Acids and Bases: Properties and Equilibria

• The provided text covers acid-base equilibria, properties of acids and bases, and related concepts

Properties of Acids and Bases

• Acids have a sour taste and react with active metals to produce hydrogen gas
• Acids turn certain organic compounds characteristic colors
• Bases have a bitter taste, feel slippery, and turn organic compounds different characteristic colors

Common Acids and Bases

• Acetic acid, citric acid, and phosphoric acid serve as flavoring or preservatives
• Boric acid functions as a mild antiseptic and insecticide
• Sodium hydroxide (lye) is used as an oven and drain cleaner
• Ammonia serves as a household cleaner
• Sodium carbonate is used as a water softener and grease remover
• Sodium hydrogen carbonate can be used as a fire extinguisher and mild antacid

Acids and Bases in Water

• Acids and bases neutralize each other forming water and a salt
• Strong acids and strong bases yields water as the net ionic product
• Acids in water leads to solvent participation e.g. hydronium ion formation

Arrhenius Acid-Base Definition

• Acids produce H3O+ in water while bases produce OH- in water
• Arrhenius acids contain covalently bonded H atoms that ionize in water
• Neutralization in this definition occurs when H+ and OH- form H2O, releasing -55.9 kJ per mole of water

Acid Strength and Ka

• Acid strength is determined by the extent of dissociation into ions
• Strong acids dissociate completely in water while weak acids dissociate slightly
• The acid-dissociation constant (Ka) quantifies acid strength
• A larger Ka value indicates a stronger acid as more hydronium ions are present at equilibrium

Range of Ka values

• Ka values reflect the fraction of acid molecules that dissociate into ions
• High (~10-2) Ka corresponds to 10% dissociation, and Low ( 10-10) corresponds to 0.001% dissociation
• Acetic acid has a Ka of 1.8 x 10-5, with a 0.42% dissociation
• Smaller Ka values means lower percent dissociation, which indicates a weaker acid

Autoionization of Water and the pH Scale

• Water self-ionizes to form H3O+ and OH-
• The ion-product constant for water (Kw) is [H3O+][OH-]
• At 25°C, Kw = 1.0 x 10-14
• In pure water at 25°C, [H3O+] = [OH-] = 1.0 x 10-7 M

Consequences of Water Autoionization

• Changes in [H3O+] result in inverse changes in [OH-]
• Both H3O+ and OH- are present in all aqueous systems

Relationship Between [H3O+] and [OH-]

• In acidic solutions, [H3O+] > [OH-]
• In basic solutions, [H3O+] < [OH-]
• In neutral solutions, [H3O+] = [OH-]

Expressing Hydronium Ion Concentration: The pH Scale

• pH is the negative logarithm of [H3O+]
• pH = -log[H3O+]
• The higher the pH, the lower the [H3O+] concentration
• Acidic solutions have pH < 7.00, basic solutions have pH > 7.00, and neutral solutions have pH = 7.00

Other p-Scales

• pOH is the negative log of [OH-]
• pOH = -log[OH-]
• Acidic solutions have higher pOH than basic solutions

pK

• pK is the negative log of K
• pK = -log K
• Low pK means high K
• pKw = pH + pOH

Brønsted-Lowry Acid-Base Definition

• Acid is a proton (H+ ion) donor
• Base is a proton (H+ ion) acceptor
• Acid-base reaction is a proton transfer process

Amphiprotic Substances

• A substance that can act as either an acid or a base is amphiprotic
• H2O can act as either an acid or a base

Conjugate Acid-Base Pairs

• An acid and its conjugate base differ by one proton
• Every acid has a conjugate base, and every base has a conjugate acid
• The net direction of an acid-base reaction depends on the relative strengths of the acids and bases involved

Predicting Net Direction

• The favored direction of the reaction is from the stronger acid and stronger base forming weaker acid and weaker base
• For equal concentrations, direction is to the right if Kc > 1

Solving Weak-Acid Equilibria

• Write the balanced equation and Ka expression
• Define 'x' as the change in concentration
• Construct a reaction table (ICE table)
• Make assumptions to simplify calculations

Weak Bases

• A base accepts a proton from water to produce its conjugate acid and hydroxide ions
• Base-dissociation constant (Kb) measures base strength

Amines as Weak Bases

• Amines are derivatives of ammonia with one or more H atoms replaced by organic groups
• General formula: RNH2, R2NH, or R3N

Anions of Weak Acids as Weak Bases

• Anions of weak acids can act as weak bases by accepting protons from water
• The acidity or basicity of the solution depends on the relative concentrations of HA and A

Relationship between Ka and Kb

• Ka x Kb = Kw for a conjugate acid-base pair

Strong Acids and Strong Bases

• Common strong acids include HCl, HBr, HI, HNO3H2SO4, HClO3, and HClO4
• In a solution of a strong monoprotic acid, [H+] = original concentration of the acid

Strong Bases

• Common soluble strong bases are ionic hydroxides of alkali metals(NaOH, KOH) and heavier alkaline earth metals (Sr(OH)2)
• The hydroxide is the only available base in the solution

Acid-Base Properties of Salt Solutions

• Acid-base properties of salt solutions are due to the behavior of cations and anions
• Many ions react with water to generate H3O+ or OH- ions, a process called hydrolysis

Salts that Yield Neutral Solutions

• A salt with the anion of a strong acid and the cation of a strong base yields a neutral solution

Salts that Yield Acidic Solutions

• A salt with the cation of a weak base and the anion of a strong acid yields an acidic solution, cation act as weak acid

Salts that Yield Basic Solutions

• A salt with the anion of a weak acid and the cation of a strong base yields a basic solution, anion act as a weak base

Salts of Weakly Acidic Cations and Weakly Basic Anions

• Solution acidity depends on the relative Ka and Kb values of the ions
• If Ka > Kb, the salt yields an acidic solution: if Ka < Kb, the salt yields a solution.