Worksheet: Hess Cycles Using Reaction Energies

In this worksheet, we will practice calculating reaction enthalpies by applying Hess's law to reaction cycles based on standard enthalpy changes.


Calculate the standard molar enthalpy of formation of NO()g from the shown data. N()+2O()2NO()kJmol2NO()+O()2NO()kJmol22222ggggggΔ𝐻=66.4/Δ𝐻=114.1/


Calculate Δ𝐻 for the process shown. Sb()+Cl()SbCl()sgg5225 Use the following information to help you. Sb()+Cl()SbCl(),kJmolSbCl()+Cl()SbCl(),kJmolsggsgg32Δ𝐻=314/Δ𝐻=80/23325


The sequence of reactions shown occurs in the commercial production of aqueous nitric acid. 4NH()+5O()4NO()+6HO(),kJmol2NO()+O()2NO(),kJmol3NO+HO()2HNO()+NO(),kJmol32222223ggglggglaqgΔ𝐻=907/Δ𝐻=113/Δ𝐻=139/ What is the total energy change for the production of one mole of aqueous nitric acid by this process?


Using the data below, calculate, to 4 significant figures, the standard molar enthalpy of combustion of ethanol.



The standard molar free energy changes for three reactions are shown in the table.

Standard Molar Free Energy Change Δ𝐺 (kJ/mol)2,697.00457.18428.66

Calculate, to the nearest kilojoule per mole, the standard free energy of formation for phosphoric acid.


A Hess cycle may be constructed from the standard reaction enthalpies Δ𝐻r of the three related reactions shown. NH()+HCl()NHCl()kJmolN()+3H()2NH()kJmolN()+4H()+Cl()2NHCl()kJmol34r223r2224rggsggggggs,Δ𝐻=176/,Δ𝐻=92/,Δ𝐻=629/ Using this thermochemical data, calculate the Δ𝐻r value for the follwoing reaction. H()+Cl()2HCl()22ggg

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