Surface Chemistry part 4

Added: 2026-05-18

[00:00:01]now we will continue with the surface chemistry in the surface chemistry the next part is your adsorption isotherm absorption isotherm so what is adoption isotherm absorption isotherm also called as as what you can say describes the also called as adsorption isotherm same thing so ah describes the equilibrium of the softgen of a material at a surface more general at a surface boundary at constant temperature you can see it is there on the screen at a constant temperature it represents the amount of material bound at the surface as a function of the material present in the gas phase and or the solution okay now what what do you mean by this the meaning is that the extent of adsorption on a given surface generally it increases with certain factors increases or decreases with certain factors listen carefully the extent of adsorption on a given surface generally increase or decrease certain factors now what happens is here that suppose if you increase the pressure of gases or the concentration of solution at a constant temperature two different things one is for the pressure of gas another is for the concentration of solution it is two different things all right at constant temperature now what you find is at low temperature the physical absorption of a gas increases rapidly as the pressure rises why low temperature because in adsorption process is exothermic in nature see if we increase the temperature the reaction instead of going in the forward direction will become reversible it will go in the backward direction that means adsorption is going to decrease so if you want to increase the physical absorption of a gas then the temperature has to be kept low remember this always because the absorption process already we have discussed it is exothermic in nature so at low temperature the physical adsorption will increase when the pressure increases got it this is you have to remember it when the temperature is high just now i said you that the pressure when the even if you increase the pressure the absorption process is going to decrease it will be relatively less so to say all right so the surface area per gram of the adsorbent means the adsorbent is a surface where it is absorbed now it is it is in the form of powder or it is in the form of porous substance we can say that porous you know holes are there all right or in the powdery substance is called its specific surface area that is on a specific surface area the surface area per gram of the adsorbent per grams simply if i say surface area then it is on a surface area only but when i say per gram got it then it is on a specific surface because i am specifying it per gram got it it is uh expressed in square meters per gram also yeah that's that's also can be uh taken off now highly active solids with large surface area its acts as an adsorbent we can say that it acts as an absorbent so this absorption data got it we are representing it by some regression analysis the most frequently one is your linear isotherm the one which you are going to study is your frenulic isotherm or langmuir isotherm and the bt model so we don't have to study the language and the bt model now we are going to study only the friend lich isotham we are going to study about it all right so the next is your what is adoption isotherm we will come to the definition now we have understood what is adsorption isotherm that is nothing but uh the extent of adsorption on a given surface it depends upon certain factors one is pressure another is your ah pressure of the gas and there is your concentration of a solution at a constant temperature now when a graph is between the amount of the gas at zord per gram just now i said you that is specific absorption specific surface area a graph which is between the amount of the gas absorbed per gram of the adsorbent which is x by m and the equilibrium pressure of the adsorbed at constant temperature is called the absorption isotherm that means when you are drawing a graph between x by m and pressure now see the graph that is this particular graph see the extent of adsorption x by m and pressure is in the x axis when you are drawing a graph between x by m and pressure then that particular isotherm is known as absorption isothermals remember isotherm isothermal temperature remains constant here the temperature has to be constant so if the temperature has to be constant then it will depend upon pressure suppose when i say absorption isobar then what it will be pressure remains constant when i say adsorption isobar let us say i am saying absorption isobar then what do you mean by that so understand it the things it is a graph between the amount of gas absorbed per gram of the adsorbent x by m and the equilibrium temperature and what you can say off that is nothing but equilibrium pressure at a that is um at a constant pressure we can say that equilibrium temperature of adsorbent at a constant pressure is called the absorption iso bar so instead of temperature you will be using the pressure there in case of isobar so that you understand it first so here i am drawing a graph between extent of adsorption on the surface all right that is x by m per gram we can say that in respect to pressure now what do you see in this particular graph see what you see in this graph the graph which you are seeing in front of you on the screen what do you see here you can see that is one ps is written there what is that it is equilibrium pressure that is known as x by m already said you now it is nothing but the amount of ah substance or the gas which is absorbed per gram of the absorbent that i have already said now see what happens at value ps of equilibrium pressure x by m reaches its maximum value can you see there that is by m is maximum there the graph is increasing increasing increasing so maximum it reaches at that particular point after that you can see the graph becomes almost a straight line can you see that is ps when the xbm reaches the maximum value so hence that particular pressure is known as saturation pressure that means you are increasing the pressure what what i said just now when you increase the pressure keeping the temperature constant the absorption rate increases but it will increase to a certain extent no it is not going to infinitely it is going to increase it will increase to a certain extent so that particular point is known as in terms of pressure is known as equilibrium pressure or saturation pressure or saturated state whatever you can say it is represented by ps where the x by m value is maximum that is the amount of gas absorbed per gram of absorbent is maximum we can say that clear now next is based upon this based upon this particular graph we will now try to draw a relationship between them you can see the three areas i have marked it here one is x prime is equal to k p one is x prime is equal to k p one by n another is x by m is equal to k p to the power zero so what you find is that here it depends upon the pressure and the friendly absorption isotherm you can see the pressure is changing now what do you find the first one that is the first one x y m is equal to k p that i am discussing with you now see when the pressure is low when x y is kp the pressure you can see it's very low when the pressure is low the graph is nearly a straight line the graph is nearly straight line we can say that clear clear so in this way now this is represented by the following equation x by m is actually proportional to p because if it is a straight line means x prime is greatly proportional to pressure all right so hence to remove the proportional sign we are taking a constant this is nothing but known as k so x y is equal to k p done fine now at high pressure at high pressure when the pressure increases increase increases when the pressure is high that is saturated ps when the pressure is high high pressure then what you find is that listen carefully x by m becomes independent of the value how you say it is independent because if you increase the pressure the graph is not increasing the graph is a straight line can you see ps got it ps you have drawn we will i have drawn a straight line there after that you can see the graph becomes a straight line got it that means if you increase the pressure further beyond ps if you go then x by m value is not going to increase that means here the pressure becomes independent because by increasing the pressure listen carefully by increasing the pressure x y m value is not increasing it is a straight line it is independent because that clear so hence here p to the power 0 in the first case the dotted line you can see here the graph is almost a straight line that means pressure is increasing x y is increasing so hence it is directly proportional in after ps if you see the graph is a straight line here now this straight line this straight line difference is there here it remains constant x by value remains constant that means if we increase the pressure x value does not increase that means pressure is independent here so p to the power 0 got it now in the intermediate range all right between the dotted line you can see a straight line is there there's an intermediate range all right between ah pressure is equal to 1 and pressure is equal to 0 between 1 and 0 what do you find an intermediate range all right so there is a small change in pressure if it is taking place then it is a fractional that is a p is raised to between 1 and 0 the value p remains because in the first case it is one in the second case in the last case it is zero so in between that one and zero the value of p will remain so hence it is one by n for that particular purpose is it clear that is x by m in between the straight line which you can see which is not a dotted line the value will remain between 1 and 0. so 1 and 0 means it will be fractional since it is said to be as 1 by m so here that is x by m is equal to k p 1 by n we can say that where n is a positive integer got it k is a constant of course you know that clear so in this way this is adsorption isotherm we came to know that is when the initially when the pressure is low the extent of absorption is higher so it is directly proportional after that when the pressure increases increase increases at a saturated pressure even if you increase the pressure further then the x band value does not increase that means the extent of adsorption does not take place it is becoming independent so in between what you find is that between 1 and 0 the value is fractional which value the p value is fractional temperature is constant here remember it is an isotherm temperature remains constant so hence what we get it an equation ultimately x by m is equal to k p to the power 1 by and we got it clear so this is known as friendly absorption isothermal now this particular one we can show it that is graphically we can show it this particular in the form of log we can show this particular value yeah it is here in the form of log you can see this i have shown it here that if you take log both sides then what you will be getting log x by m is equal to log k plus 1 by n log p got it what is the equation you got x prime is equal to k p to the power 1 by n take log both sides log x by m is equal to log k plus 1 by n log p so that i have represented in a graph again all right i have i have taken log x prime on the y axis and log p on the x axis i have taken it you can see the intercept is log k and the slope will be 1 by n because you just equate it now y is equal to m x plus c when you equate it to y is equal to m x plus c then y is your log x by m and c is your log p so m will be what 1 by n all right sorry x is your log p sorry i am very sorry y is equal to mx plus c y is your log x by m x is your log b and c is your log k as you can see intercept y intercept that is log k then what is the slope so slope is equal to y is equal to m x plus you see if you do then m is equal to 1 by n you got it so in this way we can convert the friendly ah that equation into the logarithmic form in this way we can convert it done we can convert it in the logarithmic form we can do it all right now now this particular one if you see now previous equation which you have seen it in the previous equation the i have given you a relationship between pressure between pressure pressure and x by m there was a relationship x prime is the amount of gas absorbed got it per gram isn't it the surface area the specific surface area we have taken it the amount of gas that is absorbed per gram of the absorbent but it is that is nothing but x binding was taken there got it clear so there is amount of gas absorbed here instead of that the gas what i am saying instead of gas i am taking here concentration so when will take concentration x by m k p to the power 1 by n instead of that what you will have x by m is equal to k c to the power 1 by m c x by m is equal to k c to the power 1 1 you will say that from that q e is coming here q e is taken to be as x by m instead of x by m you here it is taken to be as q e because there x by 1 m was the amount of gas absorbed per gram of the what you can say absorbent here q e is nothing but the amount that is nothing but amount of solution absorbed per gram of the adsorbent you can say that the amount of the solution absorbed per gram of the adsorbent which is represented by q you can also put x by m no problem but you can also take a different unit also symbol also you can take it that is q e so what is a formula now instead of x prime is equal to k into what you can say k into p to the power 1 by m instead of that what we will say that is nothing but q e is equal to or x prime is equal to k into c to the power 1 by m or as you can see in this particular on the screen q e instead of x prime i have taken it as q e is equal to k kf again is a constant proportionality constant into c to the power 1 by n we can say that c to the power 1 by n and here e what you are seeing it what does it e stands for equilibrium that is the equilibrium when the amount of substance which is absorbed okay there we are saying equilibrium pressure so it is nothing but when it attains equilibrium when the system attains equilibrium isn't it what was the absorption isotherm definition a graph between the amount of gas exhaust per gram of the adsorbent and the equilibrium pressure so here instead of that you will be saying the equilibrium that is nothing but concentration understood so in this way this is another case of friendly absorption isotherm when instead of gas a solution was being taken you can see it is written there for the special case of heterogeneous surface energies particularly good for mixed weights in which the energy term kf varies at a function of the surface so same thing it is the meaning is the same here n and kf are what that is nothing but n is the positive integer and k is a constant instead of k it is written as kf and e stands for equilibrium we can say that clear so this is a graph we have understood and this again when you convert that q e is equal to k f into c to the power 1 by n when you convert it into logarithmic form it comes to be this that is log q e is equal to log k f plus 1 by n log c understood it is converted the same equation q e is equal to k f into c to the power 1 by m that equation again comes to be as log q e is equal to log k f plus 1 by n log c when you convert in the logarithmic form and same graph you can draw it here instead of log x by m what do you write it at in the y axis log q e instead of log p in the x axis what you write it as log c because concentration so when you compare this equation with y is equal to mx plus c what you will get log kf as the y-intercept that is your c and 1 by n is the slope slope as 1 by n when you equate it y is equal to y is equal to m x plus c then you will get this particular value now what are the factors which affects the absorption from solution nature of the absorbent at the adsorbent definitely it depends upon the nature of the adsorbent and the adsorbent means what now solid surface can also absorb solute from the solutions we know that solid surface it can absorb solute from the solutions so suppose example you have taken a litmus solution and it is taken with charcoal listen carefully you have taken a litmus solution that is absorbent you have shaken with charcoal adsorbed it becomes colorless litmus solution suppose if it is a blue litmus it becomes colorless or red litmus it becomes colorless because the dye of the litmus solution is as absorbed by the charcoal the dye the color of the litmus solution is absorbed by the charcoal we can say that similarly when a solution of acetic acid is shaken with charcoal some amount of acetic acid from the solution is absorbed on the charcoal absorbed and hence the concentration of acid acid in the solution will decrease because if acid will be absorbed then this concentration is going to decrease all right so in this way it depends upon the nature of the adsorbent and the absorbance the how good the adsorbent and the adsorbent is suppose instead of charcoal if i use another and definitely the absorption power is going to decrease another means if you take silica gel or if you take let us say that is starch or something like that then definitely is going to decrease it is going to change the absorption decreases with increase of temperature that already we have discussed because adsorption process is exothermic so if temperature will increase absorption is going to decrease greater the surface area of the adsorbent greater is the adsorption definitely because if the surface area is more the absorption power will be more because adsorption depends upon the surface area we have just learned learned its specific surface area means per gram got it so a graph between the amount of the gas at per gram of the absorbent experiment temperature t at a constant equilibrium pressure of the absorbed gas is known as adsorption isobar now this is adsorption isotherm i was talking i've already told you earlier also suppose if it is absorption isobar then what should be the definition i said you at that particular time that instead of pressure will be using the word your temperature now we are having two absorption isobar sorry it is physical absorption or so bar it has become it is not also bar it is isobar instead of o it will be i that's a spelling mistake you can correct it so physical and chemical adsorption isobar which is known as fizzy option and chemise option these two graph represents physio option and chemist option so what you find is that here when the graph is drawn against temperature in both the graph you can see pressure remains constant it is drawn against temperature because there is an iso bar remember it so in case of physio option the value decreases when temperature is increasing the x by m value decreases just now i have told you why because the reaction is exothermic instead of the reaction if you increase the temperature then what will happen is that the reaction favor in backward direction so hence x by m that is the amount of gas absorbed per gram of the absorbent is going to decrease with the increase in temperature you can see that is called physio option the first graph in the second graph what you see first it increases and second it decreases can you see there x by m first it increases in chemical adsorption first increases the graph and see the graph reaches the peak then after that it decreases why why it is so because in case of chemistry initially an increase with temperature increases the activation energy all right increase here when the body cancer temperature increases you reach to the that is you provide the activation energy so hence there is the graph shows an upward trend but after that when the activation energy is being used up when you reach to the activation level will you reach the threshold level if you increase the temperature further then the amount of adsorption is going to decrease all right again i repeat here in case of chemistry isobar you can see first it increases initially it shows increase with temperature and then it decreases why the initial increase is because of the fact that the heat which you are supplying it it is used for the activation energy to reach to the threshold level after it reaches thruster level it is uh what you can say the reaction it's reaching to the peak all right then after that the reaction does not carry out in the forward direction the reaction it carries out in the backward direction now if you increase the temperature for that purpose first the graph increases then the graph decreases so initially white increases because the heat is required which acts as an activation energy to in to reach to the thirst level once it reaches threshold level even if still you provide you know the reaction is exothermic even if still then you provide the reaction will proceed in the backward direction since it shows in the backward curve we can say that the downward curve rather not backward downward curve you see it here so this is your graph absorption isobar for physio option or physical adsorption and chemistry or chemical adsorption okay so this is all about