Ch.18 - Free Energy and Thermodynamics

Tro - Chemistry: A Molecular Approach 4th Edition

Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831당신이 사용하는 게 아니라요?교과서 변경

Tro 4th Edition

Ch.18 - Free Energy and Thermodynamics

문제 93

18장, 문제 93

These reactions are important in catalytic converters in automobiles. Calculate ΔG° for each at 298 K. Predict the effect of increasing temperature on the magnitude of ΔG°.
a. 2 CO(g) + 2 NO(g) → N₂(g) + 2 CO₂(g)
b. 5 H₂(g) + 2 NO(g) → 2 NH₃(g) + 2 H₂O(g)
c. 2 H₂(g) + 2 NO(g) → N₂(g) + 2 H₂O(g)
d. 2 NH₃(g) + 2 O₂(g) → N₂O(g) + 3 H₂O(g)

검증된 단계별 안내

Calculate \( \Delta G^\circ \) using the equation \( \Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \).

Find \( \Delta H^\circ \) (standard enthalpy change) for the reaction by using the standard enthalpies of formation: \( \Delta H^\circ = \sum \Delta H^\circ_f (\text{products}) - \sum \Delta H^\circ_f (\text{reactants}) \).

Find \( \Delta S^\circ \) (standard entropy change) for the reaction by using the standard entropies: \( \Delta S^\circ = \sum S^\circ (\text{products}) - \sum S^\circ (\text{reactants}) \).

Substitute the values of \( \Delta H^\circ \), \( \Delta S^\circ \), and \( T = 298 \text{ K} \) into the equation \( \Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \) to find \( \Delta G^\circ \).

To predict the effect of increasing temperature on \( \Delta G^\circ \), consider the sign of \( \Delta S^\circ \): if \( \Delta S^\circ > 0 \), \( \Delta G^\circ \) becomes more negative with increasing temperature; if \( \Delta S^\circ < 0 \), \( \Delta G^\circ \) becomes less negative.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Gibbs Free Energy (ΔG)

Gibbs Free Energy (ΔG) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It is crucial for predicting the spontaneity of a reaction; a negative ΔG indicates a spontaneous process, while a positive ΔG suggests non-spontaneity. The standard Gibbs free energy change (ΔG°) is calculated under standard conditions (1 atm, 298 K) and is essential for understanding reaction feasibility.

Temperature and Reaction Spontaneity

The relationship between temperature and the Gibbs free energy change is significant in determining reaction spontaneity. According to the Gibbs-Helmholtz equation, increasing temperature can affect ΔG, particularly if the reaction is endothermic (positive ΔH). For reactions where ΔS (entropy change) is positive, increasing temperature generally decreases ΔG, making the reaction more favorable. Understanding this relationship helps predict how temperature changes influence reaction behavior.

Catalytic Converters and Reaction Mechanisms

Catalytic converters are devices in automobiles that facilitate chemical reactions to convert harmful gases into less harmful emissions. They typically involve multiple reactions, including the conversion of nitrogen oxides (NO) and hydrocarbons into nitrogen and carbon dioxide. Understanding the reaction mechanisms and the role of catalysts is essential for analyzing the efficiency and effectiveness of these converters, as they lower the activation energy and allow reactions to proceed at lower temperatures.

관련 실천

교과서 질문

Living organisms use energy from the metabolism of food to create an energy-rich molecule called adenosine triphosphate (ATP). The ATP acts as an energy source for a variety of reactions that the living organism must carry out to survive. ATP provides energy through its hydrolysis, which can be symbolized as follows: ATP(aq) + H₂O(l) → ADP(aq) + P_i(aq) ΔG°_rxn = -30.5 kJ where ADP represents adenosine diphosphate and P_i represents an inorganic phosphate group (such as HPO₄^2-). b. The free energy obtained from the oxidation (reaction with oxygen) of glucose (C₆H₁₂O₆) to form carbon dioxide and water can be used to re-form ATP by driving the given reaction in reverse. Calculate the standard free energy change for the oxidation of glucose and estimate the maximum number of moles of ATP that can be formed by the oxidation of one mole of glucose.

All the oxides of nitrogen have positive values of ΔG°_f at 298 K, but only one common oxide of nitrogen has a positive ΔS°_f. Identify that oxide of nitrogen without reference to thermodynamic data and explain.

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교과서 질문

The values of ΔG°_f for the hydrogen halides become less negative with increasing atomic number. The ΔG°_f of HI is slightly positive. However, the trend in ΔS°_f is to become more positive with increasing atomic number. Explain.

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교과서 질문

Calculate ΔG° at 298 K for these reactions and predict the effect on ΔG° of lowering the temperature.

a. NH₃(g) + HBr(g) → NH₄Br(s)

b. CaCO₃(s) → CaO(s) + CO₂(g)

c. CH₄(g) + 3 Cl₂(g) → CHCl₃(g) + 3 HCl(g) (ΔG°_f for CHCl₃(g) is -70.4 kJ/mol.)

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