Acid-Base Reactions

Objectives Calculate the hydrogen ion and hydroxide ion concentrations in water Calculate pH values Explain why indicators change color

Self Ionization of Water H2O  H+ + OH- A reaction in water that produces ions. In pure water, the concentration of water molecules, [H2O] is constant at constant temperature.

Calculating Molarity of [H+] or [OH-] [H+] [OH-]= Kw Kw is the ion product for water If [H+] increases, then [OH-] decreases to keep Kw constant. At 25 C, Kw has been found by experiment to be 1.0x10-14. This is always true!

A mass of 4g of NaOH (0.10mol) is dissolved in water to make a solution with a volume of 1.0dm3. What is the molar concentration of hydrogen ions in this solution [H+]=? What is the molarity of [OH]? [OH-]=0.10mol/1dm3=0.10M Substitute into the formula [H+] [OH-]= Kw Remember Kw=1.0x10-14 [H+] [0.10]= 1.0x10-14,, [H+] =1.0x10-13M

The pH of a Solution pH=-log [H+] pH + pOH = 14 Don’t forget: pH < 7 is acidic pH = 7 is neutral (pure water) pH > 7 is basic

Example The pOH of a base is 9.3. Please calculate the pH of the acid. pH + pOH = 14 14 - 9.3 = 4.7. 4.7 would be the pH of a solution whose pOH was 9.3.

Example pH=-log [H+] For a 0.1 M solution of an acid: 0.1 M = 10-1 M The negative log of 10-1 = 1 For a 0.01 M solution of an acid: 0.01 M = 10-2 M The negative log of 10–2 = 2

Look at the Relationship What is the pH, if [H+]= 1.0x10-5=5 1.0x10-6=6 1.0x10-7=7 1.0x10-8=8 1.0x10-9=9 pH=-log [H+]

What is the hydrogen-ion concentration of a solution with a pH of 8 What is the hydrogen-ion concentration of a solution with a pH of 8.00 at 25 C? pH=-log [H+] 8.00=-log [H+] [H+]=10-pH [H+]=10-8 [H+]=1x10-8 M

Acid-Base Indicators Chemists often need to know how acidic or basic a solution is. A pH meter gives an accurate measurement of the acidity of a solution If less accuracy is needed, an acid-base indicator can be used to measure acidity Indicators change color over a relatively narrow pH range.

Range If a given solution makes methyl orange turn yellow and blue litmus turn red, then the solution has a pH of between 4.4 and 5.5. pH paper can approximate acidity

Acid-base indicators Many acid-base indicators are dyes from common plants, such as beets, red cabbage, and black cherries.