This month’s blog post aims to follow on from last month’s post that was an introduction to acids and bases. This has some (but not all) of the more advanced stuff. The idea behind this post is the same as last month’s, namely, to help you revise some content you may have covered remotely or to give you an introduction to the acids and bases topic which you will probably start next term.
This month I will cover the pH of water, a HL concept.
By Matthew Bowden www.digitallyrefreshing.com – http://www.sxc.hu/photo/158242,
So, to begin with we need to take on board the idea that water will exist in a natural equilibrium with itself:
H2O ⇌ H+ + OH–
The equilibrium for this reaction lies well over to the left.
As this is an equilibrium, it is possible to write an expression for this reaction in terms of Kc(‘Kc’ means the equilibrium constant which is constant at a fixed temperature)
Kc = [products] / [reactants]
Kc = [H+] x [OH–]/ [H2O]
Kc x [H2O] = [H+] x [OH–]
At this point, because Kc is fixed and the concentration of water is very high, we let
Kc x [H2O] = Kw
Kw = [H+] x [OH–]
Kw is referred to as the ionic product of water and at 25 oC, Kw = 1 x 10-14
Now, recall that:
pH = – log 10 [H+]
So, to determine the pH of water
Kw = [H+]2 (as the [H+] = [OH–])
So √Kw = [H+]
pH of water = -log 10 (√Kw)
= -log 10 (√1 x 10 -14)
= -log 10 (1 x 10 -7)
= pH 7 (but you knew this, didn’t you!)
Kw values at different temperatures and these can be found in the data booklet.
We find that as the temperature increases, Kw increases. This means that the [H+] will also increase.
Does this mean water becomes more acidic?
Well, the answer is no. The water stays neutral because [H+] = [OH–] and this fits with the definition of neutralization.
However, using this information we can make some deductions regarding DH for the dissociation of water, prompting us to ask the question, is the dissociation of water exo or endothermic?
To answer this, we need to use our existing knowledge on equilibrium. As the temperature increases, the equilibrium will shift to the right (because the concentration of H+ increases).
This is the same process that we find in an endothermic reaction. The reaction moves in this direction to absorb the excess heat.
But remember, the water does stay neutral although if you were to drink hot water you could possibly burn yourself from the temperature (but not the acid) so please still be careful!