For a certain chemical reaction, ΔH° = -35.4 kJ and ΔS° = -85.5 J/K. (c) Calculate ΔG° for the reaction at 298 K. (d) Is the reaction spontaneous at 298 K under standard conditions?

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To calculate the standard Gibbs free energy change (ΔG°) for the reaction, use the equation: ΔG° = ΔH° - TΔS°. Here, ΔH° is given in kJ, so convert ΔS° from J/K to kJ/K by dividing by 1000.

Substitute the given values into the equation: ΔG° = (-35.4 kJ) - (298 K) * (-85.5 J/K / 1000).

Perform the multiplication: (298 K) * (-85.5 J/K / 1000) to find the TΔS° term in kJ.

Subtract the TΔS° value from ΔH° to find ΔG°.

To determine if the reaction is spontaneous at 298 K, check the sign of ΔG°. If ΔG° is negative, the reaction is spontaneous under standard conditions.

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Key Concepts

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

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 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 a reaction is spontaneous under the given conditions.

Enthalpy (ΔH°)

Enthalpy (ΔH°) is a measure of the total heat content of a system and reflects the energy required to break and form bonds during a chemical reaction. A negative ΔH° value, such as -35.4 kJ, indicates that the reaction releases heat, making it exothermic. This release of energy can contribute to the spontaneity of the reaction.

Entropy (ΔS°)

Entropy (ΔS°) is a measure of the disorder or randomness in a system. A negative ΔS° value, like -85.5 J/K, suggests that the reaction leads to a decrease in disorder, which can hinder spontaneity. In thermodynamics, the balance between enthalpy and entropy changes determines whether a reaction will occur spontaneously at a given temperature.

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Related Practice

Textbook Question

(b) For a certain process that occurs at constant T and P, the value of ΔG is positive. Is the process spontaneous?

Textbook Question

For a certain chemical reaction, ΔH° = -35.4 kJ and ΔS° = -85.5 J/K. (b) Does the reaction lead to an increase or decrease in the randomness or disorder of the system?

Textbook Question

Use data in Appendix C to calculate ΔH°, ΔS°, and ΔG° at 25 °C for each of the following reactions.

a. 4 Cr(s) + 3 O₂(g) → 2 Cr₂O₃(s)

b. BaCO₃(s) → BaO(s) + CO₂(g)

c. 2 P(s) + 10 HF(g) → 2 PF₅(g) + 5 H₂(g)

d. K(s) + O₂(g) → KO₂(s)