Hydrogen Ions and Acidity The Ionization of Water and pH

Hydrogen Ions in Water We are used to thinking of water as a pure liquid that contains only H 2 O molecules. However, as a pure liquid, water ionizes into hydrogen ions and hydroxide ions: H 2 O(l) → H + (aq) + OH - (aq) H 2 O(l) + H 2 O(l) → H 3 O + (aq) + OH - (aq)

Hydrogen Ions in Water The symbol for the concentration of hydrogen ions is [H + ] and for hydroxide ions is [OH - ]. We can measure the concentration of the ions in pure water. [H + ] = 1.0×10 -7 M [OH - ] = 1.0×10 -7 M By stoichiometry, H 2 O(l) → H + (aq) + OH - (aq), we see that [H + ] = [OH - ] in pure water.

Hydrogen Ions in Water We also know that the product of the concentrations is a constant, K w. K w = [H + ][OH - ] = 1.0× This means that as we change [H + ], we change [OH - ]. If [H + ] goes up, [OH - ] goes down. If [H + ] goes down, [OH - ] goes up.

Hydrogen Ions in Water The higher the value of [H + ] and lower the value of [OH - ], the more acidic the solution. The lower the value of [H + ] and higher the value of [OH - ], the more basic the solution. For example: [H + ] = 1.0×10 -6 is acidic. For example: [H + ] = 1.0×10 -3 is more acidic. For example: [H + ] = 1.0×10 -8 is basic. For example: [H + ] = 1.0× is more basic.

Hydrogen Ions in Water Example 1: Colas are slightly acidic. If a cola solution has [H + ] = 1.0×10 -5 M, what is the [OH - ]? [H + ] = 1.0×10 -5 M K w = [H + ][OH - ] = 1.0× K w = [H + ][OH - ]  [OH - ] = KwKw [H + ] = 1.0× ×10 -5 [OH - ] = 1.0×10 -9

The pH Concept pH is another way to measure [H + ] and/or [OH - ]. pH = -log[H + ] pOH = -log[OH - ] If [H + ] = 1.0×10 -7 then pH = -log(1.0×10 -7 ) = 7.00 If [OH - ] = 1.0×10 -7 then pOH = -log(1.0×10 -7 ) = 7.00

The pH Concept To find pH on the TI-83 or TI-84 Plus... press the “(-)” button press the “LOG” button enter the value for [H + ] press the “)” button press “ENTER”

press the “ctrl” button The pH Concept To find pH on the TI-Nspire... press the “10 x ” button enter the value for [H + ] press the “)” button press “ENTER” press the “(-)” button

The pH Concept If [H + ] = 1.0×10 -5 then pH = -log(1.0×10 -5 ) = 5.00 If [OH - ] = 1.0×10 -8 then pOH = -log(1.0×10 -8 ) = 8.00 If [H + ] = 3.1×10 -2 then pH = -log(3.1×10 -2 ) = 1.51 If [OH - ] = 4.5× then pOH = -log(4.5× ) = 10.35

The pH Concept We can also convert from pH and pOH to [H + ] and [OH - ] respectively. [H + ] = 10 -pH [OH - ] = 10 -pOH If pH = 7.00 then [H + ] = = 1.0×10 -7 If pOH = 3.25 then [OH - ] = = 5.6×10 -4

The pH Concept To find [H + ] on the TI-83 or TI-84 Plus... press the “2ND” button press the “LOG” button press the “(-)” button enter the pH value press the “)” button press “ENTER”

The pH Concept To find [H + ] on the TI-Npire... press the “10 x ” button press the “(-)” button enter the pH value press the “)” button press “ENTER”

The pH Concept If pH = then [H + ] = = 2.78×10 -3 If pOH = 9.27 then [OH - ] = = 5.4× If pH = then [H + ] = = 7.6× If pOH = then [OH - ] = = 2.10×10 -7

The pH Concept pH + pOH = 14 if pH = 2.0, then pOH = = 12.0 if pH = 3.250, then pOH = =

The pH Concept Solutions with pH = 7 (pOH = 7) are neutral. Litmus paper remains colorless. Solutions with pH 7) are acidic. Litmus paper turns red. Solutions with pH > 7 (pOH < 7) are basic. Litmus paper turns blue.

Measuring pH Indicators may be added to a solution to tell us the range of pH of the solution.

Measuring pH Indicators are most often used where pH may change, such as in a titration. But they are also used to tell us where we are in a particular range of pH. For example, if we wanted to tell if a solution has a pH greater than or less than 7... we might use Bromthymol Blue (which changes color at about pH = 7).

Measuring pH Indicators are most often used where pH may change, such as in a titration. But they are also used to tell us where we are in a particular range of pH. For example, if we wanted to tell if a solution has a pH greater than or less than 7... we would not want to use Methyl Red (which changes color near pH = 5).

Measuring pH Some meters provide quick and accurate readings of pH. They vary from the simple to the very complex.