Exothermic and Endothermic Reactions

Overview

• Exothermic Reactions: Reactions that release energy to the surroundings, usually in the form of heat.
• Endothermic Reactions: Reactions that absorb energy from the surroundings.

Energy in Chemical Bonds

• Different chemicals store different amounts of energy in their bonds.
• Important to compare total energy of reactants vs. products.
  • Example: Methane + Oxygen → Carbon Dioxide + Water
  • Products have less energy than reactants in exothermic reactions.
• Energy Transfer: Energy cannot be created or destroyed; it is transferred.
  • Heat is a common form of energy exchange.

Reaction Profiles

• Y-Axis: Total energy of molecules
• X-Axis: Progress of reaction
• Exothermic Reaction Profile:
  • Products are placed lower on the y-axis than reactants.
  • Energy lost by chemicals is released to surroundings.
• Endothermic Reaction Profile:
  • Products are higher on the y-axis than reactants.
  • Energy is absorbed from surroundings.

Examples of Reactions

• Exothermic Examples:
  • Combustion reactions (e.g., burning fuels in oxygen)
  • Neutralization reactions between acids and bases
  • Most oxidation reactions
• Endothermic Examples:
  • Decomposition of calcium carbonate (CaCO3 → CaO + CO2)

Activation Energy

• Definition: Minimum energy required for reactant particles to collide and react.
• Importance: Higher activation energy means more energy is needed to start the reaction.
  • Shown on reaction profiles as the peak of the curve between reactants and products.
  • Even exothermic reactions require some activation energy to initiate.

Drawing Reaction Profiles

• Exothermic: Draw curve with peak higher than reactants, but products lower than reactants.
• Endothermic: Draw curve with peak higher than both reactants and products.
• Adjusting Activation Energy:
  • Higher activation energy: raise the curve peak.
  • Lower activation energy: lower the curve peak.
• For specific reactions, replace generic labels with actual chemical names from the reaction equation.

These notes summarize key concepts related to exothermic and endothermic reactions, their representation in reaction profiles, and the role of activation energy as discussed in the lecture.