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The Dynamic Nature of Chemical Equilibrium

Let us examine the hypothetical, elementary, reversible reaction taking place at constant temperature, [Pg.58]

The reactants A and B combine to form the products C and D. In this reaction, a moles of A combine with b moles of B to form c moles of C and d moles of D. If we were to introduce A and B into a suitable reaction [Pg.58]

The unit of concentration as indicated by [ ] is moleAiter, but other concentration units also can be used. From this discussion we learn that the equilibrium state can be approached from both directions. [Pg.59]

When we investigate the rate at which the equilibrium condition is approached, we can deduce that the equilibrium condition is a dynamic one, not a static situation. The interaction of reactants and products does not cease when equilibrium is reached. The forward and reverse reactions proceed at such a rate that the ratio of concentrations of products to reactants (as described by the equilibrium constant, Eq. 3-2), remains constant. Another way of stating this is that a chemical reaction is at equilibrium if its forward rate of reaction, Vf, is equal to the rate of the reverse reaction, v,.. For example, we have seen previously in Chapter 2 [Pg.59]

Initially, if only A and B are present, the forward rate of reaction will proceed at a finite rate while there will be no reverse reaction because no C and D are present. However, as soon as the reaction of A and B produces C and D, they will combine, and by the reverse reaction produce A and B. The reaction will proceed until the opposing reaction rates are equal, and [Pg.60]


Experimental Verification of the Dynamic Nature of Chemical Equilibrium. [Pg.404]

The dynamic nature of chemical equilibrium can be illustrated by placing equal masses of iodine crystals in two interconnected flasks, as shown in Figure 18-4a. The crystals in the flask on the left contain iodine molecules made up entirely of the nonradioactive isotope 1-127. The crystals in the flask on the right contain iodine molecules made up of the radioactive isotope 1-131. The Geiger counters indicate the radioactivity within each flask. [Pg.562]

The dynamic nature of chemical equilibrium can be proved experimentally by inserting radioactive atoms into a small percentage of molecules and following them through the reaction. Even when the initial mixture is at equihbrium, radioactive atoms eventually appear in both reactant and product molecules. [Pg.709]

The Law of Mass Action is thus essentially the statement that the equilibrium composition of a reaction mixture can vary according to the quantities of components that are present. This of course is just what Berthollet observed in his Egyptian salt ponds, but it was now seen to be a consequence of the dynamic nature of chemical equilibrium. [Pg.6]

In introductory chemistry lessons, chemical reactions are usually associated with observable phenomena (e.g., change of colour, evolution and absorption of heat, precipitation of a solid, evolution of a gas) and chemical reactions are presented as proceeding to completion, taking place in one direction (Andersson, 1990). The introduction of chemical equilibrium at a later stage, however, demonstrates the reversibility of chemical reactions and the possibility that chemical reactions do not proceed to completion. Moreover, the dynamic nature of chemical equilibrium requires students to assume that two opposite chemical reactions are taking place, in spite of the fact that this cannot be deduced from observation. As a consequence, the introduction of chemical equilibrium requires students to revise their initial conception of chemical reactions. This is illustrated in Table 1. [Pg.276]

Guldberg and Waage also initially experienced difficulties in finding the proper exponents involved in the description of the variations in the concentrations of the different substances this problem was resolved in 1887, in terms of molecular kinetic theory. However, far more importantly, these authors did not manage to distinguish the rate laws (what we would call today the initial conditions) from the derivatives of the equilibrium conditions. This considerably complicated and delayed the future development of chemical kinetics. The dynamic nature of chemical equilibrium was never in doubt. However, the complexity of the systems was far from being considered and the link between equilibrium and kinetics was weak. The works of Harcourt and Esson are models of meticulous experimental and theoretical work, but on reading them, it is also obvious that these authors had to confront many conceptual and technical problems. Their kinetic studies... [Pg.2]

The end product of the dehydroxylation of pure phases is, in all cases, hematite, but with lepidocrocite, maghemite occurs as an intermediate phase. The amount of water in stoichiometric FeOOH is 10.4 g kg , but adsorbed water may increase the overall amount released. Thermal dehydroxylation of the different forms of FeOOH (followed by DTA or TG) takes place at widely varying temperatures (140-500 °C) depending on the nature of the compound, its crystallinity, the extent of isomorphous substitution and any chemical impurities (see Fig. 7.18). Sometimes the conversion temperature is taken from thermal analysis data (e. g. DTA), but because of the dynamic nature of the thermoanalysis methods, the temperature of the endothermic peak is usually higher than the equilibrium temperature of conversion. [Pg.367]

The dynamic nature of molecules can be troublesome for students. To aid in understanding molecular behavior, a number of games mimic the activities that chemical compounds may undergo. To duplicate chemical equilibrium, students throw dice in one published game (Edmonson and Lewis 1999). By participating in the equilibrium firsthand, students may better comprehend the system. [Pg.272]

Students often fail to understand the dynamic nature of the chemically equilibrated state. Instead, many students beMeve that at equilibrium no reaction is taking place (Griffiths, 1994) or that nothing happens in a... [Pg.277]

Teaching chemical equilibrium. With these ideas in mind, this study reports an expert chemistry teacher s use of analogies when teaching chemical equilibrium. This topic is central to chemical education, and is considered complex because it includes important sub-topics such as reversible reactions, reaction rates, chemical kinetics, and the dynamic nature of equilibria. Many students misunderstand chemical equilibrium believing that the forward reaction finishes before the reverse reaction commences, and that at equilibrium the reaction stops and nothing... [Pg.354]

Classroom context. The lessons that we studied occurred over two days. On the first day, a double lesson (80 minutes) was given, starting with a recapitulation of the particulate nature of chemical reactions and factors that influence reaction rate, followed by activation energy, reaction profile diagrams, and the conditions for chemical equilibrium. The next day, a single lesson (40 minutes) elaborated the concept of dynamic equilibrium. No teaeher demonstrations or student practical work were included in the lessons. The topies were presented and discussed in an interactive way with Neil and his students asking many questions. [Pg.355]

The perplexing difficulties that arise in the crystallization of macromolecules, in comparison with conventional small molecules, stem from the greater complexity, lability, and dynamic properties of proteins and nucleic acids. The description offered above of labile and metastable regions of supersaturation are still applicable to macromolecules, but it must now be borne in mind that as conditions are adjusted to transport the solution away from equilibrium by alteration of its physical and chemical properties, the very nature of the solute molecules is changing as well. As temperature, pH, pressure, or solvation are changed, so may be the conformation, charge state, or size of the solute macromolecules. [Pg.23]

An understanding of chemical equilibrium requires that students understand reversible reactions and their dynamic nature. In the pre-test and the pre-interview, students had the pre-instructional conceptions that reactions go in one direction and that at least one of the reactants would completely react. The first lesson used experiments (van Driel et al., 1998) resulting in anomalous data. Experiment 1 involved the iron(lII)-thiocyanate equilibrium and was intended to show the presence of unreacted iron(lll) and thiocyanate ions and the incompleteness of the reaction. The use of anomalous data or discrepant events to create cognitive conflict causing students to be dissatisfied with their current conceptions and to adopt a target concept has been advocated by many educational researchers (Posner, Strike,... [Pg.461]


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