Hello everybody welcome back today we're going to be starting a new unit Unit Nine which is the last unit in the AP Chemistry curriculum so I hope you're ready for it let's get started okay so for our final unit in this AP Chemistry course here's our little factoid which I think is just interesting that when astronauts come back from space because there is less gravity in space their spines actually expand they are a little bit taller when they come back of course that doesn't last but I just think that's interesting so Unit Nine guys is about thermodynamics and electrochemistry thermodynamics just means the exchange of different types of energy we're going to um pull multiple Concepts back from unit six which was on thermochemistry and then at the end of unit 9 there's a whole section on electrochemistry which I imagine will be new to just about everybody in this course new material so what we're going to be looking at a great deal in Unit Nine is looking at different chemical reactions or physical processes and deciding making a decision or a prediction on whether that process is spontaneous or not and thermodynamics the exchange of different types of energy is going to let us predict whether that process is going to occur it does not give us any information about how long it's going to take but that's what we studied in the kinetics unit we're not doing that here but as I said one of the things you're going to be asked to predict is whe whether a process is spontaneous and the AP exam doesn't usually like that word spontaneous so much they prefer the term thermodynamically favored meaning a process that occurs on its own it occurs without any outside intervention without any help you put the two reactants together and stand back and vaa they produce products just as a an example of you know a thermodynamically favored process if you place a ball at the top of a hill a ball will spontaneously roll down the hill without any intervention however if you have the ball at rest a ball at rest is not going to spontaneously move up a hill in order to do that you have to apply work you have to expend energy so we're going to be making a lot of predictions in this unit and one of the concepts that we're going to use to help us make that prediction is a concept called entropy which I imagine is probably a new term to most of you so let's define it entropy which has a variable of a capital S is a measure of Randomness or disorder and you'll see that I've put this term in bold degrees of Freedom that's the term that the AP graders like the most from what I have heard so I'm going to try to use that that term as much as I can but let me just give you an example let's just talk about simple states of matter solid versus liquid versus gas and I want you to picture in your mind think back to unit three when we talked about the different states of matter what do the particles look like what is their behav behavior in the solid state versus the liquid state versus the gaseous State and the entropy of a solid is relatively low solid particles are very often locked into a rigid structure they have very little degrees of freedom very little ability to move around freely liquids have a little bit more and of course gases gaseous particles are moving all over the place like crazy gases have the highest entropy because they have the greatest degrees of freedom okay so in general when we're talking about entropy of the different states of matter gases will always be greater than liquids which will be greater than solids and one of the driving forces of a thermodynamically favored process is an increase in entropy typically speaking if we're looking at a process and the entropy is increasing meaning becoming more chaotic more disordered in general things are increasing in their degrees of freedom that usually will lead to a thermodynamically favored process and we'll talk more about that uh in the next subunit but one of the things that you will be asked to do both in multiple choice and pre response is to make predictions about the changes in entropy for a particular process so some of the things on the screen here are simple phase changes some are chemical reactions and just to be clear what you're being asked to do is say whether the change in entropy that's what I mean by Delta s is going to be positive or negative for each process on the screen we're going to talk through this I want you to ask yourself is this process becoming more disordered or more organized if it's becoming more disordered the degrees of freedom must be increasing we're going to say that Delta s is a positive value whereas if the process is becoming more organized degrees of freedom are decreasing Delta s will be negative so let's look at each one individually first one says a cube of sugar dissolves in hot tea so think about the particles of the sugar they're going from a solid form to a dispersed aquous form degrees of freedom are definitely increasing so the Delta s uh sign would be positive again guys if this was a free response situation and you are asked to justify your answer you want to use that term degrees of freedom and in this case are increasing next one says a lake freezes over in Winter okay solid no excuse me liquid to solid okay crystallization is taking place degrees of freedom are decreasing so that's going to give us a negative value for Delta s rainwater on the pavement evaporates again this is another phase change we're going from the liquid phase to the gaseous phase degrees of freedom are increasing that's going to be a positive value now these next three might look a little bit more complex so let's talk about how do you make a prediction if we're looking at an actual chemical reaction first thing that comes to my attention is the states of matter okay we've got solid lead plus gaseous oxide and then on the product side we've just got the one product and it is in solid form I can see that we've got some different states of matter here solid and gas going to one solid to me that looks like there's a decrease in the degrees of freedom on the reactant side we had some gas on the product side we have no gas this looks like a decrease in entropy to me now this next one everything is in the same state of matter we have all gases and if that is the case ladies and gentlemen what I want you to focus your attention on are the coefficients okay on the reactant side we have three total moles of gas producing just one mole of gas so everything is gaseous but we have fewer moles of gas on the product side that tells me that there's going to be a decrease in the degrees of freedom there's simply less gas so I'm going to predict this would be a negative Delta s last one we have a solid dissolving into its aquous ions this is an ionic solid solid dissolving into its aquous ions it tells me that that is an increase in the degrees of freedom so I'm going to predict that that's a positive Delta s so that is a skill that you could absolutely be asked to do both in multiple choice or free response if it's free response you want to justify your answer using that phrase degrees of freedom so let's look at a practice problem for you to try on your own I'll set it up for you we have a uh a heating curve given to us and it says at one atmosphere a sample of ice is heated to liquid water and then to water vapor the heating curve is shown in the graph above predict the signs for the changes in enthalpy now wait a minute let's stop there for a second let's review that word that's calling back to unit six guys remember enthalpy means change in heat that was Delta H what they're really asking for is whatever process they're going to be showing us is it endothermic or exothermic remember those terms so they want to know is Delta H positive or negative they also want to know if the entropy Delta s for the process corresponding to segment X okay so right up here is the sign for Delta s positive or negative going from left to right and this is a free response so you do need to justify your answer pause the video see how you do with it okay so first thing to make sure we know is what is even going on in this segment X and if you go back and review things from unit six heating curves what's happening in that segment X is a phase change from liquid to gas the vaporization process or boiling okay so we're answering questions about the boiling process so let's see what did I say so the enthalpy the Delta H will be positive because vaporization is an endothermic process a process that requires heat energy okay so that's a review back from unit six how about the change in entropy I said it's going to be positive because the phase change from liquid to gas is an increase in the degrees of freedom remember that's the term you want to use okay so that would be a great sample free response kind of question and then just one final thing and we're going to talk more about this incoming subunits but in general guys when there're is an increase in entropy meaning for whatever process you're looking at if the Delta s is positive that is usually please notice I say the word usually not always but usually probably a thermodynamically favored process if the Delta s is negative usually probably that process is not going to to be thermodynamically favored and in fact would be favored in the opposite direction we're going to come back to this concept uh as I said in coming subunits but I just want to make sure that that concept is clear in general in the UN in nature in the universe processes tend to become more chaotic if you think about if you were to not clean your room for a month and I mean like really not clean your room like never pick anything up off the floor not don't make your bed don't put your clothes away you know leave trash on the floor if you expended Zero Energy over the course of 30 days your room would become a very disordered chaotic place things in nature tend to favor or tend to increase in chaos in disorder that's just how the universe tends to work but again we'll talk more about that incoming subunits so that concludes subunit 9.1 I hope you guys have learned a little something about the concept of entropy and I look forward to seeing you next time