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Video transcript

when you're studying chemistry you'll often see reactions in fact you always see reactions for example if you have hydrogen gas it's a diatomic molecule because hydrogen bonds with itself in the gaseous state plus iodine gas I - that's also in the gaseous state it's very easy to discern oh you know if you put them together they're going to react and form the product if you have a well you have two moles of hydrogen two moles of iodine so it's going to form two moles of hydrogen iodide that's all nice and neat and it makes it seem like it's a very clean thing that happens without much fuss but we know that that isn't the reality and we also know that this doesn't happen just instantly it's not like you can just take some hydrogen put it with some some some iodine and it just magically turns into hydrogen iodide that there's some process going on that you know these gaseous state particles are bouncing around and somehow they must bounce into each other and break bonds that they were in before and form new ones and that's what we're going to study now this whole study of how the reaction progresses and the rates of the reactions is called kinetics which is the very fancy word but you're probably familiar with it because we've talked a lot about kinetic energy kinetics kinetics which is just the study of the rate of reactions how fast do they happen and how do they happen so let's just in our minds come up with a intuitive way that hydrogen and iodine can combine so let's think about what hydrogen looks like so if we get our periodic table out hydrogen's got one valence electron so if they have two hydrogen atoms it can share them with each other and then iodine iodine has seven valence electrons so if they share one they get complete as well so let's just review that right now so hydrogen this hydrogen might have one we'll have one electron out there and then you could have another hydrogen that has another electron out there and then if they form a bond they share this this hydrogen can pretend like he has this electron this hydrogen could pretend like she has that electron and then they're happy they both feel like they've completed their 1s she´ll same thing on the iodine side where you have to iodine's they both have seven valence electrons they're halogens you know that already halogens are the group seven elements so they have seven electrons this guy's got one here this guy's got one here if this guy can pretend like he's got that electron he's happy he has eight valence electrons if this guy can pretend like he's got that one same thing so there's a bond right here and this is why hydrogen is a diatomic molecule molecular gas and this is why iodine is the same now when you when they're in the gaseous state you have a bunch of these things that are moving around bumping into each other I'll do it like this so the hydrogen might look something like this the hydrogen or C's two atomic spheres that are bonded together they have these electrons in between that are keeping them bonded the iodine might look something like this it's a much bigger molecule or it's bonded together like this it's also sharing some electrons in a covalent bond everything's probabilistic so in order for these two molecules to turn to this somehow these bonds have to be broken and new bonds have to be formed and what has to happen is these guys there's ton of these guys I mean I could draw a bunch of them all right could copy and paste so there's a bunch of there's a bunch of hydrogen molecules around and some of these iodine gas molecules around so it has to happen in order for us to get the hydrogen iodide is they have to collide and have to collide in exactly the right way so let's say this guy check it sure let's say he's moving this is neat I'm just dragging and dropping but he's moving he has to he has to hit this hydrogen molecule just right and maybe just right if he just happens to hit it and bounce it with enough energy then all of a sudden let's say we get to this point right here these electrons are going to say hey you know I you know it's nice to be shared this way we're in a stable configuration we're filling the 1s shell but look at this there's this ayah ayah Dyne that's close by and they really want me this is it they're they're much more electronegative than me the hydrogen so maybe they're kind of attracted here they don't know whether they want to be here between that hydrogen and this right here between that and so they kind of enter this higher energy state and similarly you know these guys they say hey wouldn't it be nicer I don't have to be here I could kind of go back to back home to my to my home atom if this guy comes in here because then we're going to have then we're going to have eight valence electrons and the same things happening here and this complex right here this this this kind of you know right when the collision happens this is actually this is actually a state it's the high energy state of the transition state of the reaction and this is called an activated complex activated complex activated deviated complex sometimes you know I just drew it kind of visually but it could be you could draw it like this so hydrogen has a covalent bond with another hydrogen and then here comes along some iodine that has a covalent bond with some other iodine but all of a sudden these guys like to bond as well so they start forming so there's kind of eight eight you know there's there's a little bit of an attraction on that side too so this is another way of drawing the activation complex but this is a high energy state because in order for the electrons the way you can think of it to kind of go from that bond to this bond or this bond to that bond or to go back they have to higher enter into a higher energy state a less entered stable energy state than they were before but they do that if there's enough energy because you can go from from so you're going from both of these things separate let me just draw them separate so you have both of them separate you have the hydrogen separate plus the iodine separate they go to this which is a higher energy state but if they can get to that higher energy state if there's enough energy for the collision and they they have enough kinetic energy when they hit in the right orientation then from this activated complex or this higher energy state it will then go to the lowest energy state and the lowest energy state is the hydrogen iodide whoops to draw iodide and then the hydrogen and then the hydrogen this is actually this is actually a lower energy state than this but in order in order to get here you have to go through a higher energy state and I could do that with an energy diagram so if we say that let's say the x-axis is the progression of the reaction progression progression of the reaction and actually you know we don't know how fast it's progressing but this is you could kind of view it as time and some on some dimension and let's say this is the potential energy now I want to draw thicker lines say this is the potential energy right there let me make this line thicker as well so this is the potential potential energy so initially you are at this this reality and we can kind of view it as the combined potential energy so this were eventually we start off here and this is the h2 plus i2 and a lower potential energy is when we are in the hydrogen iodide so this is the lower potential energy down here lower potential energy down here this is the 2h I right but to get here we have to enter this higher activation energy where the electrons have to get they have to have some energy to kind of be able to at least figure out what they want to do with their lives and so you have to add energy to the system you'll always have to add it but if we if it doesn't happen spontaneous you're going to have to add some energy to the system to get to this activated state to get to this activated state right so this is when we are at this thing right here we're there so some energy has to be in the system and this energy the difference between the energy we were at when we were just hydrogen molecules and iodine molecules and the energy we have to get to to get this activated state this distance right here this is the activation energy activation energy if we're able to to get to somehow put enough energy in the system then this thing will happen they'll collide with enough energy and bonds will be broken in reformed activation energy sometimes it's written is EA energy of activation and in the future will maybe do reactions where we actually measure the activation energy but the important thing is to conceptually understand that it that it's there that that things just don't spontaneously go from here to here and I won't go in deeply into catalysts right now but you've probably heard of the word catalysts or something being catalyzed and that's something some other agent some other thing in the reaction so right now so right now we're doing we have h2 plus i2 yielding two H hydrogen iodides now you could have a catalyst and I'll just say plus C plus C and I actually don't know what a good catalyst would be for this reaction and how a catalyst operates is it can actually operate in many many different ways so that's why I don't want to do it in this video but what a catalyst is is something that doesn't change it doesn't get consumed in the reaction the catalyst was there before the reaction the catalyst is there after the reaction but what it does is it makes the reaction happen or either faster or it lowers the amount of energy for the reaction to happen which is kind of the same thing so if you have a catalyst then this activation energy will be lower and what it does is it makes it it might easily it might be some molecule that allow some other transition state that it has less of a potential energy so that you require less heat or less concentration of the molecules for them to bump into each other in the right direction to get to that other state so you require less energy so given how we understand how these kinetics occur these molecules interact with each other what do you think are the things that will drive whether a reaction happens or not I mean we already know that if we have a positive catalyst or something there's something called a negative catalyst that will actually slow down a reaction but if we have a positive catalyst it lowers a positive catalyst obviously it lowers the activation energy so this so this makes reaction faster reaction faster more molecules are going to bump into each other just right to be able to get over this hump because the hump will be lower when you catalyst also if you increase the concentration right if you put if you increase your concentration of molecules if the concentration goes up then you just have more stuff to bump into each other right there's just the likelihood everything is probabilistic when you know when people write these these reaction equations it all seems nice and simple and very clear and it happens but no in the real world that you just have things bumping into each other and you know when we when we do biology videos will be fascinating to talk about because all of every biological process is really just a chemical process and it's really just the byproduct of all of these things bumping into each other and you can imagine the more concentration you have of the things that need to bump into each other the more likely you're going to get just that perfect bump and that perfect amount of kinetic energy for the reaction to happen and actually I'll make a little other note here this reaction you might say okay I have some let's say I'm at this energy how do I ever get over this how does this ever react well remember in a gas the kinetic energies of all of the molecules they're not uniform some gases have some molecules will have higher kinetic energy some will have lower temperature just gives you the average so there's always some probability that two may be high kinetic energy molecules will bump into each other just perfectly surpass the connect so they have enough kinetic energy to get into the activation state and then they can go to the lower state which is the hydrogen iodide so even at a at a at all temperatures this will occur but obviously if you increase the temperature if you increase the temperature that reaction is more likely so that's the other one so temperature temperature is actually probably the biggest temperature is probably the single biggest thing that will make the reaction happen faster so all of these things you want higher temperature higher reaction and then if you just want to think about the molecules itself if you have molecules where their original bonds are weak they're more likely to be able to interact and there's other things you could talk about the molecular shape how available certain atoms are to interact with other atoms and that really becomes significant when we start going into biology and then the last one you probably realize this is just the surface area if you increase the surface area so that we were just doing gas gasps interactions which almost by definition have pretty good surface area interactions but if the surface area goes up then the reaction also goes up the reaction rate and how do you think about that well think about the reaction of think about the reaction of you know we've done this multiple times sodium chloride solid so solid salt plus liquid water leads to sodium well we can think of it a lot of different ways but we could think of it as sodium ion aqueous plus chloride anions is a cation anion aqueous so it gets dissolved and how does that happen if you have a big block of ice or not ice of salt if you have a big I'll do salt and gray if you have a big block of salt in there and you have you know so there's a bunch of sodium and chloride atoms in it and you have water all around it the water is only going to be able to interact with the surface molecules and slowly dissolve away the salt slowly make make polar bonds these are actually up well the polar dipole bonds with the different the different sodium or chloride ions but if you were to break this up into smaller cubes if you were to break it up or really crush it into really small pieces then all of a sudden the surface area that the water molecules can interact with it can actually interact with more of the sodium chloride so the reaction will happen faster so surface area you increase the surface area interaction and you'll also increase the reaction rate if you're trying to do it with two fluids what you could do is you can kind of spray one fluid into the other so you have little droplets so you also increase the surface area so anyway this is kind of an introduction to the idea of kinetics but hopefully gives you a sense that you know these reactions and I want you to really think about chemistry this way not think about it as oh it's just some formula have to remember that these really are bumps and and bruises between atoms it's probabilistic and it's messy and and we really have to think about what will make it more likely that these things collide in just the perfect way for the reactions to happen
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