Hi. It's Mr.Andersen and this is chemistry essentials video 70. It's on solubility. You've
probably noticed that if we add salt to water it will dissolve. But if we add more salt
and more salt and more salt eventually it doesn't dissolve anymore. And so what's going
on? Well if we look down at the atomic level we're adding a salt grain. And that's made
up of all of these ionic bonds, sodium and chloride. And they're going to break apart
into their aqueous ions. They're going to be ushered away by the water. That's going
to increase the concentration of those ions in solution. And so if we increase, increase,
increase the amount of ions, eventually no more ions can be added. And so that's going
to push it back to this more solid form of salt. And so if we look at a solution, remember,
we're dissolving a solute in a solvent. And so you can think of that solute as a solid.
If it's all chemically bonded together and if we can break it apart into its ions then
it's dissolved in solution. And so if we think of on the left side of our equation we have
the solid form and on the right side we have it broken apart into its ions. And so really
is a solution we have a reversible reaction. It's going to have its own equilibrium constant.
And we'll call that Ksp which is essentially the solubility product constant. And that's
a way to measure the solubility of any kind of a salt. And so there are a few things that
are going to affect the solubility. One of the big ones is going to be delta G. In other
words it's going to be the enthalpy. It's going to be the entropy of the system. Let's
say that they're really intense bonds between these in the solid form, it's going to be
hard for those to break apart into their ions. Likewise as we change what is in the solvent,
if we add more ions to it, it's harder for more ions to be added. We could also change
the pH. And that's going to affect what can be dissolved as well. Now there are a few
things that dissolve readily in water that you should simply know. Those are going to
be the ammonium ions, potassium ions, sodium ions. And then also the nitrate anion as well.
And so if we look at sodium chloride breaking down in water as a reversible reaction, on
the left side we're going to have the solid form. On the right side we're going to have
the two ions that are in aqueous environment. Remember we're breaking those apart. A wonderful
simulation to look at this is the PHET on salts and solubility. And so I'll put a link
down below. But basically I'm going to add a little bit of salt and we're going to look
at this at the atomic level. And so what's going to happen is the water is ushering those
ions away. And so we're going to break it apart into its cation and its anion. Let's
say I radically increase the number of ions, what's going to happen? Well you can see that
most of them are going to be in solution. But some are going to go back to that solid
form. There's simply too many ions out there. And so we've pushed it more towards the right
side of this reversible reaction. Now let's look at a different salt. So now we've got
silver bromide. If I were to add 200 of the cations to this we're going to see a different
kind of an effect. What's happening is that they're all going back to that solid form.
So if we were to look at this equation up here, it's shifting it back more towards the
left. And so how do we measure, does the reaction go to the right or does it go to the left?
Remember that's going to be our equilibrium constant. And so if we were to look at the
equilibrium constant of sodium chloride being dissolved in water we could set it up like
this, where it's going to be the concentration remember of the two products over the concentration
of that reactant. Now we can get rid of that reactant since it's a solid. So our Ksp value
is going to be 37.3. What does that mean? Since it's much greater than 1 that's going
to shift it way towards the right. We're going to have way more of those ions. If we compare
that to the silver bromide, what's our Ksp value here? It's going to be much less than
1. What does that mean? Our equation is going to be shifted way towards the left. And so
we're going to have more of that solid form. And so the neat thing about this simulation
is you can play around with the Ksp values. Now I've got a theoretical salt with cations
and anions and I'm simply going to lower the Ksp value. You can see as I lower it it's
starting to move back more towards the left. It's moving back more towards that solid form.
So if you have a really low Ksp value it's going to be shifted towards the left. If I
radically increase Ksp and it goes way back more towards the right or in aqueous solution.
And so that's what Ksp affects. But you should be thinking why does it affect that? What
affects Ksp? Well it's mostly going to be delta G. So changes in enthalpy and changes
in entropy. And so if we were to look at the solid form, remember there are going to be
ionic bonds between all of these, and the greater those bonds are it's harder to break
them apart and then move it away into solution. And so also they're going to be interactions.
Once it does get broken away it's going to interact with a solvent itself. And let's
say there are already a lot of ions in solution, it's harder to be added. And we could also
change, for example, the pH of that solution, increase or decrease the number of protons,
and that's also going to change how it interacts with the solvent. Now we basically just talked
about enthalpy. Remember entropy is going to be an important part of this as well. And
that's why it's better to look at the whole delta G. And so what are some applications
of this? Well let's say we have sea water and fresh water. And we're going to try to
dissolve salt in each of those. It's going to be different. Because in sea water we're
going to have all of these ions already present. Sodium, chloride, sulfate, magnesium. All
of these are already there. And so it's hard for us to add more of those salts into solution.
What's another application of this? Acid mine drainage. So how did this get so nasty? Well
there was probably a coal mine at some point that had a lot of sulfur in it, which combined
with water and made sulfuric acid. And as it drained through this area that decreased
the pH and so there is all of a sudden a lot of ions in solution. Mostly iron hydroxide.
What happens as it now settled out is we're adding more fresh water to it. It's increasing
the pH at this point. We can't hold those in solution and now all of these ions, all
this nasty iron is coming out of solution. Or we could look at acid rain. What's happening
there is we're decreasing the pH and that changes the amount of carbonate that we can
actually dissolve. And so did you learn to predict the solubility of a salt or rank its
solubility given the Ksp values? Remember the lower it is the more likely it will be
in the solid form or shifted more towards the left. The greater it is the more it's
shifting toward the right. Also could you interpret data based on a Ksp value? How is
that going to behave? And then finally, can you look at what's going on inside the solid
and inside the ions in relation to the solvent to predict our Ksp values? I hope so. And
I hope that was helpful.