 Ok guys welcome to all of you today we are going to study Leach-Atelier's principle right chemical equilibrium all other stuffs we have already gone through we have already said the only thing which is left is Leach-Atelier's principle ok. So guys this topic Leach-Atelier's principle is the most important topic of this important chapter that is chemical equilibrium ok. So you will get questions from this topic only this is the like I said it is the most important topic in any exam if you see or previous year if you see of any exams they have asked question on this topic Leach-Atelier's principle ok. So without any delay let's start with Leach-Atelier's principle ok. So first of all what is Leach-Atelier's principle ok what is Leach-Atelier's principle Leach-Atelier's principle it deals with the equilibrium state of any system ok since it is a chapter chemical equilibrium then again we will talk about equilibrium only right. So any system if it is an equilibrium right that means what suppose if I write down a reaction say A gives B this is the reaction right. So initially what happens at time t is equals to 0 we have only A present right suppose the initial concentration of A is A naught and there is no B so B is 0 right. So when the reaction is starts what happens is A starts converting into B right. So this is the forward reaction conversion of A into B and as soon as B starts forming this reaction for B also has tendency to convert back into A through this backward reaction ok. So hence it is a reversible reaction correct. So the rate of forward reaction as we know RF is the rate of forward reaction decreases with time and the rate of backward reaction that is conversion of B into A increases with time right. So we will have a time or after some time what happens both rate of forward reaction is equals to the rate of backward reaction and this time we call it as t equilibrium t equilibrium means what it is a time required for equilibrium to achieve right. So after t equilibrium the reaction maintains this relation RF is equals to RB that is rate of forward reaction is equals to rate of backward reaction and this state this condition is the condition of equilibrium right. Now the question is do we have this state always in this reaction right means if you do some changes into this after this state if you do some change over here what kind of change means suppose reaction condition you are changing you are increasing pressure you are decreasing pressure you are increasing temperature decreasing temperature or adding some you know reactant molecule or removing some product molecule means whatever change we will do here at this state right then what happens there is disturbance in the equilibrium right equilibrium will get disturbed suppose you if you increase the pressure because this equilibrium is defined at a given condition right we can say RF is equals to RB at this pressure and this temperature with this number of moles obviously we have constant number of moles because the system is the reaction is taking place in a closed system right. So what we are going to discuss here that when you change the reaction condition right if you change the reaction condition do we have the same equilibrium state or no that is the question right obviously if you change the pressure change the temperature the equilibrium state will be changed right the equilibrium will get disturbed into this reaction but since the equilibrium the reaction always try to maintains its equilibrium so whatever change you make right whatever change you make the system or the reaction will change itself in such a way so that the equilibrium state will maintain again okay like if I tell you the property or equilibrium is what A and B will have constant concentration when the equilibrium is achieved the concentration of A and B won't change it will be constant under a given condition right when the equilibrium is achieved it does not mean that the reaction is the reaction stops over there reaction does not stop reaction is going in forward and backward direction right but the rate of forward reaction is equals to the rate of backward reaction right and that is why the concentration of A and B won't change once the equilibrium is achieved right so so that is the you know the whole thing we have for at equilibrium now like I said if you change the equilibrium state then the reaction shift in such a direction in such a way the shifting of reaction will be in such a way so that the effect of change is minimized okay so what is the statement of Lee-Charles principle this kind of change if you do at equilibrium of any reaction this kind of discussion will discuss or do under Lee-Charles principle correct so what is the statement of Lee-Charles principle whenever a system is in equilibrium state system is in equilibrium state and we try to change the equilibrium or disturb the equilibrium state of a reaction then the reaction will shift in a direction so that the effect of change is minimized okay so for example if I take the simplest one here if you increase the concentration of A at equilibrium so this reaction is taking place and we have equilibrium is achieved now now we have a system in which this A is equals to B the reaction we have and the equilibrium is achieved now what we do we add some more amount of A into it right so when you add some more amount of A into this reaction vessel what happens the concentration of A is increasing right and hence the equilibrium disturbs right so concentration of A is increasing equilibrium gets disturbed so reaction has tendency to maintain its equilibrium state according to Lee-Charles principle hence the reaction direction or the reaction will shift in one particular direction since we are adding A so reaction will shift in such a direction or in a direction so that the concentration of A will decrease and hence the reaction will go in forward direction correct to maintain the equilibrium state again so like this I have just taken an example okay if you add B then it will go into backward direction if you remove B again forward direction if you add A backward direction if you remove A again backward direction okay so anything if you do then the reaction also will change in thus in that manner only so that the equilibrium will get established again right so all these factors will study under Lee-Charles principle effect of pressure effect of temperature effect of inert gas effect of catalyst there are many things that we study under this particular topic okay so the first you know the first condition we are going to discuss here that is the effect of change in concentration okay this one we have already discussed okay so first factor here we have effect of effect of concentration so what happens here the first case if the concentration of reactant increases and we are talking about this particular thing at equilibrium when the concentration of reactant increases then we will have backward shift so that so that the concentration of reactant decreases and hence the equilibrium state will maintain again right so reactant concentration is increasing so reaction will shift in such a direction so that the reactant concentration decreases right so reactant concentration my mistake just you do this change reaction concentration we are increasing so it will be in forward direction okay it will be in forward direction backward decreases and forward reaction proceeds in forward direction correct reactant concentration increases so on react so the equilibrium will shift in forward direction and the amount of product increases okay if the reactant concentration decreases then it will be what backward shift the product converts into reactant to maintain the equilibrium if if the concentration of product increases then what happens you are adding being to this product then again we have backward shift okay if the concentration of reactant sorry concentration of product decreases then we will have forward shift this is of four condition or four different effects we have of concentration concentration of reactant product okay second one what is the second factor we have effect of temperature effect of temperature effect of temperature okay this effect of temperature we define for exothermic exothermic and endothermic reaction exothermic or endothermic reaction okay so exothermic reaction you know energy releases and delta H is less than 0 endothermic reaction energy gets consumed delta H greater than 0 okay so to understand the effect of temperature in these two kind of reaction we should know the relation of equilibrium constant and temperature okay so the relation is log of K2 by K1 is equals to minus of delta H divided by 2.303 into R into 1 by T2 minus 1 by T1 this is the relation we have okay K2 and K1 is the equilibrium constant and we know equilibrium constant depends only upon temperature okay this equation we call it as van hoff equation van hoff equation okay now one thing I would like to add over here which is not related to this particular topic but you you must have seen this kind of you know equation in chemical kinetics also okay chemical kinetics and there you have EA written over here instead of delta H it is EA so once you see this equation and if EA is written over here then this K1 K2 is not equilibrium constant but it is rate constant okay so if it is EA then rate constant if it is del H then equilibrium constant this you must remember so here we are talking about equilibrium constant not rate constant we know as equilibrium constant increases reaction has tendency to go into forward direction more value of K more forward direction will be okay now what happens and one more thing you see here this K2 is the equilibrium constant at temperature T2 and K1 is the equilibrium constant at temperature T1 okay so if the reaction is exothermic first of all we will discuss for exothermic reaction and effect of temperature we have we are looking at so what happens if temperature increases or decreases okay as the first case here as temperature increases temperature increases so what we can write exothermic means means delta H is less than 0 and as temperature increases it means T2 is it means T2 is greater than T1 which further means 1 by T2 minus 1 by T1 is less than 0 this is the relation we have okay and delta H is less than 0 we already know for exothermic reaction so what we see here delta H for this condition exothermic it is negative 1 by T2 minus 1 by T1 is already 1 by T2 minus 1 by T1 is negative again why because the temperature is increasing okay so what happens here log of K2 by K1 is negative negative negative negative positive positive negative negative it means what we can write in this that the log of K2 by K1 is less than 0 which means K2 is less than K2 K2 is less than K1 okay which means K2 is less than K1 so what does it mean that in exothermic reaction in exothermic reaction as temperature increases equilibrium constant decreases and when equilibrium constant decreases will have backward shift right so exothermic reaction temperature increases backward shift we can also write the exact opposite of it exothermic reaction temperature decreases forward shift okay similarly we can do this for endothermic reaction okay so I have explained this here that what happens in exothermic reaction you can try this at home that what happens of the effect of temperature when the reaction is endothermic so I will summarize this here for exothermic reaction what we can write exothermic reaction the first case as temperature increases equilibrium constant decreases K2 less than K1 equilibrium constant decreases means it is backward shift backward shift okay in this only we can write as temperature this is A and this is B as temperature decreases K2 increases just opposite of it and when equilibrium constant increases it is forward shift okay these two things we have discussed here okay so what I want you to do is for endothermic reaction I will write down the final result here I want you to try this on your own like we have discussed this one okay so what is the result here endothermic reaction A as temperature increases then K also increases K increases means forward shift so here as temperature increases backward so here it will be forward as temperature decreases K also decreases and this is backward shift backward shift okay so I want you to try this thing this endothermic reaction on your own delta H greater than 0 and delta H less than 0 okay I want you to try this similarly based on this pent up equation you can try