A melt mechanism for the thermal decomposition of ammonium dichromate

Abstract

Microscopic examination of (NH₄)₂Cr₂O₇ crystallites, previously decomposed to measured extents, has given strong evidence that reaction proceeds through the intervention of a melt. This reaction has previously been accepted as occurring in the solid phase and the use of higher magnifications than were applied previously revealed the progressive development of foam-like zones of active reaction. These structures are termed ‘fusion nuclei’. We conclude that an initial, and probably reversible, dissociation of the reactant (NH₄)₂Cr₂O₇⇌ 2NH₃+ H₂O + 2CrO₃ is followed by a sequence of steps by which NH₃ and/or NH⁺₄ is oxidized in the CrO₃ melt to N₂ and N₂O, possibly involving the intervention of NH₄NO₂ and NH₄NO₃ respectively. Since these various consecutive and concurrent reactions contribute to the overall chemical change we conclude that the quantitative characterization of their kinetics and stoichiometry requires the separate investigation of selected simpler reaction mixtures. Some observations on the reaction of ammonia with molten CrO₃ are discussed.