Strong and Weak Acids and BasesTopic 8.3

ACIDS• when a STRONG acid dissolves- all, or nearly all, of

the acid molecules react to produce H+ or H3O+ ions (reaction goes to completion)– have a very high Kc value• HA H+(aq) + A-(aq)

0% ~100%• HA + H2O(l) H3O+(aq) + A-(aq)

0% ~100%– equilibrium is so far to the right for strong acids that we use

a yields symbol () instead of an equilibrium symbol (⇌)

HA H+ A-Initial amount of HA

At Equilibrium

common strong acids (need to know)

• hydrochloric acid: – HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)

• nitric acid: – HNO3(l) + H2O(l) H3O+(aq) + NO3

-(aq)

• sulfuric acid: – H2SO4(l) + H2O(l) H3O+(aq) + HSO4

-(aq)

• when a WEAK acid dissolves, very few acid molecules react to produce H+ ions (equilibrium lies to the left)– have a very low Kc value• HA ⇌ H+(aq) + A-(aq)

99% ~1%• HA + H2O(l) ⇌ H3O+(aq) + A-(aq)

99% ~1%

HAInitial amount of HA

At EquilibriumH+ A-HA

common weak acids (know these)

• carboxylic acids (contains one or more carboxyl groups, COOH):– acetic acid• CH3COOH(l) + H2O(l) ⇌ CH3COO-(aq) + H3O+

• carbonic acid: (CO2 in water)– CO2(aq) + H2O(l) ⇌ HCO3

-(aq) + H+(aq)

Characteristics of strong vs. weak acids• equal [conc] of strong and weak acids can be easily

distinguished:– acid strength does not change when a solution is diluted,

only the concentration (molarity) does

• DO NOT confuse strong and weak, with dilute and concentrated– concentrated = high molarity (M)– 0.1 M HCl (strong) vs 0.1 M CH3COOH (weak)

• same concentration, different strength0.1 mol dm-3 HCl(aq) 0.1 mol dm-3 CH3COOH(aq)

[H+(aq)] 0.1 mol dm-3 ~0.0013 mol dm-3

pH 1.00 2.87

Conductivity High Low

Reaction Rate with Mg Fast Slow

Reaction Rate with CaCO3 Fast Slow

– strong acids• have more H+ ions, hence lower pH• higher conductivity• react more vigorously with metals, metal

oxides, metal carbonates and bicarbonates• have a more (-)∆H of neutralization (more heat)• dangerous

–weak acids• opposite of strong acids

BASES• when a STRONG base dissolves, nearly all the

base molecules react to produce hydroxide (OH-) ions solution– have a very high Kc value• BOH + (aq) OH-(aq) + B+(aq)

0% ~100%– equilibrium is so far to the right for strong bases

that we use a yields symbol () instead of an equilibrium symbol (⇌)

common strong bases

– all group I hydroxides and barium hydroxide• NaOH(s) + (aq) Na+(aq) + OH-(aq) • KOH(s) + (aq) K+(aq) + OH-(aq)• Ba(OH)2(aq) + (aq) Ba2+(aq) + 2OH-(aq)

• when a WEAK base dissolves, very few base molecules react to produce OH- ions– have a very low Kc value• BOH + (aq) ⇌ OH-(aq) + B+(aq)

99% ~1%

common weak bases

• ammonia– NH3(g) + (aq) ⇌ NH4

+(aq) + OH-(aq)

• amines (contain NH1,2, or 3) :– ethylamine• C2N5NH2(g) + H2O(l) ⇌ C2H5NH3

+(aq) + OH-(aq)

Strong Bases Weak BasesLiOH NH3

NaOH Amines (ex: C2H5NH2)KOH

RbOHCsOH

Ba(OH)2

– strong bases• has more OH- ions, hence higher pH• higher conductivity• have a more (-)∆H of neutralization (more heat)• dangerous

–weak bases• opposite of strong bases

strong vs. weak base characteristics

• http://phet.colorado.edu/en/simulation/acid-base-solutions

Using experimental data to determine acids and bases

• Given the following are 0.1 M solutions, determine whether an acid or base, and strength– ph of 1: strong acid– poor conductor with pH of 6: weak acid– fast reaction with magnesium: strong acid– [H+] = 10 -10M: weak base– [H+] = 10 -4M: weak acid– neutralized 0.1M HCl quickly: strong base– good conductor with pH of 13: strong base– slow reaction with calcium: weak acid– has a high -ΔH when reacting with a base: strong acid