Endothermic and Exothermic Reactions

Definitions

• Endothermic Reaction

  • ΔH (Enthalpy Change): Positive
  • Heat energy is absorbed by the system.

• Exothermic Reaction

  • ΔH (Enthalpy Change): Negative
  • Heat energy is released into the surroundings.

Potential Energy Diagrams

• Endothermic Reaction Diagram

  • Products have more energy than reactants.
  • ΔH is positive.
  • Example: The energy of the products is higher than that of the reactants.

• Exothermic Reaction Diagram

  • Products have less energy than reactants.
  • ΔH is negative.
  • Example: The energy of the products is lower than that of the reactants.

• Multi-step Potential Energy Diagram

  • First Step: Reactants to intermediates (endothermic, ΔH positive).
  • Second Step: Intermediates to products (exothermic).
  • Overall Reaction: Reactants to products is exothermic.

Phase Changes

• Solid to Liquid (Melting): Endothermic, ΔH positive (heat absorbed).
• Liquid to Gas (Vaporization): Endothermic, ΔH positive (heat absorbed).
• Gas to Liquid (Condensation): Exothermic, ΔH negative (heat released).
• Solid to Gas (Sublimation): Endothermic.
• Gas to Solid (Deposition): Exothermic.
• Liquid to Solid (Freezing): Exothermic (heat released).

Bond Breaking and Forming

• Breaking Bonds: Endothermic process, requires energy input.
• Forming Bonds: Exothermic process, releases energy.

Examples of Reactions

• Highly Exothermic Reactions:

  • Combustion of hydrocarbons like methane.
  • Dissolution of calcium chloride (releases heat, can vaporize water).
  • Dissolution of sodium hydroxide (increases temperature).

• Endothermic Reactions:

  • Some dissolution reactions (specific examples not provided).

Summary

Endothermic reactions absorb heat while exothermic reactions release heat. The direction of phase changes and bond interactions (breaking and forming) dictates the flow of energy. Understanding potential energy diagrams helps in analyzing these thermodynamic processes.