Bond energies

• What are bond energies used to determine?

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

  The energy needed to break one mole of a bond
• If the bond energy of hydrogen chloride is 431 kJ/mol, how much energy is needed to break one mole of these bonds?

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

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

  Energy is released to the surroundings
• In the reaction of hydrogen and chlorine forming hydrogen chloride, what bonds need to be broken?

  Hydrogen-hydrogen and chlorine-chlorine bonds
• How do you determine if a reaction is exothermic or endothermic?

  Compare energy required to break bonds and released energy
• What is the formula to calculate overall energy change?

  Energy required to break bonds minus energy released
• What is Avogadro's constant?

  6.02 times 10 to the 23
• In the example reaction, 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?

  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?

  941 kilojoules
• How do you calculate the overall energy change for the nitrogen and hydrogen reaction?

  Plug in bond energies into the formula
• What is the overall energy change for the nitrogen and hydrogen reaction?

  -16 kilojoules per mole
• What indicates that a reaction is exothermic in terms of energy change?

  A negative energy change value
• What are the steps to analyze a chemical reaction using bond energies?

  1. Identify bonds broken and formed.
  2. Determine bond energies from a table.
  3. Calculate total energy required to break bonds.
  4. Calculate total energy released from forming bonds.
  5. Compare the two to find overall energy change.
• What are the key concepts of exothermic and endothermic reactions?

  • Exothermic: releases energy, negative energy change
  • Endothermic: absorbs energy, positive energy change
  • Bond breaking: endothermic process
  • Bond formation: exothermic process