Influence of Temperature and Catalyst on the Decomposition of Potassium Chlorate in a Simple Erwin Wiederholt Universitat-GesamthochschuleWuppertal, West Germany
The catalytic decompmition of KC103 is commonly described by the following equation:
Several features of this equation seem to he incorrect. Before the turn of the century, the oxygen produced was known t o he contaminated by small amounts of chlorine-containing gases, a fact rediscovered by Bostrup, Demandt, and Hansen in 1962 (I).One year later Gaidis and Rochow (2) identified these byproducts with mass spectrometry as C12, CIOz and ClzO?. In addition, further problems are associated with this well-known reaction. Markowitz. Borvta. and Stewart (3) investigated some that catalytical decompositioncommences at aLout 350°C; that is, after the mixture is fused. But no author mentioned that the 150°C reaction temperature, which is often cited in the literature, is about 200" too low. Catalysts are important chemicals for scientific investigations and technical processes. Catalysis is also treated in chemical education. In this contribution we will describe the use of a simple DTA-apparatus in demonstrating the catalytic effects of MnOz and A1203on the decomposition temperature of KC103.
Le Chatelier's thermal analysis method, is not well-known in teaching chemistry. Proposals for construction of DTA-apparatus for teaching purposes have been described in this journal ( 4 4 ) . We constructed a simple DTA apparatus for demonstrating the principle and for teaching experiments by converting an electric hammer solderine iron into a DTA-stove as follows (Fig. I I. Two synunetric twrings (30 mm deep, 5.1 mm wide) are made in the ureviousl,, shmtentvl solderinrc hit (1.5mm u,). Glass tubes (35mm long, 5 mm wide) whicg have one end sealed, are charged with sample and reference (10-200 mg) and inserted into the horings. Two Cromel-Alumel Coax thermocouples, which are used for measuring the temperature, are put into each vessel. The thermocouples, arranged in a simplified mimic diagram, are used for measuring the A T signal and the temperature of the reference (Fig. 2). Both signals are recorded with a two-channel ylt plotter. If a commercially purchased electric soldering tool is used, one can frugally change a well tested and inexpensive tool into a stove. However, at full voltage the temperature of the shortened soldering.. hit areatlv exceeds the normal nueratine. temnera.. . lure fur such R drvi;e. Sinre the hit is shoriened, l:ttle lossof heat occwli. Thcrefurc thr n,lwce should he nvitrhed off uoon reaching the desired temperature or the maximum tempera-
DTA-Principle DTA is a technique with which the temperature difference between a sample and reference is measured as a function of temperature while hoth are subject to a controlled temperature program. Filled in different crucihles hut in the same stove, sample and reference are treated under equal conditions. In the classic arrangement of Roberts-Austen, the temperatures of hoth suhstances are measured directly by inserting thermocouples into the material in the crucibles. The thermocouples are switched against another, the temperature difference (AT) being recorded with a plotter. Simultaneously, the absolute temperature of the sample, reference, or stove is also registered. So long the sample shows no thermic effects (i.e., no endothermic or exothermic reaction occurs), the sample and the reference will have the same temperature. Consequently AT is equal to zero and there is no differential thermal voltage. If an endothermic or exothermic reaction occurs, the respective temperature of the sample will he below or ahove the temperature of the reference. These temperature differences cause thermal voltages which will he recorded as negative or positive peaks, respectively, by the plotter. DTA may he carried out on inorganic and organic suhstances which show uhvsical chanees and/or chemical reac~tl.~ all sub.itanceh are tions. If heated s u f f i ~ i t ~ hiRh,';learly suitable fur i u h~ meawrementi. Therefon:, lYl'A is doolied .frequently. A Simple DTA-Model Unfortunately, modern DTA apparatus are expensive; thus the DTA method, developed in 1899 by Roberts-Austen from
Figure 1. DTA-model
Volume 60
Number 5
May 1983
43 1
ture of the device. Generally one would apply a lower voltage I,y regulating with :I triinitornmr
