18.1 Rates of Reaction

Connecting to Your World

• Corrosion can be damaging but useful in ready-to-eat meals, using the heat from iron-magnesium alloy reactions with saltwater for cooking.
• Reaction rates can be increased using specific methods.

Collision Theory

• Reactions can vary from fast (striking a match) to extremely slow (coal formation).
• Speed or rate is a change in distance over time.
• In chemistry, reaction rate is usually the amount of reactant changing per unit time.

Rate of Chemical Change

• Expressed as the change in concentration over time.
• Collision Theory: Atoms, ions, and molecules must collide with sufficient energy to react.
• Activation Energy: Minimum energy required for particles to react.

Energy Changes

• Depends on whether energy is released or absorbed.
• Activated Complex: Temporary arrangement of atoms at the peak of the activation-energy barrier.

Factors Affecting Reaction Rates

1. Temperature: Higher temperatures increase reaction rates.
2. Concentration: More particles in a fixed volume increase collision frequency.
3. Particle Size: Smaller particles have larger surface areas, increasing reaction rates.
4. Catalysts: Lower activation energy, increasing reaction rates without being consumed.

18.2 Reversible Reactions and Equilibrium

Reversible Reactions

• Reactions can proceed forward and in reverse; equilibria occur when the forward and reverse rates are equal.
• Chemical Equilibrium: No net change in the amounts of reactants/products.

Factors Affecting Equilibrium

• Le Châtelier’s Principle: A system will adjust to relieve stress and restore equilibrium.
• Stresses include changes in concentration, temperature, and pressure.

Equilibrium Constants (Keq)

• Ratio of product concentrations to reactant concentrations at equilibrium.
• Keq > 1: Products favored.
• Keq < 1: Reactants favored.

18.3 Solubility Equilibrium

Solubility Product Constant (Ksp)

• Product of concentrations of ions in a saturated solution.
• Lower Ksp values indicate less soluble compounds.

Common Ion Effect

• Adding an ion common to both salts reduces solubility.
• Used to predict precipitation formation.

18.4 Entropy and Free Energy

Spontaneous Reactions

• Occur naturally, forming products and releasing free energy.
• Entropy (S): Measure of disorder; systems naturally increase in disorder.

Enthalpy and Entropy

• Both determine reaction spontaneity.
• Gibbs Free Energy (ΔG): Negative in spontaneous reactions.

Factors Affecting Spontaneity

1. Enthalpy Change (ΔH): Heat release/absorption.
2. Entropy Change (ΔS): Disorder change.
3. Temperature (T): Affecting balance of ΔH and ΔS.

18.5 The Progress of Chemical Reactions

Rate Laws

• Express reaction rate in terms of reactant concentration and a specific rate constant (k).
• Order of Reaction: Depends on concentration exponent.

Reaction Mechanisms

• Elementary Reactions: Single-step reactions.
• Reaction Mechanism: Series of steps in complex reactions.
• Intermediates: Products of one step and reactants in the next in a reaction mechanism.

Reaction Progress Curves

• Show energy changes, with peaks (activated complexes) and valleys (intermediates/products).
• Catalysts lower activation energy, affecting reaction rates.