 Welcome to this next segment where we are going to discuss about the other periodic property known as hard soft acid base rule. So, when we discuss about acid and base, we can define an acid or a base in different ways. One of the way is to define with respect to the bronze state theory which says if a system can donate a proton that is going to known as bronze state acid and if a system can accept a proton it is known as bronze state base and we have discussed about this acid base system of bronze state theory in the acid base equilibria with respect to the thermodynamics. Now, we take another look how alternatively we can discuss the acid base properties and another way to define acid base is with respect to the electron pair donation which is much more generalized. So, if there is a system which can accept an electron pair that is going to known as the Lewis acid generally metal ions and then the entity which is supplying this electron pair is going to known as the Lewis base generally ligand donors. So, this is the definition with respect to the Lewis acid base theory which says a system that can accept an electron pair is an acid a system which can donate an electron pair that is the base. Now, when we talk about Lewis acid and Lewis base city we need to know what is the strength of this acid base city and when they are interacting between themselves a set of Lewis acid or Lewis base how they interact and whom they actually prefer. So, for an example if you have a Lewis acid a metal ion and I give them a large number of choices of bases which of them you would like to prefer and can we connect that with respect to a little bit of idea about thermodynamics that is where this discussion about this hard base acid base theory come into the picture. Scientist R. G. Pearson actually coined these terms hard acid hard base systems and soft acid soft base system and he was actually looking at the time on the stability of different salts prepared by the different combinations of acid and bases and he defined hard acid hard base soft acid and soft base as following. So, accordingly hard acid he says it is a small cation which has a very high charge density generally combined with high positive charge and they are generally weakly polarizable. So, if we connect the dots from the polarizability that we have covered in a different segment if you have very small electron density which is actually present around the system is a very high charge that means your charge density is actually very high then it will be very less it will be very difficult to polarize it and this less polarizable system is known as a hard acid. Similarly hard base is a anionic counterpart of it if there is an anion small anion small in size which contains some charge such a way that the charge density is pretty high and it is also less polarizable and around those kind of systems are generally tied to come together and form a very stable salt. So, this hard acid and hard base would like to interact with each other why they interact with each other very easily because of their non polarizable system they actually try to keep their charge very distinctly and those to charge system between the hard acid or a cation and hard base or anion come together in the form of a Coulombic or ionic interaction and that is the reason hard acid hard base system are very stable. Another system comes in the name of the soft acid and soft base what is a soft acid generally a cation but with a very high surface area where we have a very low charge density and generally the overall positive charge is actually quite low so that the charge density is like the charge divided by the area area is very high however the charge is pretty low and they are polarizable systems. So, again connected to the polarizability we have now two sets of ions which are very highly polarizable if it is a cation we call them the soft acid in the case of soft base that is very similar to the soft acid but instead of a cation now it is an anion it is going to be a large anion very low charge density comes up with some negative charge which is very low in magnitude and this is also very polarizable system. So, soft acid and soft base also come together and interact with between very strongly why because the polarizable character of the electron density means the electron density can adapt and form new bonds. So, this soft acid and soft base can form covalent interaction which also becomes very stable. So, that is why either hard acid hard base through columbic or ionic interaction or salt bridges or soft acid or soft base through covalent interaction gives us very stable salt formation. So, that is why if we give different system and ask them to react the hard acid will like to find out the hard base and soft acid will like to find out the soft base and form the salts and drive the reaction on to that particular direction. Now, we will follow some question to understand how this soft acid hard acid soft base hard base system works. So, over here we have taken example of four anions fluoride chloride bromide iodide and try to find out the nature of the soft base which is the most stronger soft base and which is the hardest base. So, soft base city as we discussed it is directly connected to the polarizability more the polarizable the system is soft at the system is and we have discussed earlier in the polarizability section as we go down to this period to this particular group over there as we go down this particular group we found fluoride chloride bromide iodide has the same charge, but the volume increases as you go from fluoride to iodide and with that the polarizability also increases. So, iodide is the most polarizable system then bromide then chloride then fluoride and similarly the soft base city also for the same trend that means iodide will be the softest base then the bromide then the chloride then the fluoride. So, fluoride will be the least soft or we can say in the other way the hardest base in this series where iodide is the softest base. So, if we want to look into soft nature fluoride to chloride to bromide to iodide it soft nature increases from fluoride to iodide and hard nature increases as we move from iodide to fluoride. Now, instead of anion we take a range of metal ions cobalt plus 3 nickel plus 2 copper plus 1 silver plus 1. So, the first 3 cobalt plus 3 nickel plus 2 and copper plus 1 are in the same period over here there is the first source anion elements and silver ion is actually in the same group of the copper. So, let us take a look into how we can monitor that. So, as we know the soft nature of acid or base directly coordinates with the polarizability. So, this correlation we can expand in this particular way. So, polarizability we know is connected with the charge density. So, it depends on the amount of charge and what is overall volume. So, now among all these ions cobalt plus 3 has the highest charge then the nickel plus 2 then copper plus 1 then it has the same charge as silver plus 1. So, charge wise we can say cobalt plus 3 has the highest charge then nickel plus 2 and then copper plus 1 and silver plus 1 has the same charge. So, how the charge density differ? So, now cobalt nickel and copper belongs to the same period. So, they have the same outer sphere of electrons a 3D electrons. Over here the number of electrons are actually differ it is actually a different system it is a D6, it is a D8 and it is a D10 system. So, as the number of D electrons slowly increase it increases the overall volume of the system and this higher charge also contracts the D electrons. So, the D electron density among these first potential element is actually such that it is the smallest volume in the terms of cobalt plus 3 then it is larger in the nickel plus 2 and the largest among the first potential element is the copper plus 1. So, we have not only have the higher charge but also the lower volume. So, obviously cobalt plus 3 has going to have the highest charge density then the nickel plus 2 then the copper plus 1. So, that will be the trend of the polarizability order cobalt plus 3 will be the least polarizable then nickel plus 2 copper plus will be the most polarizable. Now, we have to distinguish between copper plus 1 and silver plus 1. Copper plus 1 and silver plus 1 have the same charge and same outer sphere electron density D10 system but over here copper in the first row say it has the 3D 10 electron and silver plus in the next way. So, it is the 4D 10 electrons. So, over here 4D electrons are actually present outside. So, it is much more larger in size compared to copper plus 1. So, that is why silver plus 1 the volume is much larger than the copper plus 1. So, the charge density is much lower in silver. So, now if we combine all these things together this will be the polarizability order cobalt plus 3 less than nickel plus 2 less than copper plus 1 less than silver plus 1 and because it is a D10 system the charge density is way too low compared to this highly charged ions. So, there is a huge gap over there and this particular trend is going to follow even for the acidity. So, silver plus 1 will be the softest acid then the copper plus 1 which will also show a very good amount of soft nature whereas this charge system with very high charge density it will be more on the harder side. So, there will be more hard acid. So, their soft nature will be pretty weak. Now, coming back to another question when we discussed the solidability of this particular salts sodium fluoride, sodium chloride, sodium bromide and sodium iodide. What is the common factor? Sodium is same in all of them and we are adding the anions fluoride to chloride to bromide and iodide. So, when you talk about the solubility we are trying to find out we take this salt in a solid form put that in water and then water they actually break the bond present in the salt between the cation and anion and the water dissolve them separately. Now, stronger is the bond between this salt harder will be to break this bond and lower will be the solubility. If a salt is easily breakable it will dissolve in their particular cation and anion very easily in the water. So, that will going to have a higher solubility whereas if a salt is very strong in their bond it will take very high amount of energy and in the water they may not even break down. So, that will have very low solubility. So, to understand that we have to now find out which of this bond sodium fluoride, sodium chloride, sodium bromide and sodium iodide is the strongest. Sodium is common the variation is in the anion. Now, sodium itself present on the over here which has relatively high charge density. So, we can say sodium ion is a very less polarizable ion. So, we can say it is generally a hard ion. So, it is a hard acid. So, it will prefer a hard base. So, among the fluoride chloride bromide iodide we have already discussed what is the combination? Now, fluoride has the highest charge density because it is the lowest volume over there we can follow that from the periodic table. Then the chloride then bromide then iodide, iodide has the highest volume. So, the least amount of charge density highly polarizable. So, it is a soft ion soft base. So, the hardest is the fluoride softest is the iodide. So, generally what will happen the sodium will form which is a hard acid the strongest bond with the fluoride because it is a hard hard interaction. Then it will having interaction with the sodium chloride which will be less than the fluoride, but higher than the bromides or iodides. Then comes the bromide which will be less than chloride or fluoride, but higher than the iodide and sodium iodide will be the complete mismatch because the hard acid and soft base. So, when we discuss about the basicity of this particular anions fluoride is the hardest. So, it will prefer to bond with the hard acid sodium. So, this sodium fluoride bond will be the strongest. Sodium fluoride bond will be weaker than sodium fluoride because chloride is a little bit less hard compared to the fluoride that will be strong compared to bromide and iodide. Sodium bromide will be weaker than fluorides and the fluorides, but stronger than the iodide and sodium iodide will be a strongly mismatch because it is a interaction between hard acid and soft base. So, now, sodium fluoride the most strongest bond it will be the least soluble because water cannot break it very well because the bond strength is so well that it cannot break it to sodium ion and fluoride ion. Whereas sodium iodide very weak bond. So, it breaks down very easily. So, that will be having the highest solubility. And a very similar question than the last one, but over here we change the ion from sodium ion to copper ion copper plus 1 ion. And as you have discussed earlier copper plus 1 ion is actually a soft ion because it is a detain system and it will prefer a soft base for generating the salt. And as you have just discussed in the earlier answer also we go from fluoride to iodide in this particular group fluoride is the hardest because it has the highest charge density lowest volume and you go to the iodide it has the least charge density but the highest volume. So, most polarizable and the soft state ion. So, over there copper because it is a soft acid it will prefer the soft base. So, the strongest bond it will form with the soft base that is the copper iodide. Whereas the weakest bond it will form with the fluoride because it is a complete mismatch it is a hard ion hard base and a soft acid. So, copper iodide will have the strongest bond and the least solubility where the copper fluoride is the least stable and it will be the highest solubility. So, again solubility is connected directly with the bond strength where we find the highest bond strength it will be the least soluble while find the least bond strength it will be the highly soluble and the bond strength over here between the copper and the halides goes in the trend of copper iodide is the strongest soft nature copper bromide is still stopped but not as soft as the iodide. Copper chloride now we start moving to the hard system. So, it is a mismatch it will be much more weaker copper fluoride is the most weak or the weakest link over here because it is the hardest base present in this system. Now, the last question of this segment we are looking into this particular reaction over here there is a phosphine bromide bond and there is a amine borane bond. So, it is actually so over here we are looking into a reaction there is a phosphine and borane bond where the borane content is bromide there is amine and borane bond the borane content fluoride and then there is exchange they are exchanging the borane counterpart. So, the phosphine is now connected to the borane content fluoride and the amine is containing the borane content the bromide. We want to find out where this equilibrium lies this reaction goes to the forward side or it is happy to stay back in the left hand side. Can we understand this fate of the reaction with respect to the hard soft acid base system? To understand that we have to look into this reaction and find out is an acid based reaction there is a phosphine present over here and which is actually reacting with our base in the form of the borane. So, over there is an amine system present with two electron pair and there is a borane present like this which has a vacant p orbital this is an empty orbital and this nitrogen amine transfer the electron to the borane. So, from the Lewis acid base theory this borane is accepting the electron pair. So, it is a Lewis acid and this amine is supplying the electron pair. So, it is a Lewis base and the same thing can happen not only on the nitrogen, but also with the phosphine. So, either nitrogen the amine or the phosphine are actually the base and the borane system is actually the acid. Now, it is much easier we have to just find out what is the soft and which is the hard acid based system. Now, amine is a hard base compared to phosphine why because both amine and phosphine actually has a lone pair, but this lone pair stays in 3 p orbital or nitrogen on 4 p orbital in phosphorus. This is much more diffused higher volume and the same two electrons is spreaded over a larger volume larger space. So, this is going to be more polarizable. So, that is why phosphine is actually a softer base compared to amine. So, amine is hard phosphine is soft. Now, with respect to the borane the borane seems same, but there is a difference which atoms are bound to it. We can have either fluoride bound to it or we can have either bromide bound to it. So, here is the lone pair of the bromine present over there which has the capacity to get electrons over there. Now, what happens the fluoride has some lone pairs of orbitals present over there which can interact with this orbital and present some electron density over there. Same thing happens with respect to the bromide also. Now, bromide when it is interacting with this borane it is actually interacting its 4 p orbitals whereas fluoride is using the 2 p orbitals. So, during through this interaction this is actually going to spread the overall electron density out. But with respect to 4 p when it is interacting it is going to show movement over a larger space because 4 p orbitals are much more larger. So, that means when the bromides are interacting with this borane center it is going to show a soft nature with respect to the acidity because the orbital will be much more polarizable through this interaction in the bromide. Whereas in the interaction in the fluoride because it is in using small and less polarizable 2 p orbitals that is also going to reflected on the boron center. So, the boron center acidity will be strong acidity when it is connected to the fluoride. So, that is why BF3 is less polarizable or strong acid compared to BBR3. So, now we know BF3 is the strong acid BBR3 is the soft acid. So, now we have to just combine them together. So, a hard acid hard base system will be preferred. So, amine to BF3 will be preferred and a soft acid soft base will be preferred phosphine to the boron connected to the bromine and these two systems will be more stable. This is through the ionics interaction this is through the covenant interaction and this particular combination is present you can see on the left hand side of the reaction. On the right hand side it is a phosphine and BF3 soft base strong acid amine to the BBR3 strong base soft acid. So, they are not preferable this side is preferable. So, the equilibrium for this side will lie on the left hand side. So, over here now we would like to conclude this particular section of hard soft acid base theory. Hard soft acid base theory is directly connected to the polarizability. A hard acid or a hard base is so systems those are less polarizable generally small in size and which has high electron charge density. On the other hand, soft acid and soft base systems are such which are actually larger in size and whose electron density is much more polarizable which has very low charge density. Hard acid and hard base actually prefer each other through the ionic interaction. Soft acid and soft base prefer each other because of their covalent interaction and this hard acid hard base interaction is very important when you talk about the formation of stronger bonds like in salt into their solubility and also even equilibration when we actually put an acid with a combination of base or vice versa. So, all this fate of the reaction will be controlled by the hard acid hard base and soft acid soft base nature of the systems. So, over here we conclude the hard acid hard base system discussion. Thank you.