Note: If you don't understand Le Chatelier's Principle, you should follow this link before you go on. Using the simpler version of the equilibrium:Īccording to Le Chatelier's Principle, if you make a change to the conditions of a reaction in dynamic equilibrium, the position of equilibrium moves to counter the change you have made. The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process. The variation of the pH of pure water with temperature That's where the familiar value of 7 comes from. Taking the square root of each side gives: That means that you can replace the term in the K w expression by another. For every hydrogen ion formed, there is a hydroxide ion formed as well. In pure water at room temperature the K w value tells you that:īut in pure water, the hydrogen ion (hydroxonium ion) concentration must be equal to the hydroxide ion concentration. To find the pH you need first to find the hydrogen ion concentration (or hydroxonium ion concentration - it's the same thing). That question is actually misleading! In fact, pure water only has a pH of 7 at a particular temperature - the temperature at which the K w value is 1.00 x 10 -14 mol 2 dm -6. Try it on your calculator! Notice that pK w doesn't have any units. The K w value of 1.00 x 10 -14 mol 2 dm -6 at room temperature gives you a pK w value of 14. The relationship between K w and pK w is exactly the same as that between K a and pK a, or and pH. Multiplying mol dm -3 x mol dm -3 gives you the units above. Each of these has the units of mol dm -3. The units of K w: K w is found by multiplying two concentration terms together. In fact, this is its value at a bit less than 25☌. Its value is usually taken to be 1.00 x 10 -14 mol 2 dm -6 at room temperature. Like any other equilibrium constant, the value of K w varies with temperature. K w is defined to avoid making the expression unnecessarily complicated by including another constant in it. So little of the water is ionised at any one time, that its concentration remains virtually unchanged - a constant. You may wonder why the water isn't written on the bottom of these equilibrium constant expressions. Whatever version you come across, they all mean exactly the same thing!
You may find them written with or without the state symbols. You may meet it in two forms:īased on the fully written equilibrium. K w is essentially just an equilibrium constant for the reactions shown. This is OK provided you remember that H + (aq) actually refers to a hydroxonium ion.ĭefining the ionic product for water, K w You may well find this equilibrium written in a simplified form: It turns out to be 1.00 x 10 -7 mol dm -3 at room temperature.
Further down this page, we shall calculate the concentration of hydroxonium ions present in pure water. The net effect is that an equilibrium is set up.Īt any one time, there are incredibly small numbers of hydroxonium ions and hydroxide ions present. As fast as they are formed, they react to produce water again. However, the hydroxonium ion is a very strong acid, and the hydroxide ion is a very strong base. This will be happening anywhere there is even a trace of water - it doesn't have to be pure.Ī hydroxonium ion and a hydroxide ion are formed. One water molecule (acting as a base) can accept a hydrogen ion from a second one (acting as an acid). Water molecules can function as both acids and bases. It looks at how the ionic product varies with temperature, and how that determines the pH of pure water at different temperatures. This page explains what is meant by the ionic product for water.
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