chem1212 week 1c part 1

Slide 41 - Electronegativity Affects p K Values

oxoacids

when on the same column, electronegativity goes by size

chem1212 week 1c part 1

Transcript

Nothing that we cover today is technically going to be on the test.

However, all we're talking about today is dealing with acid and bases still same with Wednesday.

Right. So we're going to be talking about acids and bases, which is part of the test.

So the more context that you can get from the material in this class, it may still help you on the testing material.

But I'm not going to ask you specific questions I give you today, just any questions on any kind of housekeeping stuff right now.

Okay? All right. So we're going to start talking about how, just based on the structure of chemical compound, we can determine if that is going to be more or less acidic.

So we know if we look at the KA value of an assay, the larger that KA is, the more acidic that pka.

The smaller the pka, the more acidic. Right. It's just that our scale is different than that at the normal scale, but is going to be related to the structure of the compounds, whether or not an acid will be stronger or weaker than another.

So there's kind of three broad sets of rules and there's two types of compounds that we apply those rules to.

So the first one we're going to look at connected to an O and that O is connected to other stuff.

For our oxo acids, the two rules are if we have more oxygens, it is more acidic, and if those oxygens are connected to a more electronegative atom that is more acidic.

So we can keep track of this a little bit more clearly and all of these rules are going to apply this way.

If you think about your six strong acids, right? Perchloric acid is a strong acid. Hypochlorous acid is not because perchloric acid has a bunch of oxygen.

Right? So keeping that in mind, there's a reason for the strong acid that we have and the generic form, right, is that we have our H attached to O, attached to something the more electronegative and the more oxygen that has the Morse.

So that's kind of our first go. Right. Again, another way to look at it, if we have sulfuric acid, H2SO4 versus sulfurous acid, HSO3, sulfuric acid is strong.

Sulfur doesn't mean again, number of oxygens in that.

So that is our oxo acid. Questions about oxidized 4 oxygen farther to negative 4 Sigma.

Alright? The other rule is only going to affect what we refer to generally as binary acids.

So our binary acids are when we have generally the best way to think about these guys are going to be that we have our H, our H directly attached to X.

Right. For our OXO acids, what we had was H attached to O attached to X.

Right. That's where the next starts changing because that X position.

So for our binary acids, we're explicitly attached to the thing that might change.

When we're comparing our acids, we generally also kind of refer to these in their simplest form as we just take one of these elements and we put enough H's on that we don't need any more bonds.

Right? So carbon makes four bonds. I take the carbon, we put four Hs on it. That's our kind of binary compound. For carbon, for nitrogen it would be MH3. For oxygen it's H2. Right. So those are kind of our just simplest forms. But the rules will still apply if, like, let's say we had H attached to o attached to Ch3 and then H attached to S attached to Ch3.

So those would technically not be in this simplest form, but they would use the same rules as our binary assets.

Whenever we have the thing we're comparing being the thing that's directly attached to H, that's where a binary asset rules come in.

The binary acid rules depend on the elements that we are comparing pairing and how they relate to each other if they are in the same row.

So when you're cross periodic table, we care about the electronegativity, right?

Same rule we solve for. But for a binary acid to work with pairing across a row, the electronegativity change is going to be more dramatic than the other thing we're going to look at, which is the size of the ass.

So if we compare down a group, we care about size across a row, it's electronegative.

And again, the reason for this is what we're actually comparing.

Right? So if we look at carbon and nitrogen, the difference in electronegativity there is like 0.5.

Right. So carbon is roughly like two and a half. Nitrogen is roughly. Oxygen is three and a half, four horses. Right. Whereas if we look at how big one of them is compared to the other one, we are adding one proton and one electron and that's it as we go from one to the other field.

Right. So the difference size is pretty small when we go across a row, whereas the electronegativity is pretty dramatic in terms of its change.

When we go down in group and we look at size, remember, the size is going to be impacted not just by like we added the proton electrons, we can go around the entire graph.

Difference in size between F and chlorine. Is an entire extra 8 protons and electrons between chlorine and bromine.

That's an entire 18 extra protons and electrons.

Right. Same with either. So we get way, way, way, way bigger when we go down a group than when we go across a roof.

And essentially that way more dramatic change in size just beats the electronegativity change.

So, again, if we remember strong acids and strong bases, there's a way to keep that in mind.

Hydrochloric, hydrobromic, and hydroiodic acids are all strong.

HF is not. Even though it's the most electronegative, it's the smallest.

Right. And when we go down a group, size is the one that we care about.

And so HF being the smallest means it's actually a weak acid, not a strong acid.

And we told you it's the scariest acid. We tweeted that earlier. Yes. What's the limit? Each of them. Okay. I would rather deal with HCL than with hl. Anyway. So if we're going across a row, electronegativity going down a group that sucks.

Again, our kind of differentiation here are do we have the H directly attached to the thing that's changing, or is our H attached to something that's attached to the thing that change?

Usually it's oxygen, but in theory we can have like H attached to in attached to something, attached to chlorine, H attached to intensity.

Still, same rules would apply, which are our oxoacid rules where we care about number of oxygen atoms and electronegativity.

So whenever we have anything where we have our H attached to O attached to X, this thing is electronegativity.

That is what we're looking at. And then extra. Extra oxygen atoms or extra electronegative atoms could be helpful too.

What we're really looking at when we're comparing how strong our acids are, remember we have ha plus and a minus.

That's our equilibrium. If our acid is stronger, our equilibrium looks more like that.

We say that the side would be H plus. You think back to our equilibrium thermodynamics, that would mean that our equilibrium.

Right. Thermodynamics would look something like this.

Right. Where our products are more stable than our reactants.

That's how we get more H plus and a minus. That's our thermodynamic equation. So if a minus is more stable, then ha is more acidic.

That's our general rule. And the way that we know if a minus is stable is those rules.

We just said size, electronegativity, number of oxygens, any Questions?

Carbon, Sorry. In general, black is carbon, whites, hydrogen, red is oxygen, and blue, the rest vary those four level.

You know, I'm going to go ahead and say, well, so here's the deal.

We come back and we have two more classes. We have a Monday schedule now, I guess, so there has to be an instructor.

So, like, cover. All right, so when you're trying to answer questions like this, the best way to go about it, right.

We're trying to look at all five things to figure out which is the most.

It's going to be a lot more helpful to look at two things and figure out which of those is more acidic and then take the winner and go on to the next one.

Right. So we'll start and we'll look at carbon, and we'll look at A and B, which for some reason are on top of each other.

Tonight I will fix that. But we have carbonated magnitude. Are those things in the same row or the same group?

Rows are this way and groups are this way. All right, we got these two right here. Are they next to each other or they on top of each other?

So we're in the same row, Right? All right, so we're in a row. Do we care about size or electronegativity? We care about electronegativity if we're in a rub, right?

So which is more electronegative, nitrogen or carbon?

Nitrogen. Great. All right, we got nitrogen, we got oxygen. So this is what we're taking one at a time. Nitrogen, oxygen, rho or group? Rho. So we care about electronegativity. Which one's more electronegative, Nitrogen or oxygen?

Oxygen. Oxygen. And sulfur. Rho or group. Remember, we've now gone down. So do we care about size or electronegativity? Size. Which one's bigger? Sulfur. And we have sulfur and selenium. Which one of those are group or rho. Groove. Which one's bigger? Selenium. There's our answer. So that's how we're. When you get a bunch of options, right? That's how we can. We could. That was the question. But just one at a time. All right, and then our second question. Which of these has the strongest acid? Was first. So we're comparing selenium and arsenic. Are these guys binary acids or oxoacids? They're binary, right? We just put the elements. We slapped some Hs on it and we said that's our. So if they're binary acids, we're going to look and see if they're in the row or in a Group.

Right. Because we need to figure out what we're comparing.

So selenium, arsenic, rho or group? Rho. So we're looking at electronegativity or size. Electronegativity. And which one is more electronegative? Selenium. Selenium. So that is our stronger acid. We can look at the other four. Right. We have. This is gonna kind of answer the question. We have aluminum hydroxide and boronic acid. Which of those is more safe? All right, so what we have here, right, is H attached to O attached to al or H attached to O attached to beam.

Right. They have the same number of oxygens. But if we have this compound, are we going to use our binary rules or oxoacid rules?

Our oxo acid rules. Right. We are attached. Our H is attached to specifically an oxygen in this case, that is then attached to an element that's different.

So for our oxo acid rules, do we care about size at all?

No. So we don't even need to look and see if we're comparing rho or group.

We just don't even care about size. We care about electronegativity, and that's it.

Assuming they have the same number of oxygen, which they do.

So which is more electronegative or under aluminum gives you a little PR table and it draws four.

We increase in electronegativity that much. So boronic acid is the more acidic. All right, we have HBRO and HBRO 2. Which of those is going to be more acidic? The one with two of those, right? Yes. Generally, oxygen's wins. Usually we don't ask you that because of that wiki H that we look up, but usually undone and we'll see.

I'll show you why on the next slide. It's going to be correlated to some of the organics that we'll talk about.

So we got this guy here, hbar and hi. Which of those? Varsity. All right, so are we using binary or oxo acids? Binary. Are we comparing or. Or a group? Are we comparing a row or a group? Group. So do we care about size or electronegativity? Size. And which one's bigger? I bet. All right, and then E is going to be similar to C. We got HNO2 and HNO3. Which is the more acidic compound? HNO, which again, HNO3, strong acid. HNO2, not a strong acid for that reason, for the oxygen.

So the only answer here is going to be any questions on those guys.

All right, so we're going to take it a slight step further and Get a little bit more complicated.

We're going to talk about specifically organic acids.

Now, I've already shown you carboxylic acids. Those are kind of our generic weak acids. Carbosylic acids are generally found in organic chemistry, although they can be found in other stuff, too.

But they're incredibly common in organic chemistry.

And the reason that they are particularly acidic is going to be similar to our oxyacid rules.

So we're going to get a little bit more in depth and why those rules work the way that they do.

So we have here acetic acid, sometimes referred to as ethanoic acid and ethanol.

The naming. Nobody calls it ethanoic acid because it was named acetic acid, like 2,000 years ago.

So that name is really stuck. But it's the primary ingredient that occurred. But we have two, this extra C double bond O that is missing here.

And otherwise that's kind of the only difference in our structures.

Right. That difference accounts for a magnitude of 10 to the 11th difference in the K.

Right. That is the difference in the essentiality of these two compounds is, you know, more than a trillion.

So why what we're seeing here is not just that we have an extra oxygen, but specifically the way that it's attached when we draw our compound, our A minus.

Right. We have that guy right there. So we have a carbon, we have RCH3, that. So we have our acetate ion. We put the negative charge on one of those oxygens.

But remember, electrons don't ever just sit still.

They're not sitting in one place if they don't. And if they don't have to even be on one atom, they're not even on one atom.

Right. That's what a bond is, is those electrons being shared.

So even when we draw the electrons as a lone pair, an extra lone pair, that's they're still not necessarily stuck on that end.

And so when we have a situation like this carboxylic acid, what we can do is we can take these electrons that are in that lone pair and we can actually turn this single bond between our oxygen and carbon into a double bond.

When we do that, we take the double bond that's already there and we turn that into a load pair and we get this other structure here.

These are resonance structures of each other. You guys learned about resonance structures and Jenkin, one kind of phrase maybe at some point.

So these guys are resonant structures of each other.

What that means is that in reality, while we draw the structure like this, it doesn't actually look like this Right.

The reality of it is a little bit fuzzier, right? We don't just have electrons on one atom, and that is they are constantly moving back and forth between these two oxygens.

So instead of actually having this negatively charged oxygen, what we have instead is something that looks a little bit more like this.

That's kind of our real story, right, is this. Both of them are singly bound and they kind of have like a half of a double bond and a half of a negative charge on each oxygen.

On each oxygen. Another way that we can kind of think about this is a double bond is shorter than a single bond.

That's a fact that you can just memorize. If you would like to understand why, you can ask me after class.

I will bring up molecular orbitals. So if you want to avoid that, we just don't have to talk about it.

But a double bond is shorter than a single bond, right?

And so we would expect that between carbon and oxygen one, we have a shorter distance between carbon and oxygen two, or vice versa.

What we actually see is each of these bonds is the same length, and that length is in between a one and a two.

So we expect it to be this long or this long is actually this long, and they're both the same.

So in reality, what we actually have here is something more complicated than just one of our structures.

But the complexity allows for that negative charge to be essentially fully cut in half across each of those octanes.

And that makes a minus so much more stable that it's roughly 10 to the 11th more stable.

Right? That's our ka difference between this guy and our ethanol over there.

Right. Remember, we think about our ethanol. Our other option here is we just put that negative charge on the oxygen.

That's we're done. There's no sharing, there's no moving it around.

Right? So the reason why putting more oxygens on our atoms for our oxo acids is better is because those more oxygens, based on the way our oxo acid structures are, allow for more resonance.

So that's actually why the oxygens are helpful. It's not just having O is better, it's because having oxygens gives us these more potential resonance and that helps with the stability of our conjugates.

Right? That helps with A minus B Corsica. Another example of these resonance structures is there's a compound density just like that.

This is a reminder for how these structures work, right?

That's our benzene structure, and it's actually in its resonance structure.

Right. Each of these double bonds and single Bonds, we could kind of alternate all of them.

Right. Another way that people often draw benzene is just like that.

Again, we would expect short bond, long bond, short bond, long bond.

Right. Instead they're all the exact same and it's empty.

So we have a resonance structure with our benzene, which means that if we did that guy right there, where we put an oh on there, right.

So we have an oh on our carbocylic acid, we have an oh on our ethanol.

If we put our oh next to our benzene ring, we can also have resonance structures associated with that compound there.

Except that the resonance structures for this guy are going to involve putting that partial negative charge on a bunch of carbons instead of on an oxygen.

Oxygen will share the negative charge here. 50 50. And they're both electronegative atoms, so they're both pretty happy with negative charge.

They're fine helping out with that, stabilizing that Carbons don't like negative charges.

Right. So even though they don't have to get the full negative charge, they don't even really want part of it if they can help it.

So they do help out enough, but they don't help out that much.

Right. Our ka for this guy is going to be roughly the PKA is 9.8.

What is it actually tend to do? 1.58, 10. Right. That is our ka for this compound. Right. So it is more acidic than just our regular ethyl. Right. So just our normal oh with nothing else helping out.

It's about 100,000 times more acidic than that. But it's a million times less acidic than our lascivious.

These are all logarithmics. So we have our kind of using our resonance structures, we have condoms that are more or less acidic based on how many resonance structures we have and how we get them.

This depends about the most complicated going to see most of the time they look because again something like sulfuric acid.

The reason sulfuric acid is such a strong acid is when we get this hydrogen sulfate.

Right. We have resonance structures here, four there. Right. We have three different resonance structures where each of our oxygens is only getting one third of the negative charge.

Right. So we have more options. That is similar to the Sophia acid, but even better because there's three times and because that sulfur is a little bit more helpful in this process than for reasons we don't need to learn.

Any questions? Alright, what is our other rule for oxoacids More oxygens is better.

What's the other thing is better electronegativity.

Right. So that rule is also going to apply for our organic acids with another little kind of complication.

So the more electronegative, the better. It doesn't even actually have to be like right next to where the H is, right?

So the oxygen acid we were looking at before, right?

We had like H and then O and then C, Right. For our organic acids we usually have like H and then O and then carbon and then carbon and then carbon and then maybe we have a co or something like that.

Right? Because it's organic, we like our carbons. We stick them on everything. So for these acids, generally if we have an electronegative atom, it's not necessarily right next door.

It can still help, it's just not going to help as much.

So when we start seeing things like this here we have a fluorine way over here versus fluorine versus a rhoam.

The difference in the KA values of these two guys is probably like five.

And I don't mean 10 to the fifth, I mean like the number five, five times 10 to the zero, right?

It's just very small, right? It's not going to have that big of an impact. It's one atomic. It's pretty far away from the actual where that negative charge is going to be.

It does still have an impact. We can still measure that. It's just not going to be the biggest deal. If you remember one of the first weak acids that I showed you, the difference there was actually acetic acid, the one we just saw, and trifluoroacetic acid, where we have those guys didn't.

And the difference in the KA's of acetic and trichloroacetic acid was like 10 to the 3, 10 to the 4, right?

So the kind of most impact that this can have is still going to be less than just this guy by a huge, huge.

Right. Having one resonance structure value way more dramatically than even having the most possible electronegative atoms as near to our acid.

So it's a lesser effect. The electronics are. Adding oxygens is more important, adding those resonance structures is more important.

But also one thing that we can do with this is if I need an acid that has a KA value that's like, you know, five large, right?

This will be. It's very easy to take an acid and tweak it ever so slightly to get a very specific ka if I need to do that.

Because I can take something like this carboxylic acid and I can slap a fluorine like three parts in, right?

And that might be enough to tweak it just to that exact number.

I need to know why I might want an exact number. The other thing up here is the more electronegative is better is that the more close the electronegative atom is to where the actual charge is.

That's better. Right. Once we get past the immediate atom with that negative charge, if we're not making resonance structures, it's all just going to be that the chlorine or the fluorine or whatever is kind of pulling electrons a little bit more towards where they are.

So just these kind of electrons are constantly moving around and we get to pull them a little bit closer away from that atom that is having the vast majority of the electron density on it.

So we think about the electron setting. Instead of it having to spend 100% of its time on that oxygen, it's going to spend 94% of its time around that oxygen and then get to spread out a little bit more.

It's not a big change. But, you know, it's something. So that's what the more electronegative atoms can do.

And so the further away that they get, the less they can help with kind of pulling the electrons a little bit off that atom.

So for our organic, we're taking the same idea. More electromagnetic atoms, better. More oxygen is better, especially because of our resonance structures.

Right. We're just applying them specifically to that resonance.

So it doesn't have to just be oxygen. It can be anything electronegative that gives us a resonance structure.

And it doesn't have to just be that H attached to O attached to chlorine.

It can be H attached to O attached to a bunch of different stuff.

And it's not where we have chlorine, but better than not having.

What's the difference between the top. These guys are the same. Those guys are the same. The difference here is that we've taken that F and we've moved it slightly further away from our oh, so this distance is more than that one.

At the top of what we're comparing is fluorine to fluorine to bromine.

So fluorine is the most electronegative chlorine and then bromine and then bromine is not very electronegative, but it's technically more than carbon.

So I think it's slightly better, just not very much.

If we were to try and compare, like this compound and this compound, we would probably just need the KAs, right.

We would know they would be pretty close to each other and we wouldn't be able to actually tell which one's better without looking something up?

Because they're probably only one's probably only better by.

They probably both have the same exponent in their 10 to the whatever.

And it's actually just a number that's changing and that's really hard to guess.

Which also means we wouldn't have any other questions.

I just want to emphasize again, right. Whenever we have. This is the kind of mistake that I see the most people make.

We can always essentially classify these acids that we're looking at.

We're trying to use these rules to determine which of these is stronger.

We always classify them into one of two. We either have our H directly attached to the element that is chain.

We use our binary rules if that's what's what we're comparing.

Right. We're always comparing H attached to something and H attached to something else.

That's what we're looking at. If what we have is our H attached to, we'll use O as a standard.

But note that this could be any element. And then the thing that's different is one removed.

That is when our we use our oxo axis. Okay. Those oxo acid rules for oxygen. So these are our two groups. Right. We could say that we have H attached to an attached to F.

H attached to an attached to C. Right. This would also fall into that category. Even though they're not oxygens, we would use the same rules as though they were oxygens.

Does that make sense? Whenever we're comparing acids, either we have the exact same compound or something is different.

If something is different, we need to look at what is different if it's directly connected to Nate.

If it's not, we use the other rules. Okay, well, we're going to briefly introduce the next topic and then class will get out there like as far as I got.

Okay. Yes. I'll give you a brief overview of what we're going to start on Wednesday.

We'll do some practice questions with this acid based stuff at the beginning of class on Wednesday.

And then we'll really get into kind of the last little bit of this chapter.

The last bit is going to be the acid base properties of salts.

So when you react weak acid and a weak base with water.

Right. We get a conjugate acid and a conjugate base. Those conjugates are acids and bases. They're generally going to be in the form of something charged, which means it's an ionic compound.

Another term we have for ionic compounds are salts.

So that's where this is coming from. So There, broadly speaking, are three categories of salts that we're going to ask you to know what to do with.

Neutral, acidic and basic salts. Right? Neutral is pretty easy. When you put them in water, the PH of that water does nothing.

A neutral salt comes from the conjugate of a strong acid and generally just one of the elements in the first two rows or first two groups.

If we have like lithium or sodium or magnesium or calcium, any of those guys paired with the anion left behind with a strong acid, that's a neutral salt.

So if we stick a chloride with any of those first two elements, nothing's happening from.

So sodium chloride not going to make a ph sensitive solution.

Right? There is no PH change when we put chloride. So these are the five that we care about. Remember we talked about how sulfuric acid is weird?

The conjugate of H2, so 4 is HSO 4 minus, which is a weak acid, right.

So we can't use that as our. The hydrogen sulfate and the sulfate ions are a conjugate acid base pair, weak conjugate acid base pair.

So they kind of don't align. Another way we usually do is if we react, right, this is back to JK1.

If you react a strong acid with a strong base, you get water in the neutral.

So anything that reacts with comes from that strong acid.

Strong base solution is our neutral. The other two are the parts where things are going to get a little bit more complicated.

So I'll just give you a brief introduction. When we have something that was a base like ammonia, NH3, and we react that with say HCl, right?

We get a conjugate acid, ammonium, something like ammonium or ammonium is an acid.

And so we took ammonium chloride and we added that to water, it would produce A plus an ammonia and the chloride would be a spectator.

So if we add this to water, this is capacity and we will name a three

Made With Glean | Open Event

Made With Glean | Open Event