Thermal decomposition of ammonium chromate

Abstract

Kinetic evidence has shown that the decomposition of ammonium chromate [→(NH₄)₂Cr₂O₇+ H₂O + 2NH₃] obeys the contracting cylinder equation. Evidence that a reaction interface advances progressively inwards from the original crystal surfaces was obtained from microscopic examination of cross-section surfaces revealed by fracture of the lath-shaped crystals of reactant after partial decomposition. Consistent and complementary results were obtained from quantitative studies using three different salt preparations. It is concluded that two distinct chemical steps contributed to reaction. These were reversible proton transfer, which was opposed by higher pressures of accumulated product gas, followed by water elimination, which was not reversible under reaction conditions. The decomposition is very slightly endothermic but is irreversible, an unusual combination.