Autoionization of Water - Video Tutorials & Practice Problems
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1
concept
Water Autoionization
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4m
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Video transcript
In this video, we're gonna talk about the autoionization of water. So water has a slight tendency to autoionize, and all that really means is that water can react with itself to form ions. And those ions are gonna be hydronium cations, or H three O plus, as well as hydroxide anions, or Oh minus. And so recall that anions always have a negative charge, whereas cations always have a positive charge. And so the water autoionization reaction is actually a reversible reaction. So that means that the reaction can occur in both directions. And it takes place very, very, very rapidly or quickly. And so let's take a look at our example below, and what we'll see is that we've got 2 water molecules on the reactant side. And these water molecules are able to react with themselves to form the hydronium cation and the hydroxide anion. And again, this reaction can occur in both directions. It's a reversible reaction. And so you can see that we've got our equilibrium arrows here. Now what's important to note is that H three O plus and Oh minus, concentrations are always going to be very, very, very low, very small concentrations. And so so we've drawn our equilibrium arrows here to represent that. So the top arrow here showing the conversion of water into the ions is very small, showing that the concentrations of H three O Plus and Oh minus are gonna be very small. And the reverse reaction arrow here, showing the conversions of these ions back into water, is much larger in comparison to the top arrow, showing that the concentrations of water are always gonna be much larger than the concentrations of the ions. Now, what's also important to remember from our previous chemistry courses is that in pure water, the concentrations of H three O plus are always gonna be equal to the concentrations of o h minus. And so this is gonna be important as we move forward into our next video. And so, keeping, something to remember from your previous, chemistry courses is that free protons, hydrogen ions, and h plus, they're all essentially synonyms to one another. And in your textbook and your professor, they're probably going to use all of these terms here interchangeably with one another. So it's important to keep that in mind. Now, another important thing to note is that H three O plus or this hydronium cation up here, is commonly simplified to H plus, which again, usually suggests free protons. However, free protons themselves are actually nonexistent in aqueous systems. So whenever water is around, whenever, water is around, free protons are non existent. They actually exist as H3O + hydronium ions. And so, it's important to note that even though I'm going to simplify H3O+H+H+, and your professor's gonna do it and it's also in your textbooks, it's important to note that free protons are nonexistent and that really these h plus, ions when they're in water, they're going to exist as h three o plus. And so keeping that in mind, we can move on to our example, which shows an alternative depiction to the water ionization, the same one that's shown up above. And so notice here that we've simplified the H three O plus here as an, H plus, ion here. And so, when we do that, we're able to simplify this entire reaction even further and remove one of the entire water molecules. And so, what you'll see is that water is able to auto ionize and react with itself to form the hydronium cation and the hydroxide anion. And so, if we apply our equilibrium constant to this reaction here, we're able to get the ion constant of water or k w, and we'll talk about what that is and why that's important in our next video. So I'll see you guys then.
2
concept
Ion Constant of Water (Kw)
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4m
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Video transcript
So because h plus and o h minus are present in pretty much every biological solution and they participate in so many different biochemical reactions, we actually care about their concentrations, and their concentrations are actually relevant to us. And so the concentrations of H+ and Oh- can actually be determined by the equilibrium constant, which we're already familiar with from our previous videos. And recall that the equilibrium constant is simply concentration of products at equilibrium over the concentration of reactants at equilibrium. And so, what I want you guys to focus on in this video is actually the ion constant of water. And so it turns out that the ion constant of water can be abbreviated with the symbol k w, and it's just a simple rearrangement of the equilibrium constant, which we're already familiar with. And the ion constant of water k w is equal to the product of the concentration of h plus times the concentration of o h minus. So let's take a look at our example below to clarify, the difference between equilibrium constant and the ion constant for the auto ionization of water. Let's start with the equilibrium constant since we're already familiar with that. And so again, recall that the equilibrium constant is simply the concentration of products at equilibrium over the concentration of reactants at equilibrium. And in our previous video, we talked about how the autoionization of water can be depicted as so. And so we've got 2 different products here. We've got h+0h minus. And so if we go ahead and plug those in, we can put in h+ for our equilibrium constant and o h minus. And our reactant is already in here. So this is our equilibrium constant for the autoionization of water. Nothing new here. We kinda already knew this from our previous videos. So how does this tie in to the k w? And so what I want you guys to know that the k w is that the k w is equal to the concentration of h plus times the concentration of o h minus, and that is always equal to 1 times 10 to the negative 14th molar squared. And so if you know this, then you guys will be good in all of our practice problems. And so, if you're curious about how in the world did they get this number, and the the way that they got it is by a simple algebraic rearrangement of this equation, of the equilibrium constant. And so we take the h 2 o in the bottom, and we multiply it by both on both sides of the equation just to move it up here. Notice that that's exactly what we have shown here, equilibrium constant times H two O, and that gives us our K w. And so, by memorizing this one number here, what that does is it saves us from having to memorize 2 different numbers. So we don't have to memorize equilibrium constant or H two O concentrations just because of the k w. So the k w is actually a good thing to help limit our memorization. So this is this green shaded region that I highlighted here is the only region you guys really need to know about k w. Now, what's interesting is that kw actually varies with different conditions and it changes with different temperatures, just like the equilibrium constant does. And so, the good thing is is that in biological systems, we always assume that the temperature is gonna be right around 298 Kelvin. And so what that means is that k w is always going to be assumed to be 1 times 10 to the negative 14th molar squared in biological systems. So it's that same number here. So just by memorizing this value k w, that allows us to calculate either the h plus concentration or the Oh minus concentration when we're given the concentration of just one of these ions. And again, these concentrations here, we actually care about them. They're relevant to us. And so, we'll get some more practice calculating these concentrations in our practice video. So I'll see you guys then.
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Problem
Problem
Calculate [H+] in a solution given that [OH-] is 1.0 x 10-10 M.
A
1.0 x 10-2 M
B
1.0 x 10-3 M
C
1.0 x 10-4 M
D
1.0 x 10-5 M
4
Problem
Problem
Calculate [OH-] in a solution given that [H+] is 1.0 x 10-11.8 M.
A
1.0 x 10-11.8 M
B
1.0 x 10-6.8 M
C
1.0 x 10-4.5 M
D
1.0 x 10-2.2 M
5
concept
Proton Hopping
Video duration:
2m
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Video transcript
So even in pure water, we know that water molecules have the tendency to auto ionize and to form H three O plus ions and o h minus ions. And because water molecules have the ability to interact with each other in this kind of way, this allows for something known as proton proton hopping to occur. And so proton hopping is essentially when you have hydronium ions or H3O plus ions and hydroxide ions, or Oh minus ions, that can diffuse much more rapidly than other ions in aqueous solutions, or in solutions that have water in them. And the way that this works is that the protons or the hydrogen atoms that are on an h three o plus or a water molecule can actually hop or jump on to neighboring water molecules or o h minus ions in order for these 2 ions to be able to diffuse very, very rapidly. So let's take a look at our example below to clear this idea of a proton hopping. And so over here in our the image on the left, we have an h three o plus ion up here, and it's surrounded by a bunch of water molecules. And so this h three zero plus can donate its hydrogen to a neighboring water molecule so that it becomes an h three zero plus, and it can can continuously donate the hydrogen ion over long stretches to, more and more water molecules so that the h three o plus eventually shows up in a completely different area. It's diffused. And so this diffusion, this type of proton hopping, is much, much faster than if this h three o plus were to try to physically diffuse and make its way over to that same area. Now if we take a look at the image on the right, what we have is an Oh minus ion here, and the idea works very similarly. So we have a bunch of water molecules surrounding the Oh minus ion, and the water can donate a hydrogen to the Oh minus and become an Oh minus itself. And so this donation can continuously happen over long stretches so that the Oh minus ends up in a completely different area. And again, this is much faster, this proton hopping is much faster than the physical diffusion of this Oh minus to a different location. And so other ions don't have this ability to do proton hopping, but these H three O plus and Oh minus can. And so, this concludes our lesson on the auto ionization of water and I'll see you guys in our practice videos.
6
Problem
Problem
Which of the following ions is likely to diffuse the most rapidly in biological systems?