Understanding Acid-Base Properties

1. Properties of Acids • Taste sour • pH < 7 • Turn litmus red • Colorless with phenolphthalein • Neutralize bases • React with metals to produce H2 gas • React with carbonates to produce CO2, H2O, and a salt.

2. Properties of Bases • Taste bitter • pH > 7 • Turn litmus blue • Bright pink with phenolphthalein • Neutralize acids • Dissolve wool • Feel slippery

3. Acid Base Definitions • Originally recognized by properties like taste, feel, reactions with indicators

4. Arrenhius Definition • Acids produce excess Hydrogen ions when added to water • Bases produce excess Hydroxide ions when added to water • Issues: • Very limited definition • Very few substances can actually be classified by this definition.

5. Bronsted-Lowry Definition • Acids are proton victims, “donors” • The molecule loses an H+ • Bases are proton thieves, “acceptors” • Steal an H+ from another molecule • More frequently used

6. Conjugate base • Ion that is formed when an acid donates a Hydrogen ion (proton) • Examples: Acid Conjugate base H2SO4 HSO41- HNO3 NO31- HC2H3O2 C2H3O21-

7. Conjugate base • If original acid is a strong acid, the conjugate base is so weak that is does not behave as a base. • It behaves as a neutral species • If original acid is a weak acid, the conjugate base behaves as a weak base • (Important for equilibrium considerations later)

8. Conjugate acid • Ion that is formed when a base accepts a Hydrogen ion • Examples: Base Conjugate acid NaOH H2O NH3 NH41+

9. Conjugate acid • If original base is a strong base, the conjugate acid is so weak that is does not behave as an acid. • It behaves as a neutral species • If original base is a weak base, the conjugate acid behaves as a weak acid

10. Amphoteric Substances • A species that can behave as either an acid or a base • Water is the best example of an amphoteric substance

11. Strong Acids • Acid that dissociates completely in water • 100% of the sample breaks apart into ions • Six strong acids: • HCl - Hydrochloric acid • HBr - Hydrobromic acid • HI - Hydroiodic acid • HNO3 - Nitric acid • H2SO4 - Sulfuric acid • HClO4 - Perchloric acid * HClO3 - Chloric acid is borderline

12. Strong Base • Base that dissociates completely in water • 100% of the sample breaks apart into ions • Strong bases • Hydroxides of the metals in group 1A and 2A (not Be or Mg)

13. Weak acid or weak base • Do NOT dissociate completely in water • In water, establishes equilibrium between the molecular form and ionic form • Any acid or base that is not a strong acid or base is weak • Example: Acetic acid HC2H3O2 + H2O  C2H3O21- + H3O1+

14. Autoionization of water • The transfer of a hydrogen ion from one water molecule to another water molecule, • results in the formation of a hydroxide ion and a hydronium ion. • Equation: 2 H2O  H3O1+ + OH1- • Equal amounts of hydroxide and hydronium are formed, • so water remains neutral.

15. pH system • The pH of a system is an indication of the [H3O1+]. While it is based on the autoionization of water, it works for all acid-base systems.

16. pH system • Definitions: • pH = -log [H3O1+] • pOH = -log [OH1-] • pKw = -log Kw Since Kw = 1.0 x 10-14, pKw = -log (1.0 x 10-14) = 14 • Kw = [H3O1+] [OH-1]  pKw = pH + pOH = 14

17. pH Strong Acids and Bases • [H3O1+] = initial concentration of acid • To find the pH of a strong acid, use the initial concentration of the acid as the concentration of H3O1+ • [OH1-] = initial concentration of base • pH (strong acid) = -log (initial conc.)