Autoionization of water

- [Voiceover] I have two water molecules right over here, and typically the water molecules, as they interact with each other, they form these hydrogen bonds that's due to the polarity of the water molecule. We've talked a lot about that, they slide past each other, these hydrogen bonds give them all these neat properties of water. But chemistry is much messier, than sometimes our diagrams or explanations show. There's all sorts of crazy interactions. All of these things are bumping into each other in all different ways. And not only are the molecules bumping in different ways, but any given moment, the electrons are jumping around and, on average, they might spend more time, they might spend more time around the oxygen forming a partially negative charge at that end, and then a partially positive charge near the hydrogens, because the hydrogens are having their electrons hogged away from them. In fact this is what forms the hydrogen bonds. But there's always constantly change there. Because they're all just jumping around. It's all very probabilistic. And so you can imagine, under just the right conditions, one oxygen or one water molecule, might just graze this water molecule in the right way, that these electrons, that these electrons, get close enough to nab, to nab this hydrogen. But it doesn't nab the entire hydrogen. It doesn't nab the nucleus and the electron, and a typical hydrogen atom, a typical hydrogen atom, actually let me draw it. A typical hydrogen atom is just a proton, is just a proton in the nucleus. Actually the most typical isotope of hydrogen has no neutron, so it's just a proton in the nucleus. And an electron orbiting around it. So this right over here is positive, actually maybe I'll draw it that way, you have a positive proton, and then you have a negative electron, you have a negative electron orbiting around it. Actually it's more of a orbital. So it's really this electron is jumping all around it. But you could imagine, these electrons in this covalent bond, they were already being, these we already being hogged, these were already being hogged by this oxygen, in fact that's what was forming this partial negative charge over here, and the partial positive charge over here. So these would be attracted to this partial positive charge. Remember there, you have a partial negative charge over here, this is actually what's forming the hydrogen bond. And it actually could bond to the hydrogen proton, while both of these electrons, including one of these electrons that used to be part of this hydrogen, or you could consider used to be part of that hydrogen, are nabbed, are nabbed by this oxygen. And in this circumstance, and I'm not saying that this happens all the time, but under just the right conditions, this actually can happen, and what would result, so let me, what result is, this thing over here, instead of just being a neutral water molecule, would look like this. So you have your oxygen, you have, not only your two hydrogens now, you now have a third hydrogen. You now have a third hydrogen. So you have these two covalent bonds, these two covalent bonds, this lone pair. And now this lone pair, which I have circled in blue, is now being shared with this hydrogen proton. This electron right over here of the hydrogen got nabbed by this oxygen. So now you've formed another covalent bond. And now this character over here, he's lost the hydrogen proton, but he's kept all of the electrons. So this character over here's gonna look like this. You're gonna have your oxygen, and now it's only going to only be bonded to one hydrogen, only bonded to one hydrogen. Has these two original lone pairs. These two original lone pairs right over here. And then took both of the electrons from this covalent bond. And took both of the electrons from this covalent bond. And so it has another lone pair. So this molecule gained just a proton without getting any electrons. So if you do that, you're now going to have a net positive charge for this one over here. And this molecule over here, actually let me, let me, ugh, let me just write it. I wanna write it a little bit neater. And this molecule over here, so we have this molecule plus this one, this one lost a proton, without any other changes. So it now has a negative charge. So just like that, you went from two neutral water molecules, to two ions. And these ions, this one over here, the one on the left, the one that is now H three O, H three O, H three O, and it now has a positive charge, positive charge, actually I put that O in a different color. H three, H three O. It's a positive charge, this is called the "hydronium ion." Hydronium, hydronium. And this one over here, that is OH minus, so it's OH, O, let me get the colors right. OH minus. This is called the "hydroxide ion," or since it's negative you can just call it an "anion." I'll just write "hydroxide," hydroxide, hydroxide ion right over there. So you have this water and it's just kind of automatically under the right circumstances, this isn't happening a lot, but under the right circumstances, you could have one of the water molecules nabbing just the hydrogen proton, from another water molecule. And that water molecule is gonna keep both of the electrons, and then they ionize. They have autoionized, and this phenomenon, this is called "the autoionization of water." Let me write that down, it's a nice big word. Autoionization. Autoionization of, of water. And I really want to make it clear what happens. This hydrogen over here, that you can imagine at first was a proton and an electron, the typical isotope of hydrogen actually does not have a neutron. But then this electron got swiped. This electron, this electron was part of this bond and it gets swiped away, and so all you're left is with this proton, and this proton goes to this other water molecule, giving that a positive charge. And so you might say, "Well how frequently would "I find hydronium ions in water?" Well the concentration, let me actually draw a little tub of water here, let's say this is a liter of water. This is a liter, this is a liter of water. The concentration of hydronium in typical water, the concentration of H three O, the concentration of H three O in typical water, and you put brackets around something to denote "concentration," is one times ten to the negative seven molar. And molar, this just means "moles per liter." This is the same thing as one times ten to the negative seven moles, moles per liter. And now you might be saying, "Well, what's a mole?" Well I encourage you to watch the video on what a mole is, but a mole is a quantity. It's like saying, "a dozen." But it's a much larger, a dozen is equal to 12 of something. A mole is roughly equal to, let me write it. A mole is approximately equal to 6.02 times ten to the 23rd. Ten to the 23rd of something. And you're typically talking about molecules. A mole of a substance means approximately 6.022, it actually keeps going, times ten to the 23rd molecules of that thing. So you might say, "Hey, one times ten to the negative seven "times 6.02 times ten to the 23rd, "that would still get us," well let's see, this, let me actually, let me write it down. One times ten to the negative seven moles per liter, times, times, I'll do it this way. Times six, I'll just go with six, since we're gonna go approximately. So approximately, six times ten to the 23rd. Six times 23rd molecules, molecules per mole. Molecules per mole, well these two would cancel out, and you would multiply these two numbers, you would get six times, let's see. Ten to the negtive seven times ten to the 23rd, that's still gonna be ten to the 16th power, molecules per liter. Molecules per liter, so your first reaction is, "Oh my God!" "I'm gonna have six times," or roughly, I'll say roughly. "Approximately six times ten to the 16th "molecules of hydronium in this?" "That's a lot, we should see it all the time." But we have to remind ourselves. There's just a lot of molecules of water in there as well. In fact, a liter of water is roughly, so one liter of H two O, contains, contains approximately 56, 56 moles, moles of H two O. So one way to think about it is, "I have one, I have one times," and if I'm thinking about a liter of water, I have, I'll do it over here, "I have one times ten to the negative seven "moles of, moles of H three O for every, "for every 56 moles, for every 56 moles, "moles of H two O." So if you look at this ratio, then you start to appreciate. The ratio of one times ten to the negative seven to 56. Let me do it down here. So this is the same thing as, one times ten to the negative seven to 56, is the same thing as, let's just multiply both side times, or the numerator and the denominator, times ten to the seventh. So if we do that, this is the same thing as one, one, the ratio of hydronium to regular water, to H two O is gonna be one to, let's see if I multiply 56, times ten to the seventh, I'm gonna have five, let me get, write in that same color. I'm gonna have five six, then I'm gonna have, I'm gonna throw seven zeros at the end of it. Let me do that. One, two, three, four, five, six, seven. So the ratio of hydronium to regular H two O is one for ever five hundred and sixty million. So even though you might say, "Oh wow, look." "We're gonna have a huge number of molecules "of hydronium in this liter of water." For every one of them, you actually have roughly five hundred sixty million molecules of H two O. So that should give you an appreciation for the fact that this isn't that typical. In fact, you're gonna see this much more often than you see this over here. In fact, if you wanted to make these arrows kind of show which direction the equilibrium sits in, it's actually much further, it's actually much further to the left. So we could make this arrow much bigger. But it also gives you an appreciation for just how many molecules you have sitting in a liter, in a liter of water.