Enthalpy and Bond Enthalpies

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  • More on enthalpy
    • Hess's Law
      • Hess's law states that 'the enthalpy change in a reaction is independent of the route taken by the reaction, provided that the initial and final conditions are the same'
    • Bond enthalpy
      • Covalent bonds are in general strong, but they can be broken and the bond enthalpy is a measure of the strength of a covalent bond
      • Bond enthalpy is the enthalpy change when one mole of bonds are broken, in the gaseous state
        • Bond breaking is always an endothermic reaction and so takes in energy
        • Bond forming is always an exothermic process and gives out energy
      • Calculating enthalpy change using bond enthalpies
        • 1. Always write the equation showing all the bonds in the reactants and products
          • 2. Add up the enthalpies of all the bonds that must be broken and separately, the enthalpies of all of the bonds that are formed
            • 3. Calculate: enthalpy change = +(enthalpy of bonds broken) - (enthalpy of bonds formed)
    • Accuracy of enthalpy change values calculated from bond enthalpies
      • Bond enthalpies are average values, so then mey not be exactly correct for the actual bonds in a particular molecule
      • Bond enthalpies are not determined under standard conditions, so they only give approximate predictions of standard enthalpy changes
      • Bond enthalpies can only predict gas phase enthalpy changes. If any reactants or products are not gases, there will be additional contributions to enthalpy change which are not included in predictions using bond enthalpy


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