Le Chatelier’s principle applying

The solubility of most ionic compounds increases with temperature, despite the fact that the standard heat of solution (AH°) is negative for about half of them. Discussions of this seeming contradiction can be found in G. M. Bodner, On the Misuse of Le Chatelier s Principle for the Prediction of the Temperature Dependence of the Solubility of Salts, J. Chem. Ed. 1980,57, 117, and R. S. Treptow, Le Chatelier s Principle Applied to the Temperature Dependence of Solubility, J. Chem. Ed. 1984,61, 499. 

In (19-6), n has been made dimensionless. For a half-reaction, n is the number of electrons in the half-equation for the whole-cell reaction, n is the number of electrons in one of the multiplied half-equations before canceling the electrons. The Nernst equation is closely related to the laws of chemical equilibrium. Le Chatelier s principle applies to the potential of a cell in the same sense as it applies to the yield of an equilibrium process. Since Q is a fraction that has product concentrations in the numerator (top) and reactant concentrations in the denominator (bottom), an increased concentration of the product reduces the potential and an increased concentration of reactant raises the potential. 

Ans. Nothing. Le Chatelier s principle applies only to a system already at equilibrium. 

Le Chatelier s principle applies to these equilibria, as it does to all equilibria. One way to exert a stress on a solubility equilibrium is to change the amount of solvent. Adding solvent reduces the concentration of dissolved substance more solid then tends to dissolve to restore the concentration of the dissolved substance to its equilibrium value. If an excess of solvent is added so that all of the solid dissolves, then obviously the solubility equilibrium ceases to exist and the solution is unsaturated. In a vaporization-condensation equilibrium, this corresponds to the complete evaporation of the condensed phase. Removing solvent from an already saturated solution forces additional solid to precipitate in order to maintain a constant concentration. A volatile solvent is often removed by simply letting a solution stand uncovered until the solvent evaporates. When conditions are right, the solid forms as crystals on the bottom and sides of the container (Fig. 16.1). 

The thermodynamics of these reforming reactions and the different equilibria which may be established were discussed in the earlier review. In full-scale reformers, conversion by reactions (1)—(3) is virtually complete and the shift and reforming equilibria (4)—(6) are established, so it is possible to calculate the composition of the product gases. Other similar methods have been described recently. The thermodynamics of carbon formation are discussed in Section 6. The effect of temperature on gas compositions is illustrated by Figure 1, indicating that less methane and more CO and H2 are obtained at higher temperatures from the endothermic reforming reactions (5) and (6) and the exothermic shift reaction (4), respectively, Le Chatelier s principle applied to reaction (5)... 

Le Chatelier s principle applies to a heterogeneous equilibrium, but with a few new twists which follow from the preceding discussion ... 

According to Le Chatelier s principle, a system at equilibrium adjusts so as to mini mize any stress applied to it When the concentration of water is increased the system responds by consuming water This means that proportionally more alkene is converted to alcohol the position of equilibrium shifts to the right Thus when we wish to pre pare an alcohol from an alkene we employ a reaction medium m which the molar con centration of water is high—dilute sulfuric acid for example... 

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. 

How might we solve equation 6.34 if we do not have access to a computer One possibility is that we can apply our understanding of chemistry to simpKfy the algebra. From Le Chatelier s principle, we expect that the large initial concentration of Pb will significantly decrease the solubility of Pb(I03)2. In this case we can reasonably expect the equiKbrium concentration of Pb to be very close to its initial concentration thus, the following approximation for the equilibrium concentration of Pb seems reasonable... 

Le Chatelier s principle provides a means for predicting how systems at equilibrium respond to a change in conditions. When a stress is applied to an equilibrium by adding a reactant or product, by adding a reagent that reacts with one of the reactants or products, or by changing the volume, the system responds by moving in the direction that relieves the stress. 

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... 

A new state of equilibrium is then attained in which more FeSCN+2 is present than was there before the addition of SCN-. Increasing the concentration of SCN- has increased the concentration of the FeSCN+2 ion. This is in accord with Le Chatelier s Principle. The change imposed on the system was an increase in the concentration of SCN-. This change can be counteracted in part by some Fe+3 and SCN- ions reacting to form more FeSCN+2. The same argument applies to an addition of ferric ion from a soluble ferric salt. In each case, the formation of FeSCN+s uses up a portion of the added reactant, partially counteracting the change. 

Of course, the usual equilibrium considerations apply. For example, if we add the substance methanol, equilibrium conditions will shift, consuming the added reagent (methanol) and acetic acid to produce more methyl acetate and water, in accord with Le Chatelier s Principle. Thus a large excess of methanol causes most of the acetic acid to be converted to methyl acetate. 

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. 

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. 

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. 

Because this problem asks for a qualitative answer, we do not need to do calculations. It is sufficient to apply Le Chatelier s principle to determine the direction of change and draw the new picture that shows the result of the change. 

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. 

Le Chatelier s principle states that if a stress is applied to a system at equilibrium, the equilibrium will shift in a tendency to reduce that stress. A stress is something done to the system (not by the equilibrium reaction). The stresses that we consider are change of temperature, change of pressure, change of concentration(s), and addition of a catalyst. Let us consider the effect on a typical equilibrium by each of these stresses. 

Le Chatelier s principle also applies to biology. Homeostasis is the tendency of a body system to remain in a state of equilibrium. Examples of homeostasis include the maintenance of body temperature (homeothermy) and the pH balance of blood. 

Many important equilibrium systems involve ions in aqueous solution. The common ion effect applies Le Chatelier s principle to ions in aqueous solution. As its name suggests, the common ion effect involves adding an ion to a solution in which the ion is already present in solution. It is really a concentration effect. The equilihrium shifts away from the added ion, as predicted hy Le Chatelier s principle. 

In this section, you learned that the expression for the reaction quotient is the same as the expression for the equilibrium constant. The concentrations that are used to solve these expressions may be different, however. When Qc is less than Kc, the reaction proceeds to form more products. When Qc is greater than Kc, the reaction proceeds to form more reactants. These changes continue until Qc is equal to Kc. Le Chatelier s principle describes this tendency of a chemical system to return to equilibrium after a change moves it from equilibrium. The industrial process for manufacturing ammonia illustrates how chemical engineers apply Le Chatelier s principle to provide the most economical yield of a valuable chemical product. 

Another crystallization technique is used when the isolation of a highly water-soluble compound in its salt form is required from aqueous reaction mixtures. This technique takes advantage of the common-ion effect and is based on the le Chatelier s principle, which states that, if, to a system in equilibrium, a stress is applied, the system will react so as to relieve the stress. Thus, in aqueous solutions, the solubility of the compound in salt form can be reduced by adding large amoimts of a common ion which is more soluble than the salt of the compoimd. 

The relation between the equilibrium constant and the standard reaction free energy in Eq. 10 applies to any temperature. Therefore, we ought to be able to use it to relate the equilibrium constant at one temperature to its value at another temperature. In this way, we obtain a quantitative version of Le Chatelier s principle for the effect of temperature. 

In applying Le Chatelier s principle to a heterogeneous equilibrium, the effect of pressure changes on solids and liquids can be ignored because the volume (and concentration) of a solid or a liquid is nearly independent of pressure. Consider, for example, the high-temperature reaction of carbon with steam, the first step in converting coal to gaseous fuels ... 

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