Thermochimica Acta 429 (2005) 57–72 A comparative study of the thermal reactivities of some transition metal oxalates in selected atmospheres Mohamed A. Mohamed a , Andrew K. Galwey b,∗ , Samih A. Halawy a a Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt b 5, Regents Wood, Malone Road, Belfast BT9 5RW, UK Received 10 March 2004; received in revised form 5 August 2004; accepted 9 August 2004 Available online 30 December 2004 Abstract A comparative investigation has been made of the nonisothermal, solid-state thermal decompositions of the oxalates of six divalent transition metals (cations: manganese, iron, cobalt, nickel, copper and zinc) in alternative flowing atmospheres, inert (N 2 , CO 2 ), reducing (H 2 ) and oxidizing (air). Derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) response peak maxima, providing a measure of reaction temperatures, have been used to determine salt reactivities and thus to characterize the factors that control the relative stabilities of this set of chemically related reactants. Two trends were identified. Trend (1): in the inert and reducing atmospheres, the decomposition temperature (salt stability) increased with rise in enthalpy of formation of the divalent transition metal oxide, MO. It is concluded that the rupture of the cation-oxygen (oxalate) bond is the parameter that determines the stability of salts within this set. Trend (2): the diminution of decomposition temperatures from values for reactions in inert/reducing atmosphere to those for reactions in an oxidizing atmosphere increased with the difference in formation enthalpy between MO and the other participating oxide (MO 3/2 or MO 1/2 ). The change of cation valence tended to promote reaction, most decompositions in O 2 occurred at lower temperatures, but the magnitude of the effect varied considerably within this set of reactants. Observed variations in stoichiometric and kinetic characteristics with reaction conditions are discussed, together with the mechanisms of thermal decompositions of these solid oxalates. This approach to the elucidation of crystolysis reaction mechanisms emphasizes the value of comparative investigations within the group of chemically related reactants. Previous isothermal kinetic studies had been made for each of the reactants selected here. From these, much has been learned about the form of the (isothermal) solid-state yield–time curves, often interpreted to provide information about the geometry of interface development for the individual rate processes. However, identification of the controls of reactivity, reaction initiation (nucleation) and advance (nucleus growth), is much more difficult to characterize and less progress has been made towards elucidation of the interface chemistry. The trends of reactivity changes with salt compositions, identified here, offer a complementary approach to that provided by the study of single salts. Much of the recent literature on thermal decompositions of solids has been concerned with individual reactants, but many results and conclusions are not presented in the widest possible perspective. Comparisons between systematically related reactants are identified here as providing a chemical context for the elucidation of the chemical steps that participate in interface reactions. The article advocates the use of a more chemical approach in investigations of crystolysis (solid-state chemical) reactions. © 2004 Published by Elsevier B.V. Keywords: Crystolysis reactions; Oxalate decompositions; Reaction kinetics; Reaction mechanisms; Solid-state chemistry; Solid-state reactions ∗ Corresponding author. E-mail address: andrew@galwey.freeserve.co.uk (A.K. Galwey). 1. Introduction By focussing interest on the relative thermal reactivities of a series of transition metal oxalates, this study is primarily concerned with the chemical properties, including the mech- anisms, of a set of selected crystolysis reactions [1,2]. Many 0040-6031/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.tca.2004.08.021