Hess's Law of Thermodynamics

Hess's Law of Thermochemistry The enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction Enthalpy is a state function, enthalpies can be derived Elements in Preference States - ΔH° (reactants) reactants elements ΔH° (products) elements reactants [ΔH°_rxn] OVERALL: reactants →products [ΔH'_rxn = ΔH''_rxn + ΔH'''_rxn] [ΔH°_rxn = ΔH°_f(products) - ΔH°_f(reactants)] more than one product/reactant enthalpy change [ΔH°_rxn = ΣΔH°_f(products) - ΣΔH°_f(reactants)] [ΔH°_rxn] [aA + bB → cC + dD] [ΔH_rxn] [ΔH°_rxn = [cΣΔH°_f(C) + dΣΔH°_f(D)] - [aΣΔH°_f(A) + bΣΔH°_f(B)]] [ΔH°_rxn] same change in enthalpy must be obtained in direct conversion from solid to vapour indirect conversion of solid to liquid to vapour combustion → heat of combustion is the energy. given table released when 1 mole of fuel in its standard state undergoes complete combustion w/ oxygen to form products in their standard state •typically forms CO2 & H2O (if incomplete, CO) in [CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)] ΔH = -890 J/mol bond enthalpy required to break 1 mole of a specific bond blw 2 gases energy required bond formation (exothermic) breaking bonds (endothermic) total bond enthalpy enthalpy of atomization: total energy to break all bonds in 1 mole of d ho bonds blw gaseous gascous atoms on product side Enthalpy Average bond enthalpy by dividing the TBE of a molecule (w/ 1 type of bond) by total # of bonds (ex) TBE of CH4 is 1661.8 kJ/mol 1661.8 kJ/mol / 4 = 415.5 kJ/mol Determination of total bond enthalpies METHOD 1 TBE reaction can be treated like enthalpy TBE_oute = ΔH_rxn = ΣΔH°_f(products) - ΣΔH°_f(reactants) METHOD 2 Summing up enthalpies of all bonds in molecule, using avg bond enthalpies methods give slightly diff. values bcs molecular structure affects enthalpy ΔH°_f method is more accurate ΔH of gas-phase reactions determined using ΔH°_f or average bond enthalpies ΔH_rxn = ΣΔH°_f(products) - ΣΔH°_f(reactants) ΔH_rxn = enthalpies of bonds broken - enthalpies of bonds formed TBE_reactants - TBE_products using gas-phase reactions enthalpies of formation & bond enthalpies can be used to determine ΔH for gaseous rxns both values can be used interchangeably