Lesson: Hess Cycles Using Free Energies Glencoe โ€ข Chemistry โ€ข Matter & Change Modern Chemistry Glencoe Science โ€ข Chemistry โ€ข Matter and Change

Chemistry

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

Worksheet

Q1:

The standard entropies and enthalpies of formation for methane and other materials are shown in the table.

MaterialStandard Molar Entropy ๐‘†โฆต๏Šจ๏Šฏ๏Šฎ (J/Kโ‹…mol)Standard Enthalpy of Formation ฮ”๐ปโฆตf (kJ/mol)
CH()4g186.3โˆ’74.6
C()g158.1716.7
C()๐‘‘๐‘–๐‘Ž๐‘š๐‘œ๐‘›๐‘‘2.41.9
C()๐‘”๐‘Ÿ๐‘Ž๐‘โ„Ž๐‘–๐‘ก๐‘’5.70.0
O()2g205.20.0
HO()2l70.0โˆ’285.8
HO()2g188.8โˆ’241.8

The incomplete combustion of methane generates graphite and steam as the only products. Calculate, to the nearest kilojoules per mole, the standard change in free energy for this reaction, ฮ”๐บโฆต๏Šจ๏Šฏ๏Šฎ, expressed per moles of methane reacted.

Q2:

The standard entropies and enthalpies of formation for lithium carbonate and other materials are shown in the table.

MaterialStandard Molar Entropy ๐‘†โฆต๏Šจ๏Šฏ๏Šฎ (J/Kโ‹…mol)Standard Enthalpy of Formation ฮ”๐ปโฆต๏Œฟ (kJ/mol)
LiCO()23s90.2โˆ’1,216.0
LiOH()s42.8โˆ’487.5
CO()g197.7โˆ’110.5
CO()2g213.8โˆ’393.5
HO()2l70.0โˆ’285.8
HO()2g188.8โˆ’241.8

Calculate ฮ”๐บโฆต๏Šจ๏Šฏ๏Šฎ, the standard change in free energy at 298 K, for the reaction of lithium hydroxide with carbon dioxide to form lithium carbonate and steam, expressed per moles of lithium carbonate produced.

Q3:

The standard entropies and enthalpies of formation for manganese(IV) oxide and other materials are shown below:

MaterialStandard Molar Entropy ๐‘†โฆต๏Šจ๏Šฏ๏Šฎ (J/Kยทmol)Standard Enthalpy of Formation ฮ”๐ปโฆตf (kJ/mol)
O()2g205.200.00
Mn()s32.000.00
MnO()s59.71โˆ’385.20
MnO()2s53.05โˆ’520.03
MnO()23s110.46โˆ’958.97
MnO()34s155.64โˆ’1,378.83

Manganese(IV) oxide decomposes into manganese and oxygen. Calculate, to 3 significant figures, the standard change in free energy at 298 K, ฮ”๐บโฆต๏Šจ๏Šฏ๏Šฎ, expressed per mole of manganese produced.

Assuming the thermodynamic parameters do not vary with temperature, calculate the minimum temperature at which the decomposition of manganese(IV) oxide into manganese and oxygen would be spontaneous.

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