Free Chemistry NEET Video Lectures for Chemical Kinetics

00:00:21 [Music] Welcome students to today's uh lecture. My name is Promeit Churri and I am a faculty in the department of chemistry at Indian Institute of Technology IIT Delhi. So the topic we are going to discuss in today's lecture and in the fourth of my lectures is chemical kinetics as is written on this piece of paper. Now before we go on to the details of chemical kinetics let's try to understand the importance of chemical kinetics. So when we say chemical kinetics both of these words have very significant implications.

00:01:15 So for example when we are talking about chemical kinetics that means we are talking about processes processes related to chemistry or chemical processes. For example, one kind of change might be a reaction of A plus B say going to C where A and B are the products and rather sorry are the reactants and C is the product also. Now this is a this is a case where there are two reactants which are combining to give a product. Right? Now there can be another case where I have just a transformation for example

00:02:03 a state A right see in one phase phase one to the same A but in phase two so in the second case what has happened is I had a phase transformation for example say I'm going from you know ice to liquid water or I'm going from liquid water to water vapor. That's what the second one is talking about. Now see you know like this there are many many examples in chemistry right so if you consider these two what they're representing is they're representing representing change or changes

00:02:46 happening in whatever you're dealing with or whatever is in front of you or whatever you're working on. Then what about kinetics? Kinetics is going to tell you how fast or how quickly this change happens. Right? So if you write this then what kinetics refers to is how fast or how quickly the particular process is going on. Then essentially what we are saying is we are saying what is the rate of that process. Now if you think about this why do we need chemical kinetics? See all almost all of you have been taught

00:03:53 or you yourself have studied about thermodynamics. The importance of thermodynamics and chemistry. Now if you talk about thermodynamics in chemistry, what does thermodynamics tell you? Is there any need for us to go for chemical kinetics? Can we not get it from thermodynamics itself? So let's talk about thermodynamics a little bit so that we understand the necessity of this topic or the significance of this topic to chemistry. So in thermodynamics when we are talking about thermodynamics remember

00:04:27 if we write and think about thermodynamics then this is what we are focusing on it is about the initial state the initial state right of your reaction or whatever and we refer to it as I then you have the final state which you refer to as F. So thermodynamics mainly deals about these two states only. The initial state when you start a reaction and the final state what is the final state? The final state is when you have when you have reached a chemical equilibrium when you have reached a chemical

00:05:24 equilibrium and that's why it's also referred to as chemical thermodynamics. But see what is happening is yes you are talking about the initial state you're talking about the final state good but what is happening in between you're not being able to focus too much on for example you know think about a certain process say ice going to water liquid right now what will thermodyamics tell you thermody tell you that if I have to make this transformation from ice to water, right? I would need to supply

00:06:09 heat so that this transformation can be brought about which tells me that the process thereby this process is endothermic. Similarly, if I go from liquid water to water vapor, right? Which is the gaseous state. Again, what you're doing is you're transforming the molecules, the same water molecules from their liquid state to a gaseous state. Again, you're supplying energy. So, this process is also endothermic. And thermodynamics tells you that that you have to supply heat so that this

00:07:03 process or this transformation is being brought about along with this. So this is only a part of it. Along with this there are some other very common thermodynamic parameters that you obtain or get from these sort of reactions. So common things you ask for is or you know parameters you ask for like the free energy change of the reaction or process right so this free energy change is often referred to as delta G and we know so I'm not going to write that you know that delta G is negative negative that means the process

00:07:50 is spontaneous. If the delta G is positive the process is a non-spontaneous process also you can talk about entropy changes which is given by delta S. So, so far so good, right? You have the initial state, you have the final state. And because you're talking about delta G, delta S or delta H, right? Whether it's delta H is endothermic, so that means you're supplying heat or exothermic that means heat is getting released. So, delta H is negative, right? So for an example, if you take about if you you know if you

00:08:30 talk about dilution of concentrated sulfuric acid. Okay, let's talk about this very briefly. So you have very concentrated sulfuric acid. What you do is you take some sulfuric acid from the chemical reagent bottle and you dilute. So I'll give you some uh you know measures of the heat that's released. This is a highly exothermic process. So for example suppose you are having say this is H2SO4 in milliliters considered sulfuric acid. Remember you have taken directly from the reagent bottle. considered reagent

00:09:22 bottle. Then you have H2O in millus. Okay, you're mixing these. So how you mixing? So suppose the volume of H2O is 100 ml and the volume of H2S4 you're adding to this 100 ml of water is 10 ml. So then the delta H that means the enthalpy change of this reaction the enthalpy change of this reaction in kilogjles in kilogjles is -1 kiloj right and then the corresponding change in temperature is 25° C. What does this mean? What it means is when you're taking 100 ml of water say in a beaker or in a suitable container

00:10:16 you're adding 10 ml of concern sulfuric acid then this amount of heat is released and the temperature increases 25. So that's why the container feels so warm. So this is an exothermic process exothermic being referred to or being signified by the presence of this negative sign. Okay. Now let's increase the volume of sulfuric acid. added say if you go to 30 ml of sulfuric acid again added to the same amount of water then the heat released is about -30 kJ and the temperature change is about

00:10:54 70°C. So you can immediately understand while in one case while in one case say for example the phase transformation of ice to water liquid water and liquid water to water vapor you had to supply heat so that they can make the next transformation or go to the next phase. In this case when you are diluting sulfuric acid considering sulfuric acid water you're having a huge amount of heat coming out or getting released and that's why the container also feels very hot or warm depending upon the amount of

00:11:25 temperature change you're having. Hence this process hence this process of dilution is referred to as an exothermic process. Okay. So then this again comes under the realarm of thermodynamics because thermodyamics is telling you that okay this is the energy that is coming out because of the dilution or this is the energy you have to supply so as to bring about the phase change for the previous endothermic processes. But you have to realize one thing now if you ask the question how long does it

00:12:06 take for the phase change to happen? How long does it take for the reaction to happen? For any reaction to happen? Thermodynamics does not give you an answer for that. So then we can write from thermodynamics from thermodynamics we do not have any information. Any information about time. Okay. So if I can write it again, I can say that thermal dynamics gives me no time information gives me no information about time. So the only way I can get information about the time that is the rate at which this

00:13:09 transformation or any transformation is taking place is to resort to or take help of chemical kinetics. That's why this topic in itself is having such a significant place in the heart of chemistry or as a topic it is so very important to chemistry. Okay. Now when you talk about kinetics as we just said we are mainly interested in the time taken right how slow how fast it's one also the question is remember When we are talking about thermodynamics, we said that calculations involved when they include

00:14:13 this delta H or delta G or delta S, we said that this is the difference between the final state and the initial state. So these are the only two states we are concerned about in thermodynamics all the time. However, in case of kinetics, you start asking this question. If I have a process. So that means if I have a going to b and if this is a process then I start asking how does this process happen. So when you ask this question which everybody should right so the biggest question is how what takes place for

00:14:59 this change to happen then it's not only the time you're referring to well time is definite an aspect but what you're also referring to is the mechanism the mechanism at the molecular level that is mechanism at the molecular level you need to know if I have to go from A to B then what is taking place at the level of the molecules in that reaction system or in that container so that this transformation or following which this transformation is happening that is a is going to b so this also

00:15:48 is addressed by chemical kinetics. This you can immediately understand right the significance hopefully it is becoming a little more relevant in terms of discussion of chemical kinetics in chemistry which is not only the rate yes how fast how slow but also when this transformation is taking place or when this process is taking place what steps might come in or What is the mechanism through which that particular process is happening? All these can be addressed through kinetics. Now once you think about this, you start

00:16:46 asking other questions, right? So suppose you're thinking about the rate of the reaction of any particular reaction. Say I say rate of a reaction then immediately the question that comes to your mind can I control the rate of the reaction? If I can what are the factors? So that means the first question that comes to our mind is can I have a control on the rate of the reaction? You say I say yes. Then your next question is great. If so please tell me how I can control. That means what are the

00:17:40 factors? What are the factors? These are this is this word is factors that will control the reaction rate. Now as we will go through our lectures, we are going to spend time on this and discuss the different factors. But I'm sure most of you already know or can already realize that some of the factors are very commonly used factors that can control the rate. So one would be concentration. One would be concentration then another one would be temperature. So generally with increase in temperature the reaction rate increases

00:18:35 and then there is something which is a very unique which is having a very unique place in chemistry. It's a catalyst. A catalyst is something which increases the rate of the reaction. Okay. So that means if you have to control the rate of a reaction. So suppose you see that the reaction went too fast. You're doing a re you're doing an experiment in a certain laboratory. So you're doing a practical experiment say in one of your practical classes and then you're following this transformation say from A

00:19:13 to B and you suddenly found out oh this reaction just went too fast for me to capture what was happening or to capture the rate because it was just too fast. So how can I now decrease the rate? A I can play with the concentration. Two I can play with the temperature and catalyst also has its own unique place which we will discover later as we go through this course more and more. Okay. Now, kinetics is mainly referred to kinetics is mainly referred to as a branch of physical chemistry. Okay, but this is generally what you

00:20:15 know kinetics is uh thought as as being a branch of physical chemistry. But you know what if you really think about kinetics, it is actually a unifying topic. So kinetics I can say it's a unifying topic covering many branches. So it has relevance in biochemistry. It is applicable in biology. Right? Now talk about mechanisms in organic and in organic chemistry. The moment you talk about mechanisms that is the very moment you also start talking about kinetics. Again how fast how how slow do these things happen? Can

00:21:26 I speed the reaction up by adding a catalyst? Can I speed the reaction up by changing the concentration of this? So what it means is that kinetics the importance of kinetics is not only in the branch of physical chemistry as it's supposed to be but it really is spread over all branches and that's why the relevance of kinetics or the importance of kinetics and hence I think this is a very good starting point based on which we can build on this topic or this concept of chemical kinetics. But you know before I discuss about the

00:22:05 rate equations and other aspects or features of chemical kinetics, I would like to discuss some examples with you in a daily life where chemical reactions and that to kinetics become quite important. So as an example first I will discuss about chemistry in cars. Now you must have seen cars flying on the roads right nowadays there are many many cars on the roads and many different cars. There are many different car companies like Honda, Hyundai, you know, many many different car companies, Maruti.

00:23:01 Now, what happens is the way the cars run. So, at different places in cities or on the highways, you would see that there are petrol pumps where the car so tank needs to be filled with petrol. Now this petrol which the car runs on this petrol or gasoline named as is a mixture of hydrocarbons. Okay, it is a mixture of hydrocarbons. can say this CX H Y right so the hydrocarbon I'm referring to it's the generic symbolism where have X atoms of carbon and Y atoms of hydrogen right so if it is methane so suppose if

00:24:03 it is methane CH4 then X is equal to 1 Y is equal to 4 if it is Ethan C2H6 then I I have x = 2, y = 6 and so on. Now what happens is when you turn a car on this petrol which you had filled the tank with from the petrol pump is burnt. So this petrol during the running of the car the petrol is burnt right now when the petrol is burnt so that means the hydrocarbons are getting burnt if it is an ideal if it is an ideal condition. If it is an ideal condition then this is typically what you would get. So that

00:25:07 means C X H Y would combine with say the oxygen of air to give you CO2 and H2O. So this is what you expect under ideal conditions. If the fuel I'm taking which is composed of these mixtures of hydrocarbons are being burnt or the fuel is being burnt then their burning full burning and ideal burning I'm why I'm talking about ideal you will soon realize so the ideal burning should lead to the formation of carbon dioxide and water which are not very harmful however what happens This now this is an ideal case

00:25:57 right? Now suppose there is incomplete fuel burning. So that means the all fuel is not getting burnt. If all fuel is not getting burnt then what can happen is I can have some un burnt hydrocarbon still out there. Not only that when you burn this what happens is you give rise to a high temperature that means the temperature rises. When the temperature rises and also because of incomplete burning you can have other reactions happening. For example this incomplete burning of CXH can give rise to not carbon dioxide but

00:26:46 carbon monoxide as one of the gases is coming out. Then you also have so you know this is you're where are you getting the oxygen from? You're getting the oxygen from air. The air is also having lots of nitrogen. So what can also happen is that nitrogen can combine during burning to give rise to nitric oxides NOX. So this NO X is typically composed of N O and NO2. So this one you know is nitrogen dioxide and this one is nitric oxide. So see what has happened. The ideal condition was this. The ideal

00:27:46 condition was that you have fuel. It combines with the oxygen of the air and it gives rise to carbon dioxide and water. This is the ideal condition. Good. But then under non ideal cases, this is typically what happens. You know like when you read about ideal gas, non ideal gas, see ideal gas is an ideal condition. Mostly all gases are non ideal in nature. Similarly, here incomplete fuel burning gives rise to some gases which we do not want which are poisonous for us. I'll come to that very soon. But what are those gases? One

00:28:32 is unburned hydrocarbon. Then you also have this unborn hydrocarbon reacting with oxygen going to CO which is again incomplete combustion that means it does not go to CO2. You have nitrogen from the air now which can combine at this high temperature to give rise to oxides of nitrogen represented as NO X and under this NOX umbrella we have NO which is the nitric oxide and NO2 which is nitrogen dioxide. So then in one shot I can write that if I have air plus petrol that's what you're burning will

00:29:21 give rise to CO2 plus H2O you know these are the ideal ones plus CO plus NO X these are the ones which we do not want plus unburnt hydrocarbons. The main problem arises from these three and that's why these three are often clubbed as pollutants or environ they should read as environmental pollutants. That means they pollute the environment. So see you're talking about fuel burning, right? You're talking about an ideal combustion where I should be getting ideally carbon dioxide and water. I do not have much to worry

00:30:24 about. But then because the combustion is not ideal because of the conditions there will be some hydrocarbons which should not be burnt. There would be carbon which would be incompletely oxidized. That means it would not go to carbon dioxide. It would rather go to carbon monoxide. this carbon coming from the hydrocarbon and then you have so much of nitrogen in air. So this nitrogen can combine at this high temperature with oxygen to give rise to different oxides you know NO X under which we have NO and NO2.

00:31:00 So why are these referred to as pollutants? Now before I write something else down, let me show you something as a picture. Now if you look at this picture, if you look at this picture and if you look at my or the white pointer, what you see is at the top of this picture, it's written photochemical smog. I will come to that word later or those two words later. But remember the smog which means that you have heavy pollutants in the air. Now look at the picture below. What you see is not only

00:31:46 do you see so many cars running but if you look at the atmosphere it's very very hazy. No way can you say that it is clean air that you are breathing. It's very hazy. It is hazy. Why? One of the main reasons why we have pollutants why we have pollutants is the emission coming out from cars. So then I can write emission from cars are a huge source of environmental pollution. It's a huge source. Okay. Now what do cars do to stop this? So let's look at a picture of a car. So if you look at this car and again if you

00:32:45 follow my arrow so you can see it's written as so this is the skeleton of a car and you're seeing some components inside. I will tell you which the comp which are the main components right now we'll be discussing about or relevant for our discussion. If you look at this one this is called the exhaust manifold. Exhaust manifold means nothing. That when the engine is driving your hydrocarbon is burning that means your fuel is burning. Then whatever gases you produce they come out through these

00:33:17 exhaust pipes. Okay. So this is what exhaust pipe A. All these gases come out through the exhaust pipes. Now if you would not do anything to these gases then what would happen is these exhaust gases would go straight out into the air and pollute your environment. But that is a big no no right because pollution is very harmful for us and in big cities it is directly related that is big cities the more the number of cars you have the more number of automoils you have the greater is the pollution.

00:33:51 So then each and every car has to do something about it and this is compulsory. This is compulsory. So what do cars do? So each and every car each and every car is equipped with something referred to as a catalytic converter. referred to as a catalytic converter. If you see my pointer or arrow, I'm moving this pointer over this word catalytic then converter. Do not worry about the 3A. But what the catalytic converter is supposed to do is it is supposed to take these gases, the harmful ones, and

00:34:40 convert them to non-harmful ones. So that when finally the gases come out through this pipe you can see out here exhaust pipe tip then these pollutants like NO X CO and the unburnt hydrocarbons are not there. So this is one of the most important features, one of the most important features environmentally that a car has to have such that pollution of the environment is kept to the minimum. Okay. Now what you'll see in this picture it's a zoomed into figure of this catalytic converter. So typically if you would be

00:35:28 having access to a car or if you see what your neighbors have cars, your friends have cars and if you look at the bottom of the car you will see an object like this very much so like this you know not too much difference as design but the catalyic converters of most of the cars would be having this structure. Now let's decide or let's look at what the catalytic converter does. So it's by the name what does it mean? By the name if I say the name catalytic converter then by the name it suggests that I am

00:36:08 converting something. What am I converting here? I'm converting the gases under NO X. Then I'm converting carbon monoxide and I'm converting unborn fuel. How am I doing it? Because it's called a catalytic converter. So I say it is doing with the help of catalyst. Now if you go back to one of our discussions before you know when we were slowly moving into this concept of chemical kinetics and we said that as opposed to thermodynamics, chemical kinetics tells you about the rate of the reaction and also about

00:36:53 some idea of what goes on during the reaction. Then a question that comes to your mind automatically is can I control the rate and we discussed that it can be concentration say one it can be temperature second and it can also be a catalyst which ends up changing the rates of reactions. So that means this catalytic converter would be having some catalysts either one catalyst or a combination of catalysts which we will just see which will help converting these harmful pollutants to something which would not harm us or pollute the

00:37:44 environment. And with the idea and with the fact that the number of cars on roads, the number of automoils, not only cars, trucks, motorcycles, everything on bikes on road are increasing day by day. It just makes sense that this level of pollution coming out or being contributed by these automoils would increase if measures are not taken to control that level of pollutants being emitted through the exhaust pipe of the cars. Okay. So here as we were talking remember we looked at this catalytic converter right now what

00:38:29 I'm going to show you is the inside of a catalytic converter. Now there is a reason we are going to this because do understand that this is we are talking about a chemistry in something as modern a technology as cars with the technology improving day by day. So now if you look at the inside of this catalytic converter what do we see? So the construction is fairly simple. On the two sides you have two ports. What are these ports? If you see this red arrow, you follow, you follow my white arrow.

00:39:01 If you see this big red arrow, then this is the inlet port. So the inlet pipe, what does it do? It comes from the exhaust manifold where the gases are produced after burning on fuel. So then you can see you have those unborn CX HY then carbon monoxide then oxides of nitrogen which would enter the catalytic converter through this port. Okay. Now the inside of the catalytic converter you can see there are two slabs. Now without going into the very details also realize one thing that these slabs are built with certain high

00:39:45 temperature materials which can resist the temperature at which these fuel is being burnt so that those do not get bad or not affected. But not only that in these slabs you have catalysts embedded. So for example the first slab you can see out here this slab has roodium as a catalyst. What does roodium do? As it is said out here roodium as a catalyst it reduces the oxides of nitrogen. What does it reduce to? So NO X gets transformed to nitrogen and oxygen. So what does roodium do? That means roodium

00:40:33 is reducing N O X as oxid asen to nitrogen and oxygen gases. So that's why roodium is the catalyst. Now also if you look at this small circle which is a part of this catalyst. So what happens is the way this catalyst is made or the way this you know this this structure is made where the roodium catalyst is there it is full of it is porous. That means it is full of pores. Why do you need pores? You need pores so that the gas which is coming out or the gases which are coming out from the exhaust pipe or through the

00:41:18 exhaust pipe from the exhaust manifold can pass through this one. During passing what is happening is these are getting reduced at least no X in this case the oxidiz getting reduced to nitrogen and oxygen. Now comes the next one. Remember you have been able to take care of the oxides of nitrogen but what are you left with? You are still left with remember still left with the carbon monoxide gas and then the incompletely burnt hydrocarbons. What do you do here? So in the second slab or structure what you

00:41:59 have is you have two catalysts. What are these catalysts? The second one the two catalysts are as shown platinum and palladium. What do they do? They should be oxidizers. So they oxidize carbon monoxide and the hydrocarbons. So the next lab which is the second one out here have as catalysts platinum and palladium. Right. they oxidize CO and CX HY. Okay. So that means CO plus O2 gas gives me CO2 gas, right? And also remember from before CS X HY plus O2 gas. So this is also a gas would be giving me

00:43:04 CO2 gas plus H2 gas. Okay. So if I balance this, this is how it would come. So what has this cataly catalytic converter done for you? What this cataly catalytic converter has done is it has taken these harmful gases. The first being the oxides of nitrogen which were reduced to nitrogen and oxygen no pollutants. Then the ones which are coming out carbon monoxide and the hydrocarbons they are now being oxidized using platinum and paladium to carbon dioxide and water. By this way and by efficient design of the catalytic

00:43:50 converter you can try to minimize you can try to minimize the extent of harmful pollutants that are being given out by the car or are coming out from you can see this blue solid arrow. This is the other side of your catalytic converter through which the gases which just got converted or some percentage which did not get converted go through. So this is really fascinating right? So within that short time and I'll tell you what typically the time is within the short time the car engine is running the

00:44:31 fuels are being burnt you know these pollutants are being produced these pollutants are sent through the exhaust pipe into the catalytic converter they're passing through the catalytic converter during that time simultaneously what is happening is the nitric oxides the oxides and nitrogen are getting reduced and CXX X HY and carbon monoxide these are getting oxidized to less harmful or nonpolluting species. Okay. Now if you go by the time uh that it takes so because we're talking about kinetics it is always uh

00:45:11 you know good to give you some uh sense of time. So the time for which you know it uh kind of stays in contact you know if if you think about how fast does this whole process happen or how fast or how you know how long does this one remain you know these gases remain in contact with the catalyst then it takes about 50 to 70 milliseconds. So if I can write out here. So it takes about 50 to 70 milliseconds for the gas to go through the converter. Right? Remember the car is running. So MS stands for

00:46:06 milliseconds and during this time this whole conversion has to take place. So you realize that it is not only about the reaction that is happening under some conditions reaction or reactions when the temperature is raised because you're burning fuel and so on. But it's also that in the catalytic converter when the gases are passing through the two slabs where you had these catalysts for a short time for a very short time the gases get the opportunity to pass over the catalysts or in other words the catalysts have

00:46:54 only that much of time to make sure that the conversion can happen as efficiently as possible. Okay. Now based on this if you read newspapers then you would come across some guidelines. What are those guidelines in terms of environmental pollution? One very common guideline for automoils is going by that bhat stage say four. What does this mean? What it means is that under this every car say has to comply by the restrictions imposed under this concept or this heading bat stage four. What is it related to? It is directly related to

00:47:49 the pollutants or the amount of these pollutants which are coming out through your exhaust. So in the coming days you would see that cars would have to comply by say bar stage six. That means the amount of carbon monoxide that can come out through the exhaust which was not oxidized to carbon dioxide has to be even lower than what is permissible now or the amount of oxides of nitrogen that can come out. the permissible amount would be much lower than what is being used now which is part of stage 4. Right? So this was

00:48:40 one example where chemistry in cars along with the rates of the reaction high temperature high rates right then because of burning of fuel then also the application of catalyst everything is happening together. Right? That's why chemical kinetics is such an important concept. So what we will do in the next lecture is again before you know we delve into the real equations about the rate of chemical reactions and so on. We will look at another example and if you can work on that yourself or think

00:49:28 about that yourself I will tell you what the example is. That example is about airbags a safety feature in cars and I'll tell you how or what fascinating chemistry goes on in there as a direct relevance to our discussion on chemical kinetics. Thank you. [Music]