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Correlation between pH and temperature of pure water | |||||||||||||||||||||||||||
| Water molecules can function as both acids and bases Hierdoor is het water altijd in reactie. De reactie in het water is als volgt: H2O « H3O+ + OH- However, the hydroxonium ion is a very strong acid, and the hydroxide ion is a very strong base. As fast as they are formed, they react to poduce water again.
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pH of pure | |||||||||||||||||||||||||||
| Why does pure water have a pH of 7? That question is actually misleading! In fact, pure water only has a pH of 7 at a particular temperature - the temperature at which the Kw value is 1.00 x 10-14 mol2 dm-6. To find the pH you need first to find the hydrogen ion concentration (or hydroxonium ion concentration - it's the same thing). Then you convert it to pH. In pure water at room temperature the Kw value tells you that: [H+]2 = 1.00 x 10-14 But in pure water, the hydrogen ion (hydroxonium ion) concentration must be equal to the hydroxide ion concentration. For every hydrogen ion formed, there is a hydroxide ion formed as well. That means that you can replace the [OH-] term in the Kw expression by another [H+]. [H+] = 1.00 x 10-7 jConverting that into pH: pH = - log10[H+] pH = 7
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The variation of the pH of pure water with temperature | |||||||||||||||||||||||||||
According to Le Chatelier, if you increase the temperature of the water, the equilibrium will move to lower the temperature again. It will do that by absorbing the extra heat. That means that the forward reaction will be favoured, and more hydrogen ions and hydroxide ions will be formed. The effect of that is to increase the value of Kw as temperature increases.
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A word of warning! | |||||||||||||||||||||||||||
| If the pH falls as temperature increases, does this mean that water becomes more acidic at higher temperatures? NO! A solution is acidic if there is an excess of hydrogen ions over hydroxide ions. In the case of pure water, there are always the same number of hydrogen ions and hydroxide ions. That means that the water remains neutral - even if its pH changes. The problem is that we are all so familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that you calculate the neutral value of pH from Kw. If that changes, then the neutral value for pH changes as well. | |||||||||||||||||||||||||||
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