Chemical Composition of Aqueous Solutions

Chemical Composition of Aqueous Solutions

• Water is a universal solvent.
• Most solutes we will discuss are electrolytes.

Electrolytes

• Electrolytes form ions when dissolved in water.
• Solutions containing electrolytes conduct electricity.
• The amount of ionization depends on the strength of the electrolyte.

Classification of Electrolytes

• Strong electrolytes ionize completely in a solvent.

  • Most inorganic acids: HCl, HClO4, HNO3
  • Alkali and alkaline earth metal hydroxides
  • Most salts

• Weak electrolytes ionize partially.

  • Some inorganic acids: H3B03, H3PO4
  • Most organic acids
  • NH3 and most organic bases
  • Some salts: e.g. HgCl2

Different Definitions of Acids and Bases

• Three theories define acids and bases: Arrhenius, Brønsted-Lowry, and Lewis.

1-Arrhenius Theory

• An acid ionizes in water to give a hydronium ion: HA + H2O ↔ H3O+ + A-
• A base ionizes in water to give a hydroxyl ion: B + H2O ↔ BH+ + OH-
• This theory doesn't explain the role of the solvent in the ionization process.

2-Brønsted-Lowry Theory

• An acid is a proton (H+) donor.
• A base is a proton acceptor.
• A species can act as an acid only if a proton acceptor (base) exists, and vice versa.

Conjugate Acids and Bases:

• When an acid donates a proton, a conjugate base is formed that can accept a proton.
• When a base accepts a proton, a conjugate acid is formed that can donate a proton.
• Acid1 ↔ Base1 + Proton
• Base2 + Proton ↔ Acid2

Neutralization Reactions:

• Acid-base reactions occur when a proton is transferred from an acid to a base.
• Acid1 + Base2 ↔ Base1 + Acid2

Amphoteric Solvents

• Amphoteric species act as either an acid or a base.
• Water (H2O) is an amphoteric solvent.

3-Lewis Theory

• An acid is a substance that accepts an electron pair.
• A base is a substance that donates an electron pair.

Chemical Equilibrium

• Most analytical chemistry reactions are reversible and reach a state of chemical equilibrium.
• At equilibrium, the ratio of reactant and product concentrations is constant.
• The rate of forward reaction equals the rate of backward reaction.

Law of Mass Action

• The rate of a chemical reaction is proportional to the product of reactant concentrations, raised to the power of their stoichiometric coefficients.
• aA + bB ↔ cC + dD

Equilibrium Constant (K)

• K = [C]c[D]d / [A]a[B]b

Interpretation of K Values

• K >> 1: The reaction is product-favored.
• K << 1: The reaction is reactant-favored.

pH Scale

• pH = -log[H+]
• pOH =- log[OH-]
• pKw = pH + pOH = 14
• pH < 7: Acidic
• pH > 7: Basic

Methods for Measuring pH

• pH paper
• pH meter

Calculating pH

• Example: Calculate the pH of an ammonia solution with a hydroxide ion concentration of 1.9 x 10-3 M.
• pOH = -log(1.9 x 10-3) = 2.72
• pH = 14 - pOH = 11.28