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How does water’s self-ionisation decrease pH?
Posted: Fri May 12, 2023 7:28 pm
by bohojekek
Water self ionises to produce equal moles of hydrogen and hydroxide ions. Since they are equimolar, they should neutralise and the pH will remain the same. Yet, in the question and sample answer below, it is shown that pH decreases as water’s self ionisation rate increases. How is it possible for the pH to decrease if hydrogen ions are being produced in the same proportion as hydroxide ions.
Re: How does water’s self-ionisation decrease pH?
Posted: Fri May 12, 2023 10:51 pm
by ChenBeier
You are right the amount of H+ and OH- are the same, but with higher temperature there are more H+ and OH - present, equilibrium shift to right AND value of KW changes. The H+ you measure with the pH Meter. So you get a more acidic reading. At 25°C the amount is 1* 10^-7 mol/l means pH 7. At 50 °C it is 5.5 *10^-7 mol/l means pH 6.25.
The problem is KW depends on temperature.
The Definition of neutrality has to be changed at different temperature in this case KW = 5,5* 10^-14 instead of 1* 10^-14.