Question

For which of the following reactions would you expect the greatest difference between ΔE and ΔH?

a 2HCl(aq) + Na_{2}S(aq) → H_{2}S(g) + 2NaCl(aq)
b 2S(s)+3O_{2}(g)+2H_{2}O(l) → 2H_{2}SO_{4}(aq)
c 2SO_{2}(g) + O_{2}(g) → 2SO_{3}(g)
d H_{2}(g) + Cl_{2}(g) → 2HCl(g)

Answer 1

The greatest difference between ΔE and ΔH is expected for the reaction producing a gas from aqueous reactants, which is reaction (a) 2HCl(aq) + Na_{2}S(aq) → H_{2}S(g) + 2NaCl(aq).

Step-by-step explanation:

Among the options provided, the greatest difference between ΔE (change in internal energy) and ΔH (change in enthalpy) would be expected for reaction (a) 2HCl(aq) + Na_{2}S(aq) → H_{2}S(g) + 2NaCl(aq). This is because this reaction involves the production of a gas, H2S, from aqueous reactants. When a gas forms in a reaction, work is done as the gas expands against the surrounding atmospheric pressure, which can result in a significant difference between ΔE and ΔH since ΔH accounts for the heat transfer at constant pressure, while ΔE also includes the work done by the system. Reactions (b), (c), and (d) do not show such a phase change from aqueous to gaseous states and thus would likely have smaller differences between ΔE and ΔH.