Chapter 19 Notes: Part II Acid/Base Theories

There are three ways to define acids and bases. This reflects the fact that science is always revising itself.

1) Arrhenius Acids/Bases An acid is a chemical that gives off hydrogen ions in solution. A base is a chemical that gives off hydroxide ions in solution. Example: HCl + NaOH  NaCl + H2O **BASICALLY…acids start with H+ and bases end with OH-!!!

Arrhenius Acids/Bases There are three ways to describe an acid: Monoprotic-gives off one H+ in sol’n Diprotic-gives off two H+ Triprotic-gives off three H+

Bronsted-Lowry Acids/Bases The Arrhenius definition is a good one, but does not encompass everything that shows acidic/basic qualities. To account for this, Johannes Bronsted and Thomas Lowry proposed a new idea.

2) Bronsted-Lowry Acids/Bases An acid is a hydrogen-ion (proton) donor. A base is a hydrogen-ion (proton) acceptor.

Conjugate Acids/Bases A conjugate acid is the particle that is formed when a base gains a hydrogen ion. A conjugate base is the particle that is formed when an acid donates a hydrogen ion.

Conjugate Acids/Bases A conjugate acid/base pair consist of two substances related by the loss of a single hydrogen ion.

Conjugate Pairs

NH3 + H2O  NH4+ + OH- Which is the initial acid? Which is the initial base? What is the conjugate acid? What is the conjugate base? proton donator H2O NH3 NH4+ OH- proton acceptor formed after base gains H+ formed after acid donates H+

HCl + H2O  H3O+ + Cl- Which is the initial acid? Which is the initial base? What is the conjugate acid? What is the conjugate base? HCl H2O H3O+ Cl-

Amphoteric Substances Note that in one of the previous examples H2O was acidic and basic in the other. A substance that can act as an acid or a base is called amphoteric.

3) Lewis Acids/Bases A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond. A Lewis base is a substance that can donate a pair of electrons to a covalent bond.

**MUST draw Lewis dot structures to determine whether a compound will gain or lose e-!!! 3 7 5 1 Ex… BF3 + NH3 F3BNH3 B F x N H F B x N H x ACID (not happy) accept e- pair BASE (happy) donate e- pair

pH scale 7 = neutral Over 7 = base The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers based on the Molarity of the H+ (or OH-) ion , We use the NEGATIVE power of 10 on the Molarity of the H+ (or OH-) ion. pH = - log [H+] Under 7 = acid 7 = neutral Over 7 = base

The Inverse Relationship between the pH and pOH Scales For any neutral solution, pH + pOH = 14.00 (at 25°C)

pH of Common Substances

pH [H+] [OH-] pOH