Bond Energies

• Exothermic reaction

  Energy is transferred from the chemical to the surroundings
• Endothermic reaction

  Chemicals gain energy from the surroundings
• Calculating energy changes

  1. Look at formed energy when breaking chemical bonds

  2. Determine energy values for breaking and making bonds

  3. Add up energy required to break bonds

  4. Add up energy released when making bonds

  5. Subtract energy required to break bonds from energy released when making bonds
• Bond energy

  Energy required to break a chemical bond
• Sample reaction

  • Hydrogen + Chlorine → Hydrogen chloride
• Information needed to calculate energy change

  • Structures of molecules involved
  • Energy values for each bond
• Steps to calculate energy change

  1. Look at bonds being broken on left side

  2. Look at bonds being made on right side

  3. Add up energy required to break bonds

  4. Add up energy released when making bonds

  5. Subtract energy required to break from energy released
• Breaking bonds

  Endothermic (energy gained)
• Making bonds

  Exothermic (energy released)
• Final energy change = Energy required to break bonds - Energy released when making bonds
• This reaction is exothermic (final energy change is negative)
• Calculating energy change for a chemical reaction using bond energies

  1. Break bonds on left side

  2. Make bonds on right side

  3. Calculate total energy input

  4. Calculate total energy output

  5. Subtract energy output from energy input to get overall energy change
• Bond energy

  Energy required to break or make a chemical bond
• Bond breaking is endothermic (positive energy), bond making is exothermic (negative energy)
• The overall energy change is calculated by subtracting the total energy output from the total energy input
• Exothermic reactions release energy, endothermic reactions absorb energy