The kinetics of the isothermal decomposition in vacuu of calcite (two series of experiments, A and B) and of magnesite (one series of experiments, C) has been studied with the aid of an automatically recording sorption balance, over the approximate ranges 720" to 780" C for calcite and 540" to 600" C for magnesite. In all cases the course of the decomposition can be represented by the equation (w/wo)" = -kt + a, where W/WO = the fraction still undecomposed at time t ; k, a and n are constants. The value of (1n) varies slightly from one experiment to another, but its average value is 0-74 for series A, 0.72 for series B and 0.58 for series C. The decomposition probably occurs at an interface advancing from the outside of the lump, or of the whole sample, to the interior: this mechanism would require (1n) = 0-67 in the ideal case, and the deviations from this value are perhaps due jointly to self-cooling, to impedance in the escape of carbon dioxide and to a somewhat slow rate of formation of decomposition nuclei. The value of the energy of activation E of the decomposition are, for calcite (cal mole-1) : series A, 35,000 f 4,800 ; series By 41,600 d= 2,900, so that they do not differ significantly from the energy of decomposition (40,000) ; for magnesite E is 35,600 f 3,300, and so appears to be somewhat higher than the energy of decomposition (ca. 27,000). The rate of decomposition of both substances agrees reasonably with the Polanyi-Wigner expression.
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