 come to the second lecture of this course address corrosion and its control. Before we proceed today's topic, I like to recollect what we discussed in the last class. We define what the corrosion is. Corrosion essentially means the interaction of the material with the chemical environment leading to loss of materials, loss of function and that is the way it should be defined. We also looked at what are the implications of corrosion of engineering components. We saw that there is a huge loss for the nation in terms of 3.5 to 4 GDP for any industrialized nation. And more importantly the consequences of corrosion are very severe. It can affect the safety of the people. The environmental degradation can happen because of corrosion in the consequence of the pollution of the products with the environment. It can also affect the reliability of the components. It can also affect the product quality because for example, manufacturing a pharmaceutical drugs it is very important that these drugs are pure or you transport water for drinking the water has to be pure. It can affect the products. It can also affect the appearance of some of these you know components or maybe like you see the auto rails and so on. So, this requires a concerted effort in order to control corrosion to a large extent. We have seen in the last class also that thermodynamically the metals and alloys we deal with are unstable when they come in contact with the environment. Now, the question comes to us is can we predict whether a metal can undergo corrosion or not? Can we predict if a metal can undergo corrosion? If an alloy or a metal will suffer can we predict without doing an experiment can we predict? If we can so predict then can we really determine the rate of corrosion? These are the two fundamental questions that we need to know we need to address before we address the actual corrosion problem. Now, let us take the first one can we predict if an alloy or metal can suffer corrosion in a given environment. Before we address this question we need to go little deeper into what a corrosion means. Take an illustration if I take a beaker and I fill this with let us say hydrochloric acid and I immerse let us say steel I immerse steel into this right immerse steel. The major constituent of the steel is what is iron right. So, what do you think will happen here? What do you observe? Will there be corrosion in hydrochloric acid? Yes. So, what you visually observe? You will observe with your eyes the evolution of hydrogen right. There is a gas you may not know exactly it is hydrogen or not you will see that the gas is being evolved on the metallic surface. We have seen in the last class that the corrosion is oxidation of metal at the same time that has to be a reduction reaction. The electrons so liberated during oxidation process need to be accepted by some species. So, there is an oxidation process with reduction process. So, let us write this corrosion process in a chemical equation ok. How do you write this? Hydrochloric acid you put let us say steel I simplify it as iron and when they react with this what it forms? It forms ferric chloride plus hydrogen gas. So, this is a corrosion process. The metal is getting oxidized and the hydrogen ions in the hydrochloric acid turns into hydrogen gas by reduction process. Now, like that we can have several type of corrosion process the iron can be immersed in sulfuric acid the phosphoric acid. Can we really predict if the corrosion can occur with any chemical species? What is the first approach for you? It is a thermodynamic right you look at the thermodynamic. What is the parameters you normally look at? In a few. Your free energy change. The free energy change for the reaction has to be negative then that reaction becomes spontaneous. Please notice this is a spontaneous process. The corrosion all along we refer to a spontaneous process you do not supply external energy for the corrosion to occur. It is an external I mean it is an external agent, but a spontaneous process. So, look at the change in the free energy for a reaction that will tell you if the reaction will be spontaneous or not. You can give some examples the obvious example is magnesium in water magnesium hydroxide ok. And the free energy change for this reaction is the delta G for this is is minus 142.6 kilo kelvin. So, now, we can say that magnesium for sure will corrode but it exposed to water. I have introduced one more species here which is oxygen right water has dissolved oxygen percent. So, the react and form magnesium hydroxide it is a corrosion process. The free energy change for that is negative. So, confidently you can say the reaction occurred. We also know of metals which are not undergoing corrosion when you immerse in water. The obvious example is what? It is a gold right gold does not undergo corrosion right. So, you can write the equation for that gold immerse in water it consists of oxygen there it forms gold hydroxide right. And you can balance the equation here the free energy change if you calculate if there is this reaction has to occur is plus 157 kilo kelvin. What does it mean? If this reaction has to occur you need to supply this energy over here you derive energy energy comes out of the system. So, this reaction is non-spontaneous and so, you can conclude that it is possible to predict based on the free energy concept whether the reaction will occur or not. But think of a corrosion process like you have a steel tank holding sulfuric acid you want to know corrosion occurs or not or think of a case where the pipeline is buried in the soil. There are two types of corrosion. Corrosion occurs because of the soil interaction. The corrosion occurs because some product goes through the pipeline it will be a petrochemical product the water. How do you really predict? Can we really go and determine the free energy change for all these corrosion processes? It is not very easy ok. So, although the fundamentally the free energy change is easy to use easy to understand in practice it is not very easy to determine this actually. So, this cannot be used with comforts in predicting whether the metal will undergo corrosion or not. So, we need to go for a different criteria and that criteria is going to be what is called as electrochemical criteria. Now, we need to understand before we get into defining the criteria we need to understand what is an electrochemical reaction ok. So, that is important to understand. Let us go back to this equation here ok. Look at this equation iron I write again iron reacts with hydrochloric acid and forms ferrous chloride plus. I do not worry too much about the chemistry I think we will try to minimize as much as we can ok. But you have any doubt any time you are free to you know ask questions I will clarify that ok. But some amount of chemistry is very important in order to understand the corrosion process. Let us take this reaction. Now, look at closely you have iron it is in the metallic state. The hydrochloric acid is in what form? You have an ionic form H plus Cl minus and you have ferric chloride is in the ionic form Fe 2 plus 2 Cl minus and it is in the molecular form. With this you can understand very clearly how the charges are getting transferred ok. So, what is common here and I think I need to change this this changes the reaction here ok. I think there is a mistake over here also I think need to change that ok ok please change that it is not properly balanced. So, it should be 2 here right. Now, what happens you can spike this you can spike this water mines and interacts with 2 H plus and giving rise to what and giving rise to Fe 2 plus plus hydrogen. So, this is the actual corrosion process occurring on the metal. You call this electrochemical reaction because the charges are transferred. Let me write it again to make it clear. What happens? Iron goes in the solution as Fe 2 plus and you have 2 electron and then these electrons interact with hydrogen ions and form hydrogen molecules. Now, it is clear the corrosion process consists of what? Consists of in oxidation ok and it consists of a reduction process. We saw this pictorially in the last class that iron gets oxidized as Fe 2 plus releases 2 electrons these 2 electrons accepted by H plus and this so releases hydrogen. So, you have one oxidation and at least one reduction process. I can make it more complicated. How do I make it more complicated? Assume that the hydrochloric acid has dissolved oxygen right. Can oxygen dissolve in water? It can. Similarly, the oxygen can dissolve in this. So, you can have one more reaction. What is the reaction? It could be oxygen plus 4 electrons it is 4 H plus and it leads to water. So, you can have one more reduction reaction. So, the same electrons provided by iron can go to H plus can go to oxygen here H plus and you can form water. So, you can have many reduction reactions. You can add if you want. For example, I have a copper ion in the solution cover 2 plus you can combine with 2 electrons you can form. So, there can be another reduction reaction. So, you can make it more and more complex ok. In fact, in practice the corrosion processes are always complex. It is not that simple ok. Many reactions can really occur. It looks very difficult now. Can we really predict metal corrode or not? That is a task. That will see that how you can simplify this and you can write a simple equation to show that yes it is possible this reaction can occur. For example, I have put C 2 plus plus 2 electron gives you copper as reduction can I write like can I write say zinc 2 plus plus 2 electrons giving a zinc and the iron corrodes can I write like that? Is it possible? Can I ask questions? It may not be possible. That is what we are going to look at in the subsequent lecture ok. So, how do we really predict whether these reactions occur or not? So, far any of you have any questions? So, it is clear now. So, let us go to the next step of of our you know our understanding towards how do you predict if corrosion occurs. You are aware of what is called as equilibrium. You are aware of this what I mean here is a thermodynamic equilibrium where the rate of the forward reaction is equal to the rate of backward reaction. Now, if you deviate from the equilibrium what happens? Either the forward reaction is faster than the backward reaction or the backward reaction will be faster than the forward reaction. So, let us use that concept. Let us first define what an equilibrium is. What an electrochemical equilibrium is? Why you are defining electrochemical equilibrium? Because we consider the corrosion is an electrochemical process. So, first let us define what is an electrochemical equilibrium. Let me go to the iron dissolution in sulfuric acid. Let us look at the equilibrium of iron in the solution. But take a bigger I take in this solution consisting of Fe 2 plus iron right and then I immerse a pure iron. Nothing else Fe 2 plus ions are in the solution iron is immersed in the solution. So, what is expected to happen? What is expected to happen is that iron will go in the solution as Fe 2 plus. What will happen now? The Fe 2 plus will again go back as iron that is in equilibrium if the rate of this forward reaction and the backward reaction are equal then we say it is under equilibrium condition. Now, I I write this equilibrium here Fe 2 plus plus 2 electron. Please notice this is an equilibrium sign. Fe 2 plus is in equilibrium with a field. There is a rate of reaction forward, the rate of reaction backward and both are equal. And when this happens at this interface, the interface between what? The interface between the iron and the solution you establish a potential right. So, I I define it as I have a metal surface here and I have Fe 2 plus ions, they go and go back. And between these two interface between these two solutions ok a potential is is being established and that potential is called as equilibrium potentials that is called as an equilibrium potential. I can have two ways of looking at it. I can look at from thermodynamic point of view what is the free energy change for this equilibrium to exist delta G. And at the same time I can also define this process in terms of equilibrium potential. I define as E and there exists a relation between two. What are the two? The free energy change and the is the equilibrium potential right. There exists a relation between these two. What is this relation? Famous relation most of you would have it is long ago right, 10th or 11th we called famous Nernst equation right. The Nernst equation is known for us and one second. So, we know the Nernst equation. What is Nernst equation? Delta G is equal to minus N F E and delta G the free energy change for the reaction N is what? The number of involved involved in what? Involving oxidation and reduction process N F paradigm constant. And it is what is the value N F U and phi N E is the please notice I write this as equilibrium potential it is mass the potential that defines the equilibrium state. We are not talking about corrosion. What is corrosion? The spontaneous reaction of oxidation and corrosion and oxidation reduction process over here no we are talking about they are just in the equilibrium. Does iron corrode here? Does it corrode? It does not corrode at all you wait for one day or one year the rate of oxidation of iron is equal to the rate of reduction of iron nothing happens. So, we are now only defining the equilibrium state ok. So, you have delta G is equal to minus N F E. Now, I have a reaction I have an equilibrium for example, I have something like F E 2 plus plus 2 electron gives you F E. If I need to know the free energy change for that and related to the potentials I have delta G is equal to delta G naught plus RT ln k right be people or aware of this equation. What is delta G naught here? At this standard state ok. So, it is it is the standard state ok as a standard state delta G is equal to free energy change. So, the free energy change can be related to the delta G naught plus RT ln k. What is k here? k is equal to the equilibrium constant it is called the equilibrium constant. What is the equilibrium constant? Can somebody can you can you tell from here? How do you expand this? How do you expand this? For this equation above equation delta G is equal to delta G naught plus RT ln what happens? The activity of iron upon the activity of F E 2 plus plus activity of. Now, let us substitute this one delta G is equal to minus N F E have minus N F E is equal to minus N F E naught plus RT upon ln the activity of iron upon and I can rearrange this equal to E naught minus you can generalize this one right. How do I generalize this? I can generalize this as E is equal to E naught minus RT upon N F ln activity of the product upon the activity of the I can apply the standard state here. What is the standard state? Where the activity of the product is equal to activity of the reactant and they are unity right. So, on the under standard state it conditions I put that way. What is the standard condition? Actually standard state is slightly different from standard condition ok. The standard state is purely thermodynamic concept. The standard condition means you have also talked about temperatures right. In a standard state it can be any temperature in standard conditions we define the temperature as what temperature is 25 degree Celsius ok. In any case the activity of product is equal to reactants it becomes unity what happens? E is equal to E naught and what is E naught is equal to E naught is called standard potential are you followed? So, whether it is E naught or E both represent the equilibrium conditions I again emphasize it is equilibrium conditions we are not going to corrosion yet I define the equilibrium condition and allow the equilibrium condition to deviate then what happens the corrosion occurs. My first task is to define the equilibrium condition wherein these species involved in corrosion are well defined and defined. What are these species which we are considering in the case of iron in in hydrochloric acid? The equilibrium condition of iron in iron 2 plus H with H plus we are looking at two equilibrium conditions. So, my task is to first understand how do I define what parameter do I use to define this equilibrium and then use that to predict if the corrosion will occur or not ok. So, that is the way we are moving now. So, we have now gone into the electrochemical criteria of defining equilibrium from a thermodynamic criteria. For equilibrium to exist in a thermodynamic criteria delta G is is equal to is 0 right in this case the equilibrium a condition for that is is E naught I mean E here. So, we can able to translate the free energy change into a potential here that potential we called as electrochemical potentials or called as the equilibrium potential now ok. So, this is is the first task for us any of you have any questions on here ok. I will ask this question to you. Suppose I would like to know let us look at imagine a a thought process thought experiment right. I want to know the equilibrium potential of zinc in zinc ions I want to measure it maybe in the lab. So, what do you mean by zinc is equilibrium with zinc ions what do you mean by that physically what it means? What do you mean by equilibrium right when I say zinc or in equilibrium with zinc ions how visualize it yeah. So, I have I have a solution where I have zinc ions are present I just simply dip zinc into it establishes an equilibrium and you measure that potential and that potential is called equilibrium potential. You call a standard potential once the concentration or the activity of zinc ion are considered as unity ok. So, this is what I mean. So, you can have in extent this for almost all kinds of equilibrium systems it could be for hydrogen. Again hydrogen what do you do? You have H and H plus. How do I define the equilibrium condition for hydrogen and the hydrogen ions? Let us say H plus is in equilibrium with H here there is no metallic right how do I establish? I would take a small figure I would take H plus ions. What is H plus ions here? Let us say sulphuric acid ok and I shroud this with the hydrogen gas and bubble this and I put a platinum electrode right this is my a platinum electrode I can I can also keep in a noble metal I can have you know maybe a rhodium I can have ok and again I have gold for example, I will put it. Now, how do I establish equilibrium? Over here metal surface the hydrogen will get oxidized as H plus the electrons are released right this H plus will again get reduced here and goes as hydrogen gas. So, you need a metallic connector in order that the exchange is taking place. So, how do I determine the equilibrium potential for various systems ok that is our next task. How do you determine this? I would say I have taken copper I am going to dip copper in copper surface solution of 0.1 molar concentration. I am going to immerse it at say in 0.001 molar concentration of copper surface with the potential shown by the copper in both the solution the same or be different will be the same or different be different. What is the basis? Because it is Disney's equation. So, use the Nernst equation to determine the equilibrium potential for any electrochemical system. So, that is the first step that you should understand right. So, the first step is to calculate the equilibrium potential. Let us take let us say ion in say ferroequivalry solution that is they say calculation equilibrium potential ion in ferroequivalry solution to do that we use the Nernst equation ok E equal to E naught plus right RT by NF I slightly change it here L in activity of the reactants I can change also right I can just change this. This is always a confusion about the calculation of the potentials because we try to convert the thermodynamic parameter into electrochemical parameters. Let us go here A plus B this gives you C plus D or C plus D gives you A plus D A plus D ok. The difference is changed for that is is let us say let us say plus difference is changed for that is considered as negative here right. This is what generally we do here and we have some issues when it comes to electrochemical potential. So, I need to clarify this so that you do not make any mistake in determining the electrochemical potentials. Let us take this value here E naught what is E naught standard potentials ok. The standard potentials are listed are listed there used to two conventions earlier one called as American Convention, other was called as in American Convention they used to define the potential based on in oxidation reaction. Here they used to based on a reduction reaction for example, I have a metal M I represent this as M plus plus M electron and in European Convention they look at it differently M plus plus electron gives you as M right. If we use a Nernst equation for this the Nernst equation for this literally use literally use it ok. You will get one sign here you get another sign here. Numerically the value is going to be the same and not going to change, but the sign is going to be different. For example, if you get here positive if you get it at here it becomes negative automatically ok it becomes negative negative in this convention. So, that is a problem in actually defining what the equilibrium potential is you take the old books and you to take the old Fontana book you would write as standard oxidation potential or some other people write as standard reduction potential that clarity is very important without which you are likely to make mistake in really predicting the metal will corrode or not ok. Now, let us let us me try to explain this why we should not look at either American Convention or European Convention both are wrong it is not correct ok. What do you say so? Let us take an equilibrium any equilibrium you can take you want that is a copper I immerse copper in copper 2 plus ions solution right what is happening here there will be equilibrium right. What is the equilibrium here? The equilibrium is that copper goes as copper 2 plus and copper 2 plus it will return back and gets deposited. The oxidation reaction, reduction reaction both of them occurring on the same surface unless the rate of oxidation equal to rate of reduction you do not call it as a equilibrium process ok. So, we are now talking about equilibrium potential please look at this the potential established between the metal on the solution is equal to what is that called if I measure a potential that exists between the metal and the solution what is that potential called that is called as a equilibrium potential now. So, the potential so measured is equal to equilibrium potential. Now, let us take these two conventions right let us take an American Convention we call a standard oxidation potential, but also means it is equilibrium potential right. We call a standard reduction potential this is also called equilibrium potential either way you use ok it is it is written like this written like this. In fact, the better way of writing here will be m is going to be m plus plus electron here m plus plus electron is equal to m here I have just reversed this ok equation. But please notice if I write this way or this way that means I can write either as m going as m plus plus electron or I can write as m plus plus electron. In both the ways I represent the equilibrium whatever way I represent this is the picture please look at this is the picture is same or different say. So, you cannot have a potential for this different from this because both are describing the same manner look at it now. So, it is it is not the equation you written in a different manner, but actually equilibrium means the metal is in equilibrium with ions. No matter how you write it in the lab you do an experiment it represent a transfer of copper ions to solution and so and the copper ions from the solution to the metal. So, it is not dependent upon this does not depend upon this so, it is independent. So, that means the sign invariant actually is called sign invariant ok. So, it is a sign invariant. So, electrochemical potential is sign invariant. So, first and foremost you need to understand that electrochemical potential is sign invariant. It does not depend upon American convention, it does not depend upon European convention, you do not call as standard oxidation potential, you do not call as standard direction potentials, you simply call as standard potentials which implies it is standard equilibrium potentials. The same is true for equilibrium potential, you do not call equilibrium oxidation potentials, you do not call equilibrium production potential it is simply it is an equilibrium potential that is what is important. So, I have so far tried to explain to you what is mean by electrochemical equilibrium I suppose right. The electrochemical equilibrium exists at the solid solution interface. There is an exchange of ions or charges between the solution under the metal interface ok. And doing so, it establishes a potential we call them as equilibrium potentials. If this activity of the ions in the solution is considered as unity, it becomes a standard potentials. I did not so far mention about this here for a pure solid the activity is always considered as a one actually ok. Suppose you take a copper alloy, it is not one actually ok. So, it is generally this is considered as unity here when you talk about pure metal, pure solids be it platinum, be it nickel, be it ion the pure form the activity is assumed to be to be equal to one. So, far in a few I have any questions about what is mean by electrochemical equilibrium. Can I can I proceed further yeah yeah. How do we use Nernst equation? Ok. So, now the question comes is ok if I am going to use Nernst equation, how am I going to use Nernst equation for example, that is what the question is right. So, how do I because in Nernst equation if I see here reactants and products depends upon how do you write it right. I can write on the left side oxidized product right side is reduction product or I can write left side as reduced product right reduced yeah reduced as species as a reactants and you have oxidized one as a product right. So, you have this problem. So, that problem I think we should not be into resolve otherwise you do not know how to calculate actually right. The point you need to be clear about is it is not oxidation it is not reduction it is simply equilibrium. So, that is what I think you should be first be aware of actually ok. How to do that? I will come to the next step as to how you really calculate these these values ok. So, calculation of equilibrium potential first the basis. The basis is ok it establishes a potential here right. So, there is a potential existing between the metal on the solution. I need to measure this ok I need to measure this. So, how do I measure this potential? How do I measure this potential ok. Measuring potential you know suppose I have a resistor how do I measure the potential between these two point? I connect to voltmeter and I make a contact here I contact here I measure this. To measure the potential across a distance to different locations I need one more reference actually. So, potential is already measured with respect to the other one right. If I have to measure the potential here I need to insert another probe using right. I use a probe I can use a voltmeter I can use or more precisely we call as electrometer right. What is the difference between a simply saying voltmeter and the electrometer? Electrometer does not allow the current to pass through you know when you measure the voltage or you can use in a high impedance voltmeter right. Can you use a high impedance voltmeter what happens? Doesn't allow the current to pass through you will understand this concept when you talk about polarization later stage. At this at this point of time it is enough to understand that you need to measure the potential using an alpha meter or an high impedance voltmeter ok. Suppose I I put a probe here I measure it what do you think will happen? When I put here I am going to get another interface here right. I am going to have an equilibrium between this one and the solution. So, there is going to be a potential develop automatically right. So, you are going to develop another potential here right there is going to be another potential developed by the probe. So, you will essentially measure only the potential difference you will never able to measure the absolute value of this potential because no matter what you do you need to have one more probe to measure and that establishes a potential that is the problem that means is ok. Absolute potential cannot be measured you can measure only the relative potential. So, what do you do in this case? They use the equilibrium H plus plus electron giving rise to hydrogen in the standard state E naught is considered as 0. So, you are assuming the equilibrium of H plus and hydrogen is equal to 0 right. So, when I use an hydrogen electrode here I measure the potential and that potential is called as a simply the electrode potentials. So, so that means ok. So, all the potentials are in essence referred with respect to hydrogen H plus. So, this is the most important thing you should keep in mind I again repeat you cannot really measure absolute potential electrochemical potential of any system is referred in reference to hydrogen because it is assumed to be 0 in this standard state ok. Now, let us come back to this the measurement of high potential here how to do that right I use that say it is some metal m is in equilibrium with the m plus ok metal m is in equilibrium with metal plus and here what happens I have a platinum wire H plus and I bubble what is the pressure here p equal to 1 atmosphere and I measure this voltage ok using an electro meter. When I measure this voltage with respect to electro meter what it what it does? It does not allow the current to flow through this please notice if I have a potential 1 and the potential 2 I just electrically short what happens current will start flowing that is not going to happen here you you do not allow the current to flow that means equilibrium is maintained equilibrium is maintained ok and here also the equilibrium is maintained. What is the equilibrium here? It is nothing, but m plus plus electron gives you m what is the equilibrium here H plus plus electron gives you the equilibrium is maintained now I go to measure the voltage and that voltage is equal as the equilibrium voltage or equilibrium potential you agree can I call this as an equilibrium potential as long as the current does not flow as long as equilibrium is maintained here equilibrium is maintained over here and the voltmeter is just measuring the potential of this equilibrium with respect to this equilibrium. I have made this equilibrium as a value the potential of this is equal to 0 and so whatever value I measure here is called as the equilibrium potential. Did I make it clear to you? Did I make clear or anybody ask any question? How do we get the equilibrium potential of the metal since there is no kind of flow to the voltmeter yeah see one of the ways ideally you know how do you measure the potential using a potentiometer anybody remember the potentiometer by null diffraction method anybody did fewer experiments right I want to measure potential between the 0.1 and 0.2 I do a null diffraction method so that I keep on adjusting external resistance voltage and when the when the the current does not flow so potential measured between these two right is the actual potential right. So, I apply external voltage actually so that so that current is not going to flow. So, what we do in this case is that it is external so potentiometer is essentially is a balancing time right you balance it with external equivalent of voltage I do that. So, what I do I ensure that the potential applied externally is almost equal to these two value so that the current does not flow between these two you know normally we use the galvanometer right and keep on pressing it and find out at one point of time the null diffraction takes place. The potential measured is equal to the potential between these two points. So, when we when we say that I have potential 1 and potential 2 I use an electro meter that means the electro meter is supposed to be not allowing any current to flow through that. At that point of time you measure the potential between these two and that potential is equal to the the actual potential of the system otherwise when the current flows you will see later there is something called polarization you will not able to measure the actual values. So, what I refer here is you need to measure potential when they are under equilibrium conditions the current flows they are no more under equilibrium conditions you are not going to measure the actual value ok. So, so that is what it is. So, we are measuring the potential under equilibrium condition and so it is called as in equilibrium. But I am trying to say this potential is not only calculated using the Nancy equation this potential can also be measured in the lab you can go to the lab you can measure it. I think we will do some experiments if the in the lab to measure the potential of copper copper sulfate solution and see that the measured value is almost equal to what you calculate. So, it is possible to calculate and also possible to measure. The question is what you calculated now what you measured has to be same ok. So, that is the point we are going to come to this. Am I am I clearing this actually ok. So, how do I calculate the potential now? The calculated potential and the measured must be the same ok it cannot be different in this two. So, it becomes same if you write the equilibrium to be written the equilibrium to be written as follows. Always write oxidized species on the left hand side and reduce species on the right side. This is the first thing we should start with. You may have equilibrium what are the species under equilibrium here? This is oxidized species reduced species. Suppose an example you want I will say Fe 2 plus Fe 2 electron gives you Fe. This is the reduced species this is oxidized species right like that ok. Then you write the Ness equation right Ness equation is equal to E0 plus 0.0 E equal to E0 plus RT by N F ln activity of the oxidant upon the activity. Please write always like this ok. Then you will not have any problem whatsoever in actually calculating the value which is equal to which will be equal to the measured value. So, no matter what you know reaction we deal with the equilibrium should be already written as oxidant on the as a reactant and reductant as the product if you do that ok you should do this. So, when you do this and you will not have any problem in terms of the obtaining these are the values of this. Please notice the E is a function of what the standard potential the activity of the oxidant the activity of reductant actually. Of course, these are not going to be changing right for a given reaction N is constant, T is a given temperatures or is a gas constant, F is a faraday there right. So, they are not going to change. So, the E can be changed by changing this the activity by changing the activity of the reactants. For all practical purposes in our course we will consider the concentration equals to the activity. So, we consider consider ok concentration is equal to calculation. So, you do not have to worry about what the activity is actually what is the relation between activity and concentration in F u no yeah. So, concentration multiplied by activity coefficient is equal to activity right ok. So, the so, but you do not worry we normally consider the activity coefficient equal to unity which is not true ok, but you for all practical purposes you simplify that actually ok. So, we considered the concentration equal to activity and use this equation to to measure this. Please notice E does not look at E here when will the E go up when will the E go go up when the activity of oxidants increases whenever the E will reduce when the activity of reactants changes. So, you can change these values please understand that means, E is a function of I would use a term now concentration of the oxidant and the concentration of so, both of them can change. The first exercise for you is to learn to calculate the equilibrium potentials. Let me go to the next step I will explain and then leave it for you to to to to to think over until we meet next next. Let us have this corrosion of iron in let us say sulphuric acid. I want to predict if the corrosion occurs or not in sulphuric acid how do you predict? Let us go back to the corrosion reaction iron it is immersed in H res of 4 sorry ok. Excuse me iron immersed in H res of 4 give rise to what? Iron sulphate now you know in this case it is going to corrode. I just only want to give you you know how you can prove that the iron will corrode in sulphuric acid. Before corrosion occurs there will be 2 equilibria right. What are the equilibria we will have? One equilibria will be H plus plus electron gives you hydrogen the other equilibria will be Fe 2 plus plus 2 electron gives you these are the 2 equilibria that will be existing right. If iron has to corrode what should happen? So, what should happen is this equilibria now what happens should happen is iron should go as Fe 2 plus plus 2 electrons and this should go as H plus you know about it. And you also know that this equilibrium as one potential this potentially is referred as H plus H is one potential for this. And this has got what? E Fe 2 plus and Fe as other potentials. So, this equilibrium is not defined in terms of the 2 electrochemical potentials you know E H plus H and E Fe 2 plus and Fe and when the reaction occurs like that what will happen to this? It forms a cell it forms an electrochemical cell it forms electrochemical cell right the electrode 1 the electrode 2 you form a cell now it forms a cell. So, I have E E 1 and E 2 you have an E cell is always no matter what happens right. If you have electrode 1 electrode 2 I connect them you will get a positive potential right irrespective of irrespective of what the value of E 1 is and E 2 is as long as they are different you join them together E cell will always be positive. It can never be negative at all now the question is can this can this occur can this occur? Now, let us give some value for this ok. And now I assume that I assume now iron in equilibrium with with a Fe 2 plus in equilibrium with iron in the standard condition which is equal to E naught is is minus 0.44 volts suppose I assume this and I also have an equilibrium potential for hydrogen and E naught for this is equal to just look at this look at this and the E cell will always be positive and the E cell is equal to what is how do you define E cell in a few remember is equal to E cathode minus E anode. E cell equals to E cathode minus E anode if the corrosion to occur if the corrosion to occur this reaction has to be is it cathode or anode cathode this has to be anode. For the sake of argument for the argument sake you assume that this is the cathode and this is anode right. For the sake of argument assume this is the anode and this is the cathode. What does it mean that iron will not corrode in the sulphuric acid am I right or not? I assume that iron will not corrode in sulphuric acid what is wrong in myself I assume because I do not know. If I know I do not have to predict right I do not know. So, I assume that iron is not going to corrode in sulphuric acid then you make a calculation see what happens to the E cell. Ok. We do not talk about data G now here. Look at the E cell now what happens right what E cell now E cell in this case is equal to minus 0.44 minus of 0 0 is equal to minus 0 point. So, I assumed what is the assumption here? The assumption does not corrode assumption that iron does not corrode leads to E cell which is minus 0.44 which means I assumed it wrongly then I correct myself and I will say yes iron is anode is the cathode and it answers. What is the advantage of this? The advantage of that is that I do not have any assumption. I do not assume an oxidation potential. I do not assume to be reduction potentials. I do not assume anything we need to assume wrongly and try to calculate I can get corrected. So, you will never go wrong if you are going to understand what is equilibrium? What is equilibrium potentials? What is the cell voltage? And accordingly apply these concepts you can clearly predict that a metal can corrode or metal will not corrode ok. So, no assumption whatsoever is involved in all of these processes. That is why it is important that we be systematic ok in our own understanding of the electrochemical concept. I think we have come way you know close to the end of the session. I think we will continue this tomorrow. It is very important. What I want you to read understand during intervening period is please go through all this notes first of all understand what is electrochemical equilibrium. How do I define this electrochemical equilibrium using the Nernst equation? How do I calculate equilibrium potential using the Nernst equation? What is the relation between the the E cell and the corrosion? We did not cover much, but these are the basic things that will come to you again and again when I talk about oh will the metal corrode or not. If I know that zinc is corroding why should I calculate it right. People assume let us assume oxidation and so forth corroding states not what happens. So, no assumptions will be involved and you should go purely based on the the fundamental understanding of the electrochemical processes. I do hope that when you come next time you have read ones. We will have more discussion on this ok and thanks.