Leah here from leah4sci.com and in this video we're going to look at some practice questions
for acid base equilibrium.
In previous videos we looked at how to rank acids and bases not using pka values but instead
using the mnemonic kario which as a reminder stands for C is Charge, A is atom, R is Resonance,
I is Induction or the Inductive effect and O is orbital hybridization.
For in-depth CARIO tutorials make sure you watch the series first before trying these
practice problems and you can find them on my website along with the acid base practice
quiz and cheat sheet by going to leah4sci.com/acidbase.
Assuming you've mastered it, let's apply this to questions where you're not told specifically
here are two molecules which are stronger but instead you're provided with the reaction
and equilibrium and ask to determine if the reaction favors the right or the left, meaning
does it favor products or reactants.
Say you're faced with a problem like this and you're asked to determine if equilibrium
will lie to the right or the left, meaning does equilibrium favor the products or reactants.
Remember that in an equilibrium question or any sort of reaction, you always want to think
back to the mnemonic of Happy, Stable, Unreactive.
That means for a molecule that is relatively stable it's feeling relatively happy and doesn't
want to react.
But if the molecule is unstable, it's very unhappy and wants to react.
When trying to determine the direction of the equilibrium we wanna see if the molecules
on the right or left are more stable and choose the more stable side.
Since this is an acid base question we're going to use the mnemonic CARIO to help us
break it down.
But first let's pair up what's reacting to identify the acids and bases so we have what
to compare.
NH4Cl goes to NH3, KOH goes to H2O.
But wait a minute,where does this come from?
Anytime you see a molecule where Cl is just sitting at the end rather than attached with
the bond, recognize that it's a negative spectator ion, it doesn't do anything, it dissolved
in solution but it's simply there to balance the charge of in this case ammonium plus 1.
NaCl is simply that same salt just the two spectator ions floating around the solution.
Same thing with the group 1 metal in the beginning of the molecule such as K in front of OH,
that's a K+ positive spectator ion to balance the negative charge NOH- which is simply OH
minus in solution.
The next thing we wanna do is identify the acids and bases using the trick I teach in
the first video of the series with acid is the most positive species and the base is
the more negative.
If we're looking at ammonium and ammonia, ammonium with the charge of plus one is more
positive making this the acid.
Ammonia with the charge of zero is more negative than positive one making it a conjugate base.
Hydroxide is negative 1 and H2O is neutral.
Negative 1 is more negative than zero making hydroxide the base.
Zero is more positive than negative one making water the conjugate acid.
Now that we know what's reacting, let's use CARIO starting with C which is Charge.
The net charge on either side of the reaction is zero, so what do we do?
We're not looking for the net, we're looking for the individual molecules.
If we compare ammonium to ammonia, ammonium is positive, it's carrying a positive burden
of charge.
Ammonia is neutral, it's more happy.
This tells me equilibrium is probably gonna favor the right where I have the neutral ammonia
but if you're not sure yet, let's look at hydroxide and water.
Hydroxide is negative one, it's carrying a negative burden of charge and water is zero.
Once again, more happy, more stable than it's negative counterpart.
Justifying again that equilibrium in this case will favor the right.
Ready to try another one?
Pause the video, see what you come up with then let's break it down.
We're asked to find if equilibrium is going to favor the right or the left in this acid
base reaction so let's use the mnemonic CARIO but first identify what's going on.
But first identify the species.
NaOCH3 goes to CH3OH, NH3 goes to NaNH2.
Let's get rid of our spectators, positive Na is simply there to balance the OCH3 minus.
Positive Na is simply here to balance the NH2 minus.
What are our acids and bases?
CH3O minus is negative which is more negative than it's neutral CH3OH counterpart making
sodium epoxide and methanol the conjugate acid.
Ammonia is neutral which is more positive than its NH2 minus counterpart making ammonia
the acid and amide the conjugate base.
Let's go to CARIO starting with Charge.
In this case we have a negative base on either side and we have a neutral acid on either
side.
So we can’t use CARIO for the comparison.
Given that each side has a negative and a neutral, we need to compare the base to the
base or the acid to the acid to see which is more stable.
I personally like to compare the charge molecules because then we can ask the question which
one is more capable of holding that charge.
We're comparing OCH3 minus to NH2 minus having already determined that they're both negative,
charge doesn't help us.
So let's move on to A which is atom specifically the atom holding the charge.
In methoxide we have a negative oxygen, in Amide we have a negative nitrogen.
We have different atoms but what does this tell us?
Don't forget the ten atoms on the periodic table that I asked you to memorize and that
is HCNOF, Phosphorus, Sulfur, Chlorine, Bromine, and Iodine.
It helps us in this case because if we remember the trend for electronegativity which is up
and towards the right, we can see that oxygen is towards the right of nitrogen making oxygen
more electronegative and therefore more capable of holding the charge.
If Oxygen is more capable of holding the charge it is happier to hold the charge and therefore
it is more stable holding the charge giving us a reaction where equilibrium favors the
side that has a negative oxygen rather than the less stable more unhappy negative nitrogen.
Here's a problem.
For this problem, once again see if you can figure out if equilibrium will favor the right
or the left.
Pause the video and give it a try.
What do we have?
We're going to use the mnemonic CARIO, but first let's pair things up.
Even if the molecules all look kind of the same we have one ring with bonds in it and
one ring with no bonds in it so let's pair those up.
Aromatic ring, and the non-aromatic ring.
Next, let's find the acids and bases, the aromatic ring meaning the one that looks like
benzene with nitrogen on it is neutral on the left and negative on the right.
Neutral is more positive then minus one making this the acid and here we have the conjugate
base.
The non-aromatic ring meaning no pi bonds in the ring has a negative on the left and
neutral on the right.
The more negative is going to be the base and the less negative or more positive will
be our conjugate acid.
Once again we have a situation where there is a negative in the reactant side and the
product side so let's see if we can compare either both acids or both bases.
I'd like to compare both bases.
Which one is going to be more stable using the CARIO mnemonic.
We will start with C. Both nitrogens are attached to a six membered ring, have 1 hydrogen atom
and a charge of negative 1.
Can't use charge if they are the same.
Next we'll look at the atoms, but we already said, it's Nitrogen vs Nitrogen, can't use
the atom.
Next, we'll use resonance and this is where we want to see if the negative nitrogen is
stuck carrying the burden of charge by itself or if it's able to resonate that charge meaning
to share that charge with other atoms.
The non-aromatic nitrogen, the one with no pi bonds is sitting on an sp3 hybridized carbon.
Remember the rule that I teach in my resonance series, you cannot resonate on to an sp3 carbon
because even though it appears to have only 3 bonds there's a 4th bond hydrogen, a hidden
hydrogen which means if you try to resonate, you're going to create a 5th, violate the
octet rule and that just can't happen.
That means the molecule on the left has no resonance.
The molecules on the right is attached to an sp2 hybridized carbon.
The sp2 carbon has a pi bond which can be kicked off of that carbon and actually resonate
it around the ring.
In fact, I challenge you to pause the video and draw out all 4 resonance structures for
this molecule here.
Once you do, you'll see that this molecule has a total of 4 resonance structures, meaning
Nitrogen can share that negative charge with three other atoms, definitely reducing the
burden because it's not one atom holding the charge is four and that burden on nitrogen
is reduced due to resonance, nitrogen is happier, nitrogen is more stable and therefore less
reactive.
That means this reaction is going to favor this side that has a negative on the nitrogen
with resonance rather than the reactants which has a negative charge on nitrogen with no
resonance.
For even more practice problems on Organic Chemistry acid base reactions make sure you
visit my website for the acid base tutorial series, practice quiz, and cheat sheet leah4sci.com/acidbase.