In many textbooks one can find an empirical rule for how to evaluate quickly an increase in the rate of a chemical reaction when temperature changes. For example: "According to an old rule of thumb, the rate of a reaction approximately doubles with each 10 °C temperature rise" (1).Different authors sometimes add extra information to this rule. Thus, J. B. Russel (1), among others, pointed out that "unfortunately, the rule is so approximate as to be of limited value". It has been suggested that a reaction must proceed in a homogeneous system (2, 3) or in solution (4, 5), or "at ordinary temperature" (5), "at room temperature" (6 ). Even mention of a reaction rate could be found: "This rule… applies only to reactions that last longer than a second or two" (2).Other authors specify the temperature coefficient itself. "The specific rate is usually increased by a factor of about two or three for every 10 °C rise of temperature" (3). "In reality, the factor is usually in the range from 1.5 to 4" (7 ). "As a rough working rule a 5 per cent to 10 per cent increase in rate per degree centigrade rise in temperature may be assumed" ( 6). This gives a factor from 1.05 10 = 1.63 to 1.10 10 = 2.6 for a 10° increase in temperature.On the other hand, it is now widely accepted that over a limited range of temperature the majority of the reactions studied can be adequately described with the Arrhenius equation
This article contains a brief review of some "unconventional" applications of the Arrhenius law. One such example is proposed as a problem concerning the shelf-life of frozen food (Italian pizza) at temperatures ranging from 0 to -18 °C.
The aim of this article is to elucidate some interesting phenomena, which can be observed in the process of heating solid iodine in closed vessels at various pressures. The experiments provide useful possibilities for discussing the following items: vaporization and melting of iodine crystals at atmospheric pressure; long storage of iodine crystals in air and in vacuum; temperature at which iodine vapors above crystals in a closed vessel becomes visible; the rate of iodine sublimation in high vacuum; “jumping” of heated iodine crystal in vacuum and “jet force” which emerges at different temperatures; diffusion of iodine vapors and “random walk” of iodine molecules; experimental determination of mean free path and collision frequency; mean free path in real life; calculation of the number of iodine atoms in equilibrium with molecular iodine at boiling point; and safety considerations (strength of glass ampules of different diameters against internal and external pressure). The article and questions therein may be used in classroom discussion with students of the general, inorganic, and physical chemistry (molecular-kinetic theory).
A detailed thermodynamic analysis and an experimental study of the thermolysis mechanism of Li2CO3 + NH4H2PO4 + FeC2O4 blends were performed from the viewpoint of the usage of these compounds for the LiFePO4 solid-phase synthesis. Thermodynamic calculations of the assumed chemical reactions have been done within a practically important (for LiFePO4 synthesis) temperature range of (25 to 900) °C. The thermodynamic parameters of the related compounds (Li2O, Li3PO4, (NH4PO3)4, NaFePO4, FePO4 2H2O, FePO4, and LiFePO4) were determined more precisely than earlier. The temperature dependences of changes in the standard Gibbs energy, standard enthalpy, standard entropy, and standard molar heat capacity for the chemical reactions in LiFePO4 synthesis were calculated. Various paths of oxalate decomposition may well proceed concurrently with the predomination of this or that path under slight changes in the experimental conditions. The formation of orthorhombic lithium orthophosphate Li3PO4 was detected just in a blend grinded at room temperature. Heating up to 360 °C results in full destruction of the reaction mixture; Li3PO4 in its activated state is a crystal component at this synthesis stage. Lithium orthophosphate structurally belonging to the same spatial Pnma group as the target product LiFePO4 is the basis of its synthesis.
The paper deals with a widespread mistake in standard textbooks--an experiment with a shift of the equilibrium in gaseous mixtures of nitrogen oxides upon abrupt compression. Any explanation of this experiment should take into account two facts: noticeable heating upon compression and extremely fast chemical reactions in the system (relaxation time is of the order of a microsecond). The paper presents experimental data on kinetics in the system approaching equilibrium and the calculations of the equilibrium mixture just after compression. The paper will be suitable for an in-class lecture activity and will be useful for teachers and students in general chemistry and physical chemistry courses (gas laws, chemical thermodynamics and kinetics).
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