Lecture on Endothermic and Exothermic Reactions

Key Concepts

Endothermic Reactions

• Definition: Reactions that absorb heat energy from the surroundings.
• Enthalpy Change (ΔH): Positive, indicating energy absorption.
• Examples: Melting of ice, vaporization of water.

Exothermic Reactions

• Definition: Reactions that release heat energy into the surroundings.
• Enthalpy Change (ΔH): Negative, indicating energy release.
• Examples: Combustion of hydrocarbons, dissolution of calcium chloride in water.

Potential Energy Diagrams

Endothermic Reaction Diagram

• Characteristics: Products have a higher potential energy than reactants.
• ΔH Positive: Due to energy absorption.

Exothermic Reaction Diagram

• Characteristics: Products have lower potential energy than reactants.
• ΔH Negative: Due to energy release.

Multi-Step Reactions

• Endothermic Step: Initial increase in potential energy (reactants to intermediates).
• Exothermic Step: Decrease in potential energy (intermediates to products).
• Overall Reaction: Consider net energy change.

Phase Changes

Endothermic Processes

• Melting (Solid to Liquid): Absorption of heat required.
• Vaporization (Liquid to Gas): Absorption of heat required.
• Sublimation (Solid to Gas): Absorption of heat required.

Exothermic Processes

• Condensation (Gas to Liquid): Release of heat.
• Freezing (Liquid to Solid): Release of heat.
• Deposition (Gas to Solid): Release of heat.

Bond Breaking and Formation

Bond Breaking

• Endothermic: Energy is needed to break bonds.
  • Example: Breaking of Cl-Cl bond into chlorine radicals.

Bond Formation

• Exothermic: Energy is released when bonds form.
  • Example: Formation of a bond from radicals.

Examples of Exothermic Reactions

Combustion Reactions

• Hydrocarbon Combustion: E.g., burning methane releases CO2 and water.

Dissolution Reactions

• Calcium Chloride in Water: Releases significant heat, can vaporize water.
• Sodium Hydroxide in Water: Causes temperature increase due to heat release.

Notes

• Some dissolution reactions can be endothermic, but they are less common.

The lecture provides a comprehensive understanding of how to analyze energy changes in chemical reactions and phase changes, differentiating between processes that absorb heat and those that release it.