Defining Entropy

After reviewing last night's blog, Mr. Henderson quickly went into giving examples and rules for when Entropy (S) increases and decreases. I found it very helpful to make a T-Chart to help organize these examples. Situations that had increased entropy included diffusion of gases, dissolving a solid in a liquid, phase changes in the order of solid --> liquid --> gas, and cooling. One common factor that all of these have in common is that each of them causes something to change from fairly compact to a more dispersed state. This is a very helpful tip to keep in mind when thinking about entropy; if it becomes more spread out, it has an increase in entropy. Then we considered this idea of increasing and decreasing entropies with chemical reactions. The reaction of N2O4--> 2NO2 caused an increase in entropy. This is because an increase of moles as well as the creation of simpler molecules causes an increase in entropy. A similar reaction was given but in reverse on the side of decreasing entropy (N2+3H2-->2NH3). We saw that not only do the molecules become more complex, but they also decrease in number of moles from 4 on the reactants side to 3 on the product side.

Next we went over pages 33-34 in our packets. This was once a reading sheet for webassign so the answers are as follows:
1) Independent of external agencies
2)C
3)B
4)B
5)A,D,E
6)B
7)A
8)A
9)C
10)B
11)B
12)C
13)A
14)B
15)D
16)C
17)A
18)iNCORRECT: The student did not take into account the number of moles that pertained to each molecule
19)E

While going over the reading sheet, Mr.H told us a few concepts that we should keep in mind for the test. One of which was that when there is a negative enthalpy change (-delta H) the reaction is most likely spontaneous. We also went over how to calculate change in entropy or delta S. This took no more than 2 minutes because as you will notice in the reading, the equation for delta H and delta S are identical! This means that find the sum of all products minus the sum of all reactants (in terms of entropy) will provide you with your delta S. Another concept that is important to take note of is that if the system does not lower its energy in a reaction and the universe does not higher its entropy, that specific reaction will never take place spontaneously.

Next, we did pages 19 and 20 as a class. The questions on this sheet were very straight forward with one key concept on page 19. This is that if a reaction has a positive delta S (or increase in entropy) then that reaction is favorable to occur spontaneously under the given conditions, and vice versa. Although this was true for problems 4 a b c d and e, a positive entropy change does not automatically mean a spontaneous reaction. The two driving factors that determine whether or not a reaction is spontaneous are 1) increase in entropy and 2) decrease in enthalpy. This was a main idea of today's lesson. We continued on to finish problems 6a and 7 on page 20. The process of finding the answer of 6a (-21.2 J/K) is the exact same as finding the change in enthalpy (sum of products entropy - sum of reactants entropy). Finally, number 7 was focused on determining whether or not a reaction was spontaneous. Those that had 1 of the 2 driving factors of determining spontaneity had insufficient information to determine whether or not the reaction would be spontaneous. Those that had none of the 2 factors were not spontaneous. Those that had both, of course, were spontaneous.

Lastly, we began our TC10) Hess's Law Lab that will utilize both entropy and enthalpy change. Today we obtained only general information regarding the reaction of Hydrochloric acid and magnesium, but tomorrow we will utilize the entropy and enthalpy equations. Good Luck!