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The kinetics of the reduction of mercuric salts by molecular hydrogen in aqueous solution

University of British Columbia

The kinetics of the homogeneous reduction of mercuric salts by-molecular hydrogen in aqueous solution have been examined over a wide range of solution composition, temperature and hydrogen partial pressure. In perchlorate solutions, where Hg⁺⁺and Hg₂⁺⁺ are uncomplexed the kinetic results can be expressed by the equation: -d[H₂]J/dt = k₁[H₂][Hg⁺⁺] + k₂[H₂][Hg₂⁺⁺] where k₁ = 4.2 x 10¹⁰ exp [-18100/RT] 1.mole⁻¹ sec.⁻¹ and k₂ = 1.2 x 10¹¹ exp [-20400/RT] 1.mole⁻¹ sec.⁻¹ It was concluded that the rate-determining process of the reaction involves the bimolecular interaction of one H₂ molecule with either Hg⁺⁺ or Hg₂⁺⁺ , i.e., Hg⁺⁺ + H₂ [symobal omitted] Hg + 2H⁺ Hg₂⁺⁺ + H₂ [symbol omitted] 2Hg + 2H⁺ The Hg atoms thus formed undergo further rapid reactions to yield the observed products (Hg₂⁺⁺ or metallic mercury) the nature of which is determined by •' thermodynamic considerations. Mercuric complexes also reacted homogeneously with hydrogen but in most cases more slowly than the simple Hg⁺⁺ ions. The order of decreasing reactivity of the complexes, HgAc₂ > HgCl₂ > HgBr₂ > Hg(ethylenediamine)₂⁺⁺, is the same as the order of their increasing stability. This is attributed to the reduction of the electron affinity of Hg⁺⁺ through electron donation from the complexing ligand. The reactivity of mercuric complexes such as HgAc₂ and Hg(ethylenediamine)₂⁺⁺ is increased by anions such as 0H⁻, CO₃⁼, Ac⁻, etc., the influence of which increases in the same order as their basicity. This is attributed to stabilization, by the anions, of the H⁺ ions which are released in the rate-determining step of the reaction. Some conclusions are drawn concerning the mechanism of heterogeneous activation of hydrogen by solid catalysts.

Text

2012-01-31

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http://hdl.handle.net/2429/40391

Metals and Materials Engineering

University of British Columbia

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