So in order to express the ionic strength on the concentrations of species, we calculate its activity with the use of an activity coefficient. Now the activity coefficient uses the units of gamma. Remember, your ionic strength is calculated as follows. Here we have Ac=[C×γC]. A here represents the activity of the compound. C represents the concentration. This gamma represents our activity coefficient. The activity coefficient is a way of testing to see if our solution behaves ideally or not. We're going to say here, if our solution is behaving ideally. For example, if we have let's say a+bB→c, if our solution is behaving ideally, that would mean that our activity coefficient equals 1. So when I calculate the equilibrium expression for this, it would become C. And remember, whatever the coefficient is becomes a power, C,a⋅aa⋅bB. That's if our solution is behaving ideally. Under ideal conditions, it just means that all the species dissolved within my solution all behave in the same way and have the same level of effect. It ignores differences in size and charge. But in actuality, there are differently sized ions, different compounds within a solution. Each of them exerts a level of influence. The activity coefficient is just used to see, do we have an ideal solution where all the ions are treated the same, or do we have a nonideal solution where they are not treated the same?
Now, we're going to say the activity coefficient and ionic strength can be more closely connected and related to one another with the extended Debye-Hückel equation. Now here, the equation is logγ=−0.51⋅z2⋅I2÷1+α⋅I⋅305. Here we're going to talk about the effect of ionic strength, ionic charge, and ionic size on the activity coefficient. What we're going to say here is, as your ionic strength increases, that's going to cause your activity coefficient to decrease. There's an inverse relationship between the two. The greater your ionic strength, the lower your activity coefficient. As the activity coefficient approaches 1, which we call unity, then the ionic strength will approach 0. That's because, remember, when your activity coefficient is approaching 1, that means we're acting ideally. And that would mean that all the ions are treated the same. Ionic strength is just looking at all the ions within a solution. And if the ionic strength equals 0, that means that there is no way of differentiating the ions from one another. They all have the same level of influence. When your ionic strength is different from 0, that means that we have to take into account the different concentrations of each ion. We have to take into account the charges involved, which is why we've been using that formula from earlier. As the size of an ionic charge increases, the more the activity coefficient moves away from unity. The less likely it is going to be equal to 1. That's because a larger charge has a greater impact and causes the solution to deviate from an ideal presence. Then finally, the smaller the ionic size, or alpha, the greater the effects of the activity coefficient. That makes sense because the smaller your ionic size gets, the less of an impact and influence they can have in terms of differentiating themselves within a given solution. So just realize that this is really just theoretical in terms of how ions interact with each other. In reality, there are no ideal solutions. Ions are different shapes, different charges. Therefore, they have different influences on one another within a solution. The activity coefficient is just a way of talking about this deviation from the ideal solution where everything is treated the same, even though everything is not the same.
Now that we've talked about activity coefficients, let's look at example 1 in the next video where we just simply write out the solubility product expression for the following compound. So guys, this takes into account what we've learned thus far in terms of writing down the activity expression for a compound and relating Ksp to that concept.