Autoionization occurs when water molecules react with one another in an aqueous solution. Recall that water is amphoteric, meaning it can act as both an acid and a base. Here, one of them is going to act as a base, the other one as an acid. The acid will donate an H+ to the base. The basic water molecule that accepts the H+ becomes H3O+ and the water that donated the H+ becomes OH-. Associated with this reaction is Kw. Kw represents our ionization constant of water. It is an equilibrium constant, and like other equilibrium constants, it's a ratio of products over reactants. Remember, it does not take into account liquids and solids; it only concerns aqueous and gaseous compounds. In this equation, the liquids will be ignored, so the reactants on the bottom will be ignored. So Kw just equals the product of the concentrations of H3O+ and OH-: Kw is equal to 1.0 times 10 to the power of negative 14 at a temperature of 25 degrees Celsius. This fact connects us to the formula pH + pOH equals 14. Now, this whole idea of H3O+ and OH-, remember they are kind of like counterbalancing one another. If one goes up, the other goes down. This is a way of maintaining the acidity or basicity of any aqueous solution. Realize that if we're dealing with pure water, that's when their concentrations are equal to one another, and that's when we can talk about the aqueous solution being neutral. Keep this in mind; autoionization is the key to understanding the relationship between H3O+, your hydronium ion concentration, with OH-, your hydroxide ion concentration. Together, they help us to create this ionization constant expression for water, which then leads us into pH + pOH equaling 14.
Auto-Ionization - Online Tutor, Practice Problems & Exam Prep
In a Self-Ionization reaction two water molecules react with one another, where one acts an acid and the other as a base.
Auto-Ionization and Kw
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Kw and Temperature
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Now recall that at 25 degrees Celsius, kw = 1.0 × 10-14. This is a value you'll have to remember on your own. You're not going to be expected to memorize, or to be given a formula sheet with this value present. But remember kw is an equilibrium constant and like the other equilibrium constants, it is temperature dependent. If I play around with my temperature where it strays away from 25 degrees Celsius, then the value itself will change. We're going to say the general trend is as the temperature increases, our kw increases.
If we take a look here, we have temperatures ranging from 0 degrees Celsius all the way up to 100 degrees Celsius. And if you look, you can see that as our temperature starts to increase, going from 0 to 100, we can see that the general trend is that my kw value is increasing. Again, at 25 degrees Celsius, kw is equal to this value. This is what you're expected to remember on your own. If the temperature changes from 25 degrees Celsius, you'll be given that new value for kw because it could really be any number, so it's hard for you to memorize an entire list of kw at all these different temperatures. Okay. And remember, the general trend is as the temperature increases, our kw generally increases as well.
Auto-Ionization Example
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A particular aqueous solution at 50 degrees Celsius contains 3.7×10-4 hydronium ions. It says to calculate the hydroxide ion concentration and identify the solution as either being acidic, basic, or neutral. Alright, so hydronium ion is H3O+ and they want us to find OH-. The equation that connects them together is Kw = [H3O+] × [OH-]. Our temperature is at 50 degrees Celsius which means our Kw value changes. If you look up above you'll see that at 50 degrees Celsius Kw = 5.476 × 10-14. Plug in our number for the hydronium ion concentration, so 3.7×10-4, and then we just have to solve for the hydroxide ion concentration. Divide both sides by 3.7×10-4, When you do that, you're going to get your hydroxide ion concentration being equal to 1.48×10-10 molar. Now, how do we determine if it's an acidic, basic, or neutral solution? Well, you can see that your hydronium ion concentration is to the negative 4, but your hydroxide is to the negative 10. Since hydronium ion concentration is greater than hydroxide ion concentration, that means that we are dealing with an acidic solution. Right? So we have both the concentration of hydroxide ion, and the fact that our solution is acidic.
Chemistry student prepared an aqueous solution at 30ºC. If the solutions contains 7.42 × 10−9 M of hydroxide ions, calculate the pH.
5.703
8.130
8.300
5.980
Calculate the Kw of pure water given the pH = 6.34.
4.57 × 10−7
6.76 × 10−4
2.09 × 10−13
4.57 × 10−14
