[MUSIC] Greetings, and welcome back to Introduction to Chemistry. So far we have looked at species dissolving and precipitating, going into and out of solution, and we've learned how to write chemical reaction equations for these processes. You can think of these processes as dissociation ad reforming a bionic compound crystals, but the electrons are staying with the anions in both of these types of reactions, aren't they? In this lecture, I will introduce two big classes of reactions, that involve electrons moving between atoms. Acid, base, and redox reactions. Then, I'll focus on some types of acid-base reactions. Do you remember the demonstration involving electrolytes? We're going to use the same lightbulb apparatus to explore some chemical reactions. Just to remind you, the light bulb remained dim when the solution was a non electrolyte. Meaning that there were no free ions in the solution to conduct electricity. If there were many, many free ions in solution, then the light bulb gave a very bright light. And there was lots of gray area in between for weak electrolyte solutions where the light bulb film it came on to some extent but was fairly dim. Do you recall how well the dilute aqueous ammonia or dilute aqueous acetic acid solutions conducted electricity? First we're going to review that demonstration of the 0.1 molar solutions of ammonia or acetic acid in water. We're going to test them using the lightbulb apparatus. As you can see they both do conduct electricity to some extent. Resulted in a dim glow of the light bulb filament. We would classify both of these solutions as weak electrolytes. But now let's predict what will happen when we mix the solutions together. First let’s recall what they look like. The 0.1 molar ammonia solution resulted in a dim glow of the filament and the 0.1 molar acetic acid solution, which is essentially vinegar, also resulted in a dim glow of the filament. So now what Dr. Lyle is going to do is pour both solutions together. Then we're going to test that solution with a light bulb apparatus to see if it results in a bright glow of the bulb, no glow of the bulb, or something in between. Remember that a bright glow means there's a strong electrolyte solution and that no glow means it's a non-electrolyte solution. Here are some options for you to choose from. What do you think will happen when he mixes the solutions? Go ahead and record your prediction now. Thank you for making your prediction. Were you able to make the correct prediction on your first try? Let’s conduct the experiment now to see if the answer on the previous InVideo question matches the results. Dr. Lyle has the electrodes both submerged in the dilute ammonia solution, and now he’s going to pour in the dilute acetic acid solution. Wow the light really does get much brighter. So the resulting solution must be a strong electrolyte. What just happened in terms of the chemistry of mixing those two compounds? This is the structure of ammonia. I've drawn the nitrogen and the three hydrogens, and between the nitrogen and hydrogens I've drawn these lines which show bonds. So the lines are used to show the bonds, and this pair of dots is called an electron lone pair. If you look on the periodic table, I'm going to just show another pair of dots over here and label it, lone pair. If you look on the Periodic Table, you see that nitrogen is in group 15 or group five A, depending on which numbering system you'll use. That means that nitrogen has five valence electrons. The valence electrons are the electrons in the outermost shell, those that are involved in the bonding. So nitrogen has five valence electrons. Two of those valence electrons are showing here as dots, and the other three valence electrons were each used to make one bond. So the nitrogen made three bonds and each of those bonds used one of nitrogen's valence electrons, and it has a lone pair left over. In water nitrogen reacts to a small extent to make a very small number of free ions, that's why that lightbulb had a dim glow. But now I'm going to mix the ammonia with some acetic acid. Here's the structure of acetic acid. Similar to the structure of ammonia, we're using lines and dots to show the electrons. But we're only showing the valence electrons, or the electrons in the outermost shells of each type of atom. Hydrogen only has one valence electron in its shell, doesn't it? So the hydrogens can only have one bond to them, or they could have an unpaired electron. The carbon, if you look at the periodic table, you'll see that carbon is in group 14 or group 4A if you're numbering the main group elements that way. So carbon should have an octet of electrons around it and that comes from each of carbon's four valence electrons making one bond. So the bonds are a pair of electrons. In this case, let me just circle one, one of the electrons came from hydrogen, and the other electron came from carbon. Carbon then has its own four valence electrons surrounding it, but it also has four electrons that were donated by the other atoms that are making the bonds with the carbon. Both of the oxygens here have two lone pairs. Look at the periodic table, which group does oxygen belong to? That's right. Oxygen is in group 16, or 6A, depending on how we number it. That means Oxygen has six valence electrons. If we look at these oxygens we could count one two three, four five six. I'm counting the bonds as each being one electron came from the oxygen. Both oxygens are in the same situation here. They each have two bonds to them and they each have two lone pairs. But what happened when we mix the ammonia with the acetic acid? Well what happened is those species are mostly intact in solution. They react a little, a little bit to a small extent with water to make ions. Right now, though the species are not ions. The species are neutral compounds, but when we mix the ammonia with the acetic acid chemistry happens, and the ammonium polyatomic ion is formed as well as the acetate polyatomic anion. So lots of ions have formed every time an ammonia bumps into an acetic acid, which is what happens when we mix them together. This is an example of an acid base reaction. What's the mechanism of this reaction though? In other words how do the electrons move around to cause the formation of the ions? Well, we can show the movement of the electrons using what's called Curved Arrow Notation. If you look at the picture, one of the differences you can probably see is that the red Lone Pair on the nitrogen is going to become this red bond between the nitrogen and the hydrogen on the product. In order to do that, it needs to grab on to the hydrogen that's one the acetic acid. And we can show that by drawing an arrow that starts with the lone pair and points to this hydrogen. It's actually not taking a hydrogen atom, because a hydrogen atom would have one proton and one electron. What the nitrogen is doing, when it plucks that H off, is it's actually only taking the proton. It's deprotonating the acetic acid. Both of the electrons in the bond, these electrons right here, both of those electrons are going to stay with the oxygen. So the hydrogen's being removed, but it's not taking its electrons with it. And the way we show that is with a second curved arrow. This curved arrow right here shows that both electrons from the bond are staying with the oxygen. Now let's look at the products. Why do I have these charges written on the products? In other words on the ammonium I have a plus 1 charge, and on the oxygen I have a minus 1 charge. And there have been some excellent questions on the course forum about why polyatomic ions have certain charges. Well, for ammonium, nitrogen should have five valence electrons, shouldn't it? But this particular nitrogen, doesn't formally own five valence electrons. Let's go over here to the nitrogen on the reactant side again. The nitrogen on the reactant side, if we counted the electrons, formally owned, this electron and this electron that are shown by the dots. So it formally owns those two that I just colored green. And then it contributed one electron to each one of the single bonds so I can count those as well, so I have three single bonds, one, two, three that each were a electron. That each came from an electron from the nitrogen, and the other electron came from the hydrogen. And then I have two from the lone pair. So this nitrogen had five electrons in its valence shell, in its outermost shell. Now, let's look at the compound on the product side. In this case, we know that nitrogen should have five valence electrons. That's how many valence electrons it has if we look at the periodic table. But this nitrogen no longer formally owns five electrons. It owns one electron from each of its bonds, but it doesn't have a lone pair, so it formally only owns four electrons. So that means this nitrogen has a formal charge of plus 1, and that's what this plus 1 charge is. We can perform a similar exercise on the oxygen of the acetate anion on the product side. How many valance electrons should oxygen have? Look on the periodic table to see which group it's in. Correct. Oxygen should have six valence electrons. And if we look at the oxygen on the top of this compound we see that it does indeed formerly own six. One, two, three, four, five, six. But the oxygen on the lower right, if you count up its electrons, it has six electrons from the dot, three, four, five, six but it has a seventh electron because he gets half of the bonding electrons. There’s two electrons in each bond and oxygen formally owns one. So if we say six which is how many it should have, in it's Valence Shell, and we subtract seven, which is how many this particular oxygen does have. That subtraction gives us an answer of minus 1, which is the charge on the acetate and ion. So by mixing these solutions, we went from having a very small amount of ions that just resulted from the acetic acid and the ammonia reacting to a small extent with the water, to having a huge number of ions. Because every single ammonia molecule reacted with every single acetic acid molecule, assuming that we have an exact number. Assuming that we have exactly the same number of each type of molecule. So that's why the light bulb got very, very bright. We've made lots and lots of ions. So let's just summarize the different types of reactions that can occur when electrons are moving between atoms. So when we did the dissolution and the precipitation, recall that the electrons are staying with the species that they started with. For example, if you had sodium chloride, the electrons are all staying with the chloride and it's the cullomic attraction that holds the sodium to the chloride. But in this case, the electrons are moving to lower energy by moving between atoms. There's two ways they can do this. One way is that the electrons can be completely transferred from one atom to the other. Is that what happened in the reaction we just saw? Did we see an electron move between molecules? No, actually, we saw a proton move between molecules. One of the things that can happen, and this is not what we observed in the last reaction, is that electrons can actually transfer between atoms. If this is what's happening, we call that a Redox rxn, or an oxidation reduction reaction. In Redox rxn, the electrons can move to lower energy by transferring from one molecule to another molecule or transferring from one atom to another atom. What we just saw happen wasn't electron's transferring. Was it? What we saw happening was the proton moving, that H plus moved. And when it moved, the electrons of the nitrogen were being shared with that proton. When there's a chemical reaction that involves sharing of electrons, a species that has a lone pair shares it with a proton. That's an acid-base reaction. The base, which in this case was the ammonia, shares the electrons with the acid, and that often involves a proton transfer. Please tune in to the next lecture to learn some different definitions of acid and bases and to see more examples of acid base reactions.
