Last week we had a major focus on entropy of reactions and how entropy of reactions work. This week was probably the most difficult to understand so far. Entropy is a difficult concept for me to understand, especially with all the equations needed to calculate values. The basic definition of entropy is the number of distinguishable micro states in a given system. Evaluating if entropy increases or decreases in a reaction is relatively easy, if the number of particles or the energy of the particles increase, then the entropy increases. Entropy always increases in the universe.
In addition to entropy, we also worked on several other thermodynamic principles, one being Gibbs free energy. This free energy, as it is called, is energy in a reaction that can be used to do work on the system or for the system to do work on the surroundings. The equation for Gibbs free energy is ∆G=∆H-T∆S, where ∆H is enthalpy, T is time, and ∆S is entropy. Enthalpies and entropies are usually given in a table, but can be calculated using this equation if the free energy was known I suppose. This ∆G, or free energy, can be used to determine if a reaction is thermodynamically favored or not. If ∆G is negative, then the reaction is favored and will "go" without any outside energy needed. For example, dry ice sublimes at room temperature because it is favored. But water does not boil at room temperature because the ∆G of that reaction is positive.
Another thermodynamic principle is Hess' law. Hess' law states that in a reaction, the sum of the enthalpy of the steps of a reaction can be added up to to equal the total enthalpy of the reaction. My understanding of this is that if you add up the bond enthalpies in each different step and them cancel out the ones that are on both sides and eventually you will come to the total heat of reaction or the enthalpy of the reaction. Here's a link to a site on Hess' law.
Overall, my understanding of this unit so far is not where I would like it to be. Hess' law is confusing, although it seems simple. My major problem with all of these principles is knowing when to apply them. I think all I need to do is work more problems and I'll know whats going on. I think the lab we are doing next week will help a lot.
