Lecture Notes: Energetics

Introduction to Exothermic and Endothermic Reactions

• Exothermic Reactions:
  • Reactions that release energy to the surroundings as heat.
  • Example: Neutralizing an acid with a base; burning hydrocarbons like methane.
  • Applications: Heating homes, powering vehicles, generating electricity.
• Endothermic Reactions:
  • Reactions that absorb energy from the surroundings.
  • Example: Photosynthesis uses sunlight to produce glucose.

Importance of Understanding Reaction Energetics

• Exothermic reactions can be harnessed for energy.
• Endothermic reactions require energy input to occur.

Enthalpy (H) and Changes in Enthalpy (ΔH)

• Enthalpy: Total energy of a system, not directly measurable.
• Change in Enthalpy (ΔH):
  • Exothermic: ΔH is negative because energy of products < energy of reactants.
  • Endothermic: ΔH is positive because energy of products > energy of reactants.

Bond Energies and Reaction Energetics

• Chemical reactions involve breaking and forming bonds.
• Bond Energy: Energy needed to break a bond.
• Reaction Enthalpy Change (ΔH):
  • Calculated as the sum of the enthalpy changes for all reactant bonds broken minus the sum of the enthalpy changes of all product bonds formed.
  • Exothermic if energy released > energy required; Endothermic if energy required > energy released.

Stability and Activation Energy

• Stability: Lower energy states are more stable.
• Activation Energy: Energy required to start a reaction.
  • Even exothermic reactions may not occur spontaneously if activation energy is high.

Calorimetry and Measuring Heat Changes

• Calorimetry Experiment: Measures temperature change during a reaction in an insulated container.
• Heat Calculation Formula: Q = mCΔT
  • Q = heat absorbed/released
  • m = mass (of water/solution)
  • C = specific heat capacity
  • ΔT = temperature change

Example Calculation

• Reaction in a calorimeter raises temperature of 100g water from 20°C to 30°C.
• Calculate heat given off:
  • m = 100g, C = 4.18 J/g·K, ΔT = 10°C
  • Q = 100g × 4.18 J/g·K × 10 K = 4180 J

Conclusion

• Covered exothermic and endothermic reactions, enthalpy changes, and calorimetry.
• Further study needed for deeper understanding of energetics.