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 following data:

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 following sequence of reactions occurs in the commercial production of aqueous nitric acid: 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.

Substance H O ( ) 2 g C H O H ( ) 2 5 l C O ( ) 2 g
Δ 𝐻 f (kJ/mol) 2 4 1 . 8 2 2 7 7 . 6 3 9 3 . 5 1

Q5:

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

Reaction P ( ) + 5 O ( ) P O ( ) 4 2 4 1 0 s g s 2 H ( ) + O ( ) 2 H O ( ) 2 2 2 g g g 6 H O ( ) + P O ( ) 4 H P O ( ) 2 4 1 0 3 4 g s l
Standard Molar Free Energy Change Δ𝐺 (kJ/mol) 2 , 6 9 7 . 0 0 4 5 7 . 1 8 4 2 8 . 6 6

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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