Bond Energies

• Energy must always be supplied to break bonds
• During a chemical reaction, old bonds are broken and new bonds are formed
• Energy is released when new bonds are formed - so bond formation is an exothermic reaction

  
• Energy must be supplied to break existing bonds - so bond breaking is an endothermic reaction

  
• In exothermic reactions the energy released by forming bonds is greater than the energy used to break them
• In endothermic reactions the energy used to break bonds is greater than the energy released by forming them
• Every chemical bond has a particular bond energy associated with it. This bond energy varies slightly depending on the compound the bond occurs in
• You can use bond energies to calculate the overall energy change for a reaction. The overall energy change is the sum of the energies needed to break bonds in the reactants minus the energy released when the new bonds are formed in the products
• Example

  Using the bond energies given below, calculate the energy change for the reaction between H2, and Cl2 forming HCl: (H-H + Cl-Cl -> H-Cl, H-Cl)

  H-H: +436 kj/mol, Cl-Cl: +242 kj/mol, H-Cl: +431 kj/mol

  1)Find the energy required to break the original bonds

  > (1 x H-H) + (1 X Cl-Cl) = 436 kj/mol + 242kj/mol = 678 kj/mol

  2) Find the energy released by forming the new bonds

  > 2 x H-Cl = 2 x 431 kj/mol = 862 kj/mol

  3)Use the equation: $overall.energy.change =$$energy.required.to.break.bonds - energy.released.by.forming.bonds$

  > 678 kj/mol - 862 kj/mol = -184 kj/mol
• You can't compare the overall energy changes of reactions unless you know the numerical differences in the bond energies
• Chlorine and bromine react with hydrogen in a similar way. Br-Br bonds are weaker than Cl-Cl bonds and H-Br bonds are weaker than H-Cl bonds. So less energy is needed to break the bonds in the reaction with bromine, but less energy is released when the new bonds form. So unless you know the exact difference, you can't say which reaction releases more energy