LeChatelier Effect

The bloodstream is an aqueous environment so when a lipophilic molecule enters the bloodstream it will prefer to reside in fatty tissue. We can look at Figure 4.2.1.1 to think about this process. A lipophilic molecule will set up a dynamic equihbrium between the blood and fatty tissue (B-F), probably with the equilibrium heavily favoring the fatty tissue. This is equihbrium B-F in the figure. If more exposure occurs, then the concentration of the molecule in the blood increases and applying the LeChatelier effect we know that this will set up a new equilibrium with higher concentrations in both the blood and fatty tissue. 

Let us now examine these electronic-nuclear coupling effects in more detail. The moderating exchange of electrons between the molecule and its hypothetical electron reservoir determines the effects of the electronic-nuclear coupling in the open molecular systems. Let us assume the initial electronic and geometric equilibria in such an initially open system p° = p.rej and F° = 0. The LeChatelier stability criteria of these two (decoupled) facets of the molecular structure requires that the conjugate forces A/jl(AN) or AFS(AQS) created by the primary electronic (AN> 0) or nuclear AQs > 0 displacements,... 

Recently Lee et al (Ref 3) re-examined the behavior of PETN under 10 to 50 kbars of external pressure. They also find a reduction in decomposition rate with increasing applied pressure. HMX behaves similarly to PETN. TNT whose explosion products contain a high proportion of solid carbon, as expected from LeChatelier s Principle, shows little pressure effect on its thermal decomposition. Nitro-methane, however, appears to decompose more rapidly under an external pressure of 50 kbars than 10 kbars. This effect is not completely understood but Lee et al suggest that high pressure may favor the formation of the thermally less stable aci form of Nitromethane ... 

C. A. Snyder and D. C. Snyder, "Simple Soda Bottle Solubility and Equilibria," /. Chem. Educ., Vol. 69,1992,573. Bromocresol green indicator, added to a glass bottle of seltzer water, changes color from yellow to green to blue as carbon dioxide concentration is decreased. Observations are related to temperature and pressure effects on gas solubility, and LeChatelier s principle is applied to the equilibria involved. 

The test results with the ultrasonic nozzle were obtained with an estimated steam to copper (S/Cu) ratio of 23 and the humidified Ar was injected co-currently with the CuCl2 solution. Several variables remain to be investigated, i.e. lower S/Cu ratios, counter-current instead of co-current operation, and subatmospheric pressures. LeChatelier s Principle predicts that reducing the pressure in the hydrolysis reactor should reduce the S/Cu ratio. The effect of a reduced pressure was quantified by the results of a sensitivity study using Aspen. Aspen predicts that a S/Cu ratio of 17 is needed for essentially complete conversion at 375°C and atmospheric pressure while a S/Cu ratio of 13 is required at 0.5 bar. The conceptual process design specifies that the hydrolysis reactor be run at 0.25 bar. The pressure drop in the reactor is achieved by adding a low temperature steam ejector after the condenser at the exit of the hydrolysis reactor in the conceptual design. 

What effect, if any, is predicted by LeChatelier s principle if the temperature is increased for a mixture of nitrogen and oxygen only ... 

The overall, generally small, effect of pressure to increase H bond formation can be understood as a molar volume change. It is another fulfillment of LeChatelier s principle—H bonds reduce the space occupied by the molecules so their formation is enhanced by a process... 

The effect of pressure will be greatest when the area occupied by the products of reaction is most different from that of the reactants. If the products occupy less area, the principle of LeChatelier shows that reaction will proceed further to completion if the products are more bulky than the reactants the surface pressure will reduce the extent of reaction. 

We apply LeChatelier s Principle to predict the effect on each reaction. 

State LeChatelier s Principle. Which factors have an effect on a system at equilibrium How does the presence of a catalyst affect a system at chemical equilibrium Explain your answer. 

The operation of a buffer solution depends on the common ion effect, a special case of LeChatelier s Principle. 

If a reaction is at equilibrium and we alter the conditions so as to create a new equilibrium state, then the composition of the system will tend to change until that new equilibrium state is attained. (We say tend to change because if the reaction is kinetically inhibited, the change may be too slow to observe or it may never take place.) In 1884, the French chemical engineer and teacher Henri LeChatelier (1850-1936) showed that in every such case, the new equilibrium state is one that partially reduces the effect of the change that brought it about. This law is known to every Chemistry student as the LeChatelier principle. His original formulation was somewhat more complicated, but a reasonably useful paraphrase of it reads as follows ... 

LeChatelier principle If a system at equilibrium is subjected to a change of pressure, temperature, or the number of moles of a substance, there will be a tendency for a net reaction in the direction that tends to reduce the effect of this change. 

Consider an arbitrary mixture of these components at equilibrium, and assume that we inject more hydrogen gas into the container. Because the H2 concentration now exceeds its new equilibrium value, the system is no longer in its equilibrium state, so a net reaction now ensues as the system moves to the new state. The LeChatelier principle states that the net reaction will be in a direction that tends to reduce the effect of the added H2. This can occur if some of the H2 is consumed by reacting with I2 to form more HI in other words, a net reaction occurs in the reverse direction. Chemists usually simply say that the equilibrium shifts to the left . 

Comment This net reaction describes the dissolution of limestone by acid it is responsible for the eroding effect of acid rain on buildings and statues. This is an example of a reaction that has practically no tendency to take place by itself (the dissolution of calcium carbonate) begin driven by a second reaction having a large equilibrium constant. From the standpoint of the LeChatelier principle, the first reaction is pulled to the right by the removal of carbonate by the hydrogen ion. Coupled reactions of this type are widely encountered in all areas of chemistry, and especially in biochemistry, in which a dozen or so reactions may be linked in this way. 

Skill 2.2a-Predict the effect of temperature, pressure, and concentration on chemical equilibrium (LeChatelier s principle) and the reaction rate Introduction to dynamic equilibrium... 

Use LeChatelier s principle to predict the effects, if any, of each of the following changes on the equilibrium system, described below, in a closed container. 

We may state the principle of LeChatelier in the following way. If the external constraints under which an equilibrium is established are changed, the equilibrium will shift in such a way as to moderate the effect of the change. 

The slight difference in the average rotational energy of the two forms enhances the heat capacity due to the LeChatelier shift in the equilibrium position as the temperature is changed. This effect is exhibited in the curve for ot labeled e-H2. Equilibrium-hydrogen, e-H2, is hydrogen that is kept in the presence of a catalyst to ensure that the equilibrium between 0-H2 and P-H2 is established at all temperatures. The curve for e-H2 is typical of the heat capacity of a reactive mixture maintained in equilibrium as the temperature is changed. 

Thinking it Through Considering LeChatelier s principle first, we see from the chemical equation that three moles of gas (2 mol SO2 plus 1 mol of O2) react to produce two moles of product. The equilibrium will shift to minimize the effect of the change, producing more SO3. Choices (B) and (C) are therefore eliminated from consideration. 

When we look at the solubility product of silver bromide or silver cyanide, we presume that an equilibrium is established for the salt in pure water. What if additional cyanide ion were introduced into the system by adding NaCN How does the presence of an ion that is part of an equilibrium, but is introduced into a system from another source, influence the solubility of a salt This question is important in industrial processes because most commodities require several steps in their manufacture. The answer can be deduced from LeChatelier s principle. Ions that are part of an equilibrium, but are introduced from an outside source, are called common ions because they are common to the solution and the solid. If we were to add a common ion (CN in our example) to a solution of AgCN already at equilibrium, the resulting shift would be in a direction that consumes the added ion, forming additional solid AgCN. This is the common ion effect (Figure 12.10). If instead we add a sparingly soluble solid to a solution in which common ions are already present, their presence will suppress solubility of the solid (because they tend to stress the system toward the production of reactants). We explore this idea in Example Problem 12.12. 

In the presence of n-butane, Ky becomes smaller than in the ternary system because of the increasing activity coefficient of methanol in the presence of butane, which leads to an increase of Kx and the equilibrium conversion. But this effect is compensated by the dilution of the reaction mixture, which leads to a decrease of the equilibrium conversion following the principle of LeChatelier, since for the reaction considered Eu, is negative. 

Recall from Section 19-1 that the common ion effect is a special case of LeChatelier s Principle. Thus, the common ion effect applies to solubility equilibria just as it does to other ionic equilibria. Silver acetate, AgCH3COO, is a slightly soluble salt... 

If we add another salt that provides one of the product ions, say Ag (from AgNOs), this dissolution reaction is shifted to the kfi (LeChatelier s Principle). Example 20-4 illustrates this effect. 

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