In this unit, we've been learning all about acids and bases. So far the acids and bases we've
considered have been what's called strong acids or bases. These are acids or bases which fall apart
when you put them into water. So imagine that you have an acid: two separate parts of a molecule
put into the water then they fall apart. So when they always fall apart that's a strong acid and
base, you can imagine it like LEGO® pieces that easily fall apart in solution. On the other hand,
we have a category of acids and bases called weak acids and bases. These are ones that don't easily
fall apart so they have two parts of the molecule and maybe sometimes they come apart but most the
time they stay together. In this case, you could think about a LEGO® piece that had been glued to
another one. Sometimes maybe it'll fall apart, but lots of times it's going to stay together.
So let's learn more about this new category of acid and base called weak acids and bases.
In this lesson, we'll introduce weak acids, then we'll introduce weak bases,
then we'll practice percent ionization calculations. In order to introduce weak
acids it's helpful to consider strong acids. So here's the strong acid HBr and if I take 3 HBr
molecules and I put them down into solution what happens? Well it turns out that they dissociate,
they split apart into ions. So we get H+ and Br-. And if I put a total of 3 HBr's down there I'm
going to get 3 H+ ions and 3 Br- ions. We would say that the percent ionization here is 100%,
every single molecule I put down into solution became an ion. And that's very different than with
a weak acid like HF. If I take 3 HF's and I drop them down into solution then some of them will
split apart, but most of them stay together so maybe the first one becomes H+ and F- and then the
other 2 stay together as HF. We would say this has a 33% ionization, just one third of the molecules
have split apart. And that's the difference between strong acids and weak acids, strong
acids always split up weak acids just sometimes split up. Let's look at the formal definition.
Strong acids are acids that fully dissociate in water. An example would be hydrochloric acid which
you see in these glass bottles on your right, you would find those in chemistry labs. On the other
hand, weak acids show up in everyday life like vinegar, which is a weak acid, and they're acids
that sometimes dissociate in water. Okay, that's weak acids. Now let's talk about weak bases.
Again, to understand weak bases it's helpful to compare it to a strong base. Here we have
sodium hydroxide, it's a strong base. If I take 3 sodium hydroxides and I drop them into solution,
what happens? Just like with HBr, they all split apart. So I'm going to get out 3 total sodium ions
and 3 total hydroxide ions. Again, we would say that the percent ionization is 100%.
Now strong acids and bases almost entirely dissociate, but it doesn't mean every single
one will. So occasionally if I put like a million down into solution maybe 1 will stay together.
Okay, what about a weak base like NH3? Well these do pretty strange things. If you notice, NH3
doesn't have any ions. Those are both non-metals nitrogen and hydrogens. Turns out though that
what they make in water is if I react NH3 with water they still make ions because what happens
is a hydrogen is grabbed by the NH3 and that makes NH4, which is an ion because it's positively
charged, and OH-. So this is a very strange way that weak bases work, but what weak bases do is
they grab the hydrogens off of water and leave behind an OH. So if I put 3 NH3's into solution,
what will happen is maybe 1 of them goes out and grabs a hydrogen and gives me NH4 + OH-,
whereas the other two stay together as an H3. So that's a little stranger
but that's what happens with my weak bases. Again, they basically partially dissociate, they
partially split up. Some of them ionize. Let's look at their formal definitions. Here we see that
strong bases are bases that fully dissociate in water and we have the example of sodium hydroxide
just like we did on the previous slide. So in purple we have sodium and blue we have oxygen
and in white we have hydrogen. And when I mix them with water they just split apart into their ions.
Weak bases are bases that sometimes dissociate in water. And here we'll take a closer look at what
happens with that NH3 and water thing. If I have water, it turns out that nitrogen has some extra
electrons right here that like to go out and grab a hydrogen. And so this hydrogen comes from water,
so we take 1 hydrogen from water and go and attach it to nitrogen. What does that
make? Well that makes our NH4, now it has an extra hydrogen and it's positively charged,
and it leaves behind water, but it's missing a hydrogen so it's OH-. So weak bases are bases
that sometimes dissociate in water and sometimes they do that in pretty strange ways like this NH3
thing we see here. Okay, with that in mind let's take a look at thinking about how weak some acid
or base is. We can quantify that with what we call percent ionization which we've already referenced.
Here for bases we see that percent ionization is equal to the concentration of hydroxide,
so that's what's up here the concentration of hydroxide, divided by B which stands for base,
so this is the concentration of base. What that means is if I put one molar of a base
into a solution and it makes 0.1 molar OH in solution then I would plug those numbers in and
I would get a percent ionization of about 10%. We can do the same thing for acids.
Here we put H+, what dissociates from an acid on top, and HA, the original acid on the bottom.
Let's practice using these equations. Here we have a problem that says "Calculate the percent
ionization for a weak acid with a concentration of 0.5 molar and a PH of 2.2". So the PH is given to
us so that we can figure out H+. So ultimately, we're going to get H+ right here from our PH.
HA, the concentration of the original acid, is actually given to us it's 0.5 molar. Okay, so
what are the steps here? Well the first step is to determine H+ concentration with the PH equation,
so here's my PH equation. And if you recall, to use that to find H+ concentration I need to go
ahead and multiply both sides by the -1. And when I multiply both sides by -1 I get -PH = log[H+]
concentration. And now to get rid of my log I do 10 raised to both sides, so 10 raised to the
PH and 10 raised to the log and you may or may not remember from algebra 2 that that goes away.
Okay, so then we get 10 raised to the -PH equals my H+ concentration. And lots of people will
just go ahead and memorize that equation so that they can easily go from PH to H+ concentration.
Alright, so that means 10 raised to the -2.2. Why 2.2? Because that's the PH
and what is that going to equal? Turns out if you put that in your calculator you'll get 0.00631.
That has the units of molarity because it's a concentration
and that's equal to the concentration of our H+ ions. Okay, so now step two is calculate
the percent ionization. Here's our formula for percent ionization and we know the top,
the numerator is going to be what we just calculated the concentration of H+ 0.00631
and we know the bottom concentration, that one was just given to us in our problem because we're told
that the acid has a concentration of 0.5 molar, so we plug in 0.50 molar. And now the last thing we
do is we multiply by 100. Wen we put that into our calculator we will get 1.2%. 1.2% Okay, so
that's the percent ionization. Notice that means most all of the acid actually stays together and
just a little teeny bit falls off which is what we would expect because we have a weak acid, which
just partially dissociates. Alright, last let's practice identifying an acid or a base a strong
or weak based on their percent ionization. So go ahead and pause the video for a second and try to
guess from the percent ionizations below whether each one of those is a weak or strong acid.
When we look at a. we see that 12% is the ionization, so that means 12% of them fall
apart that means most stay together and that means that we have a weak acid.
So a weak acid because the dissociation is quite low, 99.8 that one may have given you a little bit
of trouble because you would be like "uh I don't know it's not 100%", but remember strong acids
almost entirely dissociate. We say fully dissociate occasionally just a couple of them stay
together, but that's almost 100% so that's going to be a strong acid, so that's a strong acid.
When we go to 5.05% that's pretty straightforward it's a pretty small dissociation percent, most
of them are staying together and so we know that that is a weak acid so there we have a weak acid.
Lastly, we have 100%. All of it's dissociating and there we very clearly have a strong acid
because there's a 100% dissociation. Alright, so what have we learned? We've introduced weak acids,
we've introduced weak bases, and we've practiced percent ionization calculations.