Bond Enthalpies

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  • Bond Enthalpies
    • What are bond enthalpies?
      • Energy needed to break a bond
      • Have specific values that differ
        • depending on which atoms are attached on either side of bond
    • Breaking & Making Bonds
      • When reactions happen, reactant bonds are broken & product bonds are formed
      • Need energy to break bonds
        • bond breaking is endothermic (enthalpy change +)
        • stronger bonds take more energy to break
      • Energy is released when bonds form
        • bond forming is exothermic (enthalpy change -)
      • Enthalpy change for the reaction is overall effect of making & breaking the bonds
        • if you need more energy to break bonds than released when bonds are made - enthalpy change is +, if less its -
    • Mean Bond Enthalpies
      • Energy required to break bond can change depending on where it is
      • In calculations we use mean bond enthalpies
        • average energy needed to break certain type of bond, over range of compounds
        • often given in data tables & used in calculations
    • Calculating Enthalpy Changes
      • Enthalpy change of reaction = total energy absorbed - total energy released
      • Calculate total energy needed to break bonds in reactants
        • usually be given average bond enthalpies for each type of bond
          • multiply each value by number of each bond present
      • Find total energy released by reaction
        • calculate total energy needed to form all new bonds in products
          • use average bond enthalpies
    • Different Types of Enthalpy Change
      • Standard enthalpy change of formation
        • enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
      • Standard enthalpy change of combustion
        • enthalpy change when 1 mole of substance is completely burned in oxygen under standard conditions with all reactants and products in their standard state
      • Standard enthalpy change of reaction
        • enthalpy change when a reaction occurs in molar quantities shown in chemical equation, under standard conditions with all reactants & products in their standard state


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