Ch.19 - Chemical Thermodynamics

Problem 59a

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Chapter 19, Problem 59a

Using data from Appendix C, calculate ΔG° for the following reactions. Indicate whether each reaction is spontaneous at 298 K under standard conditions. (a) 2 SO₂(g) + O₂(g) → 2 SO₃(g)

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First, identify the standard Gibbs free energy of formation (ΔG°f) values for each reactant and product from Appendix C. You will need the values for SO2(g), O2(g), and SO3(g).

Write the balanced chemical equation: 2 SO2(g) + O2(g) → 2 SO3(g).

Apply the formula for calculating the standard Gibbs free energy change (ΔG°) for the reaction: ΔG° = ΣΔG°f(products) - ΣΔG°f(reactants). Remember to multiply the ΔG°f values by their respective stoichiometric coefficients in the balanced equation.

Substitute the ΔG°f values into the equation and perform the calculation to find ΔG° for the reaction.

Determine if the reaction is spontaneous at 298 K under standard conditions by checking the sign of ΔG°. If ΔG° is negative, the reaction is spontaneous; if positive, it is non-spontaneous.

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Key Concepts

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

Gibbs Free Energy (ΔG°)

Gibbs Free Energy (ΔG°) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic process at constant temperature and pressure. It is a crucial indicator of spontaneity; if ΔG° is negative, the reaction is spontaneous under standard conditions, while a positive ΔG° indicates non-spontaneity.

Standard Conditions

Standard conditions refer to a set of specific conditions used to measure and compare thermodynamic properties, typically defined as 1 bar of pressure and a specified temperature, usually 298 K (25°C). These conditions allow for consistent calculations of thermodynamic values, such as ΔG°, across different reactions.

Reaction Quotient (Q) and Equilibrium Constant (K)

The reaction quotient (Q) is a measure of the relative concentrations of products and reactants at any point in a reaction, while the equilibrium constant (K) is the value of Q at equilibrium. Comparing Q to K helps determine the direction of the reaction and its spontaneity; if Q < K, the reaction proceeds forward, indicating spontaneity.

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Reaction Quotient Q

Related Practice

(a) For a process that occurs at constant temperature, does the change in Gibbs free energy depend on changes in the enthalpy and entropy of the system?

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Textbook Question

For a certain chemical reaction, ΔH° = -35.4 kJ and ΔS° = -85.5 J/K. (b) Does the reaction lead to an increase or decrease in the randomness or disorder of the system?

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Textbook Question

Use data in Appendix C to calculate ΔH°, ΔS°, and ΔG° at 25 °C for each of the following reactions.

c. 2 P(s) + 10 HF(g) → 2 PF₅(g) + 5 H₂(g)

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Textbook Question

Sulfur dioxide reacts with strontium oxide as follows: SO₂(g) + SrO(g) → SrSO₃(s) (a) Without using thermochemical data, predict whether ΔG° for this reaction is more negative or less negative than ΔH°.

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Textbook Question

Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontaneous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T; (iv) spontaneous at high T but not spontaneous at low T. (c) N₂F₄(g) ⟶ 2 NF₂(g) ΔH° = 85 kJ; ΔS° = 198 J/K

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Textbook Question

From the values given for ΔH° and ΔS°, calculate ΔG° for each of the following reactions at 298 K. If the reaction is not spontaneous under standard conditions at 298 K, at what temperature (if any) would the reaction become spontaneous? a. 2 PbS(s) + 3 O₂(g) → 2 PbO(s) + 2 SO₂(g) ΔH° = −844 kJ; ΔS° = −165 J/K

Using data from Appendix C, calculate ΔG° for the following reactions. Indicate whether each reaction is spontaneous at 298 K under standard conditions. (a) 2 SO2(g) + O2(g) → 2 SO3(g)

Verified Solution

Key Concepts

Gibbs Free Energy (ΔG°)

Standard Conditions

Reaction Quotient (Q) and Equilibrium Constant (K)

Using data from Appendix C, calculate ΔG° for the following reactions. Indicate whether each reaction is spontaneous at 298 K under standard conditions. (a) 2 SO₂(g) + O₂(g) → 2 SO₃(g)