1. Acid-Base Reactions

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

3. 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.

4. 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!

5. 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

6. 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

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

8. 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

9. 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+]

10. 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

11. 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.

12. 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

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