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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
Chapter 4, Problem 11a

For the following values of ∆H° , ∆S°, and T, tell whether the process would be favored.
(a) ∆H° = -15 kcal/mol ; ∆S° = +37 cal/mol•K ; T = 273 K

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1
Step 1: Recall the Gibbs free energy equation: G = ΔH - TΔS. This equation determines whether a process is thermodynamically favored. A negative value for ΔG indicates a favored process.
Step 2: Convert the units of ΔS to match the units of ΔH. Since ΔH is given in kcal/mol and ΔS is in cal/mol•K, divide ΔS by 1000 to convert it to kcal/mol•K.
Step 3: Substitute the given values into the Gibbs free energy equation. Use ΔH = -15 kcal/mol, ΔS = +0.037 kcal/mol•K, and T = 273 K.
Step 4: Calculate the term TΔS by multiplying the temperature T by ΔS. This will give the contribution of entropy to the Gibbs free energy.
Step 5: Determine the sign of ΔG by subtracting TΔS from ΔH. If ΔG is negative, the process is thermodynamically favored; if positive, it is not favored.

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Key Concepts

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

Gibbs Free Energy

Gibbs Free Energy (G) is a thermodynamic potential that helps predict whether a process will occur spontaneously at constant temperature and pressure. It is calculated using the equation ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. A negative ΔG indicates that the process is favored.
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Enthalpy (ΔH)

Enthalpy (ΔH) is a measure of the total heat content of a system. A negative ΔH value, such as -15 kcal/mol, indicates that the process is exothermic, meaning it releases heat. This release of energy generally favors the spontaneity of a reaction, contributing to a negative ΔG when combined with the entropy term.
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Entropy (ΔS)

Entropy (ΔS) is a measure of the disorder or randomness in a system. A positive ΔS value, like +37 cal/mol•K, suggests that the process increases the disorder of the system, which is favorable for spontaneity. In the Gibbs Free Energy equation, a higher entropy contributes to a more negative ΔG, especially at higher temperatures.
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