Lesson 19Temperature

and pH

Kw and Temperature

@ this temperature pH + pOH = pKw < 14

pOH < 7

pH < 7

[OH-] > 1.0 x 10-7 M

[H+] > 1.0 x 10-7 M

Kw > 1.0 x 10-14@ 50 oC shifts right

Higher Temperatures

@ 25 oC Kw = 1.0 x 10-14 

Energy+ H2O ⇋ H+ + OH-

Kw and Temperature

@ this temperature pH + pOH = pKw > 14

pOH > 7

pH > 7

[OH-] < 1.0 x 10-7 M

[H+] < 1.0 x 10-7 M

Kw < 1.0 x 10-14@ 0 oC shifts left

Lower Temperatures

@ 25 oC Kw = 1.0 x 10-14 

Energy+ H2O ⇋ H+ + OH-

Important Points

If you're not @ 25oC

[H+][OH-] ≠ 1.0 x 10-14

pH + POH ≠ 14

All other equations are valid and…….

Pure water is always neutral, but pH ≠ 7 and…….

[H+] = [OH-] and ……

pH = pOH

1. The pH of pure water is 6.50 Is the temperature higher or lower than 25 oC? Calculate the Kw.Energy + H2O ⇋ H+ + OH-

Higher

[H+] = 10-6.50 = 3.1623 x 10-7 M

[OH-] = 3.1623 x 10-7 M

Kw = [H+][OH-] = (3.1623 x 10-7)2

Kw = 1.0 x 10-13

 2. The Kw = 5.0 x 10-13 for pure water. Is the temperature higher or lower than 25 oC?Calculate the pH , POH, and pKw.Energy + H2O ⇋ H+ + OH-

Kw > 1.0 x 10-14

shifted right……temperature is higher

Kw = 5.0 x 10-13 = [H+][OH-] = x2

x = [H+] = 7.071 x 10-7 MpH = pOH = 6.15 pKw = 12.30