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.