For which of the following balanced gaseous reactions will the standard enthalpy change () be strict — Thermodynamics and Thermochemistry Chemistry Question

💡 Solution & Explanation

The fundamental relation is $\Delta H = \Delta U + \Delta n_g RT$. For $\Delta H < \Delta U$, the term $\Delta n_g$ (change in gaseous moles) must be strictly negative. (A) $\Delta n_g = 2 - 3 = -1$. (B) $\Delta n_g = 2 - 4 = -2$. (C) $\Delta n_g = 2 - 4 = -2$ (liquid water doesn't contribute). (D) $\Delta n_g = 1 - 0.5 = +0.5$. Thus A, B, and C effectively yield $\Delta H < \Delta U$.