Bond Energy Calculations

• What can you calculate using bond energies?

  Energy change for a chemical reaction
• What is the difference between exothermic and endothermic reactions?

  • Exothermic: Energy is transferred from chemicals to surroundings; energy change is negative.
  • Endothermic: Chemicals gain energy from surroundings; energy change is positive.
• How are energy changes represented in exothermic reactions?

  As negative values
• How are energy changes represented in endothermic reactions?

  As positive values
• What happens when a chemical bond is broken?

  • Requires energy
  • Is endothermic
• What happens when a chemical bond is formed?

  • Releases energy
  • Is exothermic
• What does the energy value of a bond indicate?

  The energy required to break that bond
• What information is needed to calculate energy changes in a reaction?

  • Structures of the molecules involved
  • Energy values for each bond
• Why is breaking bonds considered endothermic?

  Because it requires energy
• Why is making bonds considered exothermic?

  Because it releases energy
• How do you calculate the total energy change for a reaction?

  1. Sum the energy required to break bonds (positive values).
  2. Sum the energy released from making bonds (negative values).
  3. Subtract the total energy released from the total energy required.
• What is the energy required to break the bond between hydrogen atoms?

  436 KJ per mole
• What is the energy required to break the bond between chlorine atoms?

  242 KJ per mole
• What is the energy released when forming a bond between hydrogen and chlorine atoms?

  431 KJ per mole
• How do you account for the formation of two molecules of hydrogen chloride in energy calculations?

  Multiply the bond energy by 2
• What is the final energy change for the reaction of hydrogen and chlorine to form hydrogen chloride?

  -184 KJ
• What does a negative final energy change indicate about a reaction?

  It indicates that the reaction is exothermic
• What is the purpose of using bond energies in chemical reactions?

  To calculate the energy change for a chemical reaction
• What are the steps to calculate the energy change for a chemical reaction using bond energies?

  • Identify bonds broken and formed
  • Calculate energy required to break bonds (endothermic)
  • Calculate energy released when forming bonds (exothermic)
  • Subtract total energy released from total energy required
• How do you calculate the energy change for the reaction of hydrogen and oxygen to form water?

  By breaking hydrogen and oxygen bonds and forming water bonds, then calculating the energy difference
• What is the bond energy required to break the bond between two hydrogen atoms?

  436 KJ per mole
• Why is the energy needed to break bonds considered positive?

  Because it requires energy input
• What is the bond energy for an oxygen double bond?

  495 KJ per mole
• What does a negative sign indicate when calculating bond energies?

  It indicates that energy is released
• What is the bond energy for an oxygen to hydrogen bond?

  463 KJ per mole
• How do you determine if a reaction is exothermic or endothermic based on bond energies?

  By checking if the total energy change is negative (exothermic) or positive (endothermic)
• What is the process to calculate the energy change for the complete combustion of methane?

  • Break carbon to hydrogen bonds in methane
  • Break oxygen double bonds
  • Calculate total energy input (positive)
  • Form carbon to oxygen bonds in carbon dioxide
  • Form oxygen to hydrogen bonds in water
  • Calculate total energy released (negative)
  • Subtract total energy released from total energy input
• What is the bond energy for a carbon to hydrogen bond?

  413 KJ per mole
• Why is the bond energy for breaking bonds considered endothermic?

  Because it requires energy to break the bonds
• How do you calculate the total bond energy for breaking bonds in methane and oxygen?

  By adding the bond energies of all bonds broken
• What does the total energy change indicate about the combustion of methane?

  It indicates that the reaction is exothermic
• What are the steps to determine the overall energy change in a chemical reaction?

  1. Identify bonds broken and formed
  2. Calculate energy input for breaking bonds
  3. Calculate energy output for forming bonds
  4. Subtract total energy output from total energy input
  5. Determine if the reaction is exothermic or endothermic based on the sign of the result