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Lecture 19. Observational Chemical Kinetics

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is 1 of the whole time he reached you this is a 18 years the ball and all these years where it is the variation in all always the that was going on in the year of all years by momentarily until you converted a volume over this extended reaction into a concentration and we're never going to go back or a lot relates the concentration reactors often is difficult because and this is a great loss that's the reaction rate With the that's why it's called a rate loss break 3 lot predicts the reaction rate makes sense 48 alpha and beta independent the concentrations right that's a rate constants it's called the constant has its independent of concentration note also that alpha and beta are not necessarily stuck metric coefficients for species a and B C that's alpha and beta you might think that that's a stoic magical vision dining and that's the stoic electrical fishing on B In general that is not true UN last the reaction that you're riding the wave off war is an elementary reaction you know that if you know the the reactions in elementary reaction then that is the Stoica metrical fission Ioannidis's that is Stoker metrical fishermen be of saying more about that but in general the less you know that you can't assume that that's true instead of a bitter urging experiments and yes so from Israel or we can say the order of this reaction with respect is often the order with respect to be is made effort that means so there's an order with respected all the chemical species in this reaction suffice order with respect to a is alpha Is this the reactions first-order with respect to a a 2nd order that means Alpha is 1 or 2 becomes comely speaker reacts a first-order Rene yes that means of equals 1 hour 2nd order and a yes that means helping equals 2 and the tall order the reaction is Alpha Plus data but the overall water is the some of those documentary coefficients this some of those exponents rather so case rate constitutes depend on OK so this is important so we we mention that there's this thing called the rate constant 1 of the confusing things as the rate constant as different units depending on what type of reaction idiots and we have to be conversant with all these different units all right the rate constant tells us what the overall reaction order it's right the overall reaction order is information that's embedded In the units Of the rate constant so if tell the rate constant for the reaction was this 26 . 2 5 per 2nd I'm telling you it's a first-order reaction but you have to know those units are units of first-order reaction reaction is overall 1st or let's look at this a little more detail case reconsidered for example if Alpha is 1 data is 0 then the reaction is best because of beta 0 less 1 right the reactions first-order Renee and 1st quarter overall because 1 . 0 is 1 OK so what's cater to be you can always figure it out you take the rate you take here's your weight loss or sulfur 10 Keyes rate over a the concentration of a idle the Raiders Moeller per 2nd is Muller and so that means the use of the up the Pesek and I can just figure that out so you use if you wanna know what the units of the of the rate constant ourselves caught in the right lot plug the dimensions of the other variables and just calculate what the dimensions of K have to be just a little dimensional analysis on tape OK so if you see units of per 2nd you know immediately it's first-order reaction no 1 has to tell you that if on the other hand Elf was 1 and there is 1 then you've got this weight loss and you've got an overall 2nd order reaction identifies all 4 K here I get rate over a concentration of concentration B and the rate is always to be Muller per 2nd as I got more squared and so now the units are :colon Muller per 2nd right if if I see those units I know immediately that I've got a 2nd order reactions for the units are telling me what the overall reaction order of the reach of the reaction why what's so stood magic reactions Raila commuters from inspection yes yes the rate lock had not version that should be bolded and in my Tallis italics In other words the rate of this reaction is not there so this is what I said earlier we mention that this is a start metric reaction you might wanna just think about this as being a rate constants which it is not there is no rate constant for this arrow because there's this process never purse OK this is just expressing the overall reaction that occurs as a consequence of about 20 steps right and so it it's tempting to say all the rate the reaction to skate times concentration hydrogen squared times comes through the locks did not hold it is untrue not even close to being true but for an elementary reaction rate lock can be generated by inspection for example by look at this reaction right here which I pulled out of the mechanism for that why the reaction rate is given by Kate finds a concentration of hydrogen atoms times a concentration of oxygen right if that's a single letter there are all these products don't have anything to do with the reaction rate the single-entity Aero tells me it's an irreversible reaction and I don't have to think about what those guys are I could just like products here and I have all the information I need to know about the race right only the reactants appear in this expression often reactions is significantly reversible and both the forward and backward rates are important so if you have a double ended error now you've got a reversible reaction you do have to think about the products you do have to know what they are right in this case we study reaction from left to right removed the AGI as it's produced that's important if we remove the HIV and produced in the raid Of the reaction will be this in other words if I remove the age Isaac produced I'm basically turning off the reverse reactions and not allowing it to happen at all so this would be the rate of the forward reaction but we can also study HI decomposition in this case if the products so HID composition would mean a reaction is running in the backward direction right in there in that case these are products of the decomposition right so if we remove them then the rate of the back reaction would be best right because this is an elementary reactions that we're talking about here right it's we can write these rate lies directly from inspection that's the main point we're trying to make here we're not going to talk about reversible reactions the kinetics of reversible reactions very much for a lecture to would get to it right but I'm just pointing out that if it's some elementary reaction we can deduce what these rates are gonna look like just by looking at this Thursday this is to in front of the HI that means it's gotta be a 2 In the sex that's the rate constant for their the composition of the HI and if I get rid of these guys is going to be no better so that would be the rate of the decomposition they have these guys are removed or if they should away very quickly and no back reaction can happen but the store committed reaction for acid elder Heidi decomposition this is acetaldehyde it decomposes to give methane and CO the rate laughter this reaction is that it now Is that an elementary reaction it's not but I can tell that just by looking at the reaction right the rate lock this rate loss would never applied to this reaction if this was an elementary reaction of this is an elementary reaction what with the rate lobby that would be a lot wouldn't right because there's a 1 year that would be the stoking that would be that of exponent right and recovery that so just looking at this rate loss I can tell immediately that the South as it all the high decomposition has a complex mechanism made up of a series of elementary reactions that I don't know what they are right but that's not an elementary reaction right there tell that immediately we did not know this was struck much that if we did not know this was a stoic met Stoker metric reaction we would know once we look at that we we know it's not an elementary reaction just based on what the rate but I can ask 1 of the units that rate constant right there 1 odds units and all I have to do is solved here's the rate is the concentration concentration of 3 halves of a work that out and have Moeller to the minus 1 half seconds to the minus-1 bulls are going to be the units of the rate constants OK if I wanted to I could take those units for the rate constant and deduce that this overall reaction order is 1 and a half right it's a perfect quiz question here's the rate constant tell me what the overall order the reaction was 1 2 3 1 . 5 OK so it's a little confusing because we've been talking about big cave equilibrium constant and equilibrium constant never has all right but the rate constant below belittled the little Terry debts by then the units have contained information but now How do we experimentally determine these realize this is important we have some methods that we can use for doing that so we need to know about but 1 of the methods is the method of initial rates the idea is very simple if you've got a complex reactions a bunch of reactants agency or you want know what the Stoker metrical fish and is for it but you don't know what it is right you want to figure out but alpha beta and gamma right the way that you do that is you isolate react aid by making all of the other reactants enormous compared to a you make a tiny and B and C large if you do that what'll happen the rate of the reaction will depend on the amount of 80 that you've got right because you got an excess amount of B and C by B and C are not rate-limiting you flooded the system would be insane he put a tiny amount a other reaction will only occur at a rate that dictated by the amount of a N you can interrogate a and learn everything about how the reaction depends on it so here's how this works if I may be in the large but become Sunil constant because the amount of the reactor so B and C and are large and is a tiny amount of a it is only and allow the reaction to occur at a relatively slow rate any amount a B and C are not going to be significantly perturbed from the total concentrations because the total concentrations are enormous so I can treat them as constants I can define a K Prime it's just that they tend to be inundated see gamma right and essentially turned this into a suit L 4 order reactions we have eliminated the dependence a B and C and now that reaction only depends on a and I wanna know what Alpha itself I wanna know is I just take a lot of all sides Logar the rate equals log of K prime plus 4 times a lot of I measure now the rate 4 different initial concentrations of a where a change the initial concentration of a while maintaining a tiny compared a B and all right after use tiny concentrations of a but I can vary right and as I do that here is the lowest concentration of 8 years the initial rate that I measure right here is a higher concentration of a here's the initial rate and measured their that's that red line and use a Harkins of all of these concentrations of a here are tiny compared a B and C A 1 emphasize that and now I can just plot a lot of the rate log the initial rate as a function of log and the slope of that is Alford by golly so I have use dealt the molecular ready of the reaction with respect the reagent I I think it's a lot right but now I know what the order of the reaction is with respect to and then I may be small making CBS and repeat the experiment right so I can take the reaction apart piece by piece and figure out what the areas of every reactor looking forward to a conflict OK now how do you know whether a small enough well you know it is small enough when you change you've got big concentrations of billions a right a 1 million and B and C A 1 Mullah Hadi Nov 1 big enough you may get 1 . 1 but the reaction better not right yet you have to actually checked to see if the reaction rate depends on B and by changing their concentrations when they're big to convince yourself or write the big enough were reactions only depending on the concentration of a now I can actually run my experiment OK so big and small are relative terms the nebulous you have in the lab experiment empirically figure out if they're high enough so that is truly isolated right that the method of initial rates yes yes method to drive an integrated rail offer the reaction right integrated weight loss right we've been talking about great but we haven't said it but we've been talking about differential rate lots alright DAT tapes right now we integrate that we can determine explicitly what that time dependence of 8 the reactions of different molecular areas 1st or 2nd or the 3rd order and so on right if we integrate differential rate lot will get an integrated rate like and the integrated rate law explicitly predicts what aid as a function of time take this for example right the rate is minus AT T that equals the Times said but very simple reaction we can integrate both sides try to move the minus sign over the right hand side in a great from the initial concentration of aid to some final alright and his integrators dt and so on and I'm doing have mind Katie Wright since that's a 0 In this is going to turn into like a over a 0 I've got an equation that predicts the concentration of a is a function of time this equation here does not predict the concentration of a is a function of time predicts the rate rights of the integrated rate law tells me explicitly here's what aid as a function of time measured the concentration of a as a function of time and then see if you can say that with this equation if you can then chances are your reaction is first-order with respect to a and you can back up the rate constant from the set right now this is laborious because what if it's not first-order in a while you get a fitted to 2nd order reaction but the reaction I half water reaction the becomes a fitting exercise but eventually you figure it out the nice thing about this process so let's look at data is the integrated here's what the integrated relies for the first-order reaction and it predicts a gets smaller as the log and the slopes to be negative and the slope of this is actually might escape and so we get the rate constant from the slope and or you could just take the raw data which is occur but you could fit that directly In the old days every process had to be converted into a straight line so that you could do the fitting because there were no computers and a lot of the textbooks that we now use to study these processes continue the convention of turning every reaction plotting it in such a way that you get a straight line it's not really necessary to do that anymore we can fit curved lines trivial eh any time you want to it's 1 button in the Gordon do that and then you get the rate constant from that fate you don't need to turn it into a straight line but this was what was done Back before 1990 or because the 1990 that's pretty much what we started to use personal computers but then about 87 remember that very well sadly focus so we can use an integrated weight loss is a greater weight loss for all different molecular you've reactions but another way that we can deal With the integrated rate what is to define the half-life of the reactions the the half-light is defined using the integrated rate loss you need the integrated rate law 1st then you can define half-life and the new measure the half-light it's a lot like and you basically have to measure the concentration of the reach of the reacted the product as a function of time anyway To get the half-life so you're basically measuring the same thing but the half-light is the time needed for half of a reactant chemical reaction to beaten pleaded half of the reactant to be completed depleted completed so here's the integrated rate Lofa first-order reaction which we just arrived right the have says Hey I wanna know how long it takes for a default that half of the value of a 0 right so if that is half of that it's just half that ratio was just half OK and so this is now my expression for tea to the one-half that's the half-life T to the one-half OK so price offered to the 1 happens log 2 overcame a it's dead simple right now know that in this case of first-order reaction half-life is independent of the initial concentration of a 0 that's the hallmark of a first-order reaction you unit you start with . 2 you measure the 1st light that's how long it takes to get the point 1 now you pretend .period 1 is your starting point you measure how long it takes to get the points 5 are rather . 0 5 that same amount of time right you measure the 3rd half-life that's half the time it takes to get the . 0 2 5 that's the same amount of time that the half-light stays the same it's a first-order reaction but notice what your measuring here your measuring the concentration of your react as a function of time you could just that this green curve this equation right here and you're done France I'll there's no particular advantage to measuring the half-life but if for some reason that happened to be a convenient way to do it you can get them order the reaction that way 1 of the advantages of both of these approaches were using the integrated a lot is that were insuring that the molecular molecular 30 of the reaction is staying the same over a period of time in over a range of reacting concentrations that's not true in the method of initial rate the mission method of initial rates for measuring the initial rate following 4 different initial concentrations of the reactor that we're isolating so in principle were only getting the molecular ready right at the beginning of the reaction to something funny happens and that changes often occurs organizers were not going to know what happens to the molecular ready later in the reaction but here we can see if something funny happened In other words of something funny happened right organist C would have a good fit to our curve here and then something is gonna go wrong we're going to fall off this curve at some later point in time and that's information that we can use to understand what's going on we were not going to have that information in the method of initial rate talking about this is superior here were ensuring that we have a fit over a wide range of reacting concentrations in a range of time and if we see effect all the way to the end of our time window we can be confident that over that time window over the range of reactant concentrations react this reaction is a great lot really holds up the Saudis this is superior but I want everything original case so at and it would start for him OK so the staff we talk about today could be on the court look at me like to say that
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Titel Lecture 19. Observational Chemical Kinetics
Serientitel Chemistry 131C: Thermodynamics and Chemical Dynamics
Teil 19
Anzahl der Teile 27
Autor Penner, Reginald
Lizenz CC-Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen und nicht-kommerziellen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen und das Werk bzw. diesen Inhalt auch in veränderter Form nur unter den Bedingungen dieser Lizenz weitergeben.
DOI 10.5446/18952
Herausgeber University of California Irvine (UCI)
Erscheinungsjahr 2012
Sprache Englisch

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Dauer 48:28

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Fachgebiet Chemie
Abstract UCI Chem 131C Thermodynamics and Chemical Dynamics (Spring 2012) Lec 19. Thermodynamics and Chemical Dynamics -- Observational Chemical Kinetics -- Instructor: Reginald Penner, Ph.D. Description: In Chemistry 131C, students will study how to calculate macroscopic chemical properties of systems. This course will build on the microscopic understanding (Chemical Physics) to reinforce and expand your understanding of the basic thermo-chemistry concepts from General Chemistry (Physical Chemistry.) We then go on to study how chemical reaction rates are measured and calculated from molecular properties. Topics covered include: Energy, entropy, and the thermodynamic potentials; Chemical equilibrium; and Chemical kinetics. Index of Topics: 0:02:21 Le Chatelier's Principle 0:06:30 Van't Hoff Equation 0:08:12 Summary of Thermodynamics 0:12:30 Ludwig Wilhelmy 0:13:12 Stoichiometric Reaction 0:17:53 Extent of Reaction 0:22:44 Rates 0:34:46 Determining Rate Law by Method of Initial Rates 0:40:05 Determining Rate Law by Using an Integrated Rate Law

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