Use the data in Appendix B to determine which of the following compounds are thermodynamically stable with respect to their constituent elements at 25 °C. (a) C6H6(l) (b) NO(g) (c) PH3(g) (d) FeO(s)

Verified step by step guidance

Step 1: Understand the concept of thermodynamic stability. A compound is thermodynamically stable with respect to its constituent elements if its standard Gibbs free energy of formation (\( \Delta G_f^\circ \)) is negative at 25 °C.

Step 2: Identify the relevant data from Appendix B. You need the standard Gibbs free energy of formation (\( \Delta G_f^\circ \)) for each compound: C6H6(l), NO(g), PH3(g), and FeO(s).

Step 3: For each compound, check the \( \Delta G_f^\circ \) value. If \( \Delta G_f^\circ < 0 \), the compound is thermodynamically stable with respect to its elements. If \( \Delta G_f^\circ > 0 \), it is not stable.

Step 4: Compare the \( \Delta G_f^\circ \) values for each compound to determine their stability. A negative value indicates that the formation of the compound from its elements is spontaneous under standard conditions.

Step 5: Conclude which compounds are thermodynamically stable based on their \( \Delta G_f^\circ \) values. List the compounds with negative \( \Delta G_f^\circ \) as stable.

Key Concepts

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

Thermodynamic Stability

Thermodynamic stability refers to the condition where a compound is at a lower energy state compared to its constituent elements under specified conditions, such as temperature and pressure. A thermodynamically stable compound will not spontaneously decompose into its elements, indicating that the formation of the compound is favored energetically.

Standard State and Formation Enthalpy

The standard state of a substance is its most stable form at 1 bar of pressure and a specified temperature, typically 25 °C. The standard enthalpy of formation (ΔH_f°) quantifies the heat change when one mole of a compound is formed from its elements in their standard states. A negative ΔH_f° indicates that the compound is thermodynamically stable with respect to its elements.

Gibbs Free Energy

Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. A negative change in Gibbs free energy (ΔG) during a reaction indicates that the process is spontaneous, which is a key factor in determining the stability of compounds relative to their elements.

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