1. The strength of acids and bases
Section 6.5 pg

2. What do you remember? ACIDS BASES Tastes sour Turn blue litmus red
Fill in as many empirical properties for acids and bases as you can ...
ACIDS
BASES
Tastes sour
Turn blue litmus red
pH less than 7
Neutralize bases
React w/ active metals to produce H2(g)
React w/ carbonates to produce carbon dioxide
Taste bitter
Feel slippery
Turn red litmus blue
pH greater than 7
Neutralize acids

3. Do all acids and bases have the same properties and to the same degree?
Two different acids of the same concentration were tested with a pH meter. HCl(aq) 1.0 mol/L = pH = 1 CH3COOH 1.0 mol/L = pH = 4 ??? How does this happpen ???

4. Strong and Weak Acids
The concept of strong and weak acids was developed to describe and explain the differences in properties of acids.
An acid can be described as a weak acid if its characteristic properties (under the same conditions) are less than those of a common strong acid
There are relatively few strong acids:
HClO4(aq), HI(aq), HBr(aq), HCl(aq) H2SO4(aq), HNO3(aq)

5. The Difference: Using the Modified Arrhenius Theory
Strong Acids: have high conductivity, high rate of reaction w/ metals and carbonates and a relatively low pH
These empirical properties suggest many ions are present (lots of H3O+ ions present); which is consistent with the idea that strong acids react completely (>99%) with water to form hydronium ions
HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq)
>99%

6. The Difference: Using the Modified Arrhenius Theory
Weak Acids: have low conductivity, a lower rate of reaction w/ active metals and carbonates and a relatively higher pH
These empirical properties suggest fewer hydronium ions are present
Based on this evidence, a weak acid reacts incompletely (<50%) with water to form relatively few hydronium ions
CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO-(aq)
<50%

7. WEAK ACIDS
Because of this difference, weak acids are much safer to handle – you can even eat and drink many of them.
Tannic Acid in Tea
Stearic Acid in Animal Fat
Lactic acid in many dairy products and builds up in muscles
Citrus acid in fruits
Carbonic acid in drinks

8. Practice – Strong and Weak Acids
Pg. 255 #1-5

9. Strong and Weak Bases
Strong bases – have a high electrical conductivity, fast reaction rate and a very high pH (>>7)
Weak bases – have a low electrical conductivity (if molecular bases), slower reaction rate and a pH closer to, but greater than, 7
How do we explain the difference?

10. NH3 (aq) + H2O(aq) OH–(aq) + NH4+(aq)
Strong and Weak Bases
Strong Bases – all soluble ionic hydroxides that dissociate completely (>99%) to release hydroxide ions
NaOH(s)  Na+(aq) + OH-(aq)
Weak Bases – an ionic or molecular substance that reacts partially (<50%) with water to produce relatively few hydroxide ions
NH3 (aq) + H2O(aq) OH–(aq) + NH4+(aq)
<50%

11. Practice Explain the weak base properties of baking soda.
NaHCO3 (aq)  Na+(aq) + HCO3-(aq)
HCO3- (aq) + H2O(aq) H2CO3(aq) + OH- (aq)
Explain why sodium acetate has a pH of 8 when dissolved in water.
NaCH3COO(aq)  Na+ (aq) + CH3COO-(aq)
CH3COO-(aq) + H2O(l)  CH3COOH (aq) + OH-(aq)
<50%
<50%

12. Practice
Pg. 257 #6, 7, 9

13. Summary Strong Acids Weak Acids Strong Bases Weak Bases
Empirical properties (need same concentration & temperature)
Very low pH
Med to low pH
Very high pH
Med to high pH
High conductivity
Low conductivity
Low conductivity*
Fast reaction rate
Slow reaction rate
Modified Arrhenius Theory
Completely react with water to form H3O+(aq) ions
Partially react with water to form H3O+(aq) ions
Completely react with water to form OH-(aq) ions
Partially react with water to form OH-(aq) ions
* Applies only to weak bases that are molecular

14. Investigation 6.3
The evidence from the demonstration clearly shows that acids with the same initial concentration can have different degrees of acidic properties.
This difference was shown in the different conductivity measurements and the different rate of reactions.
What were the two important controlled variables?
concentration and temperature
Why? A very dilute strong acid could have a higher pH than a more concentrated weak acid – which is incorrect and the conductivity and rate of reaction measurements would be misleading.