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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Tutti i libri di testoMullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblema 13a
Capitolo 4, Problema 13a

A certain process has ∆H° = 11.7 kcal/mol and AS° = +33cal/mol•K . That is, this reaction has an unfavorable enthalpy but a favorable entropy term. At what temperature will the process be neither favored nor disfavored?

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1
Identify the condition for a process to be neither favored nor disfavored. This occurs when the Gibbs free energy change (∆G°) is equal to zero. The equation for Gibbs free energy is: ∆G° = ∆H° - T∆S°.
Set ∆G° to zero in the equation: 0 = ∆H° - T∆S°. Rearrange the equation to solve for temperature (T): T = ∆H° / ∆S°.
Convert the units of ∆H° and ∆S° to ensure consistency. ∆H° is given in kcal/mol, so convert it to cal/mol: ∆H° = 11.7 \, \(\text{kcal/mol}\) \, × \, 1000 \, \(\text{cal/kcal}\) = 11700 \, \(\text{cal/mol}\). ∆S° is already in cal/mol·K, so no conversion is needed.
Substitute the values of ∆H° and ∆S° into the equation for T: T = 11700 \, \(\text{cal/mol}\) / 33 \, \(\text{cal/mol·K}\).
Perform the division to calculate the temperature (T) in Kelvin. This will give the temperature at which the process is neither favored nor disfavored.

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Gibbs Free Energy

Gibbs Free Energy (G) is a thermodynamic potential that helps predict the favorability of a reaction at constant temperature and pressure. It is defined by the equation G = ∆H - T∆S, where ∆H is the change in enthalpy, T is the temperature in Kelvin, and ∆S is the change in entropy. A reaction is spontaneous when G is negative, indicating that the process can occur without external energy input.
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Breaking down the different terms of the Gibbs Free Energy equation.

Enthalpy (∆H)

Enthalpy (∆H) is a measure of the total heat content of a system. In the context of a chemical reaction, a positive ∆H indicates that the reaction absorbs heat from the surroundings, making it endothermic. This can lead to an unfavorable reaction under certain conditions, as it requires energy input to proceed.
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Calculating Enthalpies

Entropy (∆S)

Entropy (∆S) is a measure of the disorder or randomness in a system. A positive change in entropy suggests that the products of a reaction are more disordered than the reactants, which is generally favorable for spontaneity. In the Gibbs Free Energy equation, a favorable entropy can offset an unfavorable enthalpy, influencing the overall spontaneity of the reaction at different temperatures.
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Explaining what entropy is.
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