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 N O ( ) g from the following data:

Q2:

Calculate Δ 𝐻 2 9 8 for the process from the following information:

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

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

Q6:

A Hess cycle may be constructed from the standard reaction enthalpies Δ 𝐻 r of the three related reactions: Using this thermochemical data, calculate the Δ 𝐻 r value for the reaction H ( ) + C l ( ) 2 H C l ( ) 2 2 g g g .

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