Lesson: Hess Cycles Using Standard Entropies Glencoe • Chemistry • Matter & Change Glencoe Science • Chemistry • Matter and Change

Chemistry

In this lesson, we will learn how to calculate reaction entropies by applying Hess’s law to energy cycles based on tabulated standard entropies.

Worksheet

Q1:

The standard entropies of water and other materials are shown in the table.

MaterialHO()2gHO()2lHO()22lH()2gH()gO()2gO()g
Standard Molar Entropy 𝑆 (J/K⋅mol)188.870.0109.6130.7114.7205.2161.1

Calculate, to 3 significant figures, the standard entropy change Δ𝑆 for the formation of liquid water from its constituent elements in their standard states, expressed per mole of water.

Calculate, to 3 significant figures, the maximum value of Δ𝐻, the standard molar enthalpy change per mole of water, at which this reaction would occur spontaneously at 298 K.

Q2:

Lead reacts with hydrogen chloride gas to form lead(II) chloride and hydrogen. The standard entropies of these and other materials are shown in the table.

MaterialPb()sPb()gPbCl()2sHCl()gH()gH()2g
Standard Molar Entropy 𝑆 (J/K⋅mol)64.8175.4136.0186.9114.7130.7

Calculate, to 3 significant figures, the standard entropy change for this reaction, Δ𝑆, expressed per mole of lead consumed.

Calculate, to 3 significant figures, the maximum value of Δ𝐻, the standard molar enthalpy change per mole of lead, at which this reaction would occur spontaneously at 298 K.

Q3:

The standard entropies of methanol and other materials are shown in the table:

MaterialCHOH()3lCHOH()3gO()gO()2gC()graphiteCO()gCO()2gHO()2g
Standard Molar Entropy, 𝑆(J/K·mol)126.8239.9161.1205.25.7197.7213.8188.8

Calculate the standard entropy change, Δ𝑆, for the complete combustion of methanol in the gas phase, expressed per mole of methanol reacted.

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