Acids and Bases

Acids and Bases
Acid and Base Equilibrium

Definitions of Acids and Bases
Arrhenius: Acid supplies H+
Base supplies OH-
Bronstead: Acid donates protons
Base accepts protons
Lewis: Acid accepts electron pair
Base donates electron pair

Identify the Lewis acid and base in the following reaction.
SnCl4 + 2Cl- --> SnCl6 2-

Self Ionization (Autoionization) of water
H2O --> H+ + H20 Keq=1.0 x 10-14
[H+][OH-] = 1.0 x 10-14
Water is amphoteric, meaning that it can act as either an acid or base depending on the substance that it reacts with.

Conjugate Acids and Bases
Conjugates: Acids and bases that differ only in the presence or absence of a proton.
For conjugate pairs, Kb x Ka = Kw

The Power of Hydrogen and Hydroxide, pH and pOH
pH = -log[H+]
pOH = -log[OH-]
pH + pOH = 14
When pH is greater than 7, the solution is basic
When the pH is less tha n 7, the solution is acidic

Weak Acids and Bases and the Equilibrium Constant Expression
Weak acids and bases do not fully dissociate, so they have equilibrium constants to express their relative strengths

Ka = [H+][X-]/[HX] Kb = [HX][OH-]/[X-]

Polyprotic acid: acid having more than one ionizable proton. The first proton is always the easiest to dissociate, and therefore, Ka1>Ka2>Ka3

Strength of Acids and Bases
There are 6 strong acids: HBr, HCl, HI, HClO4, H2SO4, and HNO3
Strong bases include ionic hydroxides of the group 1A metals and the heavier group 2A metals (Sr, Ba, Ra)
Strong acids and bases completely dissociate in water, and therefore do not have Ka's or Kb's
Strong acids have weak conjugate bases, and strong bases have weak conjugate acids
Weak bases are molecules like NH3 and NH2 or anions like the conjugate bases of weak acids.

Factors that affect Acid Strength
The positive end of the polar H-X bond must be centered on the H atom for the hydrogen to be acidic.
Strong bonds are less easily broken, making a hydrogen that is strongly bonded weakly acidic.
The greater the stability of the conjugate base, the stronger the acid

Binary Acids
Acids consistin gof solely hydrogen and one other nonmetallic element
The greater the size of the nonmetal, the weaker the acid
The greater the electronegativity of the nonmetal, the stronger the acid.

Oxyacids
Acids in which OH groups and possibly additional O atoms are bound to a central atom
When the OH group is bound to a nonmetal, the bond is covalent, and the OH group and is not readily lost. In these cases, the molecule is either acidic or neutral.