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.

Q1:

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/

Q2:

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

Q3:

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?

Q4:

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

SubstanceHO()2gCHOH()25lCO()2g
Δ𝐻f(kJ/mol)241.82277.6393.51

Q5:

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

ReactionP()+5O()PO()42410sgs2H()+O()2HO()222ggg6HO()+PO()4HPO()241034gsl
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.

Q6:

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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