Chatelier effect

It has been shown that several polymers exhibit instabilities in their plastic deformation process. It should finally be mentioned that instabilities may also occur during the plastic deformation of metals This phenomenon which is called the Portevin-Le Chatelier effect, is generally interpreted in terms of different modes of dislocation movement depending on whether or not dislocations move by dragging along their atmosphere of impurities behind them. 

The Portevin-Le Chatelier effect with serrated yielding was observed for both the ordered and disordered state (Mohamed etal., 1974). Recovery and recrystallization have been analyzed in detail (Vidoz etal., 1963 Cahn, 1990, 1991). Experimental and theoretical studies have been directed at dislocations and antiphase domain boundaries (see, e.g. Tichelaar and Schapink, 1991 D. G. Morris, 1992 Veyssiere, 1992), grain boundaries (Yan etal., 1992), and the electronic structure (Bose et al., 1991). It is noted that disordered layers are formed in ordered CujAu on antiphase boundaries and twin boundaries just below the order-disorder transition temperature (Tichelaar et al., 1992). This may be expected in other phases, too, and may improve the ductility of less ductile phases, as is discussed for NijAl (see Sec. 4.1.2). 

Although Y ions present only weak obstacles to dislocation motion [58], they are present in high concentrations and could, in theory, yield a large contribution to the flow stress. However, the crystals presently available apparently contain very small precipitates of ZrN [71] which provide stronger obstacles to slip than do unassociated Y ions [58, 72, 73]. Nonetheless, these unassociated Y ions do cause plastic instabilities, such as dynamic strain aging or the Portevin-Le Chatelier effect in... 

In the absence of a hydrophobic anion in the organic phase, the concentrations of K+ and Cl in the organic phase are equal. However, when the concentration of K+ in the organic phase is very high becanse these ions balance the negative charge of the hydrophobic counteranions (R ), the conseqnence of (5b) is that a, ncm becomes very low. This can be considered to be a Le Chatelier effect on the KCl distribution between the two phases (5a) a high... 

Cie] observed Portevin-Le Chatelier Effect in biaxially strained Al-Fe-Si foils. This effect is attributed to the diffusion of Si rather than Fe, as the latter has much slower diffusivity and low solubiUty in (Al). [2003Mur] reported the hardness and indentation fracture toughness of T2 and T5 phases, both have hardness much higher than binary aluminides and they have very similar indentation fracture toughness. 

Cie] Cieslar, M. C., Fressengeas, C., Karimi, A., Martin, J.L., Portevin-Le Chatelier Effect in Biaxially Strained Al-Fe-Si Foils , Scr. Mater., 48, 1105-1110 (2003) (Experimental, Meehan. Prop., 19)... 

The observation that a system at equilibrium responds to a stress by reequilibrating in a manner that diminishes the stress, is formalized as Le Chatelier s principle. One of the most common stresses that we can apply to a reaction at equilibrium is to change the concentration of a reactant or product. We already have seen, in the case of sodium acetate and acetic acid, that adding a product to a reaction mixture at equilibrium converts a portion of the products to reactants. In this instance, we disturb the equilibrium by adding a product, and the stress is diminished by partially reacting the excess product. Adding acetic acid has the opposite effect, partially converting the excess acetic acid to acetate. 

The principle of Le Chatelier shows that when the pressure applied to a gaseous system is increased, dre equilibrium composition will chairge in order to reduce tire number of gaseous molecules. In the case of tire steam reforming of metlrane, the partial pressures of methane and steam will increase as the pressure is increased. In the water-gas reaction, where tire number of molecules is the same on both sides of the equation, the effect of increasing... 

An account of the mechanism for creep in solids placed under a compressive hydrostatic suess which involves atom-vacancy diffusion only is considered in Nabano and Hemirg s (1950) volume diffusion model. The counter-movement of atoms and vacancies tends to relieve the effects of applied pressure, causing extension normal to the applied sU ess, and sluinkage in the direction of the applied sU ess, as might be anticipated from Le Chatelier s principle. The opposite movement occurs in the case of a tensile sU ess. The analysis yields the relationship... 

According to Le Chatelier s principle, conversion is increased by increasing the temperature and decreasing the pressure. Figure 6-3 shows the effect of temperature on the dehydrogenation of different light paraffins. ... 

Le Chateliers principle can be used to predict the effect of a change in temperature on the position of an equilibrium. In general, an increase in temperature causes Ihe endothermic reaction to occur. This absorbs heat and so tends to reduce the temperature of the system, partially compensating for the original temperature increase. 

Le Chatelier s principle A relation stating that when a system at equilibrium is disturbed it responds in such a way as to partially counteract that change, 337-338 buffers and, 385 compression effects, 339-340 expansion effects, 339-340 precipitation equilibrium, 442 reaction conditions, 348q temperature changes, 340 Lead, 2,501 Leclanch cell, 500 Leucine, 622t... 

We are not satisfied with the conclusion that this change or that change affects the equilibrium concentrations. We would also like to predict the direction of the effect (does it favor products or reactants ) and the magnitude of the effect (how much does it favor products or reactants ). The first desire, to know the qualitative effects, is answered by a generalization first proposed by a French chemist, Henry Louis Le Chatelier, and now called Le Chatelier s Principle. 

This heat effect can be used in predicting how K changes with temperature. Le Chatelier s Principle indi-... 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Note that there is no net change in the number of moles of gas in this equilibrium. Therefore, by Le Chatelier s principle, this reaction will be independent of external pressure (ignoring second-order effects due to gas imperfections). Under these conditions the N of the expl will... 

Le Chatelier s principle When a stress is applied to a system in dynamic equilibrium, the equilibrium tends to adjust to minimize the effect of the stress. 

FIGURE 9.13 Le Chatelier s principle predicts that, when a reaction at equilibrium is compressed, the number of molecules in the gas phase will tend to decrease. This diagram illustrates the effect of compression and expansion on the dissociation equilibrium ot a diatomic molecule. Note the increase in the relative concentration of diatomic molecules as the system is compressed and the decrease when the system expands. 

What does this equation tell us Suppose that the reaction is endothermic, then AH° is positive. If T2 > T, then 1/T2 < 1/T, and the term in braces is also positive. Therefore, In (K2/K,) is positive, which implies that K,/K, > 1 and therefore that K2 > K,. In other words, an increase in temperature favors the formation of product if the reaction is endothermic. We predict the opposite effect for an exothermic reaction because AHr° is then negative. Therefore, the van t Hoff equation accounts for Le Chatelier s principle for the effect of temperature on an equilibrium. 

We can use Le Chatelier s principle as a guide. This principle tells us that, if we add a second salt or an acid that supplies one of the same ions—a common ion —to a saturated solution of a salt, then the equilibrium will tend to adjust by decreasing the concentration of the added ions (Fig. 11.15). That is, the solubility of the original salt is decreased, and it precipitates. We can conclude that the addition of excess OH- ions to the water supply should precipitate more of the heavy metal ions as their hydroxides. In other words, the addition of OH ions reduces the solubility of the heavy metal hydroxide. The decrease in solubility caused by the addition of a common ion is called the common-ion effect. 

Le Chatelier s principle When a stress is applied to a system in dynamic equilibrium, the equilibrium adjusts to minimize the effect of the stress. Example a reaction at equilibrium tends to proceed in the endothermic direction when the temperature is raised, leveling The observation that strong acids all have the same strength in water, and all behave as though they were solutions of H,Of ions. 

The effects of changes in amounts on a system at equilibrium can be summarized in accordance with Le Chatelier s principle ... 

Any change in amounts that has no effect on the value of Q has no effect on the equilibrium position. Example provides practice in applying Le Chatelier s principle at the molecular level. 

According to Le Chatelier s principle, the system will respond in the direction that reduces the amount of change. It will do so, however, only if an appropriate response exists. Changes in quantities of substances must be analyzed for their effect, if any, on the value of Q. 

Both solubilities are low, as we would expect for a salt with a small value of. S sp. Notice that PbCl2 is about 350 times less soluble in the NaCl solution. This makes sense in terms of the common-ion effect. The excess chloride ion suppresses the solubility of Pb by Le Chatelier s principle. The actual concentration of lead in seawater is much less than 4.0 X 10 M. This is because other lead salts are much less soluble than lead(II) chloride. The ocean contains carbonate, for example, and. STsp for lead(II) carbonate is quite small, 7.4 X lO ". ... 

Le Chatelier s principle demonstrates the effect of disturbances on chemical equilibrium. Le Chatelier s principle can also help predict whether or not disturbances will affect the equilibrium constant. None of these will affect the equilibrium constant EXCEPT the —... 

Chemical equilibria being of a dynamic type, equilibrium states are altered by changes in the variables controlling them. The effect of such changes can be interpreted qualitatively on the basis of a principle which was enunciated independently by Le Chatelier in 1885 and by Braun one year later. It states that when a system in a state of dynamic equilibrium is subjected to a stress imposed by variation in anyone of the variables controlling the equilibrium state, the system will tend to adjust itself in such a way as to minimize the effect of the stress. The variables of interest in this connection are temperature of the system, pressure on the system, and concentrations for the reactants and products taken individually. 

In order to illustrate this principle, let the effect of temperature on the equilibrium constant of an exothermic reaction, involving the oxidation of a metal to its oxides, be considered. Upon increasing the temperature of this reaction some of the metal oxides will dissociate into the metal and oxygen and thereby reduce the amount of heat released. This qualitative conclusion based on Le Chatelier s principle can be substantiated quantitatively from the Varft Hoff isochore. 

It may be added here that Le Chatelier s principle is quite general in nature, and that its applicability is not restricted only to chemical equilibria. It can also be applied to physical equilibria, as for example, to explain qualitatively the effects of temperature and pressure on solubility or the effect of pressure on the melting of a solid. 

A basic principle that allows the qualitative prediction of the effect of changing reactor conditions on any chemical system in equilibrium is Le Chatelier s Principle ... 

Example 6.2 shows that for an exothermic reaction, the equilibrium conversion decreases with increasing temperature. This is consistent with Le Chatelier s Principle. If the temperature of an exothermic reaction is decreased, the equilibrium will be displaced in a direction to oppose the effect of the change, that is, increase the conversion. 

The effect of the increase in moles can be artificially decreased by adding an inert material. Le Chatelier s Principle dictates that this will increase the equilibrium conversion. For example, if the above reaction is in the ideal gas phase ... 

Once we have determined the entropy and enthalpy of polymerization, we can calculate the free energy of the process at a variety of temperatures. The only time this is problematic is when we are working near the temperatures of transition as there are additional entropic and enthalpic effects due to crystallization. From the free energy of polymerization, we can predict the equilibrium constant of the reaction and then use this and Le Chatelier s principle to design our polymerization vessels to maximize the percent yield of our process. 

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