Chapter 20: Acids and Bases

Arrhenius

• acid - produces H+ when dissolved in H₂O

• base - produces OH- when dissolved in H₂O

• don’t need to show addition of water, just dissociation

Bronsted-Lowry (B-L)

• acid - proton donor

• base - proton acceptor

• must always have 4 items (acid/base on left, conjugates on right)

• the more electronegative element accepts the proton

amphoteric

can behave as an acid and a base

ion product (K_w) of water

K_w = [H+][OH-] = 1.0 × 10^-14 at 25 degrees C

arrows

• if you react a strong acid, you need one arrow

• else, you need two (equilibrium) arrows

Lewis Acid-Base

• acid - electron pair acceptor

• base - AKA ligand'; electron pair donor

monoprotic

• dissociates one H+

• HCL

• HC₂H₃O₂

diprotic

• dissociates two H+

• H₂SO₄

triprotic

• dissociates 3 H+

• H₃PO₄

Strong acids

• HCL, HBr, HI

• HNO₃

• HClO₄

• HClO₃

• H₂SO₄

• there is only one diprotic strong acid; the rest are monoprotic

The K_a of a strong acid is

a very large number

weak acids are

weak electrolytes

Strong bases

• LiOH

• NaOH

• KOH

• RbOH

• CsOH

• Sr(OH)₂

• Ba(OH)₂

• Ca(OH)₂

• Alkali bases are stronger than alkaline earth metal bases

  • NaOH has 100% dissociation

• basically any hydroxide of an alkali or alkaline earth metal

strength of a acid/base

depends on magnitude of K_a or K_b

• greater the magnitude the stronger

pH scale

• pH = -log[H+]

• pOH = -log[OH-]

• pH + pOH = 14

• 1 - 14 but pH can be negative if concentration >1

• # decimal places in pH must match the number of sig figs in the concentration

pH of strong acids/bases

For all strong acids and bases, [reactant] = [H+/OH-] because it completes dissociates

pH of weak acids/bases

Must use the acid dissociation constant formula

relating K_a and K_b

K_aK_b = 1.0 × 10^-14

notes about conjugates

• the conjugate base of a weak acid is strong

• the conjugate acid of a weak base is strong

Case of H₂SO₄

• loses one H at a time

• first dissociation is strong while second is weak

1. If concentration >= 1.0 M:

   1. calculate pH from H+ in first reaction

   2. second rxn doesn’t contribute significantly

2. If concentration < 1:

   1. must account for second dissociation

   2. use values from strong/first dissociation as the initial concentration

   3. use ICE table using initial concentrations to find K_a expression and solve for x

pH of a polyprotic acid (like H₃PO₄)

use the first K_a to calculate pH