Now, the strength of a binary acid is based on two major factors.
First, it’s based on the electronegativity or the size of the non-metal that’s connected
to my hydrogen.
In this case, this non-metal in HCl that we’re talking about is Cl.
And in HF, the non-metal that we’re talking about is fluorine.
So the strength of a binary acid is based on those two factors, electronegativity as
well as the size of the non-metal.
Now, we’re going to say for elements in the same period and remember when I say period—we
have a periodic table here.
This is period one, two, three, four, five, six, and seven.
Here we’d have boron, carbon, nitrogen, oxygen, fluorine and neon.
They’re all on period two with each other.
We’re going to say, elements in the same period, we’re going to look at their electronegativities
in order to determine acidity.
And basically, what you’re going to see here, the more electronegative the nonmetal,
the more acidic.
If we’re taking a look at certain examples, let’s say we’re looking at CH4, HF and
BH3.
So, we’re going to say that boron, carbon and fluorine are all in the same period.
In terms of electronegativity, remember, it increases going from left to right.
And because the increase is going from left to right, we’re going to say fluorine is
the most electronegative out of these three non-metals that I’ve underlined.
And because fluorine is more electronegative than boron and carbon, HF will be a stronger
acid than CH4 or BH3.
That’s the way we apply this.
If they’re on the same period, we look at electronegativity.
However, for elements in the same group, remember groups go vertical.
This is: 1A, 2A, 3A, 4A, 5A, 6A, 7A.
We’re going to say here—in this case, we’re going to say we look at their size
when they’re in the same group.
We’re going to say, the bigger, the more acidic.
Remember, when we’re talking about size, we’re really talking about atomic radius.
And we’re going to say, your atomic radius increases when you go down a group.
So we’re going to say, let’s say we’re looking at different types of acids.
Here we have: Cl, Br, and I. Let’s say, you wanted to compare HF, HCl, HBr, HI.
We’re going to say that, all four of these non-metals are all in the same group.
They’re all in group 7A.
We’re no longer going to look at electronegativity instead we’re going to look at size, atomic
radius.
Going down the group, it’s going to increase.
We’re going to say here, “I” would be the largest.
And because “I” is the largest, HI would be strongest acid out of this group.
So that’s how we look at electronegativity and how we look at atomic radius or size to
determine which binary acid is the strongest.
Now, one last thing before I’ll let you guys attempt to do the two practice questions
at the bottom.
The most common types of binary acids--we’ve already seen them.
When H+ is connected to HF, we have HCl, HBr, HI.
Group 6A will also be another common type of binary acids: H2S, H2Se, H2Te.
For all purposes, we’re going to say that, these are the most common types of binary
acids but only three of them are really strong.
The only three strong binary acids you’re going to know this semester are these three:
HCl, HBr, HI.
Those are your three strong binary acids.
For those three, we never have to do an ICE chart in order to find pH.
For all the other binary acids: HF, HCN, H2S, H2Se, H2Te.
All those others are weak binary acids.
So we would need to do an ICE chart in order to figure out their pH.
Just remember that these are the things that we use electronegativity and size to break
ties between different binary acids but there are only three strong binary acids you need
to know, HCl, HBr, HI.