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

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

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Answer 1 · 2015-12-03T18:30:31

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:

$H_{2} O ⇌ H^{+} + O H^{-}$ $H_2O rightleftharpoons H^+ + OH^-$

(These days, it's a little bit more common to speak of the hydronium ion, $H_{3} O^{+}$ $H_3O^+$ : $2 H_{2} O ⇌ H_{3} O^{+} + O H^{-}$ $2H_2O rightleftharpoons H_3O^+ + OH^-$ ) Both equations represent the autoprotolysis of water. Now, this equilibrium has been extensively studied, and under standard conditions; the equilibrium constant for the reaction, $K_{w} = 10^{- 14}$ $K_w = 10^(-14)$ $=$ $=$ $[H^{+}] [O H^{-}]$ $[H^+][OH^-]$ .

If the solution is neutral, then $[H^{+}] = [O H^{-}]$ $[H^+]=[OH^-]$ (and $p H = p O H = 7$ $pH = pOH = 7$ , where $p H = - {log}_{10} [H^{+}]$ $pH = -log_10[H^+]$ and $p O H = - {log}_{10} [O H^{-}]$ $pOH = -log_10[OH^-]$ ).

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

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

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