Chemistry 223
Worksheet 6 Notes
Oregon State University
1.
Write a balanced chemical equation (use your notes or text as a source of reactions) for:
(A)
an exothermic process in which entropy increases.
(B)
an exothermic process in which entropy decreases.
(C)
an endothermic process in which entropy increases.
(D)
an endothermic process in which entropy decreases.
2.
Identify (if possible) ΔH, ΔS, and ΔG as being (-), (+), or (?) for:
(A)
CO2 (g) → CO2 (s)
ΔH = (-), ΔS = (-), and ΔG = (?)
(B)
2 NO2 (g) → 2 NO (g) + O2 (g)
ΔH = (+), ΔS = (+), and ΔG = (+)
(C)
The combustion of methanol (liquid CH3OH) to produce CO2 (g) and steam.
ΔH = (-), ΔS = (+), and ΔG = (-)
3.
Consider the "Cold Pack" reaction, NH4NO3 (s) → NH4NO3 (aq). Do you expect ΔH to
be positive or negative? Do you expect ΔS to be positive or negative? Do you expect
ΔG to be positive or negative?
ΔH to be positive (endothermic)
ΔS to be positive (going to more disorder)
ΔG to be negative (it spontaneously goes—the salt dissolves)
4.
Consider the complete combustion of methane gas in oxygen to produce carbon dioxide
and liquid water. Determine ΔGºreaction.
5.
Which of the following produces a DECREASE in entropy of the system? The system is
shown in bold.
Dissolving sugar in a cup of coffee.
Condensation of water on the surface of a glass of iced tea on a hot summer day.
Boiling water in a pot on the stove to make macaroni and cheese.
Allowing the liquid propane in a gas grill to escape from the tank.
Producing CO2 gas from baking soda (NaHCO3) when baking a cake.
6.
What is the Second Law of Thermodynamics?
The second law of thermodynamics is perhaps the most popular outside of the realm of physics,
because it is closely related to the concept of entropy, or the disorder created during a
thermodynamic process. Reformulated as a statement regarding entropy, the second law reads:
In any closed system, the entropy of the system will either remain constant or increase.
In other words, each time a system goes through a thermodynamic process, the system can never
completely return to precisely the same state it was in before. This is one definition used for the
arrow of time, since entropy of the universe will always increase over time according to the
second law of thermodynamics.
7.
What is the Third Law of Thermodynamics?
The entropy of a perfect crystal approaches zero as temperature approaches absolute zero.
8.
Calculate the value (in kJ) of
substance
H2O2(aq)
Go at 25oC for 2H2O2(aq) <==> 2H2O(l) + O2(g) given:
Hfo, kJ mol-1
-191.17
So, J mol-1 K-1
143.9
H2O(l)
-285.83
69.91
O2(g)
0
205.14
-210 kJ
9.
5378 K
Which of the following reactions is unfavorable at low temperatures but becomes
favorable as the temperature increases? At what temperature does the process become
favored?
Ho = -566 kJ;
So = -173 J/K
2 H2O(g) --> 2 H2(g) + O2(g);
Ho = 484 kJ;
So = 90.0 J/K
(C)
2 N2O(g) --> 2 N2(g) + O2(g);
Ho = -164 kJ;
So = 149 J/K
(D)
PbCl2(s) --> Pb2+(aq) + O2(g);
Ho = 23.4 kJ;
So = -12.5 J/K
(A)
2 CO(g) + O2(g) --> CO2(g);
(B)