The values of ΔG°_f for the hydrogen halides become less negative with increasing atomic number. The ΔG°_f of HI is slightly positive. However, the trend in ΔS°_f is to become more positive with increasing atomic number. Explain.

Key Concepts

Gibbs Free Energy (ΔGf°)

Gibbs Free Energy (ΔGf°) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. A negative ΔGf° indicates that a substance is thermodynamically stable and favors formation, while a positive value suggests instability. The trend of ΔGf° becoming less negative for hydrogen halides as atomic number increases reflects the increasing stability of heavier halides.

Entropy (ΔSf°)

Entropy (ΔSf°) is a measure of the disorder or randomness in a system. In the context of hydrogen halides, the trend of ΔSf° becoming more positive with increasing atomic number indicates that the heavier halides have greater molecular complexity and thus higher degrees of freedom, leading to increased disorder. This increase in entropy contributes to the overall thermodynamic stability of the heavier halides despite their less negative Gibbs Free Energy.

Trends in Halides

The trends in the properties of hydrogen halides, such as ΔGf° and ΔSf°, are influenced by atomic size and bond strength. As the atomic number increases from fluorine to iodine, the bond lengths increase, and bond strengths generally decrease, affecting the stability and formation energies of the compounds. This interplay between bond characteristics and molecular complexity explains the observed trends in Gibbs Free Energy and entropy for these halides.