B2 High-Intermediate US 1432 Folder Collection
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Hi. It's Mr. Andersen and in this video I'm going to talk about Acids,
Bases and pH. If you ask people what pH measures, they'll usually say if something is an acid
or a base. And they might know that water has a pH of 7. That acids are generally lower
than that and bases higher than 7. But that's where a lot of people's understanding ends.
And so I kind of want to explain to you what pH is and how it's determined. But before
that I want to tell you why it's important. And I'm a biology teacher. And so everything
kind of goes back to life. And so this is a protein. It's called myoglobin. It's found
in your muscles. And it's going to be most active at a pH of 6. It's going to work at
a pH of 5 all the way up to 7. But if we start to move our pH too low or too high, that protein
is going to denature. And our muscles aren't going to work. And so it's important that
the pH levels remain relatively constant and they're not changing that much. But what is
pH? Well we've got to start by talking about water. And so this is a water molecule. Remember
we're going to have hydrogen here. Two hydrogen atoms. And then one oxygen atom. Now one thing
that you need to understand is that this is a polar molecule. And what that means is there's
a covalent bond between the hydrogen and the oxygen. Between this hydrogen and oxygen as
well. And oxygen is really greedy when it comes to electrons. It's going to pull the
electrons towards it. And so this is a sharing of electron between these atoms. But it's
a polar covalent bond. And what that means is since oxygen is pulling the electrons towards
it, it's going to have partial negative charge on this side of the oxygen. And the hydrogens
are going to have a positive charge on the other side. And so if we were to add another
molecule of water, these are not going to arrange this way. In fact what we'll have
is they'll be arranged like that. And so the hydrogen atom of one water molecule is going
to be attracted to the oxygen of another. And that bond is called a hydrogen bond. A
lot of students think that hydrogen bond is in here, but no, that's covalent. But the
hydrogen bond is going to be between the positive hydrogen, partially positive. And the negative
oxygen. And that's why if we have one water molecule and the hydrogens are like positive
and the oxygens are negative. And we have another one, they're going to line up like
this. And as I pull one water molecule, the other one is going to go along with it. And
that's why we have cohesion. And it explains a lot about water. But some weird thing happens
with water. Sometimes that attraction is so great that this hydrogen atom will actually
become detached from the water and it will be come attached on to this other water molecule.
That would be like me pulling this pinky off and attaching it over on to this other water
molecule. Leaving me just with this. And so what is that called? This is called hydronium.
Hydronium is going to be H3O and it's going to have a positive charge. What are we left
with over here? This is a hydroxide ion. And so what is pH a measure of? Well the p stands,
we think, for the power of hydrogen. In other words the amount of hydrogen. But it could
also be the amount of hydronium or the amount of just free hydrogen ion inside the water.
And so if we look at the power of that, or almost the percentage of that, that's going
to be what pH measures. And in regular water, distilled water, they amount of this occurring
is really, really rare. In other words it's a 1 in 10,000,000 chance that we're going
to have hydronium. And this is really a molar concentration. So to give you a sense of the
scale, let's say this hydronium ion right here is represented with this little cube.
And so what I'm going to do is pull back. And let's say this is one cube and 10 and
100 and 1000 and eventually what we get, if we scale that, you really can't see that cube
anymore. But this would represent 10 million cubes. And so the chances of that one hydronium
forming are going to be really really low. But even though the probability of hydronium
forming is low, it actually occurs in water and it has huge impacts on things that are
found within that water itself. And so that 1 in 10,000,000, I want you to think about
that for just a second, and let's kind of add a little bit of the equation of pH. And
so some kids get scared by the equation. It's not that scary. So pH or the power of hydrogen
is equal to the negative log of the hydrogen ion concentration. It's also the same as the
hydronium. That's that H3O plus. Those are essentially the same thing. So it's the negative
log of that. And so it's the negative log, think of this, as 1 in 10,000,000. And this
would actually be a molar concentration. But we're keeping it conceptual right now. And
so if we take the negative log of that, let's write 1 in 10,000,000 in scientific notation.
And so it's the negative log of one times 10 to the negative 7th. So this is going to
be a really small number here, and this is where the math gets really easy. If you were
to put this in your calculator, if we take the negative log of one times 10 to the negative
7, what do we get? 7. And so the pH is going to be 7. And this gives us a number that we
can actually deal with. And so what does it mean if the pH is 7? It means that the concentration
of this hydrogen ion is going to be really, really small. And if we ever vary that, then
we're going to be varying the pH. And so pH of 7 is neutral. But if we ever have a value
greater than 7, it's going to be a base. And if it's ever lower than that then it's going
to be an acid. And so let's start by dealing with the acids. What's an acid that almost
everybody is familiar with? That's hydrochloric acid. You'd find that in your stomach. And
so if we add hydrochloric acid to water, it's going to disassociate. It's going to breakdown
into hydrogen ions and chloride ions. And so you can see here that we're increasing
the amount of this H+. And so what is that going to do to that concentration? Now instead
of being 1 in 10,000,000, it might be as often or as common as 1 in 100. And so if we were
to right that as scientific notation. It's the negative log of 1 times 10 to the negative
2. So we would have a pH of 2. And so depending on the concentration of hydrochloric acid,
we could have a pH of 2 or 1 or 3. It depends on how much hydrochloric acid is in there.
Now let's look at a base. And so a base for example, this would be sodium hydroxide. Or
lye. What's going to happen to that when we add it to water? It's going to break apart
into sodium ions. And hydroxide ions. Now that doesn't help us. Remember, because pH
stands for the power of hydrogen ion. But what do you think is going to happen to that
hydrogen ion that happens to be in the water? Now we've got a hydrogen ion and we have a
hydroxide ion. And those are quickly going to combine to form water. And as it does that
it's going to gobble up that hydrogen ion. What's that going to do the amount of hydrogen
ion in the water or hydronium ion for that matter? It's going to make it even more rare.
And so now we have the PH equal to the negative log of 1 times 10 to the negative 12 for example.
And so what's that going to be? That's going to give us a pH of 12. And so what does pH
measure? It just measures the amount of hydrogen ions. Or hydronium ions. And bases and acids
are going to have different amounts of that. We measure that using a pH scale. And so distilled
water is going to have a pH of 7. If we have anything higher than that, that's going to
be a base. Anything lower than that, that's going to be an acid. But when you're taking
a test, it can be somewhat confusing. And so let's say we increase the amount of hydrogen
ions in a solution. So we're going to have more of them. What's that going to do to the
pH? It's actually going to lower it. And vice versa on bases. And so watch out for that
when you're taking a test. Why is this important? Well acid rain is one example of that. Or
the acidification of our oceans is another example. And so this is looking at the pH
in the oceans over the last couple hundred years. And what we see is that our oceans
are becoming more acidic. How does that work? You're combining carbon dioxide with the water.
And as we increase the amount of carbon dioxide in the atmosphere, that water and the carbon
dioxide are combining to make carbonic acid in the oceans. And that's increasing the acidity
of our oceans. And so we could hear, see here, the pH is decreasing. So we've seen a decrease
of around negative 0.1 on the pH scale over the last couple hundred years. And you might
think, well that's not that big of deal. But remember this is a log scale. So by decreasing
it by a small amount in the pH, we're going to increase it quite a bit in the hydrogen
ion. And that's going to effect anything living in the oceans. It could effect coral reefs.
And every time we have a massive extinction on our planet, it seems to be correlated with
the acidification of our oceans. And so that's pH. Pretty simple. And I hope that was helpful.
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Acids, Bases, and pH

1432 Folder Collection
Wayne Lin published on February 23, 2015
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