so now what we're going to do is we're going to go over potential energy diagrams so let's start with this one on the left you have the reactants and on the right products at the top of this diagram we have the transition state also known as the activated complex on the y- AIS is the potential energy and on the x axis this is the reaction coordinate so notice that the products have less energy than reactants whenever you see that this reaction is considered to be an exothermic reaction EXO means like outside exothermic means that heat is released out of the system into the surroundings so whenever heat is released the enthropy of the reaction is negative as you can see the enthalpy is the difference between the potential energy of the products and the reactants so it's products minus reactants because the products have less energy than the reactants you're going to get a negative value now what is the energy difference between the transition state and a reactance what is that called This is known as the forward activation energy the activation energy is the energy that's needed to get the reaction started without that energy the reaction will not work so one way you can reach the activation energy is by increasing the temperature of the reaction which will speed it up now what is the energy difference between the transition state and and the products this is the activation energy but for the reverse reaction so if you need to go backwards you got to go this way once you reach the transition state that's when re um the reaction takes place so you need enough energy to get to the transition state now what's going to happen if we add a catalyst to this reaction now we know that a catalyst the purpose of a catalyst is to speed up a reaction action and enzymes are a biological catalyst that speeds up reactions in living systems now if we add a catalyst we're going to have a new diagram that looks like this as you can see the activation energy is lower here the activation energy is only this high but before it was significantly higher so as you can see adding the Catalyst lowers the the activation energy and when you have a lower activation energy the reaction will proceed faster and so that's how a catalyst will speed up a reaction it's by lowering the activation energy now how can we draw the potential energy diagram for an endothermic reaction if you wish to draw it for an endothermic reaction you need to draw the products with a higher energy than reactants so the products have to be above the reactants and now it's endothermic so as you can see Delta H is going to be positive since Delta H is products minus reactants if you take a large number and subtract it by a small number you're going to get a positive result for example 10 - 4 is POS 6 in the left side if you take a small number subtracted by a large number you're going to get a negative result like 3 - 7 is4 so because the products have more energy than the reactants what we have is an endothermic reaction so in this reaction the system absorbs heat energy it gains energy and so the eny of the reaction is positive now it turns out that the eny is really the difference between the activation energy of the forward reaction and reverse reaction so let's put some numbers to this so let's say this is at the products is at a value of 100 the reactants is at 300 and the transition state is at 600 so if we want to calculate let's say the Ford activation energy the forward activation energy is the difference between the energy of the transition state and the energy of the reactants so in this case that's going to be uh 600 minus 300 so the activation energy for the forward reaction is positive 300 now if we wish to calculate the reverse activation energy is the difference between the energy of the transition state and the energy of the products so that's going to be 600 minus 100 which is positive 500 now if we wish to calculate the enthropy of the reaction it's the uh products minus the reactants so the products have a value of 100 and the reactant is 300 so because the products are in they're lower in energy it's going to be a -200 that's the enthropy of the reaction so notice that the anthropi reaction is also the forward activation energy minus the reverse activation energy the forward activation energy is POS 300 the reverse is 500 if we subtract these two we're going to get the eny of the reaction which is -200 which is the same as this one so you can remember these four equations so that's the enthalpy you can find it by subtracting the two activation energies or you can take the energy of the products minus that of the reactants and you can calculate the forward activation energy using those two and here's the reverse so four useful equations that you can get from the potential energy diagram now what about a two-step reaction how can we draw the potential energy diagram for that so let's try it so if you have a two-step reaction there's going to be two transition States ts1 TS2 the transition state is also known as the activated complex so here's the reactance and here's the product and this is the intermediate which is like it's in the middle so overall is this reaction exothermic or endothermic notice that the products have less energy than the reactants so therefore the enthropy of the reaction overall is negative it's an exothermic reaction now what about the enthropy for the first step let's call it Delta H1 is an endo or exothermic so for the first First Step we're going from the reactant to the intermediate notice that the intermediate has more energy than reactant so we're going up as you go up that means the system has to gain energy so therefore um Delta H is positive now as you go from the intermediates to the products you have to go down in a potential energy diagram so the enthalpy for the second reaction is negative so for the first step it's endothermic the second step is X exothermic and overall it's an exothermic reaction now which step is the slow step or the rate determinant step is it the first step or is it the second step what would you say now notice that the first step has a higher activation energy than the Second Step so because the first step has a higher activation energy because the transition state is higher than the second transition state this is going to be the slow step because it's going to be a lot harder to go up this mountain than it is to go up this mountain so this is going to be a slow step it's going to take a longer time to get through ts1 TS2 is easy to pass to because it's not so high in energy so the transition state that has the highest energy it's going to be associated with the slow step and so that's what you want to know for these types of diagrams so here's uh an example problem for you draw a potential energy diagram that has three steps where the second step is rate determinant and the overall reaction is endothermic and let's say the first step is endothermic the second step is EXO and the third step is endothermic how can we draw such a potential energy diagram okay so let's be careful with this so overall it's endothermic and we said the first step is an endothermic step so let's draw something like this the second step is the rate determin step and also it's going to be exothermic so it has to be very high if it's going to be the rate determinist step if it's exothermic has to lower than the than what it was previously now the third step is endothermic and it's endothermic overall so this has to be higher than reactants so this energy diagram meets the characteristics this is uh ts1 TS2 since that's the highest the second step is the slow step and here we have the reactants and the products because the products is higher than the reactants overall it's an endothermic reaction now intermediate one is higher than the reactant so the first step is endothermic intermediate 2 is lower than intermediate one so the second step is exothermic and the third step is endo because the product is higher than intermediate 2 and so that's how you can draw a potential energy diagram with three steps and the characteristics that we mentioned before so that is it for this video thanks for watching and I hope you have a better understanding of potential energy diagrams