Acids and Bases

Arrhenius Acid

Donate H⁺ (hydrogen ion) in water and have H in their formula

Arrhenius Base

Donate OH^- (hydroxide ion) in water and have OH in their formula

Arrhenius Neutralization

The mixing of an arrhenius acid and arrhenius base to produce a salt and water. All net ionic equations for arrhenius neutralizations is the formation of H₂O

HA(aq) + BOH(aq) → BA(aq) + H₂O(l)

Strong vs. Weak (Arrhenius)

Arrhenius acids and bases can be strong or weak.

• Strong acids and bases are strong electrolytes meaning that they fully dissociate into ions when in aqueous solution.

• Weak acids and bases are weak electrolytes meaning that they only partially dissociate into ions in aqueous solution

Common Strong Arrhenius Acids

HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄ (all aqueous)

• Only the first dissociation or ionization of H₂SO₄ is considered strong

Common Weak Arrhenius Acids

CH₃COOH, RCOOH, HF, HNO₂, H₃PO₄, H₂S, HCN (all aqueous)

Common Strong Arrhenius Bases

NaOH, KOH, LiOH, RbOH, CsOH, Na₂O, K₂O (all aqueous)

Common Weak Arrhenius Bases

Ba(OH)₂, Ca(OH)₂, Sr(OH)₂, NH₄OH (all aqueous)

• NH₄OH does not truly exist, in reality it is NH₃ (aq)

Bronsted-Lowry Acids

Donate H⁺

Bronsted-Lowry Bases

Accept H⁺

• Bases will have some sort of lone pair and can be neutral or anions

Bronsted-Lowry Neutralization

Mixing a bronsted-lowry acid and a base produces the conjugate acid and a conjugate base

• Conjugate pairs differ by the H⁺, the conjugate acid has an extra H⁺ which the conjugate base lacks

• Water is still the solvent, but neutralization reactions can take place in any solvent or even gas

HA + B → A^- + HB⁺

Strong vs. Weak (Bronsted-Lowry)

Depends on pKa or pKb value

A more negative pKa/pKb indicates a stronger acid/base

A more positive pKa/pKb indicates a weaker acid/base

Common Bronsted-Lowry Acids

HCl, H₂SO₄, H₃PO₄, HF, H₂CrO₄, RCOOH, H₂S, RNH³⁺,HCN, H₂O, ROH, NH₃, R-NH₂

Common Bronsted-Lowry Bases

R-Li, NaNH₂, KH, RONa, NaOH, KCN, NaHCO₃, RCOONa,

KHSO₄, NaCl

Lewis Acids

Accept a lone pair of electrons into an empty orbital

• Generally cations or trigonal planar 6e^- species

Lewis Bases

Donate a lone pair of electrons into empty orbital

• Bases will have some sort of lone pair and can be neutral or anions

Neutralization (Lewis)

Mixing a lewis acid and a lewis base produces a new covalent bond between the acid and the base

• Product is an adduct

• Neutralization reactions can take place in solution or in gas phase and the solvent may or may not be water

A + B → A-B

Strong vs. Weak (Lewis)

Strength of a lewis acid or lewis base depends on the enthalpy of formation of the adduct

• A more exothermic reaction implies stronger acids/bases

Common Lewis Acids

H⁺, Zn²⁺, Fe³⁺, CH₃⁺, BH₃, BF₃, AlCl₃, FeBr₃

• H⁺ is a lewis acid, but HCl is not → HCl just produces H⁺

Common Lewis Bases

R-Li, NaNH₂, KH, RONa, NaOH, KCN, NaHCO₃, RCOONa,

KHSO₄, NaCl

pKa/pKb and Ka/Kb

Typically for Bronsted-Lowry and Arrhenius acids/bases

• Ka (acid dissociation constant) and pKa for acids

• Kb (base dissociation constant) and pKb for bases

• Stronger acids/bases have large Ka/Kb and small pKa/pKb

Ka = [H⁺][A^-]/[HA] pKa = -log(Ka)

Kb = [HB⁺]/[H⁺][B] pKb = -log(Kb)

Ka vs. Kb

The stronger the acid the weaker the conjugate base and vice versa

• Stronger acids want to lose H⁺, so its conjugate base is not eager to get the H⁺ back → leading to a weak conjugate base

• A weak acid is not willing to lose its H⁺ → so its conjugate base will be willing to regain the H⁺ → making the conjugate base stronger

• As Ka increases, Kb decreases

Strong vs. Weak (General)

The term "strong” and "weak” acids is typically used with Arrhenius acids and bases

• Strong acids will have a pKa<0 and Ka>1, while strong bases will have a pKb<0 and a Kb>1

• However, with bronsted-lowry acids and bases there can exist a "stronger” acid/base in comparison to another while both acids/bases are weak