A melt mechanism for the thermal decomposition of ammonium dichromate
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.