When gaseous sulfur trioxide is dissolved in concentrated sulfuric acid, disulfuric acid forms:
SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l) Use bond energies (Table 9.2, p. 371) to determine ΔH°rnx . (The S atoms in H₂S₂O₇ are bonded through an O atom. Assume Lewis structures with zero formal charges; BE of S=O is 552 kJ/mol.)