For the highly specific atom-combination reaction $2\mathrm{H}_{(g)} \rightarrow \mathrm{H}_{2(g)}$,

Question

For the highly specific atom-combination reaction $2\mathrm{H}_{(g)} \rightarrow \mathrm{H}_{2(g)}$, which of the following combined sets of thermodynamic parameter signs is unconditionally correct?

Professor PC Thomas Classes · Answer

(c) : The reaction represents the formation of a stable chemical bond (an $\mathrm{H}-\mathrm{H}$ bond) from two highly energetic free isolated atoms. Bond formation is universally an exothermic process, meaning it rigorously releases energy, thus $\Delta_r H < 0$. Furthermore, the reaction converts two independent gaseous atoms into one single gaseous molecule. This significant reduction in the total number of gaseous particles ($\Delta n_g = 1 - 2 = -1$) drastically decreases the system's randomness, demanding that $\Delta_r S < 0$.