Chemical Energetics: Exothermic & Endothermic

Chemical Energetics

• Thermochemistry: The study of heat energy changes during chemical reactions.
  • Reactions are classified as exothermic or endothermic.

Exothermic Reactions

• Definition: A chemical reaction where heat is released from the system to the surroundings.
• Temperature Change: The surrounding temperature increases.
• Energy Level Diagram: Products have less chemical energy than reactants.
• Energy Transfer: Chemical energy converts to heat energy.
• Examples: Combustion, respiration, neutralization, dissolving acids, dissolving alkalis, rusting, oxidation of metals, nuclear reactions.
• Magnesium and Hydrochloric Acid Example: The reaction between magnesium and hydrochloric acid produces heat, causing the reaction mixture to get hot.

Endothermic Reactions

• Definition: A chemical reaction where heat is absorbed by the system from the surroundings.
• Temperature Change: The surrounding temperature decreases.
• Energy Level Diagram: Products have more chemical energy than reactants.
• Energy Transfer: Heat energy converts to chemical energy.
• Examples: Thermal decomposition, dissolving some ionic salts (e.g., ammonium nitrate, potassium nitrate, copper(II) sulfate), photosynthesis, action of light on silver bromide.
• Ammonium Nitrate Example: The reaction of dissolving ammonium nitrate in water absorbs heat from the surroundings, lowering the temperature.

Energy Level Diagrams

• Exothermic: Reactants have higher energy than products.
• Endothermic: Reactants have lower energy than products.
• Diagram shows energy changes during a reaction over time.
• ΔH (Change in Enthalpy): Indicates the energy change of a reaction.
• Exothermic reactions have a negative ΔH.
• Endothermic reactions have a positive ΔH.

Bond Breaking and Bond Formation

• Bond Breaking: Endothermic (absorbs energy)
  • Requires energy to overcome forces holding atoms together.
• Bond Formation: Exothermic (releases energy)
  • Energy is released when atoms bond.
• Bond Energy: The amount of energy needed to break 1 mole of bonds.
• Stronger bonds require more energy to break.

Bond Energy Calculations

• All chemical reactions involve the breaking and making of bonds.
• The heat absorbed or released in a reaction comes from bond breaking and formation.
• ΔH° of a reaction can be calculated using bond energy values.
  • The standard enthalpy change is calculated by subtracting the sum of the bond energy of the products from the sum of the bond energy of the reactants

Radioisotopes (e.g., Uranium-235) as Energy Sources

• Fission: The splitting of an atom's nucleus, releasing large amounts of heat energy.
• Chain Reaction: A series of nuclear fissions, amplifying the energy release.
• Large amount of heat energy is released from a relatively small amount of uranium.

Hydrogen Fuel Cells

• Electrochemical cells using hydrogen as fuel.
  • Hydrogen is oxidized (loses electrons).
  • Oxygen is reduced (gains electrons).
• Uses hydrogen as fuel, oxygen as oxidant.
• Produces water (H₂O) as byproduct.
• Fuel cells are a thermodynamically open system.
• Used in various applications including vehicles, spacecraft, electronics, and emergency power supplies.

Explore the fascinating world of chemical energetics with this quiz on thermochemistry. Learn about exothermic and endothermic reactions, their definitions, and examples. Test your understanding of energy changes during chemical reactions.