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Ch.19 - Chemical Thermodynamics
Brown - Chemistry: The Central Science 15th Edition
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Tutti i libri di testoBrown 15th EditionCh.19 - Chemical ThermodynamicsProblema 94b
Capitolo 19, Problema 94b

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?

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Understand the concept of spontaneity: A reaction is spontaneous under standard conditions if it has a negative Gibbs free energy change (ΔG°). The standard conditions are 25 °C (298 K) and 1 atm pressure.
Use the Gibbs free energy equation: ΔG° = ΔH° - TΔS°, where ΔH° is the change in enthalpy, T is the temperature in Kelvin, and ΔS° is the change in entropy.
Determine the standard enthalpy change (ΔH°) and standard entropy change (ΔS°) for each reaction. These values can be found in standard thermodynamic tables.
Calculate ΔG° for each reaction using the equation ΔG° = ΔH° - TΔS°. Remember to convert temperature to Kelvin by adding 273.15 to the Celsius temperature.
Compare the calculated ΔG° values: If ΔG° is negative, the reaction is spontaneous under standard conditions. If ΔG° is positive, the reaction is non-spontaneous.

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Spontaneity of Reactions

A reaction is considered spontaneous if it occurs without external intervention under standard conditions. This is often determined by the change in Gibbs free energy (ΔG); if ΔG is negative, the reaction is spontaneous. Factors such as enthalpy (ΔH) and entropy (ΔS) changes also play a crucial role in assessing spontaneity.
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Spontaneity of Processes

Standard Conditions

Standard conditions refer to a set of specific conditions used to measure and compare the properties of substances, typically defined as 1 atm pressure and a temperature of 25 °C (298 K). Under these conditions, the standard enthalpy and Gibbs free energy of formation can be used to evaluate the spontaneity of reactions, providing a consistent framework for analysis.
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Thermodynamic Data

Thermodynamic data, including standard enthalpy of formation (ΔH°f) and standard entropy (S°), are essential for calculating the Gibbs free energy change of a reaction. By using these values, one can determine whether the overall reaction is energetically favorable and thus spontaneous. This data is typically found in tables and is crucial for predicting reaction behavior.
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First Law of Thermodynamics
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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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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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(a) For each of the following reactions, predict the sign of ΔH° and ΔS° without doing any calculations. (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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(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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