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Ch.4 - The Study of Chemical Reactions
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoWade 9th EditionCh.4 - The Study of Chemical ReactionsProblema 5a
Capitolo 4, Problema 5a

The following reaction has a value of ΔG° = –2.1 kJ/mol (–0.50 kcal/mol).
CH3Br + H2S ⇌ CH3SH + HBr
a. Calculate Keq at room temperature (25 °C) for this reaction as written.

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1
Understand the relationship between ΔG (Gibbs free energy change) and the equilibrium constant (Keq). The equation to use is ΔG = -RT ln(Keq), where R is the gas constant and T is the temperature in Kelvin.
Convert the temperature from Celsius to Kelvin. Use the formula T(K) = T(°C) + 273.15. For 25 °C, T = 25 + 273.15 = 298.15 K.
Identify the value of the gas constant R. For this calculation, use R = 8.314 J/(mol·K) (note that ΔG is given in kJ/mol, so ensure consistent units).
Rearrange the equation ΔG = -RT ln(Keq) to solve for Keq. The rearranged formula is Keq = e^(-ΔG / RT).
Substitute the given values into the equation: ΔG = -2.1 kJ/mol (convert to J/mol by multiplying by 1000), R = 8.314 J/(mol·K), and T = 298.15 K. Perform the calculation to find Keq.

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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. A negative ΔG indicates that a reaction is spontaneous, meaning it can proceed without external energy input. The magnitude of ΔG also provides insight into the reaction's favorability and equilibrium position.
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Breaking down the different terms of the Gibbs Free Energy equation.

Equilibrium Constant (Keq)

The equilibrium constant (Keq) is a dimensionless value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. It is derived from the law of mass action and provides insight into the extent of a reaction; a larger Keq indicates a greater concentration of products at equilibrium, while a smaller Keq suggests more reactants.
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Relationship between ΔG and Keq

The relationship between Gibbs Free Energy (ΔG) and the equilibrium constant (Keq) is described by the equation ΔG = -RT ln(Keq), where R is the universal gas constant and T is the temperature in Kelvin. This equation allows for the calculation of Keq from ΔG values, indicating how the spontaneity of a reaction correlates with its equilibrium position. A negative ΔG corresponds to a Keq greater than 1, suggesting that products are favored at equilibrium.
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Same atoms, same connectivity, 1 chiral center.
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The following reaction has a value of ΔG° = –2.1 kJ/mol (–0.50 kcal/mol).

CH3Br + H2S ⇌ CH3SH + HBr

b. Starting with a 1 M solution of CH3Br and H2S, calculate the final concentrations of all four species at equilibrium.

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