Acid-Base Equilibria

Arrhenius

• acids increase [H+] when dissolved in water• acids can be classified as monoprotic, diprotic

or triprotic• bases increase [OH-] when dissolved in water• bases can be classified as monobasic, dibasic,

or tribasic

A-B Strength

• strong acids & bases ionize completely and are strong electrolytes

• 7 acids & 8 bases (memorize them!!!)• diprotic acids & dibasic bases do NOT ionize

completely, only their first H+ or OH- ionizes completely.

• Strong acid + strong base = neutral salt• strong A-B are not equilibrium expressions, but

all other A-B are reversible

Bronsted-Lowry A-B

• restricts definition to H+

– acids donate H+

– bases accept H+

• allows the classification of less traditional A-B• conjugate A-B pairs = 2 formulas in an

equation whose formulas differ by a H+

What is the acid, base, and the conjugates?

HClO +H2O H3O+ + ClO-

CO32- + H2O OH- + HCO3

-

Wait, water can go both ways?

• amphoteric substances can behave as either an acid or base depending on what they react with.

• water and anions with protons (H+) attached are most common amphoterics

Weak A-B• only partially ionize in water and are weak

electrolytes• can be written as equilibrium expressions with a

Ka or Kb value• K value indicates how much the acid or base will

ionize (high K = higher ionization)• larger K values indicate a stronger acid or base• For di- and tri- protic/basic, there will be 2 K

values (one for the first ionization and one for the second)

Autoionization of Water

H2O + H2O OH- + H3O+

• reversible equilibrium where water can donate a proton to itself

• Kw = 1.0 x 10-14 at room temp.

• For any conjugate A-B pair, Kw = Ka x Kb

• What is the Ka value for NH4+?

Example

Is an aqueous solution of Na2HPO4 acidic or basic?

pH scale

pH = -log[H+]• works for pH ranges from 2-12 and

approximates pH outside that window• The exponent on the [H+] is an indicator of

approximate pH.

Strong A-B

• no equilibrium b/c all acid/base ionizes• use original acid concentration to calculate pH• Calculate the [H+] and pH in a solution of

0.37M hydrochloric acid.• Calculate the [OH-] and pH in a 0.58M solution

of NaOH

Weak Acids

• use RICE to find equilibrium concentrations:R HA + H2O H3O+ + A-

I I 0 0C -x +x +xE I-x x x

• b/c Ka for most weak acids is less than 10-3, I-x is about equal to I, so Ka = x2/I

Example

What is the pH of a 0.20M solution of acetic acid (CH3COOH)?

Weak Bases

• similar calculations as acids (replace H3O+ with OH-)R B + H2O OH- + HB+

I I 0 0C -y +y +yE I-y y y

• b/c Kb for most weak bases is less than 10-3, I-y is about equal to I, so Kb = y2/I

Example

What is the pH of a 0.68M solution of aqueous ammonia?

Classify the following as weak/strong acids/bases:

• chloric acid• ammonium chloride• calcium hydroxide• ethyl amine• sodium cyanide

Example

Measurements show that the pH of a 0.10M solution of acetic acid is 2.87. What is the Kb of potassium acetate?

A-B properties of salt solutions• for the most part anions are slightly basic (because they

attract protons) and cations are slightly acidic (because they can donate protons)

• ions from strong A-B are the only neutral ions• To determine if a salt is acidic or basic, look at the ions it

forms:– ignore any neutral ions– if anion is left, salt is basic– if cation is left, salt is acidic– if both cation & anion are neutral, salt is neutral– if both cation & anion are not neutral, the A-B-ness can’t be

determined from the formula

Classify the following salts as acidic, basic, or neutral:

• NaNO2

• CH3NH3Cl

• NaCl• MgSO4

• Al2(SO3)3

A-B-ness & chemical structure

3 factors affect attraction of electrons (acidity increases with stronger attraction of electrons)1. ionic charge – when comparing similar atoms, more

positive ions are stronger acids2. oxidation # on central atom – when comparing

similar formulas with the same central atom, the higher the ox#, the stronger the acid

3. electronegativity – when comparing similar formulas with different central atoms, the higher the EN, the stronger the acid

Common Base Reactions• Strong bases also include hydrides (H-), nitrides

(N3-), and carbides (C22-)

– NaH + H2O H2 + Na+ + OH-

– Mg3N2 + 6H2O 2NH3 + 3Mg2+ + 6OH-

– Ca2C2 + 2H2O C2H2 + Ca2+ + 2OH-

• strong bases also include oxides of groups 1&2 metals– Li2O + H2O 2Li+ + 2OH-

– CaO + H2O Ca2+ + 2OH-

Common Acid Reactions

• nonmetal oxides (aka. acid anhydrides) turn into acids when placed in water– SO2 + H2O H2SO3

– CO2 + H2O H2CO3

– Cl2O7 + H2O 2H+ + 2ClO4-