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Ch.19 - Chemical Thermodynamics
Chapter 19, Problem 95a

Consider the following three reactions: (i) Ti(s) + 2 Cl2(g) → TiCl4(1g) (a) For each of the reactions, use data in Appendix C to calculate ΔH°, ΔG°, K, and ΔS ° at 25 °C.

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Identify the standard enthalpies of formation (ΔHf°) for each reactant and product from Appendix C. For the reaction Ti(s) + 2 Cl2(g) → TiCl4(l), find the ΔHf° values for Ti(s), Cl2(g), and TiCl4(l).
Calculate the standard enthalpy change (ΔH°) for the reaction using the formula: ΔH° = ΣΔHf°(products) - ΣΔHf°(reactants). Plug in the values you found for each substance.
Identify the standard Gibbs free energy of formation (ΔGf°) for each reactant and product from Appendix C. Use these values to calculate the standard Gibbs free energy change (ΔG°) for the reaction using the formula: ΔG° = ΣΔGf°(products) - ΣΔGf°(reactants).
Calculate the standard entropy change (ΔS°) for the reaction using the formula: ΔS° = ΣS°(products) - ΣS°(reactants). Use the standard molar entropy values (S°) from Appendix C for each reactant and product.
Determine the equilibrium constant (K) at 25 °C using the relationship between ΔG° and K, which is given by the equation: ΔG° = -RT ln(K), where R is the gas constant (8.314 J/mol·K) and T is the temperature in Kelvin (298 K for 25 °C). Solve for K.

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Key Concepts

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

Thermodynamics

Thermodynamics is the study of energy transformations and the relationships between heat, work, and energy. In chemical reactions, it helps determine the changes in enthalpy (ΔH°), Gibbs free energy (ΔG°), and entropy (ΔS°). Understanding these concepts is crucial for predicting the spontaneity and equilibrium of reactions.
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First Law of Thermodynamics

Gibbs Free Energy

Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. The change in Gibbs free energy (ΔG°) indicates whether a reaction is spontaneous (ΔG° < 0) or non-spontaneous (ΔG° > 0) under standard conditions.
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Gibbs Free Energy of Reactions

Equilibrium Constant (K)

The equilibrium constant (K) is a dimensionless value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. It is related to the Gibbs free energy change (ΔG°) by the equation ΔG° = -RT ln(K), where R is the gas constant and T is the temperature in Kelvin.
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Related Practice
Textbook Question

Trouton’s rule states that for many liquids at their normal boiling points, the standard molar entropy of vaporization is about 88 J/mol‐K. b. Look up the normal boiling point of Br2 in a chemistry handbook or at the WebElements website (www.webelements.com) and compare it to your calculation. What are the possible sources of error, or incorrect assumptions, in the calculation?

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Textbook Question

(c) In general, under which condition is ΔG°f more positive (less negative) than ΔH°f ? (i) When the temperature is high, (ii) when the reaction is reversible, (iii) when ΔS°f is negative.

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Textbook Question

Consider the following three reactions: (i) Ti(s) + 2 Cl2(g) → TiCl4(1g) (ii) C2H6(g) + 7 Cl2(g) → 2 CCl4(g) + 6 HCl(g) (iii) BaO(s) + CO2(g) → BaCO3(s) (b) Which of these reactions are spontaneous under standard conditions at 25 °C?

Textbook Question

Consider the following three reactions: (i) Ti(s) + 2 Cl2(g) → TiCl4(1g) (ii) C2H6(g) + 7 Cl2(g) → 2 CCl4(g) + 6 HCl(g) (iii) BaO(s) + CO2(g) → BaCO3(s) (c) For each of the reactions, predict the manner in which the change in free energy varies with an increase in temperature.

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Textbook Question

Using the data in Appendix C and given the pressures listed, calculate Kp and ΔG for each of the following reactions:

(a) N2(g) + 3 H2(g) → 2 NH3(g) PN2 = 2.6 atm, PH2 = 5.9 atm, PNH3 = 1.2 atm

(b) 2 N2H4(g) + 2 NO2(g) → 3 N2(g) + 4 H2O(g) PN2H4 = PNO2 = 5.0 × 10-2 atm, PN2 = 0.5 atm, PH2O = 0.3 atm

(c) N2H4(g) → N2(g) + 2 H2(g) PN2H4 = 0.5 atm, PN2 = 1.5 atm, PH2 = 2.5 atm

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