Bond Energies

• What are bond energies used to determine in a chemical reaction?

  Whether a reaction is exothermic or endothermic
• What does bond energy refer to?

  The amount of energy needed to break one mole of a particular covalent bond
• If the bond energy of hydrogen chloride is 431 kilojoules per mole, how much energy is needed to break one mole of these bonds?

  431 kilojoules
• Why is breaking bonds considered an endothermic process?

  Because it requires energy from the surroundings
• What happens during bond formation?

  It is an exothermic process that releases energy
• In the reaction of hydrogen and chlorine to form hydrogen chloride, what bonds need to be broken?

  The hydrogen-hydrogen bond and the chlorine-chlorine bond
• How do you determine if a reaction is exothermic or endothermic?

  By comparing the total energy required to break bonds to the total energy released when bonds form
• What is the formula to calculate the overall energy change in a reaction?

  Energy required to break bonds minus energy released by forming bonds
• In the example given, what is the total energy required to break the hydrogen-hydrogen and chlorine-chlorine bonds?

  678 kilojoules
• What is the total energy released when forming two hydrogen chloride bonds?

  862 kilojoules
• What does a negative overall energy change indicate about a reaction?

  It means the reaction is exothermic
• In the nitrogen and hydrogen reaction, how many nitrogen-hydrogen bonds are formed?

  Six nitrogen-hydrogen bonds
• What is the energy required to break one nitrogen-nitrogen triple bond and three hydrogen-hydrogen bonds in the nitrogen and hydrogen reaction?

  2,330 kilojoules
• What is the energy released when forming six nitrogen-hydrogen bonds?

  2,346 kilojoules
• What is the overall energy change for the nitrogen and hydrogen reaction?

  • 16 kilojoules per mole
• What are the steps to determine if a reaction is exothermic or endothermic using bond energies?

  1. Identify bonds that need to be broken and their energies.
  2. Identify bonds that will be formed and their energies.
  3. Calculate the total energy required to break bonds.
  4. Calculate the total energy released by forming bonds.
  5. Subtract the energy released from the energy required.
  6. Analyze the result: negative value = exothermic, positive value = endothermic.
• What is Avogadro's constant?

  1. 02 times 10²³ particles per mole
• What is the significance of a negative energy change in a chemical reaction?

  • Indicates an exothermic reaction
  • Energy is released to the surroundings