Answer: H2(g) + Cl2(g) → 2HCl(g).
- A N2(g) + 3H2(g) → 2NH3(g)
- B PCl5(g) → PCl3(g) + Cl2(g)
- C H2(g) + Cl2(g) → 2HCl(g)
- D 2SO2(g) + O2(g) → 2SO3(g)
Correct answer: C. H2(g) + Cl2(g) → 2HCl(g)
Explanation: ΔH = ΔU + ΔngRT. Only H2 + Cl2 → 2HCl has Δng = 2 − 2 = 0, so ΔH = ΔU.
Isothermal expansion follows a gentler curve (heat flows in to keep T constant) while adiabatic expansion drops in pressure more steeply (no heat exchange, so internal energy and temperature fall as the gas does work).
Concept context
Study energy changes in chemical reactions. Understand enthalpy, entropy, Gibbs free energy, and the laws of thermodynamics that decide whether a reaction will occur spontaneously or not.