bond energy

• What should you be able to do by the end of the two-part video on bond energies?

  Use bond energies to calculate the energy change for a chemical reaction
• What happens in exothermic reactions regarding energy transfer?

  Energy is transferred from the chemicals to the surroundings
• How are energy changes represented in exothermic reactions?

  Energy changes are shown as negative
• What occurs in endothermic reactions regarding energy transfer?

  Chemicals gain energy from the surroundings
• How are energy changes represented in endothermic reactions?

  Energy changes are shown as positive
• What is the relationship between breaking chemical bonds and energy?

  Breaking chemical bonds requires energy and is endothermic
• What happens when chemical bonds are made?

  Making bonds releases energy and is exothermic
• What is the energy value associated with breaking the bond between hydrogen atoms?

  436 KJ per mole
• What is the energy value associated with breaking the bond between chlorine atoms?

  242 KJ per mole
• How do you calculate the total energy needed to break all chemical bonds in the reaction?

  Add the energy values for breaking the hydrogen and chlorine bonds
• What is the energy released when making the bond between hydrogen and chlorine atoms?

  431 KJ per mole
• Why do we multiply the energy value for making the hydrogen-chlorine bond by two?

  Because two molecules of hydrogen chloride are produced
• What does a negative sign in front of the energy value indicate when making bonds?

  It indicates that energy leaves the reaction
• How do you calculate the final energy change for the reaction?

  Subtract the total energy released from the total energy required
• What is the final energy change for the reaction of hydrogen and chlorine to produce hydrogen chloride?

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

  It shows that the reaction is exothermic
• What are the steps to calculate energy changes in a chemical reaction using bond energies?

  1. Identify the bonds broken and made in the reaction.
  2. Find the energy values for each bond.
  3. Calculate the total energy required to break the bonds (positive values).
  4. Calculate the total energy released when making the bonds (negative values).
  5. Subtract the total energy released from the total energy required to find the final energy change.
• What is the difference between exothermic and endothermic reactions in terms of energy?

  • Exothermic reactions:
  • Energy is released to the surroundings
  • Energy changes are negative
  • Endothermic reactions:
  • Energy is absorbed from the surroundings
  • Energy changes are positive