THERMODYNAMICS LESSON PART 1 ANSWERS
Question of the Day: Do you think that objects in our world tend towards order or
disorder on a daily basis? Why you think this? Provide specific examples! Think about
your room, a car over time, the population of the earth, a human body, an ice cube
on the counter, etc.!
All objects and phenomena in nature tend towards disorder, or randomness, over
time. A room becomes messy and disordered, the weather becomes chaotic, a car
rusts, its components break down, tires wear, the population of the earth increases
and overcrowding and disorder occur, a human body becomes old, its cells lose
efficiency, systems and organs fail, and an ice cube, which is an ordered crystal
arrangement, begins to melt and the molecules separate and become disordered.
Questions to answer during lecture!
1. The second law of thermodynamics states that the entropy of the entire
universe will increase over time.
2. Chemical reactions that both increase in entropy and release heat are
spontaneous!
3. If an ice cube absorbs heat in order to melt and become disordered, the
molecules around it in the surroundings lose heat and become ordered.
4. S is a positive quantity when a system or chemical reaction becomes
disordered, or gains entropy.
5. Salt dissolving into water has a S that is positive, because it is increasing in
entropy or randomness.
6. ΔGrxno = ΔHrxno - TΔSrxno or the Gibbs free energy equation, allows me to
calculate the change in the Gibbs free energy of a chemical reaction.
7. By looking at the mathematical sign of G, we can determine if a reaction will
be spontaneous or not.
8. If a reaction is endothermic, meaning H is positive, and entropy decreases,
which means S is negative, the reaction will have a positive G and the
reaction will not be spontaneous.
9. When 1 mole of salt dissolves in water, the H0 = +3 kJ/mol at 298 K and the S0 =
28.1 J/mol K at 298 K. This reaction has a G0 value of -5.37 kJ/mol, which
means it will be spontaneous!
10. If a chemical reaction has a positive H, and a positive S, it can be
spontaneous if the temperature is high enough.
11. Given the following reaction:
NH4NO3 (s) + heat  NH4NO3 (aq)
If the ΔHorxn = +25.7 kJ, and the ΔSorxn = +108.7 J/K, then what is the G0 for this
reaction? Show your work below! Is this reaction spontaneous? Why?
ΔHorxn = +25.7 kJ
ΔSorxn = +108.7 J/K or +0.1087 kJ/K
ΔGorxn = +25.7 kJ - (298 K)(+0.1087 kJ/K)
ΔGorxn = -6.7 kJ
This reaction is spontaneous, because it has a negative ΔGorxn, and the reaction is
increasing in entropy, even though it requires heat!
12. When acetylene combusts, the ΔHorxn = -2476 kJ, and the ΔSorxn = -194.8 J/K. Is
this reaction spontaneous? Show your work below, and provide an explanation!
ΔHorxn = -2476 kJ (Given in problem)
ΔSorxn = -194.8 J/K or -0.1948 kJ/K
ΔGorxn = -2476 kJ - (298 K)(-0.1948 kJ/K)
ΔGorxn = -2418 kJ
This reaction is spontaneous, because it has a negative ΔGorxn. Despite the fact that
disorder is decreasing, the reaction is exothermic, which drives the reaction.