 In this video, we are going to talk about the strength of a base. So by now you know what a base is, right? A base is simply anything that can pick up H plus ions. Now what exactly is this H plus? Well, an H plus is simply a hydrogen atom and we know that hydrogen has one proton and one electron in it. And now if I remove this electron, I'll be left with my H plus ion, right? So an H plus is nothing but a proton. So an H plus is simply a proton. Now protons are very, very reactive. In fact, protons are so reactive that they actually do not freely exist in nature. You don't find free protons. So protons do not actually exist independently in nature, independently. So a base never picks up a free proton. A more realistic scenario is actually of a base picking up a proton from an acid. We know that acids are H plus donors, right? So acids have what we call an acidic hydrogen that they can give up in the form of H plus. So a base never reacts with a free proton but it instead abstracts a proton from an acid. Now it turns out that this ability of a base to abstract a proton from an acid is actually different for different bases. For example, some bases like the amide ion, amide is NH2 minus as well as the hydroxide ion. It turns out that bases like the amide and the hydroxide, such bases can easily abstract a proton from an acid. So these are what we call strong bases. So these are our strong bases. While on the other hand things like ammonia or water or even something like say aniline, it turns out that these bases are actually not that good at abstracting a proton from an acid. So these are relatively poor or relatively weak bases. So the strength of a base is nothing but the ability of a base to pick up a proton and protons are nothing but H plus. So the strength of a base is the ability of a base to pick up an H plus from an acid. Now you might be asking how do we know that these bases the amide and the hydroxide are better at abstracting a proton compared to ammonia, water or aniline. In other words, how exactly do we measure the strength of different bases? So how do we measure the strength of a particular base? Let's see. Now how easily a base abstracts a proton from an acid also depends upon the strength of the acid. For example, HI out here is in fact a strong acid. The hydrogen iodine bond is relatively weak and easy to break. So a base can easily abstract a proton from HI compared to something like say HF. HF is in fact a weak acid. The hydrogen fluorine bond is actually very strong and difficult to break. So even if I have a very strong base, it will still be difficult to abstract this hydrogen in the form of H plus from HF. So if you want to compare the relative strength of different bases, we first need to keep the acid from which the base is abstracting the proton to be constant, right? Now because chemical reactions are generally carried out in water, so the relative strength of different bases is actually calculated with respect to water. So what we do is we take some water and to this water we add different bases and the base that can most easily abstract the proton from water will be our strongest base, right? Now how exactly do we measure this? How do we measure how easily the base abstracts the proton from water? Now when we add a base to water, the base can abstract this hydrogen in the form of H plus to form BH and OH minus. Since the hydrogen is being lost in the form of H plus, so both the electrons that are present out here will be our oxygen atom. So we are going to get OH minus. Now if you look carefully, you realize that this OH minus also has a lone pair of electrons. So even this OH minus can act as a base and it can in fact abstract this hydrogen of BH and in this way we will get back our original reactants. So whenever we add a base to water, there is actually a reversible reaction that is going on. Now if this reaction is more in the forward direction, then we can say that the base is actually good at picking up the proton from water. So the strength of the base will be higher. However, if this reaction is more in the backward direction, that is if it doesn't move much in the forward direction, then this means that the base is not so good at picking up the proton from water. So it's a relatively poor or a relatively weak base, right? Now the extent of any reversible reaction can actually be calculated by calculating the equilibrium constant K of the reaction. Now we know that for a reversible reaction, the equilibrium constant K is actually the concentration of the products by the concentration of the reactants. So for this reaction, the equilibrium constant K will be the concentration of BH multiplied by the concentration of OH minus divided by the concentration of B minus into concentration of water at equilibrium. Now for a dilute solution, the concentration of water is almost constant and we actually assume it to be equal to 1. So the equilibrium constant for this reaction is generally written in this way without including water and we call it the base dissociation constant Kb. Now we can actually carry out experiments with different bases and calculate their Kb values at equilibrium. Now if this reaction is more in the forward direction, so if this reaction moves forward, then in that case the concentration of the products that we get will be high while the amount of reactant that will be left will be low, right? So in this case the value of Kb that we get will come out to be high. Now on the other hand if this reaction doesn't move much forward, so if this reaction doesn't move much forward, so if it's in the backward direction, then in that case the amount of products that we get will be low while the concentration of the reactants that will be left will be high, right? So in this scenario the value of Kb that we get will turn out to be low. So we can now go ahead and say that a base whose Kb value is high is a stronger base as it moves more in the forward direction. So if the Kb value is high, the strength of the base will be high while a base whose Kb value turns out to be low doesn't move much in the forward direction. So its strength will be low and it is going to be a relatively poor base, right? Let us now take a look at the Kb values of a few bases. So we have our hydroxide ion out here that has a Kb of 1. Now this is a really nice number, right? So what happens is that if I add hydroxide to water and if this hydroxide abstracts the proton from water then what will I get? Well I will get water and I will be left with another hydroxide ion, right? So now as you can see in this reversible reaction both forward as well as the backward reaction are identical to each other, right? So it's not like one of the reaction is going to be favored over the other, both of them are equally likely to happen. So the concentration of the products and the concentration of the reactants will always be equal, right? So the ratio of the concentration of the products by the concentration of the reactants which is nothing but the equilibrium constant K will come out to be equal to 1. Let's take a look at some other base. So we have Amide out here and Amide has a Kb value of 1 into 10 to the power 21. So the Kb of Amide is clearly much much greater compared to the hydroxide ion. So this means that this reaction, the reaction of Amide in water to give ammonia and hydroxide since this Kb is much much greater than 1. So this means that in this case the forward reaction is much more favored compared to the backward reaction, right? In fact this Kb value is so high that if we do our calculations we will find out that for all practical purposes this reaction is almost 100% in the forward direction. Let's now take a look at some of the bases on this side of the table. So we have Amide out here and Amide has a Kb of 1.8 into 10 to the power minus of 5, okay? I can see water, water has a Kb of 1 into 10 to the power minus of 14. Similarly C6H5NH2, C6H5NH2 is basically aniline, aniline has a Kb of 7.4 into 10 to the power minus of 10. Now which of these three do you think is the strongest base? Well ammonia has a Kb of almost 10 to the power minus of 5, right? While aniline is at almost 10 to the power minus of 10 while water is at 10 to the power minus of 14. So clearly ammonia has the highest Kb of these three. So ammonia clearly is the strongest base. However if you check the value of Kb of ammonia you can see that it's 10 to the power minus of 5. So this is clearly less than 1. So this means that if we add ammonia in water, so if we add ammonia in water because the Kb is less than 1. So this means that in this case the reaction is more backward than forward, right? So therefore bases that have a Kb value much, much greater than 1 like these bases. These are what we call the strong bases. While bases whose Kb values are much less than 1 these are more in the backward direction. So these are relatively weak bases.