Bond energies

• What is the main focus of the page on bond energies?

  Energy changes in chemical reactions
• What happens to chemical bonds during a reaction?

  Old bonds are broken and new bonds are formed
• What type of process is bond breaking?

  Endothermic process
• What type of process is bond formation?

  Exothermic process
• How do exothermic and endothermic reactions differ in terms of energy?

  Exothermic releases more energy than it uses
• What is the relationship between bond energies and chemical reactions?

  • Every bond has a specific bond energy
  • Bond energy varies by compound
  • Known bond energies help calculate overall energy change
• How do you calculate the overall energy change for a reaction?

  Sum of energies to break bonds minus formed bonds
• What are the bond energies for H–H, Cl–Cl, and H–Cl?

  H–H: +436 kJ/mol, Cl–Cl: +242 kJ/mol, H–Cl: +431 kJ/mol
• What is the energy required to break the original bonds in the reaction between H₂ and Cl₂?

  678 kJ/mol
• What is the energy released by forming the new bonds in the reaction between H₂ and Cl₂?

  862 kJ/mol
• What is the overall energy change for the reaction between H₂ and Cl₂?

  –184 kJ/mol
• How do Cl–Cl and Br–Br bonds compare in terms of energy?

  • Cl–Cl bonds are weaker than Br–Br bonds
  • Less energy is needed to break Cl–Cl bonds
  • Less energy is released when forming new bonds
• Why is it important to know bond energy differences?

  • To compare overall energy changes of reactions
  • Helps predict reaction feasibility
  • Essential for understanding reaction dynamics
• What is the relationship between bond breaking and energy?

  Energy must be supplied to break bonds
• What is the relationship between bond forming and energy?

  Energy is released when new bonds are formed
• Why is bond breaking an endothermic process?

  Because energy must be supplied to break existing bonds
• Why is bond forming an exothermic process?

  Because energy is released when new bonds are formed
• What determines the overall energy change in a chemical reaction?

  The difference between the energy required to break bonds and the energy released by forming new bonds
• In an exothermic reaction, which is greater: the energy released by bond formation or the energy required for bond breaking?

  The energy released by bond formation is greater
• In an endothermic reaction, which is greater: the energy released by bond formation or the energy required for bond breaking?

  The energy required for bond breaking is greater
• What is the formula to calculate the overall energy change in a reaction?

  Overall energy change = energy required to break bonds - energy released by forming bonds
• Using the given bond energies, calculate the energy change for the reaction between H₂ and Cl₂ to form HCl.

  <latex>

  \begin{align*}

  \text{Energy required to break bonds:} &\quad (1 \times \text{H–H}) + (1 \times \text{Cl–Cl}) = 436 \text{ kJ/mol} + 242 \text{ kJ/mol} = 678 \text{ kJ/mol} \\

  \text{Energy released by forming bonds:} &\quad 2 \times \text{H–Cl} = 2 \times 431 \text{ kJ/mol} = 862 \text{ kJ/mol} \\

  \text{Overall energy change:} &\quad 678 \text{ kJ/mol} - 862 \text{ kJ/mol} = -184 \text{ kJ/mol}

  \end{align*}

  </latex>
• How do the reactions of chlorine and bromine with hydrogen differ in terms of energy changes?

  Cl-Cl bonds are weaker than Br-Br bonds, and H-Cl bonds are stronger than H-Br bonds, so less energy is needed to break the bonds in the Cl-H reaction, but less energy is released when the new bonds are formed
• What are the key steps in calculating the overall energy change for a chemical reaction?

  1. Find the energy required to break the original bonds in the reactants
  2. Find the energy released by forming the new bonds in the products
  3. Calculate the overall energy change by subtracting the energy released from the energy required