Leah here from leah4sci.com/MCAT, and in this video we'll look at the Hydrolysis of Salt
Solutions particularly the solutions of weak acids and weak bases.
To understand how it dissolve salt will impact the pH, you must remember what happens when
an acid or a base dissociates in solution.
If we have a strong acid like HCl, it'll dissociate nearly 100% to give you H+ and Cl- ions in
solution.
A strong acid wants to dissociate, that means it'll form a weak conjugate base that does
not want to grab that H+ and reform the acid.
So if you have a Cl- in solution or another conjugate of a strong acid it wants to just
remain in solution and not react with anything.
But if you have weak acid for example acetic acid written as HCH3CO2, this will dissociate
in equilibrium to give us H+ and acetate which is CH3CO2-.
But acetate is not happy to just remain in solution as spectator, some of the acetate
will grab that H+ and reform acetic acid that's why we have the two way arrows to show that's
an equilibrium.
So now what happens if you dissolve a salt of the conjugate in solution?
If I think of salt with the cation that is unreactive with NA+ and I dissolve NACl in
solution.
This will break up to give me NA+ and Cl-.
This ions are spectators, they're not reactive, they won't do anything to the pH but if I
take the conjugate of a weak like acetate and dissolve that as a salt, for example sodium
acetate, this dissolves to give me NA+ and acetate CH3CO2- . But just like the acetate
from the acetic acid dissolving is not happy to stay a hundred dissolved in solution.
The acetate from sodium acetate is also going to react.
But if it can't with H+ that came off of the acetic acid it's going to react with water
pulling the H+ off that water molecule.
If we write out the reaction, we'll see that we have CH3CO2- the starting base in solution
reacting with H2O to give me an equilibrium that has acetic acid formed from the hydrogen
of water and acetate to starting base and then OH-.
If we now analyze what we have in solution, we have some acetic acid which will be in
equilibrium between acetic acid and acetate but we also have Hydroxide and hydroxide is
a base which means the pH of the solution will be greater than seven.
In fact, if you want to calculate this you would have to write out a ka expression and
potentially set up a whole ICE Chart.
We'd use the initial molarity of the sodium acetate as the starting concentration, water
is a liquid so we don't include it in a kaq expression and we'll assume zero and zero
for acetic acid and hydroxide.
Acetate reacting with water will be X, so we shall minus X for the acetate disappearing
plus X for the acetic acid and hydroxide for an equilibrium concentration of the molarity
that we started with minus X and then X and X.
We're not gonna go nuts solving this because remember the shortcut I showed you, in this
case it's a base so we'll write this as kb for equilibrium expression is equal to X squared
representing acetic acid and hydroxide divided by molarity initial because we're estimating
that this is negligible and therefore drops out.
But you'll notice that when we solve for X and X is equal to the OH- concentration we
have a basic solution.
We'll get a pOH we can turn that into a pH but we have a basic solution.
We can say the same thing for strong and weak bases.
For example if I have a strong base like Barium Hydroxide and I dissolve that in solution,
I'll get BA2+ and 2OH- that's how we know it's a strong base.
If I take a weak base like ammonia and react it or dissolve it in water, then I will get
at equilibrium NH4+ as the conjugate acid, OH- as the conjugate base or ammonium hydroxide
if you want to put it together as one molecule.
But now the question is, what if I start with just the conjugate Barium and ammonium and
dissolve them in salt version, what happens to the pH?
Let's use an anion but know that it's not reactive like Nitrate.
How do we know Nitrate is not reactive?
If HNO3 is a strong acid and dissociates to give H+ and NO3-, we know that NO3- is a very
very weak base because it's the conjugate of a strong acid.
So let's see what happens if we take Barium Nitrate BA(NO3)2 and dissolve that in solution.
We'll get BA2+ and 2NO3-.
Well they're weak or we're not gonna do anything to the solution.
But what happens if I take the conjugate acid of my weak base ammonium and dissolve that
as ammonium nitrate in solution.
Ammonium Nitrate will break down to give me ammonium and nitrate.
But because ammonium is the conjugate acid of a weak base, it pass acidic properties.
In fact ammonium will react in water or think of it as reacting with water to give NH3 plus
Hydronium which is H3O+ where the third Hydrogen came from ammonium.
We have 4 hydrogen here and 2 for a total of 6.
We have 3 and 3 once again for a total of 6.
Hydronium is an acidic species.
If you take up the water and just write ammonium as breaking up you'll show it is NH4+ goes
to NH3 plus H+ but H+ and H3O+ are interchangeable that's why I'm showing it as Hydronium and
this is an acidic species which means you'll have an acidic solution or pH that's less
than 7.
But now what happens if I give you a salt like KBr?
We know that it breaks up to give K+ and Br- but is this considered acidic or basic?
And how do you know?
The key here is to ask yourself if each ion dissolved and reacted with water, what would
happen and is it strong enough to do that?
In the beginning you have to go through this mentally and actually work through the reaction
but after a while you'll get pretty confident so that you can simply look at it and understand.
If K+ reacted with water we would get kOH plus H+ that's because the water broke apart
to give me an OH- and H+.
The problem is that KOH is a strong base and the reaction will actually favor the reactants,
the K+ and H2O which means that K+ is neutral in solution because it's not strong enough
to obstruct hydroxide from the water molecule.
Now let's look at the reaction for Br-.
If we dissolve Br- in solution, it's negative so it'll grab an H+ giving me HBr plus H-
meaning it would be basic but once again HBr is something you should recognize as a strong
acid not something that goes at equilibrium.
Once again we favor the reverse reaction making Br- inactive in solution.
So if we have a salt that consist of unreactive cation the positive ion, an unreactive anion
the negative ion then we're going to get a neutral solution because it does not react.
So how do you know which ions to look for?
Go back to the video on strong acids and strong bases and look at how the ions are setup.
Once you understand the acids and bases just look at their conjugate and recognize that
those are not going to be reactive.
You can find all videos along with the practice quiz and cheat sheet by visiting my website
leah4sci.com/MCATAcidBase.
When the salt in question has an anion that is the conjugate of a strong acid it won't
impact the solution because strong acids give weak unreactive conjugate bases.
Those are negatives so they're anions.
But when the cation is the conjugate of a weak base we get something positive that is
reactive in solution and that will resolve in an acidic pH.
One more time let's look at the example of ammonium nitrate.
Ammonium Nitrate breaks up to get NH4 plus NO3-.
NO3- of the conjugate of Nitric Acid which is a strong acid giving me a weak unreactive
conjugate base.
But ammonium is the conjugate of Ammonia, a weak base.
An ammonium will break up further in solution to give me H+ or Hydronium making it an acidic
salt.
Ammonium Nitrate and similar salt are considered acidic salts.
Basic solutions are the exact opposite.
If the cation in the basic solution is the conjugate of a strong base, that means it's
going to give you a weak conjugate acid or a weak positive ion for the cation that will
not be reactive in solution.
And if the anion is the conjugate of a weak acid, well then the conjugate of a weak acid
is reactive in solution and it will be basic in solution.
For example, if we have a salt like KCN that will break to give us K+ and CN-.
K+ would react in solution to give KOH, but KOH is too strong that means K+ is weak not
reactive, that's our cation conjugate of a strong base.
But CN- is the conjugate of a weak acid.
HCN will break up in solution and add equilibrium give us H+ and CN-.
Equilibrium tells us it's not strong enough to completely dissociate and instead goes
back and forth.
So if we take this CN- in solution, it will react with water and at equilibrium will give
us the weak acid HCN plus the leftover OH- hydroxide will give us a POH or simply a pH
that is greater than 7, that is a basic solution.
Now what happens when both the cation and anion are the conjugates of weak acids and
bases?
For example Ammonium cyanide will break up in solution to give NH4+ and CN-.
If we look at each one, ammonium is the conjugate of a weak base ammonia that would give me
an acidic solution.
But cyanide is the conjugate of a weak acid, cyanic acid or hydrogen cyanide which means
we would expect to have a basic solution.
So which one wins?
Now if it comes to Math, I don't think this would show up on the MCAT because the calculations
would be due involve.
But if you're given values of ka and kb, you can compare them to determine which one is
going to win.
In this case which one is stronger and therefore wins the acidity or basicity of the solution.
Since ammonium is an acid, we would have to look at the ka and remember that ka is proportional
to acid strength.
The higher the ka, the stronger the acid and therefore if we have a high ka, we're going
to have a strong acid.
Cyanide CN- is a base, that means we have to look at the kB remembering that kB is proportional
to OH- concentration telling me that the higher the kb, the stronger the base.
If you're given these two values, simply compare ka to kb.
If the ka is greater than the kb, it tells you that the ka is more powerful at directing
this solution and therefore it's going to be acidic.
But if the kb is greater than the ka, that means the base is stronger, the base wins
out and it'll be a basic solution.
Again how to calculate the actual pH or pOH requires too much math and I doubt it'll show
up on the MCAT, but you should understand the concept in case you're asked to analyze
a question without numbers but simply ask for concept.
This concludes the MCAT Acid Base series, remember you can catch the entire series along
with the practice quiz and cheat sheet by visiting my website leah4ci.com/MCATAcidBase.
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