Hey, don't give Rand his cookie yet
As temperature rises, that equilibrium is driven to the right, making more H+ and OH-.
Since H+ rises as the temperature rises, the pH of pure water drops.
But if both rised the same the pH would be the same, as the H+ will be canceled out by a OH-.
I think what he meant to say was this;
pH can indeed vary with temperature. The reasons why
depend on the context, but even a simple solution of a weak
acid (HA) will exhibit a (weak) temperature dependence. The pH is
given by the Henderson-Hasselbalch equation:
pH = pKa + log { [A-]/[HA] }
where Ka is the equilibrium constant for the reaction
HA ---> H+ + A-
( ka = [H+][A-] / [HA] )
and pKa = - log Ka .
Ka is itself a function of temperature, since it is related to
the Gibbs free energy of reaction (delta G) by the equation
delta G = - RT ln Ka = -2.303 RT log Ka = 2.303 RT * pKa
so we have
pkA = delta G / (2.303 RT)
delta G is itself given by
delta G = delta H - T * delta S
where delta H is the enthalpy of reaction and delta S is the entropy
of reaction. Combining these, we get
pKa = (delta H / (2.303 RT)) - (delta S / (2.303 R))
If we assume for the sake of simplicity that delta H and delta S
are approximately independent of temperature T (constant), then
the variation with temperature is determined by the sign of delta H.
For example, if delta H is positive (endothermic dissociation),
pKa gets smaller as the temperature gets larger. A
decrease in pKa amounts to an increase in Ka, which means that the
reaction favors dissociation more as temperature increases (in
agreement with LeChatelier's principle). This increases [H+] and
decreases the pH. If the reaction is exothermic the opposite effect will
be observed. Either way, we expect the pH to depend on temperature.
These arguments can be extended to strong acids too. Things get
complicated when there are multiple chemical reactions. Biological
systems can use enzyme-catalyzed reactions to keep the pH constant even
when T varies (within limits, of course).
Boom of course did not wrtie this :lol:
