The Energy Change of Reactions

• Why is it important to know the bond energies in chemical reactions?

  To calculate how much heat is released or absorbed in a reaction
• What is the difference between endothermic and exothermic reactions based on energy changes?

  Endothermic reactions absorb more energy than they release, while exothermic reactions release more energy than they absorb
• How would you classify a reaction that requires energy to break bonds?

  Endothermic
• How can you determine if a reaction is exothermic or endothermic using bond energies?

  By comparing the total energy absorbed to the total energy released during the reaction
• What is the formula to calculate the energy change in a reaction?

  Energy change = Energy taken in - Energy given out
• If the energy in a reaction is 678 kJ and the energy out is 862 kJ, what is the energy change?

  • 184 kJ
• In the reaction of hydrogen and chlorine to form hydrogen chloride, what does a negative energy change indicate?

  That the reaction is exothermic and energy is released
• How do you calculate the bond energy of the Br–Br bond in the decomposition of hydrogen bromide?

  By rearranging the energy change equation to solve for Br–Br
• What is the significance of writing a displayed formula equation before bond energy calculations?

  It helps identify the type and number of bonds involved in the reaction
• What is the bond energy of the H–H bond?

  436 kJ
• What is the bond energy of the Cl–Cl bond?

  242 kJ
• What is the bond energy of the H–Cl bond?

  431 kJ
• What is the bond energy of the H–I bond?

  366 kJ
• What is the bond energy of the I–I bond?

  151 kJ
• What is the bond energy of the H–Br bond?

  366 kJ
• What is the overall energy change for the decomposition of hydrogen bromide?

  +103 kJ
• Why is it important to consider balancing numbers when calculating bond energies?

  To accurately account for the number of each type of bond being broken or formed
• How does the energy change in a reaction relate to the stability of the products compared to the reactants?

  A negative energy change indicates that the products are more stable than the reactants
• What does it mean if a reaction has a positive change in energy?

  It indicates that the reaction is endothermic
• What is the bond energy of the Br–Br bond calculated in the example?

  +193 kJ