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Ch.18 - Free Energy and Thermodynamics
Tro - Chemistry: A Molecular Approach 4th Edition
Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831Not the one you use?Change textbook
All textbooksTro 4th EditionCh.18 - Free Energy and ThermodynamicsProblem 55
Chapter 18, Problem 55

Use data from Appendix IIB to calculate ΔSrxn ° for each of the reactions. In each case, try to rationalize the sign of ΔSrxn ° . a. C2H4(g) + H2(g) → C2H6(g)

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1
Identify the substances involved in the reaction: C2H4(g), H2(g), and C2H6(g).
Use Appendix IIB to find the standard molar entropy (S°) values for each substance: S°(C2H4), S°(H2), and S°(C2H6).
Apply the formula for the standard entropy change of the reaction: ΔS°_rxn = ΣS°(products) - ΣS°(reactants).
Substitute the S° values into the formula: ΔS°_rxn = [S°(C2H6)] - [S°(C2H4) + S°(H2)].
Rationalize the sign of ΔS°_rxn by considering the change in the number of gas molecules: The reaction goes from 2 moles of gas (C2H4 and H2) to 1 mole of gas (C2H6), which typically results in a decrease in entropy (ΔS°_rxn < 0).

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Entropy (ΔS)

Entropy is a measure of the disorder or randomness in a system. In chemical reactions, it reflects the number of ways the particles can be arranged. A positive ΔS indicates an increase in disorder, while a negative ΔS suggests a decrease. Understanding how the number of gas molecules changes during a reaction is crucial for predicting the sign of ΔS.
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Standard Reaction Entropy (ΔSrxn°)

Standard reaction entropy (ΔSrxn°) is the change in entropy for a reaction under standard conditions (1 atm, 25°C). It is calculated using the standard molar entropies of the reactants and products. The formula ΔSrxn° = ΣS°(products) - ΣS°(reactants) helps quantify the change in disorder associated with the reaction, which is essential for understanding thermodynamic favorability.

Molecular Complexity and Gas Behavior

The complexity of molecules and their states (gas, liquid, solid) significantly affects entropy. Gaseous molecules generally have higher entropy than liquids or solids due to their greater freedom of movement. In the reaction C2H4(g) + H2(g) → C2H6(g), the decrease in the number of gas molecules (from 2 to 1) suggests a decrease in entropy, which can help rationalize the sign of ΔSrxn°.
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Related Practice
Textbook Question

Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn. d. 2 H2S(g) + 3 O2(g) → 2 H2O(l) + 2 SO2(g)

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Textbook Question

Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn. b. Cr2O3(s) + 3 CO(g) → 2 Cr(s) + 3 CO2(g)

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Textbook Question

Rank each set of substances in order of increasing standard molar entropy (S°). Explain your reasoning. a. I2(g); F2(g); Br2(g); Cl2(g) b. H2O(g); H2O2(g); H2S(g) c. C(s, graphite); C(s, diamond); C(s, amorphous)

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Textbook Question

Rank each set of substances in order of increasing standard molar entropy (S°). Explain your reasoning. a. NH3(g); Ne(g); SO2(g); CH3CH2OH(g); He(g) c. CH4(g); CF4(g); CCl4(g)

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Textbook Question

For each pair of substances, choose the one that you expect to have the higher standard molar entropy (S°) at 25 °C. Explain your choices. a. NaNO3(s); NaNO3(aq) b. CH4(g); CH3CH3(g) c. Br2(l); Br2(g) d. Br2(g); F2(g) e. PCl3(g); PCl5(g) f. CH3CH2CH2CH3(g); SO2(g)

Textbook Question

Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn . b. C(s) + H2O(g) → CO(g) + H2(g)

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