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Lesson: Hess Cycles Using Reaction Energies

In this lesson, we will learn how to calculate reaction enthalpies by applying Hess's law to reaction cycles based on standard enthalpy changes.

Worksheet • 6 Questions

Q1:

Calculate Ξ” 𝐻 ⦡ 2 9 8 for the process from the following information:

Q2:

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?

Q3:

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

Q4:

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 Ξ” 𝐺 ⦡ (kJ/mol) βˆ’ 2 6 9 7 . 0 0 βˆ’ 4 5 7 . 1 8 βˆ’ 4 2 8 . 6 6

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

Q5:

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
Ξ” 𝐻 ⦡ f (kJ/mol) βˆ’ 2 4 1 . 8 2 βˆ’ 2 7 7 . 6 βˆ’ 3 9 3 . 5 1

Q6:

Calculate the standard molar enthalpy of formation of N O ( ) g from the following data:

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