Making bonds has the same value but opposite sign

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Presentation transcript:

Making bonds has the same value but opposite sign Bond Enthalpies… Bond enthalpy is: The energy to BREAK bonds It is an ENDOTHERMIC process ΔH is therefore POSITIVE Making bonds has the same value but opposite sign

Bond Enthalpies… In order to calculate the bond enthalpy of a compound we need to know the number of each type of bond.

7 Cl H O H C=C-C-C-C OH H H H H Bond Enthalpies… How many bond types are in the below compound? Cl H O H C=C-C-C-C OH H H H H 7

Bond Enthalpies… Will the carbon – chlorine bond in the previous compound require the same amount of energy to break as the carbon – chlorine bond in: Cl No – but it isn’t possible to measure the enthalpy of EVERY bond. So we use averages

Bond Enthalpies… Average Bond Enthalpy: The mean energy needed for 1 mole of a given type of gaseous bond to undergo homolytic fission

Bond Enthalpies… Average Bond Enthalpy: The mean energy needed for 1 mole of a given type of gaseous bond to undergo homolytic fission

Bond Enthalpies… Homolytic fission is simply the process where a bond is broken and each atom gains one electron to form a radical

We can use the average bond enthalpies to calculate ΔH: ΔH = Σ(reactants) - Σ(products) Σ(reactants) > Σ(products) = ENDO Σ(reactants) < Σ(products) = EXO

Bond Enthalpies… When calculating the enthalpy change you must look at the type of enthalpy you are being asked for. e.g. ΔfHӨ requires only 1 mole of product to be formed