[MUSIC] In this video I'll be talking about solutions, and more specifically, aqueous solutions which are compounds dissolved in water. Now, the biggest aqueous solution on Earth is, of course, the oceans. The oceans contain almost entirely water and that's a chemical. But there are also other compounds dissolved in the water, such as oxygen so that the fish can breath and all kinds of salts. We know that the ocean is salt water. Those salts produce a wide variety of ions when they dissolve. Here's a table that shows some of the ions that are present in typical samples of ocean water. You can actually taste these compounds if you get a mouthful of water in the ocean. You can taste the dissolved sodium and chloride ions. Let's look at the structure of water first. And then we'll move on to look at the structures of the ionic compounds that can dissolve in water. Obviously water is the most abundant component of the ocean. It's the solvent, the species that's present in the greatest amount in the solution. So I think starting to talk about water is a good place to start. First I'm going to show you some cartoons that give the structure of a water molecule. Water, of course, has the chemical structure H2O. And then I can draw a little cartoon that shows, in this case, the oxygen is shown as yellow, and that was just my choice to pick yellow. And the two hydrogens are shown as small pick spheres, and again, pink was just my choice. In this case we know that the oxygen is more electro-negative than the hydrogens. So a water molecule has part of the molecule that has a greater density of electrons than the other part. In other words the oxygen, which here is yellow, is, pardon me, partially negative. And the two hydrogens, which are not as electro-negative, are not as attractive to the electrons. And so they're each partially positive. This little delta symbol, a Greek lowercase d, means partially. It's showing a partial charge. In bulk water, at room temperature and normal atmospheric pressure, there are billions and billions of water, which are very loosely organized. We can look at the sample over here on the left. If we were able to zoom in to look at just a few water molecules, we would see that they're grouped together, and they're not particularly well organized. They can move around. They're very fluid. Waters can spin in place, and they can easily slide past each other because it's in the liquid state. Sodium chloride, on the other hand, is a solid salt. And the particles in sodium chloride are extremely highly organized into a lattice in the solid state. Here are some cartoons depicting the sodium ion and chloride ions in a salt crystal. This is just what a, a crystal of table salt would look like if you zoomed in with a microscope. So if I take a little tiny bit of that and I am able to use a microscope to see it better, I can see that there are alternating chloride ions and sodium ions. But they're very well organized into a lattice where I alternate between chloride, sodium, chloride, sodium, chloride as I move through the arrangement in space. I'm going to show these throughout this presentation as these spheres. The sodium is going to be a pink sphere or kind of a reddish sphere with a positive charge, and the chloride is going to be a blue sphere with a negative charge. They aren't exactly drawn to scale. A sodium cation actually has a radius of 116 pica meters while a chloradine has a radius of 167 pica meters. But you get the idea, the chloride is slightly larger and it's negatively charged. Now, this pictures a little bit misleading because there's not exactly water in these empty spaces and in fact, there's not as much empty spaces as shown. The chloradines and the sodium ions are touching, along these axis like this. So they're shown much smaller than they really are in this picture. I just want people to understand that most of this base is filled up by the electrons of the ions. It's not water and it's, and some of it's empty space, but most of it is filled up by electrons. What is happening in the dissolution process of ionic compound? We can draw the chemical reaction equation for this process. And from this chemical equation, it looks like the ions are separating when the solid sodium chloride is added to the liquid water. And that's exactly what happens. This is called a dissolution equation. It's the first type of chemical equation that we're learning to write in this class. A dissolution equation shows a species that is solid dissolving and turning into aqueous species. In this case, because the solid was an ionic compound, when it dissolves it, those ionic compounds dissociated from each other to make two separate ions, the cations and the anions. So, the sodium and the chloride ions, remember initially were touching. If we were just show two of them, we could show them touching. They're touching in that lattice in a very large three dimensional array. And once we dissolve them in water, they are no longer touching each other. They dissociate from each other. So now the sodium cation and chloride anion are separate. They can move around separately and they can react separately, on the product side of this equation. So the reactant here was the sodium chloride. And the products here are these anions and cations which are both aqueous because they're now dissolved in the water. Each sodium chloride unit makes two ions. Those are the ions that form. And what happens is that those ions don't just move apart from each other and dissociate but water gets in between the ions and keeps them from going back together. We know from Coulomb's law that the opposite charges are attracted to each other. And it's only this layer of water, which might have even more water molecules than is shown there. Probably has many more actually. It's that layer of water that is able to prevent the cation and the anion from going back together and making a solid again. In fact, it's not just the pair of ions that separate, in fact, many, many ions that were in that piece of salt separate during the dissolution process. And I've shown just a few of these in a cartoon. So you can imagine this happening with billions of these sodium cations and chloride anions. I start with them all touching each other in that crystal lattice. But then, once I add water to the salt, the ions start to break apart. Here they are broken apart. And the reason they're breaking apart is that the water can get in between the ions. And there's some calculations you can do with a chemistry math called thermodynamics that you can calculate the heat that is either absorbed or evolved by this process of performing a solution. The solvation process. To summarize, the water molecules go between the cations and the anions, and keep them separated. And that is the case as long as the compound is dissolved. There's a limit to how much will dissolve in the solution and we'll talk about that a little bit later. There's an animation video that one of my colleagues made of this. And I hope you'll check that out, because it really shows the process better than I can on these two-dimensional slides. Let's do more examples of bionic compounds dissolving in water. Let's write the dissolution equations. Let's start with barium acetate as the first example that we do. Now, in order to do this problem, we need to know what the chemical formula of barium acetate is. That means we either need to look up the formula of acetate, or we need to have it memorized. It also means we need to know the charge on both the barium cation and acetate anion. So barium is an alkaline earth metal, it's in group two. It has a preferred charge of plus two. And the acetate anion has a charge of minus one each. Barium acetate is this compound. So that's the reactant, barium acetate. [SOUND] If I dissolve it in the presence of water, what will the products be? Remember, when ionic compounds dissolve in water they dissociate. They split apart. So the barium is going to split apart from the acetates to make a barium two-plus cation. And then the acetate anions will also separate. They will not only separate form the barium, but they will separate from each other. However, that chunk of polyatomic ions the C2H3O2 will stay together. So we'll show that like this. So here's the acetate polyatomic ion that has stayed in tact. And here's the barium cation that has separated, there's water in between those now because it's aqueous. How would we balance this equation? Is the equation balanced right now? Do I have the same number of each type of atom on both the product and reactant side here? In other words, where does the 2 go? I have a 2 right here, don't I? I have four oxygens on the left side but only two oxygens on the right side. I can balance it by putting the 2 out in front of the acetate as a coefficient. So, the subscript of the solid becomes the coefficient of the aqueous ion. And the only thing you need to be careful about, is recognizing those polyatomic ions that don't break apart. Remember, most of them are anions. We learned a couple of cations, but most of those that we learned were anions, like sulphate, carbonate, phosphate, nitrite. Those types of things. Those are the polyatomic anions that stay together when the species dissolves. So when one molecule of barium acetate dissolves, three ions were produced. It's a little bit different than the sodium chloride case because in that case, the sodium and the chloride broke apart to give two ions per unit of sodium chloride. And in this case, we're making three ions. That becomes important later when you do some calculations of colligative properties, for example. And we can use the stoichiometry to do more calculations. In other words, what if I dissolved four molecules of barium acetate? How many ions would be produced if I dissolved four molecules of barium acetate? And I'm looking for total number of ions. The number of barium ions plus the number of acetate ions. How many total ions would that be? That's great. You were able use the stoichiometry to calculate that it would twelve ions. You could also say if four moles of barium acetate dissolved, that would make twelve moles of ions. Four moles of ion would be barium. Cation, barium two plus. And the other eight moles of ions would be the acetate anions. So you can do this by either thinking about individual molecules and individual ions, or groups of molecules as in moles of molecules. Or you could say dozen molecules if you want, but we're going to use moles of molecules in chemistry. Here's one for you to try on your own. Go ahead and pause the video. I guess an in video question will pop up for you and pause it for you. Write the dissolution equation for solid copper II chloride in water. And that's shown here in these photographs. Here's what copper II chloride looks like. It's kind of this beautiful blue-green crystalline solid. We dissolve it in water and it makes a similarly colored aqueous solution. Hopefully, what you wrote started with the correct chemical formula for copper II chloride, which is CuCl2. You do need to write the phase, this is a solid. That's the reactant. It dissolves in the presence of water. So we need to have an arrow and have water shown, because that's what we are using as a solvent. And for products, the cation is copper 2 plus, and the anion is chloride 2 minus. But to balance it we need to put the 2 as a coefficient in front of the chloride. So again, in this case, each unit of copper II chloride that dissolves gives three units of ions. One of the things I can do is depict the product of this reaction as a cartoon, analogous to the way I showed a cartoon for the sodium and the chloride dissolved in water. In this case, I have a copper 2 cation, right there, dissolved in water. And I have two chloride anions. So, if I just dissolve one unit of copper chloride, I've been able to make three ions. One of the things that's interesting to look at in this particular cartoon that was shown fairly carefully, is the way that the water is organized around each ion type. So if I draw a little quick sketch of water up here. Here's the oxygen. Here's the two hydrogens, drawing them not quite to scale, but you get the idea. The oxygen is more electro-negative, so remember it's got a partial negative charge on it. And the two hydrogens are less electro-negative, so they're each partially positive in charge. That's because the oxygen hogs the electrons of the oxygen-hydrogen bond. So then when waters are near a copper 2 cation, for example, they want to be organized so that the negative part of the water, which is the oxygen, is closest to the copper 2 plus cation, because opposites attract. You can see that here, in, in each case, the oxygen is actually touching the copper 2 plus. We say that this is hydrated. It actually has six waters directly around that copper 2 plus. And there's one that's out in front of the screen that you can't see, and one behind. And then there's the four that are in the plane of the screen. And all of those are organized so that the oxygen is closer to the copper 2 plus than the hydrogens are. In contrast, look at how the water has organized around the chloride ions. In that case, the hydrogens, which are the more positive part of the molecule, prefer to be closer to the negative charge of the chloride ion. So even in the solution, which we think of being highly disorganized than it is because, of course, the cations and anions can move around freely, and the waters can slide past each other. There is this level of organization that comes from Coulomb's Law. From the attraction of positive and negative charges. In this case, between the solute, which here, is the copper II chloride, that's the thing being dissolved, which is in the lesser amount. And the solvent [SOUND], which for aqueous solutions, is water. Here's another one for you to try. Let's practice by writing the dissolution equation for making a solution of copper II sulfate in water. So here I have the same cation but a different anion. Go ahead and try to write that on your paper now, the dissolution equation for copper II sulfate. Hopefully what you wrote on your paper began with a reactant that is CuSO4 with a solid sign next to it. That's copper 2 sulfate. We dissolve that in water. That produced one copper 2 plus cation. And one sulfate polyatomic anion. Do we need to have any kind of coefficient in front of the sulfate? We don't actually need that in this case, do we? It's already balanced, each copper sulfate makes two ions, one copper two plus ion and one sulfate anion. In all these cases, when we dissolve the ionic compound, the cation dissociates from the anion.
