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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.9 - Thermochemistry: Chemical EnergyProblema 143f
Capitolo 9, Problema 143f

Methanol (CH3OH) is made industrially in two steps from CO and H2. It is so cheap to make that it is being considered for use as a precursor to hydrocarbon fuels, such as methane (CH4):
Step 1. CO(g) + 2 H2(g) → CH3OH(l) ΔS° = –332 J/K
Step 2. CH3OH(l) → CH4(g) + 1/2 O2(g) ΔS° = 162 J/K
(f) Calculate ΔH° for step 2.

Guida verificata passo dopo passo
1
Identify the given values and the required value. Here, the given value is the standard entropy change (ΔS°) for step 2, which is 162 J/K. The required value is the standard enthalpy change (ΔH°) for step 2.
Recall the Gibbs free energy equation: ΔG° = ΔH° - TΔS°. To find ΔH°, rearrange the equation to ΔH° = ΔG° + TΔS°.
Determine the temperature (T) at which the reaction occurs. If not specified, assume standard conditions (298 K).
Calculate ΔG° for step 2 using the standard free energy change equation for the reaction. If ΔG° is not provided, it may need to be calculated or approximated based on other given data or standard free energy values.
Substitute the values of ΔG°, T, and ΔS° into the rearranged Gibbs free energy equation to solve for ΔH°.

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Enthalpy Change (ΔH°)

Enthalpy change, denoted as ΔH°, refers to the heat content change of a system at constant pressure during a chemical reaction. It indicates whether a reaction is exothermic (releases heat, ΔH° < 0) or endothermic (absorbs heat, ΔH° > 0). Understanding ΔH° is crucial for predicting the energy requirements and feasibility of reactions, especially in industrial processes.
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Enthalpy of Formation

Gibbs Free Energy and Entropy (ΔS°)

Gibbs free energy combines enthalpy and entropy to determine the spontaneity of a reaction. The change in entropy (ΔS°) reflects the disorder of a system; a positive ΔS° indicates increased disorder. In the context of the given reactions, ΔS° values help assess the thermodynamic favorability of the steps involved in methanol production and its conversion to hydrocarbons.
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Gibbs Free Energy of Reactions

Thermodynamic Relationships

Thermodynamic relationships, such as the Gibbs-Helmholtz equation, connect ΔH°, ΔS°, and the temperature of a reaction to predict its spontaneity. For reactions, the relationship ΔG° = ΔH° - TΔS° is fundamental, where ΔG° is the change in Gibbs free energy. This relationship is essential for calculating ΔH° for step 2, as it allows the integration of entropy changes into the enthalpy calculations.
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First Law of Thermodynamics
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Methanol (CH3OH) is made industrially in two steps from CO and H2. It is so cheap to make that it is being considered for use as a precursor to hydrocarbon fuels, such as methane (CH4):

Step 1. CO(g) + 2 H2(g) → CH3OH(l) ΔS° = –332 J/K

Step 2. CH3OH(l) → CH4(g) + 1/2 O2(g) ΔS° = 162 J/K

(a) Calculate ΔH° in kilojoules for step 1.

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Methanol (CH3OH) is made industrially in two steps from CO and H2. It is so cheap to make that it is being considered for use as a precursor to hydrocarbon fuels, such as methane (CH4):

Step 1. CO(g) + 2 H2(g) S CH3OH(l) ΔS° = - 332 J/K

Step 2. CH3OH(l) → CH4(g) + 1/2 O2(g) ΔS° = 162 J/K

(e) In what temperature range is step 1 spontaneous?

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