Textbook Question
Indicate and explain the sign of ΔSuniv for each process. a. 2 H2(g) + O2(g) → 2 H2O (l) at 298 K.
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Ch.19 - Free Energy & Thermodynamics
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217
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Table of contents
- Ch.1 - Matter, Measurement & Problem Solving118
- Ch.2 - Atoms & Elements106
- Ch.3 - Molecules and Compounds131
- Ch.4 - Chemical Reactions and Chemical Quantities56
- Ch.5 - Introduction to Solutions and Aqueous Solutions82
- Ch.6 - Gases114
- Ch.7 - Thermochemistry96
- Ch.8 - The Quantum-Mechanical Model of the Atom60
- Ch.9 - Periodic Properties of the Elements90
- Ch.10 - Chemical Bonding I: The Lewis Model89
- Ch.11 - Chemical Bonding II: Molecular Shapes, VSEPR & MO Theory85
- Ch.12 - Liquids, Solids & Intermolecular Forces51
- Ch.13 - Solids & Modern Materials47
- Ch.14 - Solutions90
- Ch.15 - Chemical Kinetics115
- Ch.16 - Chemical Equilibrium68
- Ch.17 - Acids and Bases136
- Ch.18 - Aqueous Ionic Equilibrium153
- Ch.19 - Free Energy & Thermodynamics93
- Ch.20 - Electrochemistry106
- Ch.21 - Radioactivity & Nuclear Chemistry74
- Ch.22 - Organic Chemistry140
Chapter 19, Problem 100
The values of ΔG°f for the hydrogen halides become less negative with increasing atomic number. The ΔG°f of HI is slightly positive. However, the trend in ΔS°f is to become more positive with increasing atomic number. Explain.
Verified step by step guidance1
Understand the terms: ΔGf° represents the standard Gibbs free energy of formation, which indicates the spontaneity of a reaction at standard conditions. ΔSf° represents the standard entropy of formation, which measures the disorder or randomness in the system during the formation of a compound.
Recognize the trend in ΔGf°: As the atomic number of the halogen in hydrogen halides (HX) increases, the ΔGf° values become less negative, indicating that the reactions are less spontaneous. For HI, the ΔGf° is slightly positive, suggesting that the reaction is non-spontaneous under standard conditions.
Analyze the trend in ΔSf°: The entropy of formation, ΔSf°, becomes more positive with increasing atomic number. This indicates that the disorder in the system increases as the size of the halogen atom increases.
Relate ΔGf° and ΔSf° with the atomic size: Larger atoms like iodine in HI have more electrons and electron shells, which can contribute to greater molecular complexity and randomness, leading to higher entropy values.
Combine the concepts: The increasing entropy (ΔSf°) with atomic number can contribute to the less negative (or more positive) ΔGf° values. According to the Gibbs free energy equation ΔG = ΔH - TΔS, a higher entropy value can offset changes in enthalpy (ΔH), influencing the spontaneity of the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Gibbs Free Energy (ΔGf°)
Gibbs Free Energy (ΔGf°) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. A negative ΔGf° indicates that a substance is thermodynamically stable and favors formation, while a positive value suggests instability. The trend of ΔGf° becoming less negative for hydrogen halides as atomic number increases reflects the increasing stability of heavier halides.
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Gibbs Free Energy of Reactions
Entropy (ΔSf°)
Entropy (ΔSf°) is a measure of the disorder or randomness in a system. In the context of hydrogen halides, the trend of ΔSf° becoming more positive with increasing atomic number indicates that the heavier halides have greater molecular complexity and thus higher degrees of freedom, leading to increased disorder. This increase in entropy contributes to the overall thermodynamic stability of the heavier halides despite their less negative Gibbs Free Energy.
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Trends in Halides
The trends in the properties of hydrogen halides, such as ΔGf° and ΔSf°, are influenced by atomic size and bond strength. As the atomic number increases from fluorine to iodine, the bond lengths increase, and bond strengths generally decrease, affecting the stability and formation energies of the compounds. This interplay between bond characteristics and molecular complexity explains the observed trends in Gibbs Free Energy and entropy for these halides.
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Related Practice
Textbook Question
These reactions are important in catalytic converters in automobiles. Calculate ΔG° for each at 298 K. Predict the effect of increasing temperature on the magnitude of ΔG°.
a. 2 CO(g) + 2 NO(g) → N2(g) + 2 CO2(g)
b. 5 H2(g) + 2 NO(g) → 2 NH3(g) + 2 H2O(g)
c. 2 H2(g) + 2 NO(g) → N2(g) + 2 H2O(g)
d. 2 NH3(g) + 2 O2(g) → N2O(g) + 3 H2O(g)
Textbook Question
Calculate ΔG° at 298 K for these reactions and predict the effect on ΔG° of lowering the temperature.
a. NH3(g) + HBr(g) → NH4Br(s)
b. CaCO3(s) → CaO(s) + CO2(g)
c. CH4(g) + 3 Cl2(g) → CHCl3(g) + 3 HCl(g) (ΔG°f for CHCl3(g) is -70.4 kJ/mol.)
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Textbook Question
All the oxides of nitrogen have positive values of ΔG°f at 298 K, but only one common oxide of nitrogen has a positive ΔS°f. Identify that oxide of nitrogen without reference to thermodynamic data and explain.