1. Chapter 14-1 (pt 2) Properties of Acids and Bases
St. Augustine Preparatory School
April 19, 2017

2. Arrhenius Acids and Bases
Named after a Swedish chemist - Svante Arrhenius (1859 – 1927)
Arrenhius acid: a chemical compound that increases the concentration of hydrogen ions, H+(aq) , in solution
Arrenhius base: a chemical compound that increases the concentration of hydroxide ions, OH-(aq) , in solution

3. Arrhenius Acids

4. Modified Arrhenius Modified Arrhenius definition:
- The hydrogen ion reacts with water to form hydronium.
- This solved the problem as to why molecules “fell apart” (HCl -> H+ + Cl-) in solution.
Modified def’n for acids: Acids are substances that react with water to produce hydronium ions

5. Modified Arrhenius Acids

6. Acid Strength
A strong acid is one that ionizes completely in aqueous solutions.
Examples: HCl(aq), H2SO4(aq), HBr(aq), HI(aq)
A weak acid is one that ionizes only partially in aqueous solutions
Examples: HF(aq), CH3COOH(aq), HCN(aq), H3PO4(aq)

7. Modified Arrhenius Bases
Most bases are ionic compounds containing metal cations and the hydroxide ion (OH-), such as NaOH, sodium hydroxide.
Not all bases are ionic compounds though
Take ammonia for example:
NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq)

8. Strength of Bases
The strength of bases depends on how well the bases dissociates.
Strong Bases: Ca(OH)2, Sr(OH)2, NaOH, KOH, RbOH (all ionic compounds)
Weak Bases: NH3, C6H5NH2 (analine) (molecular)

9. End of Section 14-1
You should be able to answer the following questions:
Are acids and bases electrolytes?
What makes a compound/molecule an acid? What makes one a base?
What determines the strength of an acid or a base?
What are some characteristics of acids? What are some characteristics of bases?

10. Summary of Arrhenius Original: Modified Arrhenius
Acids – H+ ions from a dissociation result in a solution being acidic
Bases – OH- ions from a dissociation result in a solution being basic
Modified Arrhenius
Acids – Acids react with water to produce hydronium ions
Bases – Bases react with water to produce hydroxide ions

11. Monoprotic and Polyprotic Acids/Bases
Monoprotic – Reacts with water once to produce a hydroxide ion (bases) or a hydronium ion (acids)
Polyprotic – Reacts more than once with water. The product of the original reaction is able to react with water as well to form a hydroxide or hydronium ion

12. Percentages of Reactions
If it is a strong acid or base, the reaction will go >99%, which should be indicated above the arrow in the reaction
For polyprotic acids and bases (if they are WA or WB), it is <50%, then <1%, and if a third step is possible, it is ~0%

13. Examples
H3PO4 is a polyprotic (triprotic) weak acid. Show the dissociation equations for H3PO4.
Mg(OH)2 is a strong diprotic base. Show the dissociation equations for Mg(OH)2.

14. Chapter 14-2 Acid-Base Theories
Generally speaking, Arrhenius’ definition of acids and bases is adequate. However, his definition requires that the substances be aqueous, which isn’t always the case.

15. 1923 – J.N. Bronsted and T.M. Lowry
Brønsted (Danish) and Lowry (English) independently expanded the Arrhenius acid definition.
A Brønsted-Lowry acid is a molecule or ion that is a proton donor.
A Brønsted-Lowry base is a molecule or ion that accepts protons.
Unfortunately, this theory still was unable to explain everything.

16. Question
What is the difference between an Arrhenius acid/base and a Bronsted-Lowry Acid/Base?

17. Phosphoric acid:
Below, provide all relevant dissociations of the weak acid phosphoric acid, which is a triprotic acid when in solution

18. Acid-Base Video