The Autoionization of Water

In water, we constantlyhave (a little)...

H2O(l) + H2O(l) H3O+(aq) + OH–(aq)

Thus (@ 25oC), the eq. const. for water is:

Kw = [H3O+] [OH–] = [H+] [OH–] = 1.0 x 10–14

This equation is taken to be valid for purewater and for dilute aqueous solutions, @ 25oC.

[H+] > [OH–] [H+] < [OH–] [H+] = [OH–]

BASE NEUTRALACID

At 25oC, calculate the hydrogen ion concentration ifthe hydroxide ion concentration is 2.7 x 10–4 M. Isthis solution an acid or a base?

[H+] [OH–] = 1.0 x 10–14

[H+] (2.7 x 10–4) = 1.0 x 10–14

[H+] = 3.7 x 10–11 M

[H+] < [OH–]

base

The pH Scale

Definition of pH...

0 ACID 7 BASE 14

--

A change of 1 pH unit meansthat [H+] (and [OH–]) haschanged by a factor of ___.10

A solution with pH 2 is how many times more acidicthan one with pH 5?

pH = –log [H3O+] = –log [H+]

as [H+] / , pH /

neutral

1000X more acidic (i.e., 1000X less basic)

Soren Sorensen(1868–1939)

:

Especially in biochemical reactions, the rate law isoften dependent on the hydrogen ion concentration.If the rate is 1st order in [H+], then doubling [H+]would double the rate.

-- Human blood pH is between 7.35 and 7.45.

white blood cell

platelet

red blood cell

Scanning electron microscopemicrograph of ordinary blood.

** Significant figures rule for logarithms:

Find the pH if the hydronium ionconcentration is 5.2 x 10–9 M.

Find the pH if the hydroxide ionconcentration is 3.9 x 10–10 M.

# of sig. figs. in [ ] = # of decimal places in pH

pH = –log [H3O+] = –log (5.2 x 10–9) = 8.28

[H+] [OH–] = 1.0 x 10–14

3.9 x 10–10

So [H+] = 2.56 x 10–5 M

pH = –log [H+] = –log (2.56 x 10–5) = 4.59

Blueberries grow best in soilhaving a pH between 4.5 and 5.0.

pOH = –log [OH–]

Other equations:

Find [H+] if the pH = 5.22.

[OH–] = 10–pOH

[H+] = 10–pH pH + pOH = 14

[H+] = 10–pH = 10–5.22

= 6.0 x 10–6 M

Measuring pH

pH meter

acid-base indicators:

e.g.,

pH paper is paperimpregnated withmixtures of variousindicators.

a pair of electrodes

chemicals whose colordepends on pH

litmus,phenolphthalein,bromthymol blue,methyl red,methyl orange

Bromthymol blue isyellow at pH < 6.0and blue at pH > 7.6.