Q. The pH of water is 30* is 6.92, give a reason why it is neutral at this temp.
Ive been hardwired to write [H+]=[OH-]
but if this was the case wouldn't the pH be 7? TIA
No, because the pH of pure water is only 7 at 25°C, since [H^+] = 10^-7 mol dm^-3.
The dissociation of water is endothermic:
H2O (l) <=> H^+ (aq) + OH^- (aq)
At 30°C, the temperature is greater than 25°C, so it favours the endothermic route and more water dissociates, so [H^+] > 10^-7 mol dm^-3. As such, the pH simply cannot be 7.00
When H2O dissociates, the amounts of H+ and OH- formed will be the same. The extent to which water dissociates depends on the temperature – at higher temperatures a greater proportion will dissociate, meaning that there is more [H+] at 30°C than there is a 25°C. This means that the pH is lower at higher temperatures.