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Lecture 26. Chemical Kinetics Pt. 5.


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Title Lecture 26. Chemical Kinetics Pt. 5.
Title of Series Chemistry 1C: General Chemistry
Part Number 26
Number of Parts 26
Author Arasasingham, Ramesh D.
License CC Attribution - ShareAlike 3.0 USA:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor and the work or content is shared also in adapted form only under the conditions of this license.
DOI 10.5446/19015
Publisher University of California Irvine (UCI)
Release Date 2013
Language English

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Subject Area Chemistry
Abstract UCI Chem 1C General Chemistry (Spring 2013) Lec 26. General Chemistry -- Chemical Kinetics -- Part 5 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 Experimentally Measured Rate Law Review 0:03:02 Factors that Affect a Rate 0:09:43 Reaction Profile 0:14:13 More on Collision Theory 0:21:12 Arrhenius Equation
OK I cannot have everyone's attention please I'm so we're down to the last stretch and so I'm just going to finish up the last topic in kinetics that we will cover and then I will work in finish up that worksheet that we started looking at last week and then I'd be happy to answer any questions are right and I'll talk with me about the final exam as well OK so the last topic that will look at it as member were looking at Connecticut's last time we talked about you know the rape law and how we used experimentally measured rate long to figure out the reaction mechanisms are right to do so for the moment at the factors that affect the region reactions so what affects how fast reaction takes place and there are many factors that that affect the rate of the reaction and in the I wanted to tell you that all of these factors are related to each other Kate let's and most of them out of here and let's start with the 1st 1 and that is that the temperature affects how fast the reaction takes place so chemical reaction speed up when the temperature is raised again and the general rule of thumb is that for every 10 degrees increment temperature the rate actually doubles right approximately it doubles so that's a good rule of thumb to keep in mind the other reason that the reaction speeds up when you raise the temperature is because remember when we looked at reaction mechanisms we said her reaction to take place molecules must collide are right here it doubts whether by molecule reaction means that's where true market reaction means in reaction mechanism each step results from molecules colliding with each other so it happens in many ways raise the temperature you raise the thermal energy In the thermal energy increases how frequently the molecules collide with each other because now the molecules have greater kinetic energy they're moving around bouncing around much faster and the chances that they collide with each other increases as raise the temperature and so the collisions Greece and other molecules collide and have the right kinetic and energy the reaction speeds up again so that the 1st factor that affects the rate of reaction is the temperature all right and if you want to make a
plot How temperature affects rate is that if you plot the rate as a function of time kind of increases like that now it will move on to the 2nd property a 2nd if that that is the frequency of collisions of so it Rivera said that the rate depends on molecules colliding and so the more often the molecules collide with each other the faster the reaction would be so the rate depends on the frequency of the collisions and more frequent the collisions are then and the faster the reactions the 1st criterion for a reaction takes place is that molecules was collide I write it declined then you have a reaction taking place and as the frequency of the collision increases the rate of the reaction increases right itself 4 reactors still could molecules must fly as concentrations of reactants increased or the temperatures increased the frequency of the collision increases sell 1 way to increase the frequency of collagen is to increase the number of molecules that are there you have more molecules which would get more crowded and therefore you given have more collisions and therefore the reactors would be faster and that's why reaction rate depends on concentration because as the real as the reactants give more concentrated didn't have more molecules in there the more crowded the molecules are the chances that they will collide with each other increases are so the frequency of collision increases with concentration also you provide thermal energy that the move a lot faster and there will be a lot faster they could collide even fast remember all these interrelated with each other the 3rd factor that influences the rate Is the kinetic energy of the collisions so we look at the fact that you want the reaction take place the molecules must collide and more frequent you have the collisions the faster the reaction but it turns out just having collisions is not enough all right you have molecules colliding they also have to have just the right amount of energy right and why do do you need the right amount of energy of a minimum amount of energy because the reason is that when they collided chemical bonds must be broken and new bonds of informed all of you know to break Obama does it cost energy audits of release energy breaking bonds always custody bond formation when new bond is formed energy is released all right so if you want to break bonds they can ever really see that you have a minimal amount of kinetic energy that's required to break the response so that example if we take this reaction where we're looking at 2 and albeit not giving you to end all plus the odd to if I draw the Lewis structure can you see that the central Adam will be nitrogen and this is the lowest structure Of the reactants went to molecules of these collide with each other you end up with the products which is to end all so this is what the Lewis structure the reactants and products are a known as you can see is on electron molecule 6 The Honorable Constance sees as 1 odd electron and if you look at that as you can see that in this reaction bonds having broken so which 1 is being broken can you see that this bond is broken and this bond is broken so this said these are broken and this bond is foreign so this reaction can see chemical bonds of broken and newborns have been forms so the act of the collision when the 2 molecules collide they must have sufficient energy the Connecticut among the molecules of moving around in the energy that the molecules have by which of them motion is the kinetic energy and the kinetic energy of the molecules are moving when they collide has to be sufficient to overcome the potential energy required to break these bonds today received and so it's something like this you can't think about it as their and when use an analogy and at the same time that he was look at the minimum energy that you need it's like if you wanted to hit a golf ball Over here the city of a golf ball right here and let you hit the golf ball .period see that if I hit the ball I have hit it with sufficient energy to get it to the top that's the tipping point has sufficient if I hit the ball with just the right amount of energy to exceed the tipping point then the ball will the inside if I don't hit it with sufficient energy let's say the ball ends up here you know that's been roll back here all right likewise went to molecules collide you have to have sufficient energy to get beyond the shipping .period are right the tipping point is having sufficient energy to break those bonds aren't you at that tipping point then Everything else a downhill and therefore the 2nd point is that the molecules when they collide not every collision actually means to products the collision has sufficient kinetic energy to overcome that energy barrier and that energy barrier is 1 the breaking of the bonds so we describe In the long what we call a reaction profile when
the reaction profile is a plot of potential energy where says progress of reaction 10 and so on Over here that this is the reactant and so the reactants this is the potential energy of the reactants and in this case the reactants at 2 it will be all we hear other products and we have to NO plus B R 2 as a products and so on when reactants Gordo products this reaction it an excellent stomach reaction so this is Delta H is negative as reactants goal you start with a high potential energy enjoyed a product with little potential energy and the difference in Member you can create a destroyer energy and so on the energy is released as heat and the Delta H R right now it turns out that this tipping point is shown here there a certain amount of the amount of energy that needs to be provided this is the rule Q it is in that golf balls example this is the minimum amount of energy is the tipping point that may amount of energy that needs to be put on for to break the potential energy associated with the bonds case break bonds and call that energy so this difference in energy between here and is called the activation energy and the activation energy is due on the symbol of the subscript eh right so the 2nd factor we said what that you know we said molecules must collide with you want a reaction to take place molecules must collide and the more often they collide the better so the high the frequency of collisions the better but the 2nd point is it's just not enough for them to collide when the collision occurred as they have to have sufficient energy right and they have had sufficient energy because bonds are being broken into the kinetic energy of the molecules when they collide the kinetic energy is sufficient to overcome the potential energy required to break bonds are right and that bad energies called the activation energy I write so overall they the collision has to provide sufficient kinetic energy to get about this home notice highest point here the highest energy .period is called the transition state all right so this is a lot of potential energy versus the reaction progress is called the reaction profile in the reactor profile you always indicate with the activation energies and so do you provide the kinetic energy of the collision provides that amount of energy that's the minimum out that's a tipping point that has integrated and that great it gets over the hump and then I want to get over the hump now it's all downhill and he gets to the products and services the Manitoba Ltd that would be released so this is the amount of energy that's least but this is the amount of energy that needs to be put in at the beginning to get the process going to see that and that's called the activation energy so that the 2nd factor so you can see here went to molecules collide the kinetic energies changed a potential energy as the molecules under the bond breaking in bond formation leading to product molecules however not all collisions in the 2 products this is because the molecules need a minimum amount of energy to react this energy requirements call the activation energy or the reaction and a symbol that loses ease some aid unless the kinetic energy of the collision is equal or greater than the activation energy Noel reaction will occur itself from the back again we said If you want a reaction to take place they have to collide on Friday and more often collide the better so the more frequent the collisions the better of you make a concentrated in you increase the temperature increased efficiency collisions came 2nd but that's not good enough just because they collide is needed from the 2 products we also have have sufficient kinetic energy so when you keep the reactants now you increase the global energy and therefore the world molecules of moving much faster than it had been a kinetic energies of the raise the temperature didn't react faster because you going have so it's not really I increasing the frequency equation but also increase the kinetic energy of the molecules and therefore the rate will go up but the 3rd is so we said just collided is not good enough they have to have sufficient community and the 2nd is the minute they collide and even if they have the right to demand a comedic energy that's not good enough they also have to collide in just the right way so they ought to have what we say the White orientation so not every collision with the right amount of kinetic energy actually to products the reason has as I said before when the molecules hit each other they have to what hit each other in just the right wing and what I mean by just the right way to do that and additional space-filling model for the same reaction so let's I have a lot to do with nitrogen is a little bigger and then bromine is the biggest survived take an O B R This is what faithfully model would look quite so this is oxygen nitrogen now let's see that these molecules hit each other this way so when they hit each other the point of contact is between
oxygen and grooming so the banking job in this orientation we call this the orientation of the direction out of the way that the orientation which they if they can't tell it this way there is no reaction so they're hitting each other in the wrong orientation however if they hit each other in this way so this is oxygen and nitrogen is a little bigger bromine is the biggest 1 this is space-filling model but now it then hit each other now this way and the point of contact is like this now remembered this reaction is a new bonding form you never see them and the new bond is being formed between 2 bromine so if the kids to the point of contact contact is between the 2 brawl means can you see now this is where the new bond is being formed so what will happen is this is the new bond to be formed and you can see when that happened with the new ball is being formed you can see that now this can break up like that all right so can only see that kind of everything is set up so that instantly come into contact you have a new 1 being formed and the other 2 will break off and so now they're hitting each other just the right orientation so that you end up with the acting like being formed and the 2 the oxygen informed as well so so tender it is much more complicated than just molecules collide cell where you have to keep in mind is that they have to collide the 1st and more often they collide the better too they have have the right amount of Connecticut hedging so the more Connecticut a sufficient kinetic energy for the bonds to be broken are right and thirdly even if they have lots of collisions and even if they have the right amount Connecticut Energy you want to get the reactor taking place on unless the molecules here in just the right when and 2 in the collider have declined in the right orientation so these factors influenced how fast the reaction takes place now this orientation is called state factor missed right state the 1st to three-dimensional orientation are right and so it that the orientation depends on the 3 dimensions and how the molecules hitting each other in 3 dimensions and so that we call the systemic factors are at the orientation and molecules hit each other now so about asking what are the factors that influence of faster reaction takes place then you should know that not only temperature fixes but also the frequency of the collisions affected the kinetic energy of the collisions affected and also the orientation affects how fast the reaction proceeds and whether reaction will take place and not right no Arrhenius came up with a mathematical equation that takes all of these factors into account so if you want to come up with a mathematical equation that describes the relationship of all these factors combined it's given by this equation which is called the Ireneusz equations are a wonderful mother equation is written as a kid which is the rate constant for the reaction not only using rate constant instead of raped because the rate of the reaction remember depends on concentration we've seen that the rate varies on concentration the more concentrated as the faster the reaction would be now rate constant is independent it's a constant and so it's a better factor viewing compare different reactions it's better to look at rate constant because rate constant is independent of the concentration because it's a concentrations are right to Olivia other independence of rate constant equals what we call a constant is called eyewitness factor and this accounts for collision frequency and the state that Member the state factor is the orientation collision frequencies how often they collide right so the obvious factor a takes into account how often the client as well as the state factory and every reaction has a unique value for a is a constant and it's you know is available in tables and every unique reaction will have a unique constant that corresponds to a he the activation energy and activation energy usually it is in units of killer jewels from all all
art is the gas constant I remember I wanted to keep in mind that in this tournament activation energy is divided by the gas constant in the unit for activation
energies of jewels per mold that means they're using this are you remember I suggest considering the SI units their unit for gas constants in Julesburg Calvin per mole so I want to convert that to kill a jewels and to do that all of you recognize that 1 killer Jules is 10 to the 3 jewels so when I uses gas constant in this equation it's important that I divided by thousands of that I have unit killer jewels from all rat Jules typical informal game so we have that and of course to use the Calvin temperature so that
all of the reaction can also be written like this where Ellen K. equals not affecting the natural log Ellen equals Elin minus the able-bodied and invited rearrange this equation so that I have Ellen K. equals negative the aid over are 1 with tea last L and and is putting this in the form of white people the next policy so it might take him what the rate constant is the Y axis then this term will be the sole and that would be the x axis plus will be there in a set so if I can measure the rate constant at different temperatures cell if I make a plot of Ellen lowercase scale cannot kept OK because that include the constant rate constant experiment we can measure rate constants at different temperatures and by plot that worse as 1 over tea and reminisces in units of Calvin and buying up with then I would end up with a graft that has a negative slow where the slow will give me the activation energy divided by the gas constant and keep in mind the native slopes of this how experiment figure out the activation energy for friendly reaction said he wanted to grab the activation energy for a reaction which you do it is you measure the rate constants at different temperatures all right and applied at Elland cables 1 of witty in a figure out what this Lopez and from the slope you can divide the Newmont unit take the slope and multiplied by the gas constant was prolonged that activation and into a writer and you may have done this in the lab but when you look the kinetics experiments can now lastly if you want to look at the dependence of Ellen on temperature and let's say when looking at Celeste
take this equation where we say L and K equals there's a case escape minus the iii over are 1 over T plus Ellen agent and that several looking at this and 1 temperature so if this is K-1 disability 1
right activation energy does not change our has not changed and Allendale amendment is a constant that's unique for each reaction to might take another reaction rates Alan K to this have been negative III over R 1 nobody to plus Ellen phase now since these 2 are Illinois areas by rearrange this can never see that if I take this term over here I can see L and K 1 class he gave over are won over T-1 equals L and K 2 iii over 1 over teachers candidacy see what I'm doing because Eleanor Acer constant member is unique for a particular reactions as long as we're looking at the same reaction alright all looking at is the dependence of rate constant temperature this term Ellen a would be less plus that this
Alinghi would be this this by taking over here and since Ellen stays the same football I know these 2 identical to
each other and therefore it might take a L and K 1 minus L and K 2 now this would become EPA over are won over T 2 -minus 1 over T-1 and Alan King this would become a L and K 1 overcame a 2 would equal the over are 1 over tea to minus-1 over T-1 said here with everyone a figure out the rate constant 1 temperature anyone to figure out the rate constant at another temperature you would use that equation all right and that would help you figure it out now this this informative equation can be written like some textbooks you see this written out like this L and K 1 over to equals negative what our but now it's 31 -minus teaches you know we're became so all I'm doing is switching to sign here putting this round of opinionated sign in front what those equations of the same alright and some textbooks give the latter equation some give the former so that ,comma completes what will will look at and so the last thing that we look at is the factors that influence how fast reaction proceeds and we looked at a chemical equations that relates all the factors that influence how fast reaction takes place now with the last few minutes I want kind take an example because ultimately it's about applying reason problems so let's not
stop the 3rd today and for 1 last thing that I just wanna talk briefly about the example of the last minute and I want to remind everybody that the final exams on Wednesday it said AM so your exam time is different from the class time obtained that example the cumulative effect greater burden will be on the new material so almost half the example young Connecticut arrived the other half will include everything that we cover this quarter so please review that they enjoy equilibrium he should be able to dry women problems usually with a new equilibrium problems you should know the relationship between equilibrium and free energy memory said Dr. Julie was minus ideal and gave up you know about the relationship between the energy and equilibrium constants which goes back to that the beginning of equilibrium right up to what we've covered electric chemistry that I understand that you will not be able to study everything so for the material that's cumulative other than Connecticut I'm looking at the big picture I want to see whether you see relationships I'm not Sizemore big-picture more conceptual rather than you know sort of very detailed problems where you have to go through a lot of detail OK so I want to see whether you master just that the big picture items all right so this is the last quarter and see use of good luck I wish you all the very best the good thank you
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