A sample of pure water is found to have a pH of 6.5; which of the following is a possible explanation? (Hint: The reaction of two water molecules to form hydronium ion and hydroxide ion is an endothermic process.)?
- The sample is several miles above sea
level.
- The sample’s temperature is above 25◦C.
- The sample’s temperature is below 25◦C.
- The sample is exactly at sea level.
- The sample is in a container pressurized
to 10 atm.
- The sample is several miles above sea
level. - The sample’s temperature is above 25◦C.
- The sample’s temperature is below 25◦C.
- The sample is exactly at sea level.
- The sample is in a container pressurized
to 10 atm.
2 Answers
The
"pH" = -log["H"_3"O"^(+)]
for a higher
The autoionization of water is endothermic:
2"H"_2"O"(l) rightleftharpoons "H"_3"O"^(+)(aq) + "OH"^(-)(aq)
K_w = 10^(-14) at25^@ "C"
We can write
- An endothermic reaction favors the products at higher temperature, so an endothermic reaction has higher
[H_3O^(+)] .
Therefore, neutral
"pH" is lower than7 at warmer temperatures, NOT colder temperatures, eliminating(3) that it could be at a lower temperature than25^@ "C" .
- Since no species in the reaction is gaseous,
K_w is NOT a function of pressure. Liquids and aqueous solutions are highly incompressible...
That eliminates
(1) that it could be several miles above sea level,(4) that the sample is at sea level, and(5) that the sample is pressurized to"10 atm" .
Would it not be
Explanation:
Under standard conditions which your syllabus will detail, we represent the autoprotolysis of water as....
And this is an EQUILBRIUM RXN for which...
And clearly, this is a bond-breaking reaction....we would expect at HIGHER temperatures than
At