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Lecture 25. Chemical Kinetics Pt. 4.

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Lecture 25. Chemical Kinetics Pt. 4.
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UCI Chem 1C General Chemistry (Spring 2013) Lec 25. General Chemistry -- Chemical Kinetics -- Part 4 Instructor: Ramesh D. Arasasingham, Ph.D. Description: UCI Chem 1C is the third and final quarter of General Chemistry series and covers the following topics: equilibria, aqueous acid-base equilibria, solubility equilibria, oxidation reduction reactions, electrochemistry; kinetics; special topics. Index of Topics: 0:00:00 Brief Review of Rate Law 0:03:07 Reaction Mechanism 0:10:00 Molecularity 0:15:39 Intermediates 0:18:21 Speed of reaction 0:21:59 Overall Reaction Example 0:32:00 Kinetics and Chemical Equilibrium 0:36:54 Experimentally Measured Rate Law 0:44:26 Rate Law Example Given Experiment Concentrations
metabolic pathway rates concentration overexpression Topic Memorial chemische Reaktion end
Bohr Stöße Bis (band) man reactive molecule analogue Table wine Meeting/Interview cell overexpression Insulin level processes weight reactor type areas chemical element molecular concentration rates mode steps multiple chemische Reaktion Computer animation metabolic pathway initial Geschwindigkeitskonstante DOM materials Gamma Silver iodide oxygen
Digital elevation model species Bohr current Stöße case man Trihalomethane molecule cake Magnetometer Brown adipose tissue Butcher Medroxyprogesterone weight reactor phase rates Maische Prolin steps tuberculosis Rohtabak chemische Reaktion van Computer animation model metabolic pathway Geschwindigkeitskonstante Gamma Peroxyacetylnitrat Zigarre form sequence
Graukäse Bohr Mühle man cake race Magnetometer overexpression reactor power rates Maische ozone steps multiple chemische Reaktion flow water van Computer animation Geschwindigkeitskonstante model carriers Hab form
Maskierung <Chemie> potential Memorial Stöße chemical man restriction molecule Magnetometer Altbier storage period processes biochemistry weight fall reactor molecular power Bruise concentration rates balance Maische steps tuberculosis Topic Maskierung <Chemie> chemische Reaktion Electronegativity end chemical equilibria water systems van Computer animation Geschwindigkeitskonstante function form stuff
homo case radiation man Zeitverschiebung pharmakokinetischen overexpression period weight processes reactor type rates balance concentration Wasserscheide Maische steps Ethylen-Vinylacetat-Copolymere equilibrium constant chemische Reaktion law of mass action Computer animation Geschwindigkeitskonstante coupling Medicine form
antigenic power concentration rates Wasserscheide steps set case source end man water van Computer animation race Magnetometer Gamma molar Magma basic period
Meeting/Interview rates steps parents
OK let's go ahead and start does anybody have any questions before it begins I'm alright so were fast approaching the end of this topic and so will focused on his way to look at reaction pathways the reaction mechanism and doesn't memories said in Connecticut said we have to important roles 1 we want to look at how fast reactions to city and secondly we want to look at reaction pathways are right and sell up to this time we spent a lot of time looking at how we determined that we lock the rear lot is related to how fast the reactions proceed are right and we said we wanted to determine the rape law although rate expression there too experimental approaches that we can take on what you manager in the laboratory is in great Western's concentrations that we know we can figure out the real taking into equations and divided 1 by the other rights so that the
gear measuring rape With his concentration the alternative is we can measure concentrations versus time concentration of reactive wastes time and the 2nd approach is where your measuring concentration versus time there were to do it is you make 3 grass you plot concentration versus time Ellen concentration versus time and 1 concentration versus time and you find the plot that gives you a straight line and from that we can establish the weight loss OK so we look at that now I know what we want with the rate long Why is is it that we go through this process of trying to figure out what the order of the reaction is and trying to establish what the way the law is In reason we do that is because we want to figure out the pathway the reaction takes and the work that we used to read described the pathway is called the mechanism so we want to understand the reaction mechanism said don't look
at reaction mechanism and so on some short notes for you and it's on the class website you already have a copy of it you can see that many reactions do not occur in a single step but proceed to a series of steps are a member we looked at has small we said that know reaction can take place in a single step or you can have multiple steps it turns out many of the reactions majority reactions actually take place in multiple steps and you take an example if you look at the overall reaction shown here if you look at the overall reaction where it to reacted CEO so that is as the reactants these other starting materials that you start with it reacts actually was the product have no and 2 so we write unbalanced equation in that format where you've given the materials you started with and the products that you end up with this is the overall reaction where you look at the initial stage the initial substances that you started well in the final stage of final products are right so that this gives you the overall reaction that took place so the overall balanced equation tells us the reactants and the products in the story geometry the relative ratios in which the reactants react to the new products it does not tell us anything about the pathway or how the reaction of careers and steps soon reality if you look at the details of how this reaction takes place in reality what happened is 1 molecule in order to react to the another among them in order to to give you an old 3 plus and also the 1st ever look at that can only look at that and tell me what is really happening there you have to to in order to molecules giving you a note 3 so what's happening 1 N O 2 molecule has given up 1 an oxygen atom to the other entities in Occidental being transferred to the other so there you have it all to read and the other 1 has lost a lot of images in all that's the 1st step and the 2nd step is to see that in all 3 now what is that too it reacts with CO and what is it doing its transferring their oxygen to seal to give C O 2 so can you see the difference between the path with the reaction takes place and the overall reaction you all reactions when you take 2 steps in and demand you end up with the overall reaction so very often the amateur we look at initial stages and final states were looking at it the reactions were not looking at the mechanism we call this a mechanism of the but we're looking at what we started with and what you end up with and we do not look at the intimate details of what's actually happening as the reactants overall reactants converted to products so the status of steps call the reaction mechanisms so with the goal of kinetics is to figure out the reaction mechanism the mechanism gives you the intimate details of how exactly the reactants converted to products were not just looking at what you start with what you end up where we're looking at a series of steps that need to How the reactants converted to products so do you understand the difference between the overall reaction that we've looked at before would just looks at what is the reactant and what is the ultimate product and a reaction mechanism which looks at the series of events that lead reactants to convert the products all right so that Tuesday to recall this Tuesday mechanism and this is the 1st step in this is the 2nd step so to understand the reaction we must know it's reaction mechanism and 1 of the main purposes of studying kinetics is to learn as much as possible about the steps involved in reaction so we talk about or the mechanism that's what it means all right so each step in a mechanism so each of these steps in a mechanism is called in elementary steps and elementary step is directly caused by collisions of Adams signs of molecules the rate expression for Paul reaction cannot be derived from the study geometry of a balanced equation as we have kept emphasizing and must be determined experiments he guys remember all experiments in the lab will be measure all we have at our disposal is what we start with the initial stage of the reactors and the final products and so on when we look at reactions in the laboratory what we're looking at is the overall reaction not looking at individual steps so when you look at the overall reaction remember we kept emphasizing that we look at a reaction you cannot figure out what the rape law is going to be based on an overall reaction it turns out you are looking at a mechanism in each step in the mechanism is called the elementary steps into the EU looking at steps in a mechanism now you can actually look at the coefficient in that mechanism for each step you can write the weight loss based on the coefficients of the order of the reaction actually corresponds to the coefficient His clear so that's the difference between the overall reaction you're looking at an individual steps within a mechanism and the each element set so we called each 1 of us an elementary steps in an element to step really looks at the molecular level so what this means is that when we write elementary step in the 1st half it means that 2 N O 2 molecules are climbing so you have to to enter 2 molecules colliding the instant they collide with each other when oxygen is transferred from 1 in order to get to the other and then it breaks apart than you have in all 3 internal numbers in 0 3 now will go collided with a carbon-monoxide molecule in the instant they collide you have an opportunity transferred because remember they have to come into contact with and have Adams transferring from 1 molecule to another right and so in elementary steps always described collisions between molecules all right so the 1st step you have 2 molecules colliding you have in all 3 and the military has bounces around will collide with the sealed so again you have to mark colliding and you have an opportunity and transferred from 1 to the other so because you're looking at an elementary step because we're looking at molecules colliding recalled that a molecular areas are right and it turns out molecular Kennedy molecular refers to 1 step in the mechanism so you have a series of steps in each step in a molecule is called elementary steps or molecular area right and it turns out elementary steps on molecular can be of 3 types are right we call it a new name on and molecule 1 is where you have this is a step in a mechanism and analogy Stepanov look at the overall reaction now looking at individual steps and if you have a the reaction means it's reactant is only 1 molecule so what happens is 80 goes 2 products so now we're looking at a single step and a mechanism and we if 1 reactant in a single step goes to give new products recalled that union molecular and cell if you wanna write the the rate finesse the rate equals came rate constant k times it so this is about 1 step all right so this is 1 step in a mechanism now have to walk his clients like the 1 that we looked at where N O 2 and in order to collide and have 2 molecules colliding so we call that a Michael reactions you have been a mechanism you look at a single step and you have to molecules collide and so you have be plus it give me new products then we call them by killer because now 2 molecules are colliding with each other and you
look at the rate of this because it's a molecular collision and now the way corresponds to only the mechanism individual steps in mechanism the coefficient in the balanced equation gives you the order are right so here it's a violent reactions that you can have the same molecules colliding or you can have 2 different molecules colliding like this so if this happens you can see rates it came its 1st ordained aid and first-order and b but
overall at 2nd order still 2nd order and you can see that the rate largely of 2 molecules colliding you always colored by molecular and by Michael reaction will always be overall 2nd order yes so we're going to figure that out of the question was how do we know With its 1st order of 2nd order without experimental value it turns out we measured experimentally is the overall reaction are right today our goal is to take what we measured experimentally for the overall reaction and figure out how you come up with each of these steps I write something to talk about 1st cut into the near the fundamentals and the goal is that experiment we measure the overall rate long the use the overall rate long to figure out the individual steps in mechanism again so so we looked at by of reactions when we sent by molecule is adding 2 molecules need collide and so you can only have 2 reactants there in elementary step because you have to molecules collide in a case now you have a real molecules collide in it's called the term likely reaction OK so reactions involving the collision of 3 molecules these are rare because the probability that the molecules with collide simultaneously and that in the right orientation is rare case so that the problem is that you have 3 molecules colliding it's very rare there you have molecules of bouncing around that tree molecules will hit at the same time right and so it was we talk about Michael reactions in reality they're not experiment observed cake because I'm in reality there not observed because it's very difficult for Tremont was hit at the same time but he did have to monitor reactions then but you have this 3 molecules collide in giving new products and that would be great rate constant times aid to the par-3 because your on was colliding or you can have 2 species that you have was a plus the give me new products then rate would be k times its 2nd order in and first-order overall its 3rd order or you can have 3 species that involved 3 different molecules colliding and that would give you products and here it would be 1st story name 1st starting be 1st and see but overall the order right so like I said when we looked at I figured out orders of reactions I said comment when the 1st or 2nd order 0 order Powerade and so here we looked at the fact that you look individual stats what we commonly observed the union model which is 1st order or by molecular which will give you just for a single step it'll give you a 2nd phase no tobacco mechanism are right so we said that each individual steps In a mechanisms have you can figure out the rate and you're looking at the will step in a mechanism now you can look at the coefficient of the balanced equation to figure out the order that reaction it snows the back the mechanism that we look at it if you look at mechanism you can see that this is a molecular collision and when these 2 molecules collide what you have is you have a new species being formed that's not a reactant or product so we're just looking at reactants and products we want to see this new species that's being formed a new look at this new species that reform it's more than the rate in the mechanism and it's consumed in the mechanism so you look at an oak tree in all 3 something new that is formed in this mechanism of pathway and then not itself which concealed so you look at the overall reaction if you're just looking at reactors and this will be a products you don't see this new species that formed in consumer sentiment mechanism if you have a new species that formed and consumed it's not react and raw product recall that intermediate all right so we say to read is an intermediate and mechanism which means formed in consumer are a citizen and I have any questions at this point so we're looking at pathways and the pathways looks at individual steps just go ahead you will would take examples are sold I would show you some examples where we're going to look at your to see other things 0 order in individual elementary steps that I will show you how that comes into place can effectively take example in class and show you how 0 order comes into play again so we're looking at intermediate and said intermediates of form and consumed in a reaction and finally we want to see how this fits in with the overall weight loss that we apply itself it will look at the Westin said in this mechanism we saying that the 2 steps step 1 instead of 2 now I remember each individual steps will react at a different speed for different rate all right and so you won't hear this mechanism you can see that the 1st step is the sole staff and the 2nd step is the faster alright so what you have is is a mechanism you have reactions taking place in sequence and you know you have stepped wanting in place then stepped to interfere .period step 3 steps for you you have multiple steps of current begin you don't see that overall the
solar step is what's going to determine how fast that reaction proceeds we call that the bottlenecks are right it's much like you have 5 lanes on the freeways and that each of these violations is like 5 steps in a mechanism in various driving at the speed limit which is let's say 65 and everything normal problems the traffic flow smoothly but then someone happens there's let's the lanes are closed and the filing not close to 1 million so now what happens to begin is moving at the speed limit but once you hit that model that now what's going to happen and how fast people proceed beyond that point is going to depend on how fast the cars and moving to that single men and so that single is the bottleneck that's the slowest steps likewise when you have multiple steps and you have 1 step which is the slowest the were speed that reaction is going to be determined by that slower step because you have multiple steps everything go fast but you have 1 that is really really slow and so all overall we measured experimentally is a small step to understand that so there are a lot that we measure of the rate that we measured experimentally it is always the slowest stepped to the slowest is called the rate-limiting step while the rate determining step that's the bottleneck that's what's slowing down the whole reaction alright right so when we look at experiment what we're looking at is experimenting when we measure rape laws were looking at the overall reaction we don't see the individual stats are right so we see the overall reaction and the overall rate that we measured experimentally always corresponds to the slow step in the mechanism because that's the bottlenecks so so it it might take this two-step mechanism invited the 1st evidence biological and by right the rate low offer that the coefficient is to use its 1st step 1 rate 1 would be K 1 which is the rate constant for the 1st steps times in order to raise the party because its 2nd orders by right now stepped Is there and that rate corresponds to again remember the way always depends on rate constant times the reactants the reactants here in all 3 it's 4 starting at 3 it's 1st sealed so the faster has that 2nd using that what we measured experimentally will correspond to the slows down and so on what happens is that we carry out experiments and look at this reaction these what we measured experimentally what we need this is what we experimented measure we rate equals chaos observed in order to raise the power to do so he would carry this out in the laboratory and measure the rate this is what the rape law would come out to be all right and this always corresponds to the slowest step because remember the slowest step is the rate-limiting step and so in a mechanism you have multiple steps all right and among those multiple steps users make were all rated that reaction should depend on the slow steps and the great love that you see in the slowest step is what we measured experimentally because in the experiment but we're measuring it is the overall rate and the overall rate is going to depend on this little step so order a laboratory and look at this reaction this overall reaction overlooking an carrier the experiment will find out that the window you have an all 2 and the net result is that actually 0 order CO remember your reactions of 102 and sealed so you general form of this great along with the so by take this overall reaction and can have a look at this and tell me I had to react in some sunlight overall rate should be clear times NO to raise the power and time CEO race power and all right but will we measured experimentally In and what comes out is from this we know rate equals observed in 0 2 2nd order 2nd order C 0 0 order so that equals K observed in 0 2 the 2nd order in an to so this is what we observe experiment is from experiment now this is what we observe experiment many concede that corresponds to the at this small step is the star and that is great because K 1 times in order to raise the pod to that's what we observe as well he does get that and so what that tells us is that now we can explain so what we started measure the overall rate water that comes from experiment and then we come up with the mechanism that's consistent with that and the mechanism has to be with the slowest step corresponds to the overall rate plot got so as another example of applying this so here's another example that I have so this is the overall reaction when looking at so 2 N O 2 plus F 2 gives you 2 to act so this is the overall tells you the reactors at the start where any ultimate products that you end up with a cake laboratories and carry out this expression the experiment and determine the rate along this is what we observed experimentally and so to show that
it's experiment we set the rate constant is what we observed experimentally right so we observe experiment is rate equals King observe its 1st order in a note to and first-order in F 2 sets by molecular and that is the overall so now come up with a mechanism where what does that tell you that the small step has to involve these 2 molecules colliding with each other so that means that you have to come up with a mechanism where the slowest step is where 1 molecule of N O 2 collides with 1 molecule of F 2 are right and so on here is a possible mechanism where you have an O 2 colliding with FTD to produce an intermediate NO to F and F arrived and this would be the slowest steps that the next step would be another molecule 2 colliding with air did you know to F which is the problem all right and so the 1st step is small and determines the rate and so you can see now the mechanism that I came up with is consistent with what I have observed experimentally so it turns out that coming up with the mechanism is really a skill it's an intellectual exercise is a skill that you develop the whenever you come up with the mechanism you have to look at the overall great love and make sure they come up with a mechanism where his slowest stepped corresponds to what you see experimentally obtained so now scientists do this all the time so when you come up with a new reaction and you want to figure out what the pact isn't so remember you guys have you know you know you're a biochemistry your biologist student you look that enzymes and do you guys understand you know you mind about the lock and key mechanism you nowhere but how incensed function and that and that the Funtua lock and key sort of way all right that is a mechanism and how do we come up with that mechanism exactly this way news kinetics committed is the only way to figure out the the mechanism but it requires a lot of experience right so how does 1 go about reducing the mechanism for a given reaction the 1st thing you do is you go lab and you measure the experimental rate law you need a measure rate was concentration or concentration was looked tired and using the greater rate law and make 3 graphs and figure out at the rate lawyers all right so that standby experiment yes and the people Hong that is at it so and so the 1st step is experimentally determined the rate lock right now the next step is using chemical contusion 1 constructs all possible mechanisms that are consistent with his weight loss so now you have to just brainstorm and use of their TV at every conceivable possible way where the slowest that is going to correspond to the overall rate lower than you observed and this is just coming up with many different mechanisms all right then what you do is you carry out your list of potential mechanisms you can write to 5 to 6 different mechanisms are right now to use you for carried out experiments to see whether you can eliminate some of them 1 way to do it as you can in situ measure the intermediate you can measure the intermediate limit intermediate form and consumer sometimes intermediate forms so classes consume so fast that you can experiment senior but there are other ways where you can see it hangs around long enough that you can actually measure and see use different techniques to see whether there is an intermediate being formed in consumer and stuff like that so you do for the experiments to rule out some of the notes all right in the end but I want members mechanism can never be approved absolute and 1 can only say it is possibly correct all right so you can never 100 % proved that there is the matter because memories of individual steps and there is no way to see each individual steps separately in the laboratory do understand that it's purely intellectual exercise but you can say that in all likelihood this is the best mechanism that you can come up with to explain that I did yes :colon next to the fact that we really want to do yes which yes so you look at the overall L 1 that you measure and then you try to see which 1 would fade and that has to be the .period got it and so the where I'm going to do this because it requires a lot of skill to come up with mechanisms and if this is the 1st time seeing this topic it would be impossible for you to come up with a mechanism all right so what I would do it is that you know I have put up a worksheet on the club's website and have time and workers in class but so here is a working dinner with us and so it's going to get you to figure out the rate law and take it to the each step and lastly what we do is what I'll do it instead of asking you to come up with mechanism all you knew like 5 different mechanisms all right of blows full of being correctly 1 actual state you see experimentally are right and the see has 1 mechanism there too was the 2nd mechanism 3 the 3rd mechanism and what I will expect you to be able to do is hereby give used several different mechanisms I want to depict the mechanism that is consistent with what you see experimentally all right I will not expect you to come up with your own mechanism because that you don't need a PhD you need it requires a lot of experience right and alien and so it's not it's not practical for me to ask you to come up with a creative mechanisms design accessory but what I will expect you to be able to do is given 4 of 5 different mechanisms only 1 will actually work and is consistent with experimental data that is so now you understand why we went through this whole process of trying to figure out what the rate lobby overall rate laughter reaction is and we said when we look at the overall reaction because that's what we have and we start with you cannot look at the balanced equation to figure out why the order the reaction would be all right but once you figured that out now and you when you look at mechanisms now we know that each individual stepped in the mechanism actually the coefficient the balance equation actually gives you the water sewage arduous way each mechanism you take each step in right the rape law that corresponds to that step and then you know that the smallest step the bottleneck of the returning step of the rate-limiting step has to correspond to what you see by any questions at this point now I want to do it because I want to relate this to include have lots will look at kinetics and equilibrium right because there is a relationship between kinetics and was equilibrium interview look at kinetics and chemical equilibrium and the let's stay there were looking at consider a elementary steps a reaction mechanism when is consumed 1 step in a mechanism OK and that's where we're looking at NO plus 0 3 In equilibrium where an old shoe plus 0 2 all right so up to this point we've looked at reactions that proceed only in 1 direction when we deal with the quandary as you can see that it :colon riddles with reversible reactions all right is a Fallwood reaction and because of the rate constant associated with the forward reaction within call that
creates 1 and as a reverse reaction and they made constant associated with bad College of let's not that listed this way September went that Prince of King a negative 1 all right and that is to distinguish so that say this is 1 step in a mechanism that lets it is the 1st step of its step 1 because it's step 1 I'm going to call the forward rate constant K 1 and the reverse direction I'm going to college kid some negative 1 which is the rate constant for the rejection because there's 2 reactors going on so inviting the fall would raise memories in elementary steps I can look the balanced equation to figure out the rate long so forward rate for the forward reaction rates With equality this called that every 1 would equal the rate constant for the forward reaction the rate constant always depends on the concentration of the reactants and its 1st storing and all and 1st order in all 3 OK that I did the reverse reactions and let's call that great negative ones now the rate constant for the reverse reaction will be case of negative 1 now for the reverse reaction which of the reactants the products of the reactants for the reverse reactions of the reversed reaction takes place this way and therefore the riddle for the reverse reaction would be an 2 times 0 2 all right and we know that at equilibrium the rate of the forward reaction equals the rate of the rest reaction that's what it means when we see a system is in good memories said the rate of the forward reaction and the rate of the reaction of the same name and that's where the concentrations of reacted to products don't change any more so because this rated 1 the world's great too I can say K 1 times and all times all 3 equals can negative 1 N O 2 times 2 if I read this equation you can see that if I take the concentration of the products raised to the power of the coefficient divided by the reactants taken the pardon coefficients this really equals rate the ratio between the rate constants which is actually the quality constant all right so you wonder relate the equilibrium constants to rates it's ratio the rate constants equilibrium constant is a constant and that is the ratio Of the raid constant for Katie because constant escape cavalcade all right and if you look at that that's really the ratio between 2 constants which is the rate constant for the followed reaction divided by by the rate constant for the restrictions islamic censoring its analysis technique called mistaken example of a mechanism what we have equilibrium involved as well so let's say that we're looking at this reaction similar to the 1 example of a reaction where not only do you have just a regular reacted taking place in a mechanism for your 7 equilibrium involved so let's take this example where people were all reaction it to NO plus 0 2 giving it to an 2 a the experimentally manager great loss years rate equals came observed this is the rate constant at we of experimentally is 2nd order and N O 2 and 1st quarter In to all right so 1 way explained as 1 possible mechanism that can explain as it is in fact a few mechanism just took place someone steps are right you can see that the overall reaction is also an elementary step in other words is a one-step mechanism can that if this is an example where the this reaction took place in 1 state and therefore the overall reaction is the same as the mechanism itself which is a single step you can see 2nd order in an and 1st to our right but that's unlikely if you want it to come up with an experiment 1 possible mechanism that has been suggested for this year's that's where you have enough plus an hour giving you and to all children right where this is Katie 1 this is Katie negative ones and this is we call it fast equilibrium so this is step 1 and step 2 of this mechanism will be novice intermediate best form and to 0 2 plus 0 2 the calls
K 2 which is the rate constant for the 2nd reaction and continue to and how to and this is still staff all right so you add these 2 together you can see this intermediate will cancel out so what you end up with is to end all possible 2 gives you 2 so 2 ways in which you can explain this experiment that I say that this is actually a single step mechanism I right where it takes place in 1 step so that the overall reaction is actually just 1 single step unilateralism and it's a single step mechanisms or you can explain it by a two-step mechanism and in this case you have the 1st entry of a fast equilibrium followed by a 2nd slow steady state so now we have to do I have an expression that fits what we see experimentally this is what we see experimentally so I needed write an expression that explains this and that we start with the small step my .period is there and I know that the rate equals K 2 times it's 1st starting in 2 0 0 2 and 1st order in 0 2 are to the slow start has an intermediate can you see that I took the sole stepped in and looking at the small staff and I'm writing the weight loss corresponds to slow step yes to him it was the 2nd order of you mean this step where there is 1 of the best and 1 of its yes the rate law is first-order because the coefficient is 1 the bouncy Commission the coefficient is 1 of the 1st order to see them so a single step a mechanism the coefficient in the balanced equation corresponds to the order which resting and only for a mechanism only elementary steps you can do that OK but there is this is this is not complete because if you look at the rear love the small step an intermediate experiment can you measure intermediate no experiment which measures the reactants that you start with the receiver so that means I have to replace the terms with intermediate the term that represents intermediate with terms that represent 3 after OK and how can I do that I think the equilibrium constants and now I have this is reacted so welcome but that was when provided the 1st ever remember that it is an equilibrium and I'm OK equals in 2 0 0 2 Over In all square to see that because this is an equilibrium this is the law of mass action that tells me the equilibrium constant OK now rearranged this can you see now I have a term for this intermediate which is carried times in all square are right now in place this over there so if I take this and replace it now you can see rate equals K 2 times a constant equilibrium constant times and all squared times too and therefore radiated equals Kate observed you see this is what I see experiment as well so I can come up with the two-step mechanism all right but now Mikey observer is actually K 2 which is the rate constant the 2nd reaction times the equilibrium constant amid the constant times constant is making a constant did you that so this although those are consistent the rate not so it could be a single step mechanism where it just takes place in 1 step and the whole reaction is just 1 elementary steps in it's a single step mechanism all are you could be a two-step mechanism but if you you want to write a mechanism that is 2 steps that you see the formation of an intermediary in the intermediate is being consumed but then you can overall deriving expression that also fits with you see experiment using so your country in a mood to mechanisms that are consistent with what we see experimentally need for the experiments so it turns out is the 2nd 2nd mechanism which is what really happened and where you can check that is to see the formation of intermediate that intermediate Intuit to being formed and appearing as a reaction proceeds you can see that so once again I'm giving you the mechanisms it's very difficult for you to come up with a mechanism on Iraq all right however if I give you a mechanism and if you look at the whole work that you're getting on the online homework as well as the homework problems picked from the textbook you see all those cases there are 4 different mechanisms in your ass to fit the 1 that fits which you see experimentally OK if you know in the last couple of minutes I want to show you how this whole thing is applied and that's that wishing that I put on the class website and complaining about I would
like you to work this out because it kind of explains the process and you go along a case so let's look at themselves to begin when consider the reactions so we're looking at the overall reaction and the overall reaction is given at the top of the overall reaction was looking at is S O 2 plus 0 2 administration and what what what you're seeing is to SO 2 resulting into a 3 so that the overall reaction that you're looking at a case now the following day the foreign kinetics experiments were taken at 25 degrees Celsius so can you see what kind of measurements are making here are we using the integrated rate all all using constant at the rates versus concentration there is a great was accounts that means that you have to take 2 equations by the rate divide 1 equation by the other and then figure out the order with respect to each 1 so the 1st step is to find the order With respect to each reactor right now it's we got that then if work problems where you can determine the rate constant determine the value of the rate constant includes the units next calculate the initial rate and the disappearance of all to an experiment for you can figure it out you figured out rate constant care and forgot the order with respect to each 1 of those you can figure out what that ?question mark is in line for OK so let's quickly start by looking at the minute I get through the whole thing will completed next time but I do want to go to an exercise of how we build up and figure out the mechanism OK so let's start with this final order with respect to each reactor so we start with all the information that's given to us and will start by looking at the balance equation can everybody right down the overall rate but the general form so we don't exactly what the order is so use em and and what would the rate while look like him received radiation equal a rate constant became 1 of the reactants for this reaction we have canceled Chu let's say erase the poet
and and all too a recent power and you never to see that in this we know the concentration of great we know rate S O 2 O 2 concentrations
what we don't know this case M and in that table concentrations given and the corresponding rates given itself if we have to be announced how many algebraic equations do we need to have Sofri we need 3 equations and you can see that 3 sets of data are a source of the first one the first one would be 7 . 2 times higher than 85 malls per liter per 2nd equals came concentration of SO 2 years .period 1 will erase the power and concentration which is . 1 molar raise the power and so that's 1 equation 2nd line is 2 . 9 times tend to be negative for most polluted per 2nd equals came now it's point to Imola racer power m times .period to Imola raised to the power and equation 2 I take the 3rd equation will be 6 . 5 times tend the negative for walls pretty depressed 2nd equals Cape times . 3 smaller based upon them times .period 1 molar race apart in the 3rd iteration side-street equations now the ones that I want take picked to start where is the ones where 1 of the concentrations of the same so look at those 3 equations and which would you pick to start with which equations would you pick equation 1 and 3 embassy that because that's the only 1 that has the same concentrations are right now we always as a rule of thumb it's easiest to start with whichever has a bigger number is divided by the smaller number so this is tendency for this is terminated 5 some take equation 3 and divided by race and won a case so that means I have 6 . 5 times that the native for moles per liter per 2nd equals came times . 3 0 Morla raised apart and . 1 0 Moeller based upon and a divided by a smaller number to 7 . 2 times said the negative 5 moles per liter per 2nd case times .period 1 Imola race the power and times .period 1 molar basic power and sophistication cable cancel out .period when Muller based upon an appointment will respond and will cancel out and so if you divide this by this end up at 19 equals this is pointing to 3 racer power and so anything and comes out to be 2 OK so now that I have that now I have to chance I looked at these Jew and these dual cancel out so now I need to use these due to figure that at the next 1 a now we don't have the same concentration that doesn't matter because you I'd forget all that water is so we can put the water in their OK to know what they do is unknown figure out to the end and divide to buy 1 said to would be 2 . 9 times and the negative for Wallstreet leader per 2nd equals King times .period to racer power times .period to Mohler raise the power and divide now we've figured out that families to square foot square a case that be squared and I by 7 . 2 times and the negative 5 moles per liter per 2nd equals key times .period 1 more square because the 2nd order and 2 which is added and this would be . 1 molar in an a career it's OK will cancel out if you look at this this comes out to before he calls over here I have to raise the power to times I have to raise the power can be a case of a number of years for services for forced taken so what does it and have to come out to be 0 orders because they needed a power 0 1 can you see that so and is the order so that means my rates along with his wife Mary Lau would come out to be so that's the 1st step figure out what the rate lawyers and the rape along with the V-8 equals Cape Times SO 2 square because zeroed in on 2 so the
experimentally observed rate long is this right now can I ask you to complete the rest of that she'd because it takes a step by step in all kinds of problems that I expect to be able to do and finally the wouldn't have been about what the mechanism as a parent .period differently