# Copy_of_SCH4U_-_Hess_Law.docx - SCH4U u2013 Hessu2019s...

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SCH4U – Hess’s LawBOND ENERGIES multiple choice Bonds in compounds are broken when energies are supplied. The energy required to break a chemical bond is called a ___bond dissociation ___energy___________________________Bond dissociation energies have ___postive values ____________________Bond dissociation energies are reported as average bond energies since it depends on the __type __________________ and __numbers __________________ of bonds in the molecule. For example: a single C-H bond in a hydrocarbon has an average bond energy of 413 kJ/mol. This means it takes 413 kJ or energy to break 1 mole of C-H bonds into 1 mol of C atoms and 1 mol of H atoms!MULTIPLE BONDSMultiple bonds are generally _stronger ___________________ than single bonds.C≡C (839 KJ/mol); C=C (614 KJ/mol); C-C (347 KJ/mol)As the # of bonds _increases ___________________ , the bond length __decreases __________________ , the more the energy required to break it.Bond length (pm) Bond energy(KJ/mol)C-C 154 347C=C 134 614C≡C 120 839ENTHALPY AND BOND ENERGIESDuring a chemical reaction, the bonds in the reactants must firstbreak, energy must be added —an __endothermic __________________ process.Bond breaking has positive signsMaking new bonds in the products releases energy—an ___ exothermic _________________ processEnergy terms associated with bond making have ______negative ______________ signs.H= n x D (bonds broken) – n x D(bonds formed)ΔΣΣEnergy required energy released(sigma) means “the sum of,” n is the amount (in Σ_moles___________________ ) of a particular bond type, andD is the ___bond energy _________________ ____________________ per mole of bonds. D is obtained from referencetables.Steps in using bond energies to predict ∆H for a reaction