Why is water neutral in pH despite the fact that it produces hydrogen and hydroxide ions?

1 Answer
Dec 3, 2015

Answer:

Because at neutrality, the concentration of hydroxide ion is equal to the concentration of protium ion.

Explanation:

As you know, water undergoes the equilibrium reaction:

#H_2O rightleftharpoons H^+ + OH^-#

(These days, it's a little bit more common to speak of the hydronium ion, #H_3O^+#: #2H_2O rightleftharpoons H_3O^+ + OH^-#) Both equations represent the autoprotolysis of water. Now, this equilibrium has been extensively studied, and under standard conditions; the equilibrium constant for the reaction, #K_w = 10^(-14)# #=# #[H^+][OH^-]#.

If the solution is neutral, then #[H^+]=[OH^-]# (and #pH = pOH = 7#, where #pH = -log_10[H^+]# and #pOH = -log_10[OH^-]#).

#K_w# is quoted under standard conditions of #1# #atm# and #298K#. What do you think would happen under non-standard conditions, say at a temperature of #350# #K#; would #pK_w# go up or go down? (Be careful with the negative sign!)