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Ch.19 - Chemical Thermodynamics
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.19 - Chemical ThermodynamicsProblema 99a
Capitolo 19, Problema 99a

The oxidation of glucose (C6H12O6) in body tissue produces CO2 and H2O. In contrast, anaerobic decomposition, which occurs during fermentation, produces ethanol (C2H5OH) and CO2.
(a) Using data given in Appendix C, compare the equilibrium constants for the following reactions:
C6H12O6(s) + 6 O2(g) ⇌ 6 CO2(g) + 6 H2O(l)
C6H12O6(s) ⇌ 2 C2H5OH(l) + 2 CO2(g)

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1
Identify the reactions for which you need to compare the equilibrium constants: the oxidation of glucose and the fermentation of glucose.
Use the standard Gibbs free energy change (\( \Delta G^\circ \)) for each reaction, which can be found in Appendix C, to calculate the equilibrium constant \( K \) using the relation \( \Delta G^\circ = -RT \ln K \).
For each reaction, calculate \( \Delta G^\circ \) by using the formula \( \Delta G^\circ = \sum \Delta G^\circ_{\text{products}} - \sum \Delta G^\circ_{\text{reactants}} \).
Substitute the \( \Delta G^\circ \) values into the equation \( \Delta G^\circ = -RT \ln K \) to solve for \( K \), where \( R \) is the gas constant (8.314 J/mol·K) and \( T \) is the temperature in Kelvin.
Compare the calculated \( K \) values for both reactions to determine which reaction is more favorable under standard conditions.

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Equilibrium Constant (K)

The equilibrium constant (K) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given chemical reaction. It provides insight into the extent of a reaction; a large K indicates that products are favored, while a small K suggests reactants are favored. Understanding how to calculate and interpret K is essential for comparing different reactions, such as those involving glucose oxidation and fermentation.
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Equilibrium Constant K

Oxidation and Reduction Reactions

Oxidation and reduction (redox) reactions involve the transfer of electrons between substances. In the context of glucose metabolism, oxidation refers to the loss of electrons from glucose as it is converted to carbon dioxide and water, while reduction involves the gain of electrons by other species. Recognizing these processes is crucial for understanding the energy transformations that occur during cellular respiration and fermentation.
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Oxidation and Reduction Reactions

Thermodynamics and Reaction Spontaneity

Thermodynamics studies the relationships between heat, work, and energy in chemical processes. The spontaneity of a reaction is determined by changes in Gibbs free energy (ΔG), where a negative ΔG indicates a spontaneous reaction. This concept is important when comparing the thermodynamic favorability of aerobic versus anaerobic processes, as it helps explain why one pathway is preferred under certain conditions.
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Spontaneity of Processes
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Domanda del libro di testo

The oxidation of glucose (C6H12O6) in body tissue produces CO2 and H2O. In contrast, anaerobic decomposition, which occurs during fermentation, produces ethanol (C2H5OH) and CO2.

(b) Compare the maximum work that can be obtained from these processes under standard conditions.

C6H12O6(s) + 6 O2(g) ⇌ 6 CO2(g) + 6 H2O(l)

C6H12O6(s) ⇌ 2 C2H5OH(l) + 2 CO2(g)

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Domanda del libro di testo

(c) In each case, indicate whether K should increase or decrease with increasing temperature. (i) 2 Mg(s) + O2 (g) ⇌ 2 MgO(s) (ii) 2 KI(s) ⇌ 2 K(g) + I2(g) (iii) Na2(g) ⇌ 2 Na(g) (iv) 2 V2O5(s) ⇌ 4 V(s) + 5 O2(g)

Domanda del libro di testo

The conversion of natural gas, which is mostly methane, into products that contain two or more carbon atoms, such as ethane (C2H6), is a very important industrial chemical process. In principle, methane can be converted into ethane and hydrogen: 2 CH4(g) → C2H6(g) + H2(g) In practice, this reaction is carried out in the presence of oxygen: 2 CH4(g) + 12 O2(g) → C2H6(g) + H2O(g) (b) Is the difference in ΔG° for the two reactions due primarily to the enthalpy term (ΔH) or the entropy term (-TΔS)?

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Domanda del libro di testo

The conversion of natural gas, which is mostly methane, into products that contain two or more carbon atoms, such as ethane (C2H6), is a very important industrial chemical process. In principle, methane can be converted into ethane and hydrogen: 2 CH4(g) → C2H6(g) + H2(g) In practice, this reaction is carried out in the presence of oxygen: 2 CH4(g) + 1/2 O2(g) → C2H6(g) + H2O(g) (c) Explain how the preceding reactions are an example of driving a nonspontaneous reaction, as discussed in the 'Chemistry and Life' box in Section 19.7.

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Domanda del libro di testo

(b) Based on your general chemical knowledge, predict which of these reactions will have K>1. (i) 2 Mg(s) + O2 (g) ⇌ 2 MgO(s) (ii) 2 KI(s) ⇌ 2 K(g) + I2(g) (iii) Na2(g) ⇌ 2 Na(g) (iv) 2 V2O5(s) ⇌ 4 V(s) + 5 O2(g)

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