Lesson: Hess Cycles Using Free Energies

In this lesson, we will learn how to use enthalpy and entropy Hess cycles to calculate the free energy changes for chemical reactions.

Worksheet: 11 Questions

Q1:

The standard entropies and enthalpies of formation for disulfur dichloride and other materials are shown below:

Material Standard Molar Entropy Δ 𝑆 (J/K⋅mol) Standard Enthalpy of Formation Δ 𝐻 f (kJ/mol)
S C l ( ) 2 2 g 319.45 1 9 . 5 0
S O C l ( ) 2 g 309.66 2 1 2 . 5 5
C S ( ) 2 l 151.34 89.70
C S ( ) 2 g 238.00 116.89
C C l ( ) 4 l 214.42 1 2 8 . 2 3
C C l ( ) 4 g 309.75 9 5 . 6 8
C l ( ) 2 g 223.10 0.00

Chlorine and liquid carbon disulfide react to form gaseous disulfur dichloride and carbon tetrachloride. Calculate, to 3 significant figures, the standard change in free energy at 298 K, Δ 𝐺 , expressed per mole of disulfur dichloride produced.

Q2:

Dinitrogen trioxide decomposes reversibly to produce nitric oxide and nitrogen dioxide: The standard entropies and enthalpies of formation for dinitrogen trioxide and other materials are shown in the table.

Material Standard Molar Entropy 𝑆 (J/K⋅mol) Standard Enthalpy of Formation Δ 𝐻 f (kJ/mol)
N O ( ) 2 5 g 355.7 11.3
N O ( ) 2 4 g 304.4 11.1
N O ( ) 2 3 g 312.2 83.7
N O ( ) 2 g 220.0 81.6
N O ( ) g 210.8 90.3
N O ( ) 2 g 240.1 33.2

A sample of dinitrogen trioxide is stored under equal standard pressures of its decomposition products. Assuming the thermodynamic parameters in the table do not vary with temperature, calculate, to the nearest degree Celsius, the minimum temperature at which the sample of dinitrogen trioxide would spontaneously decompose.

Q3:

Iron(III) oxide can be produced by the reaction of iron with gaseous oxygen. The standard entropies and enthalpies of formation for iron(III) oxide and other materials are shown in the table.

Material Standard Molar Entropy 𝑆 (J/K⋅mol) Standard Enthalpy of Formation Δ 𝐻 f (kJ/mol)
F e ( ) s 27.3 0.0
F e ( ) g 180.5 416.3
F e O ( ) s 60.8 2 7 2 . 0
F e O ( ) 2 3 s 87.4 8 2 4 . 2
F e O ( ) 3 4 s 146.4 1 1 1 8 . 4
O ( ) 2 g 205.2 0.0

The standard change in Gibbs free energy, Δ 𝐺 , for the oxidation of iron at 298 K is expressed per mole of iron reacted. Calculate, to 3 significant figures, the value of Δ 𝐺 at 298 K.

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