 Now we're moving on to the third topic of chemical kinetics which is all about experimental methods. So we've done a bit of theory on what rate constant is and how it reacts to concentration. We've done quite a lot of theory on the kinetics of gases especially, how does collision affect the rate. Now we're going to move on to how would we actually go about measuring some of this in a lab. You want to go out there and measure some stuff. So this is just going to be the basic breakdown of how we go about it. I'm not going to be discussing in this screencast in particular anything about specific details of the techniques just a broad introduction to it. So we're going to start with experimental considerations about what do we really need to worry about when we're trying to get this because we want to get a rate constant to be accurate as possible and not just you know make a guess here we want to make it accurate so that means we require figuring out a couple of the limitations of the techniques. Then we're going to do two methods one mixing question and the other in situ. So these are two different approaches that we can do to get rate constants. They are quite conceptually different one involves stopping a reaction the other involves just letting the reaction go hence the word in situ it means kind of in place in the situation. So our first set of considerations when we are measuring kinetics all of our theory at this macroscopic level remember that whole triangle thing macro micro and symbolic the math sea world interpretation all of the observations of rates happen in a lab at the macroscopic wrong and that makes the assumption that I think microscopic wrong collisions are happening and their collision frequency happens at a rate determined by concentration. Now that only works if the concentration is even if it's not even then our assumptions are starting to fall down so in this case on the right here that is two sets of products or two reactants well mixed together you can see they're quite evenly distributed there's not far for any of them to travel before they actually collide with each other there is a reaction going to be happening pretty much everywhere so not those two those are the same molecule so you can see everywhere so we shine some light through here and try and figure out what the absorption is it's pretty neat. Here however that's not the case at all you can see that we've just dropped some of this green stuff into the middle and the concentration is really high here so the kinetics of what is happening is going to be based around this concentration here you can imagine that might go really rapidly out here it's going to be going really slowly so if the concentration isn't even things fall down so you will probably discover this in labs when we tell you to add things drop wise and to stir it this is sort of a related reason we want to get things evenly mixed in order to get reliable results if the concentration has something of a gradient in it and it's more concentrated in one area not in another our assumptions fall down our kinetics will not be reliable any measurement we do will be completely unreliable unless we can be sure that it is thoroughly thoroughly mixed so next we want even temperature this is for the exact same reason if things are even throughout here if we take a sample from here or we sample it here we can see that it will be running at the same rate here however is what you might expect in a lab you might be sticking something on a hot plate or something that's all done some burners these days I was going to draw this with a flame underneath and then we realized we don't actually use them do we we use hot plates so you stick a hot plate under here and it warms up here first obviously that's really hot and this is quite cold so if you sample from here the rates going to be much slower remember higher temperature means things go a bit faster the whole Horeneus equation is a function of temperature things are going to be different so if we sample from here really quick here kind of medium up here slow even if we start shining light through here the concentrations are going to be changing much more rapidly because the temperature isn't there so what we need to do is make sure that the temperature is stable throughout and even the temperature should also be as stable as possible in time as well so if you know you you can expect some kind of error in measurement here you might think that this is maybe 15.8 degrees C well actually it's probably fluctuating between maybe 16 and 15.6 okay that is a point four degree difference here maybe that's acceptable for what you're doing as an experiment maybe it's a bit too much so you have to balance out how hard is it to get the temperature even versus how much accuracy you want and the final consideration is one that's often overlooked but it's still important you need a clear way of distinguishing between your reactant and your product so if you ever look at a diagram and we talk about this schematically you're likely to see something like this the reactants if we take a spectrum of it an infrared spectrum for instance the reactant will go down and the product will go up over time and increase and that's really clear and obvious there is a distinction here and we can see one peak growing and one disappear really easy uh more likely you're gonna see something like this and this is a bit awkward isn't it um not here with this peak it's going down but this peak is coming up and you might think we are we can't kind of get these tips and figure them out but what tends to happen is if you get overlapping peaks so here is the peak representing your product and here is the peak representing your reactant they will start combining together so one side might go up and one might go down and maybe at the extremes of your reaction you'll get a reliable signal but in the middle it's going to be a bit of a convoluted mess so this is likely to what you'll what you'll see so in order to really track a kinetic experiment accurately you need to be able to distinguish between your reactant and product really clearly um if you can't you're in agony city so let's just recap those considerations we want to make sure things are mixed thoroughly if they're not mixed thoroughly concentrations differ from one part to another you don't get an even reaction temperature that also needs to be consistent it needs to be consistent in space as in your entire sample and it also needs to be consistent in time you don't want it to be wobbling too much you want to keep it very stable and then detection we need to be able to detect a real difference between the products and the reactants otherwise we are not going to be able to track any kinetics at all and that's and the difference between the products will be and reactants that you need to look for will become clear as you go into these next few methods so now i'm going to talk about quenching uh now in the simplest uh um case quenching just means stopping the reaction or we quench the reaction or stop it both mean the same quenching of the posh sounding word the easiest way to do that the absolute easiest is simply dilute it so we take some of this we dump it into another much larger beaker of solvent and the reaction slows down because you know rate is proportional to a concentration so if we lower the concentration we're going to lower the rate so while this may take several minutes to react this might take several hours so we get our reaction vessel we take a little sample out of it and we go analyze that and we've got a couple of minutes grace to go um figure it out uh we can also quench by adding something to stop the reaction so if something is very reactive we can add something that will react even faster just to mop it all up and take it out we can also mop by cooling it down as well um so if we can chill it down to stop the reaction we can dilute it to stop the reaction or we can add something in there just to physically mop up anything that's reactive and as long as we can do that we can quench it and then what do we do we take samples at different times so here we have we'll just pick a bit of it and we dump it into here and in this set of diagrams I've just assumed that we're diluting it not doing anything fancy to it and what we can then do is just take these away with five minutes 12 minutes different kind of intervals uh and then do some analysis doesn't matter what that analysis is right now we just do the analysis and we find the concentration we might get a kinetic looking graph so you can see the reactants come down here the products go up and yes this graph is actually based on the uh drawn from these molecules the number of molecules of each and these minutes so that is actually the graph representing the schematic diagram at the top so even if the schematic diagram doesn't look particularly realistic uh this at least matches it so the benefits limitation this this is something that we really need to get our heads around so the benefits we have fewer restrictions on the analysis method so that means we can take it away and we can do chromatography on it we could take our sample and run it through gas chromatography we could titrate to figure out a concentration we could go do m r i r u v whatever things that we need we can even go and do mass analysis or something and it's also relatively easy you just set your reaction away you take your action you quench it you go do your analysis so you're very likely to come and find these in the teaching labs so if you do kinetics in a teaching environment you're likely to do quenching type method but it's limited uh there are a lot of drawbacks to doing this mixing and quenching type method uh for a start we can only do it for slow reactions if your reaction is done and dusted within a minute uh you're not going to be having a good time quenching it'll take you the best part of a minute to you know do the quenching reaction so if your reaction is going to last for about two or three hours you can do this just take a sample every 15 minutes and you got enough data points uh you also must use relatively large volumes so if you're taking a couple of millilitres out for analysis every time you obviously need a couple of hundreds to have a good reaction going uh and your rapid quenching can actually be quite difficult so imagine you're doing the cooling method where you want to get it from 50 degrees to you know maybe on an ice bath how long is it going to take to quench a small sample down by 50 degrees probably a minute or so so what that does if this is your time and this is your concentration you've measured and that's your data point this quenching actually starts introducing an error in this kind of direction in the time convention so it takes you up to about a minute to quench uh you you don't have a data point that's exactly at one second you've got a one second plus or minus maybe 30 seconds at the side of it and so that when we come to kind of error analysis uh that's quite important uh so it introduces a new kind of errors that way how can we get over that oh well sorry let's just review so what we need to do is mix the reagents uh we quench by dilution lowering the temperature or adding something to mop it up and detection we can use multiple methods that's a quick review of the quenching how do we get over to limitations we do in situ measurements this means everything's still in their action vessel and then we just generate our graphs and kinetics we're not extracting individual samples here we are just looking at it so we go straight from one thing to another no sampling involved so we get over a lot of those limitations but we do introduce some new ones so how do we go about measuring it uh one thing we're going to be looking at is conductivity so if a reagent moves from uh let's just say it's a and b and then it moves to a it's a positive charge and b has a negative charge conductivity is going to change we can measure concentrations indirectly that way uh obviously spectroscopy is going to be a big one now in practice and this is probably the single biggest one that will be used for kinetics so we want to shine some light through and we see uh the intensity's gone down so remember via Lambert law we can get a concentration out of the absorbance so that's it sorry the absorbance concentration uh path length is that uh so we can figure out a concentration from spectroscopy uh and we can also do pressure so remember the ideal gas equation pressure times volume equals to the moles uh times gas constant times temperature so remember pressure doesn't really have that much to do with mass it is about the number of entities there so when we looked at that microscopically that was an individual molecule imparting some momentum um onto the side of a vessel and that's how we were detecting pressure at a microscopic level it's all to do with the number of moles so if we have in this case we have 2n2 or 5 going to 4n0 2 plus 2 we can see we go from 2 moles of gas to 5 so our pressure is going to almost double over the course of this reaction so we could track down pressure as a proxy to concentration also remember when we're doing rates on gases um you know we're looking at partial pressures not necessarily that kind of concentration so pressure is very much a direct analogy to concentration so we can get kinetic measurements off pressure so let's just kind of review this uh the method for an in-situ we introduce fewer errors we've got less time error going back and forward um so all of our measurements are effectively instantaneous it only takes a couple of seconds to run a UV spectrum or to get an IR spectrum but you do need equipment so it is a little bit of a limitation one of the reasons that you'll do a quenching method in teaching labs is simply sitting down for a couple of hours with the same spectrometer there's not enough equipment to go around the sense of undergraduates it's um even in research environment you're likely not to be able to book an instrument for that long to do a really long-term kinetics experiment you are very very unlikely to do say get a hold of an NMR spectrometer and hold it at lower temperature for 12 hours whilst you run a kinetic experiment on it um so you're likely to do quenching and monitoring instead so the methods for in-situ we use spectroscopy uh conductivity or pressure and these are the three things that we're going to cover predominantly in the next couple of lectures so how do we get spectroscopy spectroscopic measurements how do we do conductivity measurements how do we figure out pressure uh so put these slides in backwards but never mind uh the benefits and the limitations of um the in-situ methods i've kind of discussed already so let's review what we've done we are interested in the experimental considerations what do we need to think about when it comes to taking kinetic measurements we want to minimize the errors principally um so that means even temperatures even time time precise times that we want and then we want the reagents to be mixed properly uh if we start having temperature gradients or concentration gradients or we're quenching slowly for instance well we'll we'll introduce new errors that we don't want uh quenching we that this means stopping the reaction quenching just means yeah stopping the reaction we can lower the temperature we can dilute it or we can add something that physically stops the reaction by mopping up the reagents and then we measure it our main problem there is how efficient is that quenching does it take a couple of minutes or can it be stopped on a dime and then we can overcome a lot of those by saying we can do in situ measurements by spectroscopic means conductivity measurements of pressure a few others might be out there but these are the ones we're going to be interested in uh so that's it for this introductory lecture this is the first bit of the experimental considerations we're going to do we're going to do these three especially have a lot more details soon um so until then goodbye