energy change

• Endothermic reactions take in energy, usually in the form of heat
• Endothermic reactions are represented by the word "endothermic" and exothermic reactions by "exothermic"
• Exothermic reactions start with "ex" which stands for "exit" - energy leaving the system
• Temperature of the surroundings increases in exothermic reactions and decreases in endothermic reactions
• Common examples of exothermic reactions include combustion, acid-base neutralization, metal displacement, freezing, condensation, and respiration
• Common examples of endothermic reactions include dissolving of ammonium salts, photosynthesis, thermal decomposition, melting, boiling, evaporation, and sublimation
• Less common examples of exothermic reactions are dissolving of anhydrous salts and dissolving concentrated acids or alkalis
• Exothermic reactions give out energy, usually in the form of heat
• Endothermic reactions start with "en" which stands for "enter" - taking in energy
• When dissolving ammonium nitrate in water, the temperature of the surroundings initially decreases due to the endothermic nature of the reaction
• Energy level diagrams show the energy levels of reactants and products in a reaction
• In exothermic reactions, products have less energy than reactants, represented by a negative enthalpy change (ΔH)
• In endothermic reactions, products have more energy than reactants, represented by a positive enthalpy change (ΔH)
• Per mole means per mole of the reaction
• When questions provide delta H in kilojoules, final answers should be in kilojoules. If delta H is given in kilojoules per mole, final answers should be in kilojoules per mole
• In an exothermic reaction, products have lower energy than reactants
• State symbols should be included for reactants and products in energy level diagrams
• Common mistakes in drawing energy level diagrams include:

  • Arrow reaching above products
  • Small gap between arrow head and tail and energy levels
  • Using a double-headed arrow for energy change
• Identifying endothermic and exothermic reactions:

  • Melting is endothermic
  • Boiling or evaporation is endothermic
  • Combustion reactions are exothermic
  • Condensation processes are exothermic
• In electrolysis, energy is supplied to break down compounds, making it endothermic
• Forming bonds releases energy, making bond-forming reactions exothermic
• Fractional distillation and photosynthesis are endothermic processes
• Neutralization and metal displacement reactions are exothermic
• Boiling processes are endothermic
• Splitting a molecule to form atoms requires energy, making it endothermic
• Bond-forming reactions are exothermic
• Exothermic reactions have a negative delta H and increase the temperature of the surroundings. Endothermic reactions have a positive delta H and decrease the temperature of the surroundings