Equilibrium constant less than

A positive enthalpy and an equilibrium constant less than unity mean that the cis isomer is more stable relative to the trans. 

The values can also be used to estimate the equilibrium constants K. For example, if the of an acid is near zero, then the equilibrium constant for the reaction of that acid protonating water is near 1. Negative pAi values correlate to acids with equilibrium constants greater than 1, while positive values are for acids with equilibrium constants less than 1. Each single unit difference between pK values represents a tenfold increase or decrease in the strength of the acids being compared. 

Reactions for which the sign of AG° is negative are described as exergonic those for which AG° is positive are endergonic. Exergonic reactions have an equilibrium constant greater than 1 endergonic reactions have equilibrium constants less than 1. 

Arts. According to Le Chatelier s principle, raising the temperature would shift this equilibrium to the left. That means that there would be less C and more A and B present at the new equilibrium temperature. The value of the equilibrium constant at that temperature would therefore be lower than the one at the original temperature. 

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. 

Let s see how the van t Hoff equation accounts for Le Chatelier s principle for the effect of temperature on an equilibrium. Suppose that the reaction is endothermic, then AH° is positive. If T2 is greater than T, 1/T2 is smaller than 1/T, so the term in parentheses is also positive. Therefore, ln(fC2/fCj) is positive, which implies that K2/K, is greater than 1, and therefore that the equilibrium constant K2 is greater than A,. In other words, an increase in temperature favors the formation of product if the reaction is endothermic. The opposite effect is predicted for an exothermic reaction because AH° is then negative. 

Not only the internal pressure of a solvent can affect chemical reactions (see Section 5.4.2 [231, 232]), but also the application of external pressure can exert large effects on reaction rates and equilibrium constants [239, 429-433, 747-750]. According to Le Chatelier s principle of least restraint, the rate of a reaction should be increased by an increase in external pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules, whereas the rate of reaction should be decreased by an increase in external pressure if the reverse is true. The fundamental equation for the effect of external pressure on a reaction rate constant k was deduced by Evans and Polanyi on the basis of transition-state theory [434] ... 

Another disturbance is heating. If a reaction under equilibrium is heated up, how the equilibrium changes depends on whether the reaction is exothermic or endothermic. If is exothermic (that is, gives out heat), Le Chatelier s principle would predict that, since heat is consumed in the reverse reaction, more of the starting materials will be formed. Again no principle is needed—this change occurs because the equilibrium constant is smaller at higher temperatures in an exothermic reaction. Le Chatelier didn t know about equilibrium constants or about -RT In K= ATP -TAS° so he needed a principle . You know the reasons and they are more important than rules. 

Although acetic acid is normally regarded as a weak acid, it is about 34% dissociated in a 10 " M solution at 25°C. It is less than 1% dissociated in 1 M solution. Discuss this variation in degree of dissociation with dilution in terms of Le Chatelier s principle, and explain how it is consistent with the supposed constancy of equilibrium constants. 

In the case of the ammonia synthesis, the equilibrium conversion is increased by the pressure not only because of the reduction of the number of moles (Le Chatelier principle), but also because the real behavior in the gas phase leads to K values smaller than 1. This is in agreement with experimental results. While an equilibrium constant Kp of 6.59 10 atm was measured at 1 atm, a value of... 

That is, the autoionization of water is an endothermic process. According to Le Chatelier s principle (Section 10.4), the equilibrium of an endothermic process will shift in the direction of products when the temperature is increased, leading to a corresponding increase in the equilibrium constant. Therefore, the value of K will be larger than its value at 25°C (1.01 X 10" " ) for temperatures above 25°C and smaller than its value at 25°C for temperatures below 25°C. Table 11.2 shows the value of at a variety of temperatures. 

What is the role of water under these conditions It has been suggested that water suppresses the formation of proHne-oxazoUdinone, which has been considered to be a parasitic species [11]. Then, the role of water is to prevent deactivation rather than to promote activity. Studies, carried out on the proUne-catalyzed reaction between acetone and 2-chlorobenzaldehyde allow one to hypothesize a conflicting role of water. Water increases the total catalyst concentration due to suppression of unproductive species and decreases the relative concentration of productive intermediates by shifting the iminium ion back to proline [12]. Addition of water suppresses formation of both on- and ofF-cyde iminium ions 1 and 2 by Le Chatelier s principle (Scheme 24.2a). The net effect of added water on the globally observed rate will depend on the relative concentrations of iminium ions 1 and 2, which may be different for different aldehydes and can be a function of substrate concentrations and rate and equilibrium constants. Seebach and Eschen-moser have raised doubts about the fact that oxazoUdinones are unproductive and parasitic species in proline-catalyzed aldol reactions [13]. The small excess of water will potentially facilitate proton-transfer in the transition state (Scheme 24.2b), which both lowers the LUMO of the incoming electrophile as well as directs the enantioselectivity of the newly formed stereocenters. 

Our expression for K in terms of rate constants helps to explain one application of Le Chatelier s principle (Section 9.10). According to Le Chatelier, an increase in temperature shifts the equilibrium composition in the endothermic direction. We can now see why. If the forward reaction is endothermic, the activation energy will be higher for the forward direction than for the reverse direction (Fig. 13.28). The higher activation energy means that the rate constant of the forward reaction depends more strongly on temperature than does the rate constant of the reverse reaction. Therefore, when the temperature is raised, the rate constant for the forward reaction increases more than that of the reverse reaction. As a result, K will increase and the products will become more favored, just as Le Chatelier s principle predicts. 

Note that the equilibrium conversion in a flow reactor (i.e., X f = O.Sl), with negligible pressure drop, is greater than the equilibrium conversion in a constant-volume batch reactor = 0.44). RecaHing Le Chateliet s principle, can you suggest an explanation for this difference in X ... 

The principle of Le Chatelier may now be stated as follows If a system is in stable equilibrium the increase in the intensity factor Xj., corresponding to a given increase dXj. in the quantity factor Xj., is smaller for a change to another state of stable equilibrium than for a change produced by altering Xr by dxr, while all the other quantity factors remain constant. 

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