## Saturday, 22 June 2013

### THE IONIC PRODUCT FOR WATER, Kw

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.
The table below shows the effect of temperature on Kw. For each value of Kw, a new pH has been calculated using the same method as above. It might be useful if you were to check these pH values yourself.
T (°C)Kw (mol2 dm-6)pH
00.114 x 10-147.47
100.293 x 10-147.27
200.681 x 10-147.08
251.008 x 10-147.00
301.471 x 10-146.92
402.916 x 10-146.77
505.476 x 10-146.63
10051.3 x 10-146.14
You can see that the pH of pure water falls as the temperature increases.
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.
At 100°C, the pH of pure water is 6.14. That is the neutral point on the pH scale at this higher temperature. A solution with a pH of 7 at this temperature is slightly alkaline because its pH is a bit higher than the neutral value of 6.14.
Similarly, you can argue that a solution with a pH of 7 at 0°C is slightly acidic, because its pH is a bit lower than the neutral value of 7.47 at this temperature.

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