Among the following the reaction for which ΔH = ΔE, is — Thermodynamics and Thermochemistry Chemistry Question

💡 Solution & Explanation

The relationship is ΔH = ΔE + Δng × R × T.<br>For ΔH = ΔE, we must have Δng = 0.<br>Let's calculate Δng for each option:<br>(a) $PCl_5$(g) → $PCl_3$(g) + $Cl_2$(g): Δng = (1 + 1) - 1 = +1.<br>(b) $H_2$(g) + $Cl_2$(g) → 2$HCl$(g): Δng = 2 - (1 + 1) = 0.<br>(c) $C_2H_5OH$(l) + 3$O_2$(g) → 2$CO_2$(g) + 3$H_2O$(g): Δng = (2 + 3) - 3 = +2.<br>(d) $C_2H_4$(g) + $H_2$(g) → $C_2H_6$(g): Δng = 1 - (1 + 1) = -1.<br>Thus, only reaction (b) has ΔH = ΔE. (Note: Although this is a multiple-choice section, some questions may have only one correct option. The answer key on page 5.35 confirms the answer is (b)).