High-Temperature Raman Investigation of Concentrated Sulfuric Acid Mixtures:  Measurement of H-Bond ΔH Values between H₃O⁺ or H₅O₂⁺ and HSO₄⁻

Forty-two mol percent sulfuric acid is a room-temperature molten salt composed of H-bonded ion pairs H 3 O + −HSO 4 - (8.2 M) and H 5 O 2 + −HSO 4 - (5.4 M), the former breaking primarily below ∼360 °C and the latter, mainly between ∼360 °C and the critical point. These ruptures produce H 2 SO 4 and H 2 O. Iterative van't Hoff thermodynamic analysis of Raman spectra to 360 °C employing peak height or component area ratios at ∼905 and ∼1035 cm -1 or at ∼1165 and ∼1035 cm -1 yielded an average Δ H of 3.2 ± 0.2 kcal/mol corresponding to H-bond breakage between H 3 O + and HSO 4 - . A Raman Δ H of 5.9 ± 0.3 kcal/mol was measured for the rupture of H 5 O 2 + −HSO 4 - H-bonds between 360 and 513 °C versus ∼5.7 kcal/mol calculated. H-bonds exist between species other than the two hydronium−bisulfate ion pairs but are not measured by the Raman H 2 SO 4 /HSO 4 - intensity ratios. Hydrogen bonding wanes near the critical point, ∼665 °C, at which only neutral species, H 2 SO 4 and H 2 O (plus some SO 3 ), remain. A Δ H of 3.5 ± 0.4 kcal/mol was also measured from the 87 mol % acid, close enough to the 3.2 ± 0.2 kcal/mol value to assign both to H 3 O + −HSO 4 - and also because H 5 O 2 + cannot form in the 87 mol % solution. The integral heat of solution (42 mol %) calculated using Raman data for the two hydronium−bisulfate ion pairs is consistent with the calorimetric value. Values of the dielectric constant were also estimated from Raman data.