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

Explore the chemical composition and behavior of aqueous solutions in this quiz. Learn about electrolytes, their classifications as strong or weak, and the different theories defining acids and bases. Test your understanding of how these principles apply in various chemical scenarios.