— Thermodynamics Chemistry Question
Problem Context
Explain why many chemical reactions that are nonspontaneous, with ∆G˚ > 0 at room temperature, proceed to a significant extent at that temperature.
Model Answer
∆G˚ values refer to standard conditions including 1 M concentrations. Reactions that are nonspontaneous under these conditions may be caused to occur by increasing the concentration of the reactants and/or decreasing the concentrations of the products.
Account for the fact that standard enthalpies of formation of compounds at 25˚C may be either positive or negative.
Model Answer
∆Hf˚ values of compounds are relative to their elements in standards states (for which ∆Hf˚ = 0). Depending on the compound, formation may either release energy (∆Hf˚ < 0) or absorb energy (∆Hf˚ > 0).
Explain why all elements and compounds have positive S˚ values at 25˚C.
Model Answer
The standard for entropy, S˚, is a perfect crystal at 0 K, which by the Third Law of Thermodynamics is zero. As temperature increases, entropy increases, so S˚ is positive at 25˚C.
Give an example of a chemical species that does not have a positive S˚ value at 25 ˚C and explain why its standard entropy is not positive.
Model Answer
S˚ values of many ions (such as F–, Cl–, PO43–) are less than zero. This occurs because the reference for aqueous ions is the standard entropy for H+, which is set to zero. Some ions, like those listed, may organize the solvent molecules more than the hydrogen ions, so their standard entropy will be negative.