Free Chemistry NEET Video Lectures for Chemical Equilibrium

00:00:21 [Music] Hello students, my name is Shashank and uh I am associate professor at department of chemistry IIT Delhi. Today I'm going to talk about equilibrium. Generally this is a very important topic in a chemistry since suppose we are taking a reaction uh uh ammonia formation or PCL5 dissociation or any reaction which you can think of. We need to know that under what condition we can get maximum concentration of the product. So in that case we will apply concept of equilibrium. So in this chapter

00:01:14 we are going to discuss about first what is equilibrium when we can say that a reaction is in equilibrium. Then we will look for the reason why equilibrium exist. Then we can discuss about type of equilibrium. And then we will discuss about homogeneous and heterogeneous equilibrium. What do we mean by homogeneous or heterogeneous equilibrium? Then we'll discuss how to get equilibrium constant and what is the significance of this. So we'll calculate KC which is equilibrium constant using equilibrium

00:02:04 concentrations. Then we will also discuss the relationship between initial concentration and KC value types of equilibrium constant relationship between KC KP KX and reaction quotient. So we will calculate KC if initial concentration is given. We can also go in opposite direction. If KC is given, we can calculate what can be the concentration at equilibrium. What is the concentration at equilibrium? So let's first discuss about what is equilibrium. What do we mean by equilibrium? Equilibrium.

00:02:59 What do we mean by equilibrium? For that I will use this picture. Equilibrium is basically a state of balance between opposite forces or action. A state of balance. You must remember this is called a state of balance. And there are two opposing forces. So if we take a case of seessaw seesaw here you see on the both side there is a person sitting and so they are exerting force. So this person exerting force in this direction and this person exerting force in this direction they will get a balance only when the

00:04:05 force is equal here balance between two opposing forces and we can get equilibrium. Suppose we have a reaction. We can take any reaction. For example, A2 B. A to B. Now suppose I start with pure A. Suppose I start with pure A. So there are suppose six molecule of A. If I leave A alone in a container, what will happen? That A will go to B. A to BA is a spontaneous reaction. A is going to B. After some time what we will see is the number of mole of A decreasing and number of mole of B increasing. So

00:05:07 suppose we have four molecule of A left and two molecule of B is formed. Now suppose I look at the container after some time I see more changes in the number of molecules of A and number of molecules of B. So suppose now only three molecule of reactance is there and three molecules of B is formed. Now again wait for some hour and then look at the container. Now what I see is there is no more changes in A and no more changes in B. So just look at if suppose I give this a state one state two state three state

00:06:04 four. So we see that from one to two there is change in number of molecules of reactant and product. If I go from a state two to a state three again there is a change of your number of reactant and number of product. But after this we are saying there is no change. There is no change. We can wait for more hour. If we don't see any change that means equilibrium state is achieved. Equilibrium state is achieved. So three and four gives you equilibrium state. Equilibrium state. Now take the reverse reaction B2A. What will happen?

00:07:02 Again B2A is also spontaneous. So what does that mean? Delta G for this process will be less than zero. So suppose I start with B. So only B molecules are in container. and we leave it for some time. What I will see is B is going to A and suppose again two molecule of B goes to A. Now live for some more time. You will see that your more A is converting to B and we will go to the state where three molecules of B and three molecules of A is present and then we will see no change. We'll see no change. again three

00:08:09 molecules of A and three molecules of B as in case of your forward process. So in reverse process we start with six molecule of B and then we see that again we are going and getting three molecules of A and three molecules of B. And if we leave it for more time, there is no change in the number of molecules of A and B. And then this state is called equilibrium state. So suppose if I plot amount of A and B A and B with extent of reaction extent of reaction what you will see is there is a decrease of A that's

00:09:29 what we expect and there is a increase of B with time and finally there will be a plateau and concentration of A. So this is your A. This is your B. A decreases with time and there is a plateau. After that concentration of A doesn't change there increase of B and after some time there is no change in the concentration of B. So concentration of A and B does not change with change with time at this plateau. At this plateau and then we say that reaction has achieved equilibrium. On the other hand, if we start with B

00:10:54 and look at extent of reaction and amount of A and B then B will first decrease and then its concentration doesn't change after some time. A will increase and at this time the concentration of A will not change. So after this time when concentration of A and B are not changing not changing. We say that equilibrium equilibrium A state is achieved. Equilibrium has been established. Okay. So now think of another reaction. A + B giving you C. Again you will say same thing. Initially A and B will decrease. A and B will

00:12:08 decrease and C will increase. C will increase with time with time and after some time after some time your concentration of A and B A B and C will not change. it means equilibrium state is achieved. So when concentration of reactant or products are not changing with time then we say that equilibrium state is achieved. Similarly we can start from C. So we will again go to A + B. If we have a only C in the container, it will go to A + B till A, B and C doesn't change. When the change is zero and there is no change

00:13:15 happening, we say that equilibrium is achievable. So what is importance of equilibrium? That in the equilibrium all measurable property all measurable property properties of the system of the system remain constant. And what is the balancing force here? Balancing force is rate of forward reaction. When rate of forward reaction becomes rate of reverse reaction or backward reaction we say that equilibrium is HA equilibrium is HA equilibrium is achieved now let's talk about why equilibrium happens

00:14:48 reason for equilibrium. So suppose we are talking about this reaction A gas to B gas. From thermodynamics we know that this reaction is spontaneous spontaneous only when delta G is less than zero. What does that mean? GB is or less than GA. GB is less than GA. So when GB is less than GA reaction is spontaneous. Now question is if GB is less than GA why reaction does not go to completion. The second question is if GB is less than GA why reverse reaction happens if we start from start from pure D

00:16:26 for this let's again think about reaction A gas to B gas. So we started from pure A pure A and then we are going to pure B. we are going to pure B in between you have a states where there is a mixture of A and B. So sum of A exist as A and sum of A has gone to B. So this is your first state, second state and this is third state. We know that free energy of this state is GA. This is GB. This is per mole of uh per G free energy per mole of A. This is free energy per mole of B. And then you have

00:17:41 G of G of the state 2. Now how will we calculate G of estate two will be given by this. We can calculate by uh looking at this equation. So suppose a gas going to b gas we start with one mole this is zero mole and at time t this is extent of reaction is e and in that case lift a is 1 minus e mole and this is e mole per liter so g of state 2 will be equal to g of this state 2 will be equal to 1 minus E multiplied by G of A. Since there are 1 minus E mole of A in the mixture so we can just simply multiply GA by 1 minus E

00:18:51 and plus now in mixture we have E molecule of B. So we have E into G So g2 is equal to 1 - e into g a plus e into gv. So delta G for the process will be equal to delta G will be equal to 1 minus E G A plus E into GB and since we started with 1 mole of A so -1 into G A and so this is GA A minus E G A plus E GB B minus GA. This GA cancels out. So what we are left with is E GB minus GA. And we know that B is GB is less than GA. So this is always going to be negative. So what does that mean is if we plot G versus

00:20:20 G versus extent of reaction we'll get equation like this and that is quite clear from here that when we have E is equal to zero then you are left with only GA so E is equal to zero at this point then your G is equal to GA when E is equal to 1 this term goes to zero and we have G2 is equal to GB G2 is equal to GB so this is GA and GB and if I just write down this GA minus E GA plus E GB so you can see that your G2 2 is equal to simply g a plus e gb minus ga gb minus GA. So this is equation of a

00:21:26 straight line. If you plot G2 versus E, slope will be GB minus GA and intercept will be G. So that's what you are getting here. So slope is your GB minus G. Slope is your GB minus G. So what we expected since GB is equal to GB uh delta G is equal to negative at each point. So what we expect that reaction should go to completion. However, we know that reaction doesn't go to completion. So, what is the things which stops reactions to go to completion? So, let's again think about the process. We started with pure A

00:22:32 and V went to pure B. We went to pure B. Now think of what is happening in this process. You start with pure A and you end with pure B. In between there are states which have both A and B which has both A and B. If these are separate then the delta G which you calculated will be the way we got it. So delta G will be equal to E GV minus G. However, being in mixed state has contribution to entropy. Has contribution to entropy. So mixed state always have higher entropy compared to the pure state. So just if you look at the mixing

00:23:55 process and try to understand about entropy then in this first process where we are going from pure reactant to mixed state delta s will be greater than zero. However, when we go from this state, state two to state three, delta S is less than zero. This is just because of mixing. So I am talking about effect of mixing and this has contribution to delta G. And what we will write here is delta G mixture. Now we know that delta G mixture delta G is equal to delta H minus T delta S. If mixing is ideal then we take this

00:24:59 equal to zero. So basically delta G mixing is equal to minus T delta S mixing. And we know that for first step delta x mixing is greater than zero. And so this will be your delta g mixing will have your negative component. It will have negative component. But when you go from mixed state to pure b, delta s is less than zero. Delta S is a negative quantity and so this will have positive contribution. So your delta G is total delta G is we know that this is your this has negative contribution since GB is less

00:25:59 than GA. Delta G mix has both type of contribution. If you take process this, it has a negative contribution. If you take this stage, it has positive contribution. When you are working with this state delta positive, there will be a time when this factor outweighs the negative part and in that case delta G becomes positive. So if we plot G versus E again G versus E again if you don't consider mixing then you you should get graph like this but if we consider your if we consider mixing then there will be

00:27:10 minus T delta S initially It will make the process more spontaneous. But after a certain time when delta G is greater than zero you will get this kind of thing this kind of state. And so this is because of this change in entropy due to mixing that a reaction doesn't go to completion. A reaction doesn't go to completion. If there is no effect of there is no effect of mixing then we should expect a delta G should be less than always delta G should be less than zero but it is because of mixing you have two

00:28:09 different res delta G is less than zero delta G is less than zero and other res delta J is greater than zero and this is the point when delta G is equal to zero and this is your equilibrium point. This is your equilibrium point. This is your equilibrium point. And again we can answer the second question also that if we start from pure B why reaction happens although GB is greater than G. So if there is no mixing then reaction should not have happened but due to mixing entropy increases and that's gives you negative

00:29:07 contribution to G and so reaction takes place. So ent entropy of mixing entropy of mixing is responsible for reaction not going to completion. reactions are not going to complete. So we discussed first about equilibrium. What is equilibrium? Suppose we start with A and just leave it in a container for some time. There will be a time when A and B the concentration of A and B ratio of concentration of A and B will become con constant. A will not change. B will not change and during that state no physical

00:30:18 property will change. In that case you tell okay equilibrium is achieved. Reason for equilibrium is although G of B may be less than G of A, when B is getting formed, A and B will mix and there will be a time when this mixing will be will decrease delta G to such an extent that conversion further conversion of A to B is not possible. Similarly, if you start from B, B will get converted into A. Since B is getting mixed with A, entropy of mixing, entropy of mixing is increasing. Entropy of mixing is

00:31:18 increasing. So now we discussed about what is equilibrium and why that happens. Now let's go and discuss about types of equilibrium. Types of equilibrium. There are two types of equilibrium. One is your physical equilibrium and chemical equilibrium. Physical equilibrium and chemical equilibrium. So physical equilibrium is when equilibrium is established in a physical process. Physical process while chemical equilibrium is when equilibrium is established in a chemical reaction. Chemical reaction

00:32:22 So let's discuss first about what is a physical equilibrium. So let's see this is the condition which I am going to discuss is your your melting of ice or freezing of water. So freezing of water So this is the case when S2O liquid at 0° C and one atmosphere convert to solid 0° C at 1 atmosphere. Now reverse process is melting when S2O solid at 0°C and one atmosphere converts to S2O liquid at 0°C and one atmosphere. You can think about this case. Suppose you take a beaker. You have water and then you put ice. Yeah,

00:33:27 this is ice. Okay, this is water. And if we put at 0° C then either water will go to ice or ice will melt depending on concentration of water and ice. So at 0° C and one atmosphere your concentration of water divided by concentration of ice is going to be constant. Similarly we can start from a ice cube and just leave it at 0° C and one atmosphere. What you will see is that water ice is getting converted into water. So a small amount of ice will go to water. You can go in other way. Start with water put at 0° C one atmosphere.

00:34:27 you will see some freezing of water into ice. So melting and freezing of melting of ice or freezing of liquid water is a reversible process is a reversible process and equilibrium exist between your water ice and water liquid. Now next thing you can think of is boiling boiling of water molecule. Okay. So suppose I take water and put at 100° C what will happen that some molecule of water will get converted into water vapor. Not all molecule will go to your water vapor. Similarly, if I take water vapor and put at 100° C,

00:35:26 it will sum of the amount of water vapor will go to water into the liquid form. So an equilibrium is always established whether we start from liquid form or vapor form form. An equilibrium will established when there is more no more conversion between liquid and vapor. No more conversion between liquid and this is known as liquid vapor equilibrium. There are other cases of equilibrium. And for example, if you are looking at solubility of solute in solution. So suppose I take water and I want to

00:36:13 dissolve AgCl. You put some amount of AgCl. You mix it. You will see it will dissolve. But if you put extra amount then you will see it is not getting dissolved. only very small amount of AgCl goes into water. Okay. So these are the soluble form Ag+ plus CL minus. While you will see that lot of AgCl comes and settles down at the bottom at the bottom it means this is a reversible reaction and Ag+ plus CL minus is in equilibrium with AgCl solid form. AgCl solid form. Okay. AgCl solid form and so this is a different kind of

00:37:08 equilibria where we are looking at solubility of solute in solution. Now chemical equilibrium this is the till now we have discussed physical equilibrium now we can discuss chemical equilibrium. Chemical equilibrium. You can take any chemical reaction A + B going to C plus D. For example, formation of ammonia N2 + 3 S2 giving you 2 Ns3. You can look at dissociation of PCL5, PCL3 plus CL2. These are the cases of chemical equilibrium. So either we start with PCL5 or PCL3 or Cl2. There will be one point when reactant

00:38:12 will stop getting converted into product. So after a a time so 100% conversion is not possible. PCL5 doesn't goes completely into PCL3 and CL2 amount of PCL5 will change to PCL3 and Cl2. Now equilibrium can be of different types. The type of equilibrium will also depend on uh phases which kind of phases are there in the reaction. So there are two different kind of reactions. First is your homogeneous reaction. In homogeneous reaction all the components are in single phase. What I mean by singlephase is it is

00:39:28 either in solid form or liquid or gaseous form. All reactants and products. So all reactants and all component means all reactants and products and products. For example, if I take a reaction now you can see that A in gases form is going to B in gases form plus C in gases form. So all come all reactant and product A B C are in gasous phase. This kind of reaction is called homogeneous reaction. Now heterogeneous equilibria heterogeneous equilibrium. For example, if you take a solid for example calcium

00:40:34 carbonate, this is this exists in solid form and if we just dissociate it, it will give you calcium oxide solid plus CO2 gas. Now you can see that although calcium carbonate and calcium oxide are in solid phase, your CO2 is not in solid phase. So you have two phases in the reaction solid and gases phase and that kind of equilibria is known as heterogeneous equilibrium. heterogeneous equilibrium. Now what I told you about equilibrium is chemical equilibrium is the balancing force is rate of forward

00:41:47 reaction. And the other force is rate of reverse reaction. When they are equal then we say that equilibrium exist. So suppose I take a reaction A molecule of A reacting with B molecule of B to give you C molecule of C and D molecule of D. So let's calculate what is the rate of forward reaction RF. We know that RF is equal to KF which is the rate constant A power A B power B. Similarly, we can calculate rate of backward reaction and that is KB C power C and D power D. Now at equilibrium, at equilibrium

00:43:15 since RF is equal to R reverse or R backward. So we can simply write kf a power a b power b is equal to k b c c power c and d power d. In that case we can simply write KF by KB is equal to C power C D power D divided by A power A and B power B. And since these are constant KF and KB are constant you can write K K and this K is called equilibrium constant constant equilibrium constant. So you can see that if I use the condition RF is equal to RB then we get this formula where C power c D power D a

00:44:40 divided by a power a b power b will give you a constant and that constant is called equilibrium constant and since here we are taking as concentration this is also known as KC K. This is also known as KC. So suppose I take a reaction N_sub_2 + 3 S2 giving you NS3 2 NS3. Okay. So in that case K will be KC will be NS3 product NS3 is the product. So you see product squared since the strichometry is 2 divided by N_sub_2 this comes from here multiplied by S_sub_2 and since three molecules of S2 has been

00:45:48 used. So put three here. And this is how we calculate equilibrium constant. NS3² divided by N_sub_2 S23. So we can calculate KC using concentration concentration of NS3² into divided by N_sub_2 into S2 Q. You must keep this thing in mind that this concentration is the concentration of ammonia at equilibrium not at any time. This is the concentration of ammonia at equilibrium. Similarly, this is the concentration of nitrogen at equilibrium. This is the concentration of H2 at equilibrium. If you take some another point

00:46:51 where equilibrium is not achieved then the concentration ratio is not KC. In that case it is called Q which is reaction quotient. So this is different. This is not equal to KC. And when we write this, this is NS3 at any time t². This is not at equilibrium divided by n_sub_2 at any time t where equilibrium is not achieved divided by s_ub_2. But suppose this at this time equilibrium is achieved. It means the concentration ns3 at that particular time is basically equilibrium concentration of ammonia.

00:47:56 Similarly, N2 is equilibrium concentration of nitrogen and H2 at time t is equilibrium concentration of hydrogen gas. Then in that case Q becomes equal to KC. Now take uh this reaction again further. N2 + 3 S2 A 2 N S3 equilibrium can also be equilibrium can also be also be expressed in terms of partial pressure a partial pressure. Since all these three components N_sub_2 is in gas's form, S2 is in gas's form and N3 is in gases form. In that case, equilibrium can also be expressed in terms of partial pressure of various

00:49:15 gases. So for example in this reaction we can write K is equal to pressure of ammonia partial pressure of ammonia squared divided by partial pressure of N_sub_2 and partial pressure of S2 power Q and that K is called KP. P this K is called KP. So this is different from KC. In KC we use concentration whereas in KP we use partial pressure partial pressure. So we can write for example suppose we take PCL5 giving you PCL3 plus Cl2. Again these are in gasous form, gasous form, gasous form. So we can express this equilibrium in two

00:50:22 different terms. KC and KP. KC is basically PC3 into CL2 divided by PCL5. Whereas KP can be written as pressure of PCL3 into partial pressure of Cl2 divided by partial pressure of PCL5. Partial pressure of PCL5. Now once we know that what is KP KC we can also look at the relationship between this relationship what is the relationship between KP and KC so let's think of same reaction PCL5 giving you PCL PCL3 plus CL2 these all are in gases form and just I wrote down KP is equal equal to pressure

00:51:36 or KC is equal to concentration of PCL3 into CL2 by PCL5. Suppose these are gases which follow your ideal gas equation. In that case we can write simply P V is equal to NRT and P is equal to N / V RT and we know that N by V is C. So we can simply write C RT. Okay. So suppose I take KP KP is P PCL3 into P C2 divided by P PCL5. I can write this simply as P is equal to concentration of PCL3 concentration then RT. So multiply here by RT and then CL2 multiply this by RT. PCL5 divided by PCL5 multiply this by RT

00:52:54 1 RT RT cancels out. So we can simply write at KC into RT KC into RT RT power 1. So this is the way we can calculate the relationship between KP and KC. Let's take a general case. A A going to BB B uh sorry plus BB giving you C molecule of C plus T molecule of D. In this case your KC will be KP will be given as pressure of C power C pressure of D power D divided by pressure of A power A and pressure of P power B power B and we know that P is equal to C RT. So we can simply write C. This is the

00:54:02 concentration of C or let's write concentration of C into RT power C. concentration of D RT power D divided by concentration of A RT power A B RT power B. Now we can simply write C into D C power C into D power D divided by A power A B power B. Now what is left is RT and so we can just write RT C + D minus A minus B or we can simply write K C into RT delta N. So delta N is where delta N is N of product minus N of reactant. So for example in this case you can see that NP is C + D where is N R is A + B. So this is C + D

00:55:37 minus A minus B. So there is a relation between KP and KC and the relation is KP is equal to KC RT delta N RC delta N. For example, if I take 2 N 2 N2 gas to N204 N204 gas. Now what is the relationship between KP and KC? So KP will be KC RT delta N. You see N is N in product is 1 minus N in reactant is 2. So it is simply KC RT minus V. So this is the relationship between KP and KC for this reaction. for this reaction. Now suppose we take another reaction S2 plus I2 which are again gases form gives you 2 HI

00:56:55 gases form. Now in this case KP is equal to KC RT. Now you see product is 2 -1 -1 that means this is simply zero. So this is simply so for this reaction S2 + I2 going to 2 HI gas your KP is basically equal to KC. So till now we have discussed cases of uh homogeneous uh equilibrium. So whether we take PCL5 going to PCL3 plus CL2 N2 plus 3 H2 going to ammonia all components are in gases phase and that's why uh we can say that these reactions are homogeneous reactions and the equilibria established in these

00:57:56 reactions are homogeneous equilibrium. Now let's think about if suppose I have a heterogeneous system. What will happen in hetrogen? How will we express equilibrium constant in cases where system is not homogeneous? System is heterogeneous. There are more than one phases available. For example, calcium carbonate solid going to calcium oxide solid plus CO2 gas. CO2 gas. Now in this case we write KC as simply concentration of CO2. Since concentration of solids are taken as one, they are constant and taken as one. And

00:59:04 in that case, KC is equal to CO2 simply CO2 or KP. If I want to express KP, I will just neglect this solid terms. I will simply write P CO2. So KP is pressure of CO2. KP is pressure of CO2. We can again think of another equilibria like Agl going to Ag+ plus CL minus Ag+ plus CL minus. So this is in solid form and this is aquas Ag+ this is aquis CL minus. Now again Agl in solid form and so we express K C is equal to simply Ag+ plus sorry Ag+ into Cl minus. So this is your K and this is called solubility

01:00:01 product Ksp. This equilibrium constant is called Ksp and this is equal to Ag+ into Cl minus. So today I will stop here. In the next class we will do more questions and discuss what is the effect of temperature pressure on the equilibrium constant. Thank you. [Music] Oh.