Notes Oct 3 2013 Thermochemical equations balanced chemical reactions plus the Hrxn Ex C5H12 l 8O2 g 5 CO2 g 6H2O l 3523 J 1 mol 8 mol 5 mol 6 mol If the energy of the reactants is higher than the energy of the products we would get a change in enthalpy heat Exothermic H 0 Endothermic Heat released during reaction is 3523 J Energy is an additive process Hess s Law the enthalpy change for a reaction is the same whether it occurs by one step or by any hypothetical series of steps True because H is a state function ConcepTest What is the enthalpy change for the first reaction 1 2 C s O2 g 2CO g 2 2 CO g O2 2 CO2 g Hrxn 566 kJ 3 C s O2 g CO2 g Hrxn 393 kJ A 221 kJ B 221 kJ C 173 kJ Horxn np Hof products nr Hof reactants Add up Hf of products D 110 5 kJ E none of these n stoichiometric coefficients Add up Hf of reactants ConcepTest Write the balanced equation for the reaction of aluminum with oxygen to form Al2O3 A Al s O2 g Al2O3 B 2 Al s 2O2 g 2 Al2O3 C 4 Al s O2 g 2 Al2O3 s D 4 Al s 3 O2 g 2 Al2O3 s E none of these When you break molecules apart it costs energy ConcepTest Calculate the enthalpy change for the reaction in which 30 0 g of Al reacts with oxygen to form Al2O3 at 25 C and one atm 4 Al s 3 O2 g 2 Al2O3 s Hf Al2O3 1676 kJ mol A 233 kJ B 233 kJ E none of these C 466 kJ D can t be determined without more values 30 0 g x 1 mol Al 26 9815 g x 2mol Al2O3 4 mol Al x 1676 kJ 1 mol Al2O3 932 kJ ConcepTest For this reaction which Hf values do we need from a table C3H8 g 5 O2 g 3 Co2 g 4H2O l i ii iii Propane CO2 Water A i only Ignore Bond Energies B i ii C ii iii D i ii iii E none of these