all right so we looked at the potential energy diagrams for a couple of reactions both the exothermic and endothermic reaction we saw that there's going to be a point in time between the reactants and products during the reaction progression where the transition state is going to reach a peak in potential energy and for the reaction to occur the reactant molecules need to overcome that activation energy or activation barrier with enough kinetic energy to do so so before we talk about where they get that kinetic energy from we do need to talk about why there is some transition state with a very high potential energy very early on in general chemistry one we started to make the relationship that if a certain molecule atom or ion is high potential energy that means it's very unstable and that's why it's high potential energy so these transition states that occur at the highest potential energy position during a for a potential energy diagram happen to be really unstable and it turns out that during a reaction there's always going to be a transition state that is very unstable aka high potential energy for some reason as an example let's look at this reaction here we have an organic molecule where we have a ch3 group bonded to a nitrogen and then that nitrogen is triple bonded to a carbon now not shown here would be the two lone pairs or the lone pair of electrons on that carbon it turns out that that molecules fairly high potential energy it's less stable than if it were to rotate that nitrogen and carbon triple bond so that now carbon is the central atom and nitrogen is on the end and of course we'll have to write the lone pairs of electrons for that nitrogen as well so it turns out that this molecule wants to do that rearrangement it wants to put this carbon atom in the middle of nitrogen I'm on the end and that is of course because carbon is much more stable with four bonds here it only has three bonds and a lone pair it still has the octet rule eight valence electrons but it would be much more stable in this configuration where it has four bonds also the nitrogen is much more adept at handling a lone pair of electrons because it's very electronegative so nitrogen is more Sara this molecule is more stable with the lone pair of electrons on that nitrogen atom so what happens is this molecule will break the carbon nitrogen bond and start to do this rotation well when it does that when it breaks that bond it turns out that the nitrogen keeps the two electrons that were formerly this covalent bond between carbon and nitrogen and so now that bond is broken and these two atoms the carbon and nitrogen that are triple bonded can start to rotate so then that carbon comes in the middle and forms a new covalent bond with that carbon atom that's bonded to three hydrogen's but in the mean time before it forms this molecule that's lower potential energy more stable it's at this transition state where there's no bond between the carbon this carbon atom and either the carbon atom or the nitrogen atom and so as the nitrogen atom took away that lone pair or that that those two electrons in that bond and that are now a lone pair for that nitrogen this carbon atom doesn't have the octet rule it only has six valence electrons the six valence electrons that are bonded to the three hydrogen's okay since we know that the octet rule is the is what makes a lot of elements stable having eight valence electrons this position where this carbon atom only has six valence electrons is very unstable aka very high potential energy so in between the reactant and product even though the product is lower time for energy more stable this molecule has to go through this very high potential energy transition state and that is what leads to these activation energies every single reaction is going to have a a transition state that is very high potential energy for some reason or another has too few valence electrons as too many valence electrons doesn't have the right number of bonds that leads to a stable formal charge every reaction goes through this and so that is what causes the activation energy for a reaction now the next thing we want to think about is where does where would this molecule get enough kinetic energy to overcome this activation energy and it turns out that any given temperature a molecule could have enough kinetic energy to overcome an activation energy and that comes from looking at the thermal energy distribution of a given molecule okay so the thermal energy is of course related to its temperature and if you think about the temperature of a sample of some molecules we know that temperature is a measure of their kinetic energy a relative measure of their kinetic energy just because a but but because but if a molecule is at a certain temperature or sample of molecules is at a certain temperature that doesn't mean that every single one of those molecules has the same kinetic energy even though they're all at the same temperature temperature is really a relative measure of their kinetic energies and molecules actually have a wide distribution of kinetic energies that's what we call the thermal energy distribution so if we plotted the percentage of molecules that have a certain kinetic energy that plot actually takes what's called a Boltzmann distribution where there's small number of molecules that that have low kinetic energy and some molecules I have high kinetic energy and most in the middle so the thermal energy distribution looks something like this we're slowly goes off to zero alright so this isn't and actually that's a it should tail off a little bit more than that so let me rewrite that redraw that so they go up okay so what this basically tells us is that at any given temperature a small percentage of molecules are gonna have really low kinetic energy a small percentage of molecules are gonna have high kinetic energy and of course most of the molecules will have kinetic energy somewhere in the middle all right but if you think about a activation energy needed for some kinetic energy that's or for some reaction to occur that's an actual number of numerical value it is a value that we can put on this plot we can say that the activation energy is so many kilojoules per mole and that would correspond to some value on this x-axis so that's the activation energy for a reaction because of the thermal distribution of molecules it turns out that at some given temperature some percentage of molecules have enough kinetic energy to overcome the activation energy the higher the activation energy the further the activation energy goes this way the smaller number of molecules would have enough kinetic energy to overcome the activation barrier and so that might be a slower our reaction rate because less molecules can overcome the activation barrier if a reaction has a smaller activation energy and that shifts that way that would be more molecules might have enough kinetic energy to overcome the activation energy and so that would lead to a faster rate you know given all other parameters being equal so it turns out that at a given temperature some percentage of molecules have enough kinetic energy to overcome the activation energy it could be a very small percentage of molecules or it could be a bigger percentage of molecules and that would be different for each chemical reaction