Learn about Advanced Placement Chemistry, Thermodynamics 4, in this comprehensive video by bannanaiscool.
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Speaker: A lot of multiple choice questions in exams deal with your ability to think through this formula and tell if the reaction is spontaneous or not. Okay, somebody says to you you got a delta h value that's positive and a delta s value is positive, is your reaction spontaneous? Okay look at, break it down. If this is positive and this is positive then that means then remember temperature always has to be a positive number, cant be zero k or less, so it has to be above that. So, this also is going to be positive. So positive value here multiplied by this with the negative from means this is going to be a negative quantity and this is going to be positive. So, when the temperature is high the this number here or what this gets you will be bigger than this here and you will get a delta g that's negative. So, when the temperature is actually high, the t value is actually high here, you have got a spontaneous reaction and what about when both of them are negative values. When the delta h is negative and this is negative, negative times negative makes up positive. So if this is going to be positive and this is going to be negative, if this is a bigger negative than this is then the reaction is going to be spontaneous that means that the temperature has to be low to make this a small positive value and so when temperatures are low this condition here will give you spontaneous. Now what happens if your reaction is positive for delta h and negative for delta s, negative here times t which is and its negative value makes a positive, positive and positive can never give you a negative, so this is going to be non-spontaneous but it's a positive here and a negative here, it doesn't matter what the temperature is. Lastly if this is a negative value and this is a positive delta s the system has positive has positive entropy, which means its going random and whatever the temperature is it doesn't add, because as long as its exothermic can release energy, negative here and a positive here means the reaction is going to be spontaneous at any temperature. Recap, you know how to calculate delta h, don't you? If you are given any calorimetric data, you can go to the back of a book or a data booklet and if you look at the heats of formation table and the some of the heats of formation of the products like we will say X, Y minus the sum of the heats of formation of the reactants that will give you the delta h, well okay, you know this has this log type of equation here also works for, if somebody says you well, what's the delta s of this, all you have to do is go and find the chart that actually has delta s values and you know any textbook has the delta h and delta s values next to each other and this holds true to calculate the delta s for a system. There are delta S or S values for each of the chemicals, now you know that for calculating delta h, elements don't have the heat of formations, but, you've got to be careful don't take for grated that the s values for elements or zeroes, they are not, and guess what else. This is going to be real convenient, sum is not convenient, just tell me whether this is going to be spontaneous reaction or not, well, you can actually go and find that the delta g equals the delta g of the products minus the delta g of the reactants. So, all three of those s, g, and h can be calculated for an equation with a table that has those values in it. One of the things that I wanted to mention, you know this formula, delta g equals delta h minus t delta s. If somebody says I have substance X which is a solid and it is turning into X liquid what is the melting point for that? What's the temperature of the melting point, while you know what if you know the delta g in that reaction and you know the delta S and the delta H, you can manipulate to find the t, or you have to really know is this and this how much is the free energy value at a phase change, a phase change is a point of equilibrium and a delta g value at equilibr