s) + O2(g) 2MgO(s)
for which ΔH = -1203.4 kJ and ΔS = -216.6 J/K. Calculate the standard Gibb’s free energy change, ΔG, for the reaction at 500C. Is this reaction spontaneous under standard state conditions at 500C?
4)Consider a gas-container consisting of three connected 1 L flasks. (a) How many ways can 4 different gas molecules be arranged between the three flasks of the container? (b) How many ways can all 4 molecules be placed into flask “A”?
5)Is the following statement correct or incorrect? If it is incorrect, explain why.
“A reaction that results in more order in the system (Ssystem < 0) can never be spontaneous.”
Answers for Spring 2001 Acid-Base Exam Questions
1.a.1 mol Cu(s) at 105C, 1 atm
1 mol NaCl(aq) at 25C, 1 atm
1 mol C2H6(g) at 25C, 1 atm
15 mol NH3(g) at 25C, 1 atm
0.1 mol H2O(g) at 100C, 1 atm
b.<0
>0
<0
>0
0
2.a.-203.9 kJ
b.5.7 x 1035
c.forward direction
3.-1036 kJ, spontaneous
4.a.81
b.1
5.This is incorrect. According to the 2nd law of thermodynamics:
Suniverse > 0 for a spontaneous process
and
Suniverse = Ssystem + Ssurroundings
Thus, a process can be spontaneous if Ssystem < 0 as long as Ssurroundings > 0 and has a larger magnitude than Ssystem.
CH186
Thermodynamics Exam Questions
From Fall 1999 Semester
Useful constant: R = 8.314 J/K mol,
1)a. For each of the following processes, predict whether the entropy change is positive (S > 0), negative (S < 0), or approximately equal to zero (S 0).
phosphorous pentachloride decomposes: PCl5(s) PCl3(s) + Cl2(g)
CaF2(s) precipitates from a solution containing Ca2+ and F- ions
gaseous CO2 is forced under pressure into a bottle of coca cola
a bear dies and decomposes in the forest
b. Which of the following processes are spontaneous and which are nonspontaneous?
a reaction already at equilibrium shifts closer towards products
burning of a piece of wood
heat transfer from a block of ice to a room at 25C
rust, Fe2O3, decomposes to form iron, Fe, and oxygen, O2
c. Indicate whether the forward reaction tends to be spontaneous at low temperatures, spontaneous at high temperatures, spontaneous at all temperatures, or nonspontaneous at all temperatures. (Hint: How do S and H relate to G?)
4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)H < 0
2)For the following reaction, HCl(g) + NH3(g) NH4Cl(s), H = -176 kJ and S = -284 J/K at room temperature. Calculate the value of G at 25C. Is this reaction spontaneous or nonspontaneous at this temperature. At what temperature does G = 0, i.e. the crossover temperature?
3)The solubility product for insoluble MgF2(s) is 6.6 x 10-9 at 25C. Calculate G for this reaction. Is the dissolution of MgF2 in water a spontaneous process under standard state conditions?
4)Consider the oxidation of ammonia:
4NH3(g) + 3O2(g) 2N2(g) + 6H2O(l)
Calculate G for this reaction at 25C. Is this reaction spontaneous under standard state conditions?
5)How is it possible, given that the entropy of the universe is always increasing, for complex and sophisticated life forms representing decreased entropy to exist/evolve on Earth? Explain in terms of the second law of thermodynamics.
Answers for Fall 1999 Acid-Base Exam Questions
1.a.>0
<0
<0
>0
b.NS
S
NS
NS
c.Spontaneous at all temperatures.
2.G = -91.4 kJ, spontaneous; T = 619.7 K
3.G = 4.67 x 104 J/mol, nonspontaneous
4.G = -1357 kJ, spontaneous
5.Many life processes that occur on Earth are nonspontaneous and lead to increased order. According to the 2nd law, all spontaneous processes increase the disorder in the universe. However, the universe consists of the system and the surroundings. Thus, it is possible for the order in a complex system, such as the Earth, to increase as long as the disorder in the surroundings increases by an even larger amount. In terms of Gibb’s free energy, the nonspontaneous reactions occurring in nature that are vital to life must be coupled to some spontaneous processes that occur elsewhere in the universe. In other words, GsurroundingsGsystem. On Earth, the nonspontaneous reactions of life are coupled to the nuclear reactions in the sun.