Basics of Acids and Bases in Chemistry

In this video, we're going to go over a basic introduction into acids and bases, particularly with reference to organic chemistry. So first, you need to be able to identify a typical acid and a typical base. Most acids contain hydrogen. HF, hydrofluoric acid, that's an acid. HCl, hydrochloric acid, that's another one. NH4+, the ammonium ion, that acts as a weak acid. H3O+, that's another example. Acetic acid, which has a carbosilic acid function group, that's another acid that you'll see in organic chemistry. Now here's some typical bases. Hydroxide is a base, fluoride is a weak base, ammonium is also a weak base, and the conjugate of a carboxylic acid is also a weak base. So those are some common bases that you'll see in organic chemistry. Now anytime you put an acid in water, The pH of the solution will be less than 7. Whenever you put a base in water, the pH of the solution will be greater than 7. Water, being neutral, has a pH of approximately 7. So just keep that in mind. Acids, they will create a solution with a low pH, while bases will create a solution with a high pH. Now let's discuss acid-base reactions. Let's mix hydrofluoric acid with water. Now how will these two react? What will be the products of this chemical reaction? According to the Bronsolari definition of acids, acids are proton donors, bases are proton acceptors. A proton is basically a hydrogen ion. Hydrofluoric acid is going to behave as the Bronsolari acid. It's going to give up hydrogen. Water in this reaction will behave as the Bronsolari base. It's going to accept the proton. So HF is going to lose the hydrogen, it's going to become fluoride, water is going to gain the hydrogen ion, and it's going to become H3O+, or the hydronium ion. Now, HF is a weak acid, therefore it will not ionize completely. Only a small amount of HF will ionize into the fluoride ion and the hydronium H3O+. So therefore, we need two arrows for this reaction. This reaction is a reversible reaction. It can go both ways. A single arrow represents a reaction that is not reversible or an irreversible reaction. Now H3O plus is the conjugate acid of water. Whenever you want to write the conjugate acid of a molecule, simply add an H plus to it. Fluoride is the conjugate base of HF. Anytime you want to write the conjugate base of something, remove an H plus from it. So this is a typical acid-base reaction. Now sometimes you may need to write the reaction using curve-arrow notation. So here's how we can show the mechanism for this reaction. So let's put a bond between hydrogen and fluorine. And let's redraw water. The oxygen part of water has two lone pairs. Oxygen needs to have eight electrons around it. Each bond represents two electrons. Fluorine has one bond but it has three lone pairs. Now the oxygen of water is going to form a bond between itself and hydrogen. Now because fluorine is more electronegative than hydrogen, when this bond breaks, fluorine is going to pull those two electrons to itself. And so water now has the extra hydrogen. So water has three hydrogens now. It used up a loam here to form a bond between itself and hydrogen. So now it has one lone pair but a positive charge. The fluoride ion, it lost a bond but it gained two electrons. By the way, a half arrow represents the flow of one electron. A full arrow represents the flow of two electrons. So in the case of water, these two electrons was used to form one of the bonds between H and O. And these two electrons are going to go into fluorine. So fluorine now has four lone pairs. So now it turned into the fluoride ion. Now let's consider another example. Let's combine ammonia and water. So go ahead and predict the products of this chemical reaction. Now ammonia is a weak base relative to water. So ammonia is going to be the brancelari base. Water is going to act as the brancelari acid. So because ammonia is the base, it's going to accept a proton from water. So when it absorbs a proton from water, it's going to change into the ammonium ion. When water loses a proton, it becomes hydroxide. Ammonium is the conjugate acid of ammonia and hydroxide is the conjugate base of water. Now let's write out the Kerr-Varrow notation for this reaction. So first let's draw water. I'm going to draw it like this. And then we can rewrite ammonia. So ammonia is the base and this loam here is going to be used to connect a bond between nitrogen and hydrogen. So whenever you draw the arrow, the arrow is going to flow from a region of high electron density to a region of low electron density. So typically, the arrow is going to start from Basically, the base or the nucleophile. We'll talk more about electrophiles and nucleophiles later. So ammonia is going to take a hydrogen from water. But keep in mind, what's happening is that this arrow, it shows the flow of electrons. The electrons is coming from the nitrogen atom. And those electrons will be used to connect a bond between nitrogen and hydrogen, as indicated by the bond in red. Now what's going to happen with the two electrons in this bond? Where is it going to go? Is it going to go with the hydrogen atom or the oxygen atom? So once ammonia removes hydrogen, those two electrons are going to be pulled. to the more electronegative of those two atoms. Oxygen is more electronegative than hydrogen. Hydrogen has an electronegativity value of 2.1. For oxygen, it's 3.5, based on a 4.0 scale. So because oxygen is more electronegative than hydrogen, when that bond breaks, those electrons will go to the oxygen. And so we're going to get hydroxide. with three lone pairs. So that's how we could show the curve error notation for that particular example. Now, let's go back to the reaction that we had here. You need to be familiar with something called K8, which... you've seen in general chemistry. Ka is the acid dissociation constant, in this case for HF. It's equal to the products, or more specifically the concentration of the products, divided by the concentration of the reactants. So HF is dissolved in water, and the same is true for H3O plus and F minus. What is a liquid? So keep in mind liquids and solids are not included in the equilibrium expression. Now pKa is negative log of Ka. But for organic chemistry you really don't need to use these formulas. What you need to know is the relationship between acid strength Ka and pKa. So as the strength of the acid increases, the value of Ka increases as well, but the value of pKa decreases. So strong acids have large Ka values but small pKa values. So for instance, which one is the stronger acid, HF or HTL? And let's say you're given the pKa of HF. which is 3.2, and the pKa of HCl, which is negative 7. So feel free to pause the video and think about this question. So which one is the stronger acid? Remember, acid strength increases with a decrease in pKa value. So the stronger acid will be the one with the lower pKa value. In this case, it's going to be HCl. HTL is classified as a strong acid. HF is classified as a weak acid. As we said before, weak acids, they ionize partially, whereas strong acids, they can ionize almost completely. So strong acids typically have pKa values that are in the negative range and even close to zero. Now let's look at another one. Which of these two acids is the stronger acid. Acetic acid or hydrosephoric acid. The Ka4 acetic acid is 1.8 times 10 to the negative 5. And the Ka4 hydrosephoric acid is approximately 9 times 10 to the negative 8. So here we're not dealing with pKa values, but we're dealing with Ka values. So using these two Ka values, which of these acids is a stronger acid? So remember, acid strength is directly related with Ka. So the stronger acid is going to have the larger Ka value. So which number is higher, so to speak? Negative 5 or negative 8? On a number line, negative 5 is higher than negative 8. So therefore, acetic acid is the stronger acid. Now let's work on a few example problems. Draw the conjugate acids and bases for the following molecules or ions. So let's start with water. How can we draw the conjugate acid of water? Whenever you want to draw the conjugate acid of something, simply add an H plus to it. If you combine water and H plus, you're going to get H3O+. Now to draw the conjugate base, simply remove an H+. If you take away H plus from water, you're going to get, you're going to have one oxygen, one hydrogen, and the charge is going to go from zero to negative one. So H3O plus is the conjugate acid of water, and hydroxide is the conjugate base of water. By the way, feel free to pause the video and try the other examples. So for ammonia, to write the conjugate acid, add an H plus to it. To write the conjugate base, remove an H plus. So we're going to have NH2 minus, which is the amide ion. Now let's move on to the next one. So here we have hydrogen sulfate, or bisulfate, and we're going to write the conjugate acid and the conjugate base. So to write the conjugate acid, remember, add an H+. So it's going to become H2SO4. When we add plus 1 to negative 1, we're going to get a neutral compound. Now, to write the conjugate base, we're going to remove an H+. So we're not going to have any hydrogens. in this substance and we're going to decrease the charge by 1. Negative 1 minus 1 is negative 2. So, sulfuric acid is the conjugate acid of hydrogen sulfate and sulfate is its conjugate base. Now let's try hydrogen phosphate. So to write the conjugate acid, we're going to increase the number of hydrogen atoms by 1. So we're going to have H2PO4, and then we're going to add 1 to the charge. Here we have a charge of 2 minus, or negative 2. If you add 1, you get negative 1. Now to write the conjugate base, we're going to remove a hydrogen, so it's going to be PO4, and we're going to decrease the charge by 1. Negative 2 minus 1 is negative 3. So phosphate is the conjugate base of hydrogen phosphate, and dihydrogen phosphate is the conjugate acid of hydrogen phosphate. You can also call this monohydrogen phosphate. Now how would you answer this question? Which base is stronger? The methoxide ion or the ethoxide ion? How can we find the answer? What you need to know is that the stronger the acid, the weaker the conjugate base. So if you want to identify the base that is stronger, you need to look for the weaker conjugate acid. So let's write the conjugate acid of each base. All we need to do is add a hydrogen. Now we know the pKa for methanol, it's given to us, it's 15.5. And the pKa for ethanol is 15.7. So which one is the weaker acid? The stronger acid has the lower pKa value and the weaker acid has the higher pKa value. So methanol is slightly stronger in acidity than ethanol. So relative to each other, ethanol is the weaker acid, methanol is the stronger acid. So then the opposite is true for the conjugate base. If ethanol is the weaker acid, Ethoxide is the stronger base. And if methanol is the stronger acid, methoxide is the weaker base. So to answer the question, it's going to be ethoxide. Ethoxide is a stronger base than methoxide. It has a stronger affinity for a proton. Now let's review a few things. Earlier in this video, we said that a brancelari acid is a proton donor. whereas a Brontolari base is a proton acceptor. Now you need to be familiar with the Lewis definition of acids and bases. A Lewis acid is an electron pair acceptor, whereas a Lewis base is an electron pair donor. Lewis acids are electrophiles, while Lewis bases are nucleophiles. Lewis bases, they tend to have a lone pair that they can donate. So hydroxide could behave as a Lewis base. Fluoride could behave as a Lewis base because they have electrons that they can donate. Lewis acids, they are electron pair acceptors. So typically, they can have a positive charge like H+, or they could simply... just be able to accept a pair of electrons. In this case, aluminum has three bonds. It's attached to three chlorine atoms. It can accept a pair of electrons. So AlCl3 is a Lewis acid. Another Lewis acid is FeCl3 and BH3. Each of these have three bonds and they can accept a pair of electrons to make a fourth bond. So those are Lewis acids. Ammonia, which has three bonds, it's a Lewis space because it has a lone pair. Note the difference between BH3 and NH3. NH3 has three bonds with a lone pair. BH3 doesn't have that lone pair. And that difference... makes it a Lewis acid, whereas NH3 is a weak base but also a Lewis base. Electrophiles are electron poor. They lack electrons, whereas nucleophiles are electron rich. They have plenty of lone pairs, or sometimes just one. Now, if you think of the word, or the root word, fill, fill means love or attraction. Electrophiles, they're attracted to electrons, or negatively charged particles. they are willing to accept an electron pair. Nucleophiles, well they're attracted to the positively charged nucleus. So nucleophiles tend to have a lot of electrons. They tend to be electron rich. They can donate a pair of electrons. So these are some things that can help you to understand the difference between Lewis acids and Lewis bases. So remember, a Lewis acid is an electrophile, and a Lewis base is a nucleophile. Now let's work on some Lewis acid-base reactions. Go ahead and write the reaction between AlCl3 and NH3. And also, do the same for, let's say, BH3 and F-. So let's start with the first one. So first I'm going to draw AlCl3. And let's put three chlorine atoms. And then let's draw ammonia. Ammonia has a lone pair. Aluminum has a basically an empty orbital, which ammonia can fill those electrons into. So the only arrow that we need to show is one arrow that's going to go from n to al. These two electrons will be used to create a bond between aluminum and nitrogen. So the product of this Lewis, this acid-base Lewis reaction, is this. When aluminum has four bonds, it's going to have a negative formal charge. When nitrogen has four bonds, it's going to have a positive formal charge. So overall, this product is neutral. Now we can clearly see that AlCl3 is the Lewis acid. Ammonia is the Lewis base. The nitrogen atom in ammonia donated a pair of electrons, which makes it the Lewis base or the nucleophile. Aluminum. accepted that pair of electrons. So aluminum is the Lewis acid or the electrophile. So that's a typical example of a Lewis acid-base reaction. Now for those of you who want to know how we can get these formal charges, there's a formula that you could use. The formal charge is equal to the number of valence electrons minus the number of bonds and dots on that element. So let's focus on aluminum. According to the periodic table, aluminum has three valence electrons. In this particular structure, it has a total of four bonds. But it has no dots or lone pairs. Each lone pair represents two electron dots. So 3 minus 4 is negative 1. That's how we can get the negative formal charge on aluminum. Now for nitrogen, nitrogen is in group 5a of the periodic table, so therefore it has 5 valence electrons. It has 4 bonds and no lone pairs or electron dots. So it's 5 minus 4, thus we get positive 1. So this is the formula that you could use if you want to calculate the formal charge of an element. That is, when that element is in a compound. Now let's look at this example. So let's draw a fluoride and then let's react it with BH3. Now, which one is the Lewis acid and which one is the Lewis base? Well, we know the Lewis base has to be fluoride. It's electron-rich. It can donate a pair of electrons. Boron can accept that pair of electrons, so boron is going to be the Lewis acid. You can also say that BH3 is the Lewis acid because it contains boron. So fluoride is going to donate one pair of electrons, which I'm going to indicate by the line in red. And so we're going to get this structure as our product. So now fluoride, it lost a lone pair, now it has three. Now all we need to do now is add a negative charge to boron. Fluoride lost this negative charge when it gave up a pair of electrons. But boron acquired that negative charge when it... accepted a pair of electrons. Now let's move on to another topic. Consider this reaction between fluoride and methanol. What do you think the products of this reaction will be? Well, we know fluoride is a weak base. So intuitively, we can imagine that this is going to be the base in this reaction. It's going to take a proton from methanol, turn in methanol into methoxide. And when fluoride acquires the proton, it's going to become HF. But now, here's the question for you. What type of arrows should we put here? Is this going to be an irreversible reaction? Should we use a single arrow? Will it be equally reversible? Should we use a double arrow of equal length? Or will it be unequally reversible, where one arrow will be larger than the other? So just to take some notes, this is an irreversible reaction when you have a single arrow. When you have a double arrow, it's reversible. So we have three situations where it could be reversible. Any one of these three are reversible reactions. However, for this one, This would indicate a product favored reaction. And then one on the left indicates a reactant favored reaction. So sometimes in organic chemistry, when you're dealing with acids and bases, you might get a question that asks you, is the reaction product favored or reactant favored? If it's product favored, you want to use this symbol to indicate it. If it's reactant favored, you want to use this symbol or those arrows to indicate it. So how can we find the answer? What we need to do is we need to compare the pKa of the acid and the conjugate acid. So we know that the acid on the left is methanol. It's the proton donor. The pKa of methanol is 15.5. HF is a weak acid, and its pKa is 3.2. So which acid is stronger? The stronger acid is the one with the lower pKa value. The weaker acid is the one with the higher pKa value. The stronger acid is more reactive, which means that it's less stable. The weaker acid is less reactive, which means that it's more stable. So make sure you understand that. Strong acids are very reactive and they tend to be less stable. Weak acids, they're less reactive and they're more stable. So will the reaction go towards a side that is more stable or less stable? Well, in nature, things tend to go towards stability. To illustrate this, let's say, imagine you have a ball on top of a hill. This ball is in an unstable position because it can fall. It can roll down to the hill. But once it gets to the bottom, it's going to come to a stop. So at that point, it's in a stable position. So things tend to have a tendency to move towards a more stable situation. If we look at the reverse, if the ball is at the bottom of the hill, it's not going to go up the hill on its own. It's not going to go from a stable position to an unstable position without any outside intervention. The only way you could bring it up the hill is you need to apply energy to it. You have to do work on the ball. So without applying some external force, that ball is not going to go to an unstable position. So what you need to take from this is that the reaction is going to shift in the direction toward increasing stability. So it's going to shift to the side with the weaker acid or the higher pKa value. So this reaction is not completely irreversible. So we're going to use a double arrow because it could go to the right. However, it's reactant favored. So we're going to have a longer arrow pointing towards the left. So you could say it's unequally reversible.

HF, hydrofluoric acid, that's an acid. HCl, hydrochloric acid, that's another one. NH4+, the ammonium ion, that acts as a weak acid.

H3O+, that's another example. Acetic acid, which has a carbosilic acid function group, that's another acid that you'll see in organic chemistry. Now here's some typical bases. Hydroxide is a base, fluoride is a weak base, ammonium is also a weak base, and the conjugate of a carboxylic acid is also a weak base. So those are some common bases that you'll see in organic chemistry.

Now anytime you put an acid in water, The pH of the solution will be less than 7. Whenever you put a base in water, the pH of the solution will be greater than 7. Water, being neutral, has a pH of approximately 7. So just keep that in mind. Acids, they will create a solution with a low pH, while bases will create a solution with a high pH. Now let's discuss acid-base reactions. Let's mix hydrofluoric acid with water. Now how will these two react?

What will be the products of this chemical reaction? According to the Bronsolari definition of acids, acids are proton donors, bases are proton acceptors. A proton is basically a hydrogen ion.

Hydrofluoric acid is going to behave as the Bronsolari acid. It's going to give up hydrogen. Water in this reaction will behave as the Bronsolari base. It's going to accept the proton.

So HF is going to lose the hydrogen, it's going to become fluoride, water is going to gain the hydrogen ion, and it's going to become H3O+, or the hydronium ion. Now, HF is a weak acid, therefore it will not ionize completely. Only a small amount of HF will ionize into the fluoride ion and the hydronium H3O+.

So therefore, we need two arrows for this reaction. This reaction is a reversible reaction. It can go both ways. A single arrow represents a reaction that is not reversible or an irreversible reaction. Now H3O plus is the conjugate acid of water.

Whenever you want to write the conjugate acid of a molecule, simply add an H plus to it. Fluoride is the conjugate base of HF. Anytime you want to write the conjugate base of something, remove an H plus from it.

So this is a typical acid-base reaction. Now sometimes you may need to write the reaction using curve-arrow notation. So here's how we can show the mechanism for this reaction.

So let's put a bond between hydrogen and fluorine. And let's redraw water. The oxygen part of water has two lone pairs.

Oxygen needs to have eight electrons around it. Each bond represents two electrons. Fluorine has one bond but it has three lone pairs. Now the oxygen of water is going to form a bond between itself and hydrogen.

Now because fluorine is more electronegative than hydrogen, when this bond breaks, fluorine is going to pull those two electrons to itself. And so water now has the extra hydrogen. So water has three hydrogens now.

It used up a loam here to form a bond between itself and hydrogen. So now it has one lone pair but a positive charge. The fluoride ion, it lost a bond but it gained two electrons.

By the way, a half arrow represents the flow of one electron. A full arrow represents the flow of two electrons. So in the case of water, these two electrons was used to form one of the bonds between H and O. And these two electrons are going to go into fluorine.

So fluorine now has four lone pairs. So now it turned into the fluoride ion. Now let's consider another example. Let's combine ammonia and water.

So go ahead and predict the products of this chemical reaction. Now ammonia is a weak base relative to water. So ammonia is going to be the brancelari base. Water is going to act as the brancelari acid. So because ammonia is the base, it's going to accept a proton from water.

So when it absorbs a proton from water, it's going to change into the ammonium ion. When water loses a proton, it becomes hydroxide. Ammonium is the conjugate acid of ammonia and hydroxide is the conjugate base of water.

Now let's write out the Kerr-Varrow notation for this reaction. So first let's draw water. I'm going to draw it like this.

And then we can rewrite ammonia. So ammonia is the base and this loam here is going to be used to connect a bond between nitrogen and hydrogen. So whenever you draw the arrow, the arrow is going to flow from a region of high electron density to a region of low electron density.

So typically, the arrow is going to start from Basically, the base or the nucleophile. We'll talk more about electrophiles and nucleophiles later. So ammonia is going to take a hydrogen from water. But keep in mind, what's happening is that this arrow, it shows the flow of electrons. The electrons is coming from the nitrogen atom.

And those electrons will be used to connect a bond between nitrogen and hydrogen, as indicated by the bond in red. Now what's going to happen with the two electrons in this bond? Where is it going to go? Is it going to go with the hydrogen atom or the oxygen atom? So once ammonia removes hydrogen, those two electrons are going to be pulled.

to the more electronegative of those two atoms. Oxygen is more electronegative than hydrogen. Hydrogen has an electronegativity value of 2.1. For oxygen, it's 3.5, based on a 4.0 scale. So because oxygen is more electronegative than hydrogen, when that bond breaks, those electrons will go to the oxygen.

And so we're going to get hydroxide. with three lone pairs. So that's how we could show the curve error notation for that particular example. Now, let's go back to the reaction that we had here.

You need to be familiar with something called K8, which... you've seen in general chemistry. Ka is the acid dissociation constant, in this case for HF. It's equal to the products, or more specifically the concentration of the products, divided by the concentration of the reactants.

So HF is dissolved in water, and the same is true for H3O plus and F minus. What is a liquid? So keep in mind liquids and solids are not included in the equilibrium expression. Now pKa is negative log of Ka.

But for organic chemistry you really don't need to use these formulas. What you need to know is the relationship between acid strength Ka and pKa. So as the strength of the acid increases, the value of Ka increases as well, but the value of pKa decreases. So strong acids have large Ka values but small pKa values. So for instance, which one is the stronger acid, HF or HTL?

And let's say you're given the pKa of HF. which is 3.2, and the pKa of HCl, which is negative 7. So feel free to pause the video and think about this question. So which one is the stronger acid?

Remember, acid strength increases with a decrease in pKa value. So the stronger acid will be the one with the lower pKa value. In this case, it's going to be HCl. HTL is classified as a strong acid.

HF is classified as a weak acid. As we said before, weak acids, they ionize partially, whereas strong acids, they can ionize almost completely. So strong acids typically have pKa values that are in the negative range and even close to zero. Now let's look at another one. Which of these two acids is the stronger acid.

Acetic acid or hydrosephoric acid. The Ka4 acetic acid is 1.8 times 10 to the negative 5. And the Ka4 hydrosephoric acid is approximately 9 times 10 to the negative 8. So here we're not dealing with pKa values, but we're dealing with Ka values. So using these two Ka values, which of these acids is a stronger acid?

So remember, acid strength is directly related with Ka. So the stronger acid is going to have the larger Ka value. So which number is higher, so to speak? Negative 5 or negative 8?

On a number line, negative 5 is higher than negative 8. So therefore, acetic acid is the stronger acid. Now let's work on a few example problems. Draw the conjugate acids and bases for the following molecules or ions. So let's start with water.

How can we draw the conjugate acid of water? Whenever you want to draw the conjugate acid of something, simply add an H plus to it. If you combine water and H plus, you're going to get H3O+. Now to draw the conjugate base, simply remove an H+. If you take away H plus from water, you're going to get, you're going to have one oxygen, one hydrogen, and the charge is going to go from zero to negative one.

So H3O plus is the conjugate acid of water, and hydroxide is the conjugate base of water. By the way, feel free to pause the video and try the other examples. So for ammonia, to write the conjugate acid, add an H plus to it. To write the conjugate base, remove an H plus.

So we're going to have NH2 minus, which is the amide ion. Now let's move on to the next one. So here we have hydrogen sulfate, or bisulfate, and we're going to write the conjugate acid and the conjugate base.

So to write the conjugate acid, remember, add an H+. So it's going to become H2SO4. When we add plus 1 to negative 1, we're going to get a neutral compound.

Now, to write the conjugate base, we're going to remove an H+. So we're not going to have any hydrogens. in this substance and we're going to decrease the charge by 1. Negative 1 minus 1 is negative 2. So, sulfuric acid is the conjugate acid of hydrogen sulfate and sulfate is its conjugate base.

Now let's try hydrogen phosphate. So to write the conjugate acid, we're going to increase the number of hydrogen atoms by 1. So we're going to have H2PO4, and then we're going to add 1 to the charge. Here we have a charge of 2 minus, or negative 2. If you add 1, you get negative 1. Now to write the conjugate base, we're going to remove a hydrogen, so it's going to be PO4, and we're going to decrease the charge by 1. Negative 2 minus 1 is negative 3. So phosphate is the conjugate base of hydrogen phosphate, and dihydrogen phosphate is the conjugate acid of hydrogen phosphate. You can also call this monohydrogen phosphate.

Now how would you answer this question? Which base is stronger? The methoxide ion or the ethoxide ion?

How can we find the answer? What you need to know is that the stronger the acid, the weaker the conjugate base. So if you want to identify the base that is stronger, you need to look for the weaker conjugate acid. So let's write the conjugate acid of each base. All we need to do is add a hydrogen.

Now we know the pKa for methanol, it's given to us, it's 15.5. And the pKa for ethanol is 15.7. So which one is the weaker acid?

The stronger acid has the lower pKa value and the weaker acid has the higher pKa value. So methanol is slightly stronger in acidity than ethanol. So relative to each other, ethanol is the weaker acid, methanol is the stronger acid. So then the opposite is true for the conjugate base.

If ethanol is the weaker acid, Ethoxide is the stronger base. And if methanol is the stronger acid, methoxide is the weaker base. So to answer the question, it's going to be ethoxide.

Ethoxide is a stronger base than methoxide. It has a stronger affinity for a proton. Now let's review a few things. Earlier in this video, we said that a brancelari acid is a proton donor.

whereas a Brontolari base is a proton acceptor. Now you need to be familiar with the Lewis definition of acids and bases. A Lewis acid is an electron pair acceptor, whereas a Lewis base is an electron pair donor. Lewis acids are electrophiles, while Lewis bases are nucleophiles. Lewis bases, they tend to have a lone pair that they can donate.

So hydroxide could behave as a Lewis base. Fluoride could behave as a Lewis base because they have electrons that they can donate. Lewis acids, they are electron pair acceptors. So typically, they can have a positive charge like H+, or they could simply... just be able to accept a pair of electrons.

In this case, aluminum has three bonds. It's attached to three chlorine atoms. It can accept a pair of electrons.

So AlCl3 is a Lewis acid. Another Lewis acid is FeCl3 and BH3. Each of these have three bonds and they can accept a pair of electrons to make a fourth bond. So those are Lewis acids.

Ammonia, which has three bonds, it's a Lewis space because it has a lone pair. Note the difference between BH3 and NH3. NH3 has three bonds with a lone pair.

BH3 doesn't have that lone pair. And that difference... makes it a Lewis acid, whereas NH3 is a weak base but also a Lewis base.

Electrophiles are electron poor. They lack electrons, whereas nucleophiles are electron rich. They have plenty of lone pairs, or sometimes just one. Now, if you think of the word, or the root word, fill, fill means love or attraction. Electrophiles, they're attracted to electrons, or negatively charged particles.

they are willing to accept an electron pair. Nucleophiles, well they're attracted to the positively charged nucleus. So nucleophiles tend to have a lot of electrons. They tend to be electron rich.

They can donate a pair of electrons. So these are some things that can help you to understand the difference between Lewis acids and Lewis bases. So remember, a Lewis acid is an electrophile, and a Lewis base is a nucleophile. Now let's work on some Lewis acid-base reactions.

Go ahead and write the reaction between AlCl3 and NH3. And also, do the same for, let's say, BH3 and F-. So let's start with the first one.

So first I'm going to draw AlCl3. And let's put three chlorine atoms. And then let's draw ammonia.

Ammonia has a lone pair. Aluminum has a basically an empty orbital, which ammonia can fill those electrons into. So the only arrow that we need to show is one arrow that's going to go from n to al.

These two electrons will be used to create a bond between aluminum and nitrogen. So the product of this Lewis, this acid-base Lewis reaction, is this. When aluminum has four bonds, it's going to have a negative formal charge. When nitrogen has four bonds, it's going to have a positive formal charge.

So overall, this product is neutral. Now we can clearly see that AlCl3 is the Lewis acid. Ammonia is the Lewis base. The nitrogen atom in ammonia donated a pair of electrons, which makes it the Lewis base or the nucleophile. Aluminum.

accepted that pair of electrons. So aluminum is the Lewis acid or the electrophile. So that's a typical example of a Lewis acid-base reaction.

Now for those of you who want to know how we can get these formal charges, there's a formula that you could use. The formal charge is equal to the number of valence electrons minus the number of bonds and dots on that element. So let's focus on aluminum. According to the periodic table, aluminum has three valence electrons.

In this particular structure, it has a total of four bonds. But it has no dots or lone pairs. Each lone pair represents two electron dots. So 3 minus 4 is negative 1. That's how we can get the negative formal charge on aluminum. Now for nitrogen, nitrogen is in group 5a of the periodic table, so therefore it has 5 valence electrons.

It has 4 bonds and no lone pairs or electron dots. So it's 5 minus 4, thus we get positive 1. So this is the formula that you could use if you want to calculate the formal charge of an element. That is, when that element is in a compound. Now let's look at this example.

So let's draw a fluoride and then let's react it with BH3. Now, which one is the Lewis acid and which one is the Lewis base? Well, we know the Lewis base has to be fluoride.

It's electron-rich. It can donate a pair of electrons. Boron can accept that pair of electrons, so boron is going to be the Lewis acid.

You can also say that BH3 is the Lewis acid because it contains boron. So fluoride is going to donate one pair of electrons, which I'm going to indicate by the line in red. And so we're going to get this structure as our product. So now fluoride, it lost a lone pair, now it has three. Now all we need to do now is add a negative charge to boron.

Fluoride lost this negative charge when it gave up a pair of electrons. But boron acquired that negative charge when it... accepted a pair of electrons.

Now let's move on to another topic. Consider this reaction between fluoride and methanol. What do you think the products of this reaction will be? Well, we know fluoride is a weak base.

So intuitively, we can imagine that this is going to be the base in this reaction. It's going to take a proton from methanol, turn in methanol into methoxide. And when fluoride acquires the proton, it's going to become HF. But now, here's the question for you.

What type of arrows should we put here? Is this going to be an irreversible reaction? Should we use a single arrow? Will it be equally reversible?

Should we use a double arrow of equal length? Or will it be unequally reversible, where one arrow will be larger than the other? So just to take some notes, this is an irreversible reaction when you have a single arrow.

When you have a double arrow, it's reversible. So we have three situations where it could be reversible. Any one of these three are reversible reactions. However, for this one, This would indicate a product favored reaction.

And then one on the left indicates a reactant favored reaction. So sometimes in organic chemistry, when you're dealing with acids and bases, you might get a question that asks you, is the reaction product favored or reactant favored? If it's product favored, you want to use this symbol to indicate it. If it's reactant favored, you want to use this symbol or those arrows to indicate it. So how can we find the answer?

What we need to do is we need to compare the pKa of the acid and the conjugate acid. So we know that the acid on the left is methanol. It's the proton donor.

The pKa of methanol is 15.5. HF is a weak acid, and its pKa is 3.2. So which acid is stronger? The stronger acid is the one with the lower pKa value.

The weaker acid is the one with the higher pKa value. The stronger acid is more reactive, which means that it's less stable. The weaker acid is less reactive, which means that it's more stable. So make sure you understand that.

Strong acids are very reactive and they tend to be less stable. Weak acids, they're less reactive and they're more stable. So will the reaction go towards a side that is more stable or less stable?

Well, in nature, things tend to go towards stability. To illustrate this, let's say, imagine you have a ball on top of a hill. This ball is in an unstable position because it can fall. It can roll down to the hill. But once it gets to the bottom, it's going to come to a stop.

So at that point, it's in a stable position. So things tend to have a tendency to move towards a more stable situation. If we look at the reverse, if the ball is at the bottom of the hill, it's not going to go up the hill on its own. It's not going to go from a stable position to an unstable position without any outside intervention. The only way you could bring it up the hill is you need to apply energy to it.

You have to do work on the ball. So without applying some external force, that ball is not going to go to an unstable position. So what you need to take from this is that the reaction is going to shift in the direction toward increasing stability.

So it's going to shift to the side with the weaker acid or the higher pKa value. So this reaction is not completely irreversible. So we're going to use a double arrow because it could go to the right. However, it's reactant favored.

So we're going to have a longer arrow pointing towards the left. So you could say it's unequally reversible.

Transcript for:Basics of Acids and Bases in Chemistry

Transcript for:
Basics of Acids and Bases in Chemistry