Bond Energy Calculations

• 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?

  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?

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

  Energy changes are shown as positive
• What is required to break a chemical bond?

  Energy is required to break a chemical bond
• What happens when bonds are made?

  Making bonds releases energy
• What is the energy value for breaking the bond between hydrogen atoms?

  436 KJ per mole
• What is the energy value for 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 each bond
• What is the energy released when making the bond between hydrogen and chlorine atoms?

  431 KJ per mole
• How many molecules of hydrogen chloride are produced in the reaction?

  Two molecules of hydrogen chloride
• How do you calculate the total energy released when making bonds in the reaction?

  Multiply the energy value by the number of bonds made
• What does a negative sign in front of the energy value indicate?

  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 between hydrogen and chlorine?

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

  This reaction is exothermic
• 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 is the method to calculate energy changes in chemical reactions using bond energies?

  1. Identify the bonds broken and made in the reaction.
  2. Look up the bond energy values.
  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 energy change.
• What is the first example reaction discussed in the video?

  The reaction of two molecules of hydrogen with one molecule of oxygen to make two molecules of water
• What are the differences between exothermic and endothermic reactions in terms of energy transfer?

  • Exothermic reactions:
  • Energy is transferred to the surroundings
  • Energy changes are negative
  • Endothermic reactions:
  • Energy is absorbed from the surroundings
  • Energy changes are positive
• What is the process to calculate the energy change for a reaction using bond energies?

  1. Identify bonds broken and bonds formed.
  2. Calculate energy required to break bonds (positive values).
  3. Calculate energy released when forming bonds (negative values).
  4. Subtract total energy released from total energy required.
• What is the bond energy needed to break the bond between two hydrogen atoms?

  436 kJ per mole
• How much energy is needed to break the bonds in two molecules of hydrogen?

  872 kJ
• What is the bond energy required to break the oxygen double bond?

  495 kJ per mole
• What does the positive sign in energy calculations indicate?

  Energy is being put in to break bonds
• What is the total energy input for breaking the bonds in the first example reaction?

  1,367 kJ
• What is the bond energy for an oxygen to hydrogen bond?

  463 kJ per mole
• How much energy is released when forming four oxygen to hydrogen bonds in the reaction?

  • 1,852 kJ
• What is the overall energy change for the first reaction involving hydrogen and oxygen?

  • 485 kJ
• What does a negative energy change indicate about a reaction?

  The reaction is exothermic as energy has been released
• What is the second example reaction discussed in the video?

  The complete combustion of methane and oxygen
• How many carbon to hydrogen bonds are there in a methane molecule?

  Four
• What is the bond energy of a carbon to hydrogen bond?

  413 kJ per mole
• What is the total energy required to break the bonds in methane?

  1,652 kJ
• How much energy is needed to break the double bond in two molecules of oxygen?

  990 kJ
• What is the total bond energy required for the second example reaction?

  2,642 kJ
• What is the bond energy for a carbon to oxygen bond in carbon dioxide?

  799 kJ per mole
• How much energy is released when forming the bonds in carbon dioxide?

  • 1,598 kJ
• How much energy is released when forming the bonds in water?

  • 1,852 kJ