Ch.19 - Chemical Thermodynamics

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.19 - Chemical Thermodynamics

Problem 88

Chapter 19, Problem 88

For each of the following processes, indicate whether the signs of ΔS and ΔH are expected to be positive, negative, or about zero. (a) A solid sublimes. (b) The temperature of a sample of Co(s) is lowered from 60 °C to 25 °C. (c) Ethyl alcohol evaporates from a beaker. (d) A diatomic molecule dissociates into atoms. (e) A piece of charcoal is combusted to form CO₂(g) and H₂O(g).

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Identify the reactants and products in the combustion of charcoal. Charcoal is primarily carbon, so the reaction is C(s) + O_2(g) -> CO_2(g) + H_2O(g).

Consider the change in entropy (ΔS). Combustion typically involves a solid reactant forming gaseous products, which increases disorder. Therefore, ΔS is expected to be positive.

Consider the change in enthalpy (ΔH). Combustion reactions are exothermic, meaning they release heat. Therefore, ΔH is expected to be negative.

Summarize the expected signs: ΔS is positive due to increased disorder from solid to gas, and ΔH is negative due to the exothermic nature of combustion.

Reflect on the implications: The positive ΔS and negative ΔH indicate that the combustion of charcoal is both entropically and enthalpically favorable.

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Key Concepts

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

Entropy (ΔS)

Entropy, represented as ΔS, is a measure of the disorder or randomness in a system. In a combustion reaction, such as the burning of charcoal, the products (gaseous CO2 and H2O) have greater molecular freedom compared to the solid charcoal, leading to an increase in disorder. Therefore, ΔS is expected to be positive.

Enthalpy (ΔH)

Enthalpy, denoted as ΔH, reflects the heat content of a system at constant pressure. Combustion reactions are typically exothermic, meaning they release heat as chemical bonds are broken and formed. In the case of charcoal combustion, the reaction releases energy, resulting in a negative ΔH.

Gibbs Free Energy (ΔG)

Gibbs Free Energy, represented as ΔG, determines the spontaneity of a reaction and is calculated using the equation ΔG = ΔH - TΔS. For a reaction to be spontaneous, ΔG must be negative. In the combustion of charcoal, the combination of a negative ΔH and a positive ΔS suggests that ΔG will be negative, indicating that the reaction occurs spontaneously.

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Related Practice

Textbook Question

The crystalline hydrate Cd(NO₃)₂⋅4 H₂O(s) loses water when placed in a large, closed, dry vessel at room temperature: Cd(NO₃)₂⋅4 H₂O(s) → Cd(NO₃)₂(s) + 4 H₂O(g) This process is spontaneous and ΔH° is positive at room temperature.

(a) What is the sign of ΔS° at room temperature?

Textbook Question

(b) If the hydrated compound is placed in a large, closed vessel that already contains a large amount of water vapor, does ΔS° change for this reaction at room temperature?

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Textbook Question

The reaction 2 Mg(s) + O₂(g) ⟶ 2 MgO(s) is highly spontaneous. A classmate calculates the entropy change for this reaction and obtains a large negative value for ΔS°. Did your classmate make a mistake in the calculation? Explain.

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Textbook Question

Consider a system that consists of two standard playing dice, with the state of the system defined by the sum of the values shown on the top faces. (f) Calculate the absolute entropy of the two-dice system.

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