Temperature affects equilibrium constants

A comparison of Eq. (22) with Eq. (23) reveals that the apparent rate constant, k, equals the product kp K. Thus, the rate of polymerization is affected by the concentrations of monomer, initiator, and Lewis acid, as well as by the rate and equilibrium constants. Although the ionic propagation rate constant is not very sensitive to the nature of the counterion, solvent and temperature, the equilibrium constant K usually depends strongly on the temperature, solvent, Lewis acid, and the leaving group X. 

In 1888, the French chemist Henri-Louis Le ChStelier discovered that there are ways to control equilibria to make reactions, including this one, more productive. He proposed what is now called Le Chatelier s principle If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress. A stress is any kind of change in a system at equilibrium that upsets the equilibrium. You can use Le Chatelier s principle to predict how changes in concentration, volume (pressure), and temperature affect equilibrium. Changes in volume and pressure are interrelated because decreasing the volume of a reaction vessel at constant temperature increases the pressure inside. Conversely, increasing the volume decreases the pressure. 

The equilibrium constant of the aldolase reaction depends greatly on temperature. At low temperatures the condensation is more favored, whereas the amount of triose at equilibrium increases with rising temperatures. The equilibrium constant is evaluated as (dihydroxyacetone phosphate) (phosphoglyceraldehyde)/(HDP) = IT, = 6 X 10 at 28 C. At first glance it appears that this implies that very little triose exists at equilibrium at this temperature. This is an example of reactions in which one compound is converted to two, and closer examination shows that the percentage conversion in such cases is a function of the absolute concentration. Thus, with 1 M HDP, only a fraction of 1 per cent is split at equilibrium, whereas at 10 M, approximately half is converted to triose phosphates. The equilibrium constant is markedly affected by temperature. Lower temperatures favor the condensation to HDP, while higher temperatures cause the reaction to shift toward increased formation of triose phosphates. 

Pressure affects equilibrium constants in a way similar to temperature ... 

Experimental studies on how temperature affects equilibria reveal a consistent pattern. The equilibrium constant of any exothermic reaction decreases with increasing temperature, whereas the equilibrium constant of any endothermic reaction increases with increasing temperature. We can use two equations for A G °, Equations and, to provide a thermod3mamic explanation for this behavior AG = -RT x Teq AG° — AH°-TAS°... 

As anticipated, SA conversion increases with increasing residence time (1/LHSV) and with increasing temperature to a maximum of about 98%. This limit is most likely caused by equihbrium. This limit and thus the equilibrium constant were not affected by the temperature range studied, consistent with a low heat of reaction. The sum of the molar heats of combustion of stearic acid (11320 kJ/mol) and methanol (720 kJ/mol) is almost the same as the heat of combustion of methyl stearate (12010 kJ/mol), meaning that the change in enthalpy of this reaction is nearly zero and that the equihbrium constant is essentially temperature independent. 

The equilibrium constant, K, is affected by the temperature of the system but not by the pressure of the system, the presence or otherwise of inert substances or the kinetics of the reaction. 

The temperature is high enough for the gases to be considered ideal, so the equilibrium constant is written in terms of partial pressure rather than fugacity, and the constant will not be affected by pressure. Mol fraction can be substituted for partial pressure. As the total mols in and out is constant, the equilibrium relationship can be written directly in mols of the components. 

Thus, the dissociation equilibrium is affected by the ionic strength, temperature and dielectric constant of the solvent as well as by the parameter h (involved in AGf,). On the other hand, the term dG /dn does not depend on the degree of polymerization (except for very small values of n). The degree of polymerization does not affect, for example, the course of the potentiometric titration of a poly acid. 

The pH, alkalinity, and temperature of the treatment area these conditions will affect equilibrium and kinetic constants defining the extent and rate that each oxidation step can take place. 

The quantity Kf/P is constant for a given temperature and pressure. However, unlike the equilibrium constant Ka, the term Kf/P is affected by changes in the system pressure as well as by changes in temperature. 

Does this agree with LeChatelier s Principle that for an exothermic reaction the system will shift to form more reactants at higher temperatures Yes, it does. Recall, that that only temperature will affect the value of the equilibrium constant. More reactants are formed because the value of the equilibrium constant decreased when the temperature increased. 

Pressure affects the composition of an equilibrium mixture, but the equilibrium constant itself is independent of pressure. It does, however, depend on temperature.. 

At a given temperature, a reaction will reach equilibrium with the production of a certain amount of product. If the equilibrium constant is small, that means that not much product will be formed. But is there anything that can be done to produce more Yes, there is— through the application of Le Chatelier s principle. Le Chatelier, a French scientist, discovered that if a chemical system at equilibrium is stressed (disturbed) it will reestablish equilibrium by shifting the reactions involved. This means that the amounts of the reactants and products will change, but the final ratio will remain the same. The equilibrium may be stressed in a number of ways changes in concentration, pressure, and temperature. Many times the use of a catalyst is mentioned. However, a catalyst will have no effect on the equilibrium amounts, because it affects both the forward and reverse reactions equally. It will, however, cause the reaction to reach equilibrium faster. 

The esterification of TPA with EG is a reaction between two bifunctional molecules which leads to a number of reactions occurring simultaneously. To simplify the evaluation of experimental data, model compounds have been used for kinetic and thermodynamic investigations [18-21], Reimschuessel and coworkers studied esterification by using EG with benzoic acid and TPA with 2-(2-methoxyethoxy) ethanol as model systems [19-21], The data for the temperature dependency of the equilibrium constants, AT, = K,(T), given in the original publications are affected by printing errors. The corrected equations are summarized in Table 2.3. 

As you know, the value of the equilibrium constant changes with temperature, because the rates of the forward and reverse reactions are affected. 

In catalysis applications, the tunable solvent properties result in a variety of effects, such as controllable component and catalyst solubilities. Moreover, it is possible that kinetic rates are affected by both temperature and pressure effects, equilibrium constants are shifted in favor of the desired products, and selectivity and yields are increased by manipulating the solvent s dielectric constant or by controlling the temperature in highly exothermic reactions through an adjustment of the solvent s heat capacity [18-23]. 

The heat of reaction for vinyl polymers affects the thermal stability of the polymer during extrusion, and the thermal stability is related to the ceiling temperature. The ceiling temperature is the temperature where the polymerization reaction equilibrium is shifted so that the monomer will not polymerize, or if kept at this temperature all the polymer will be converted back to monomer. From thermodynamics the equilibrium constant for any reaction is a function of the heat of reaction and the entropy of the reaction. For PS resin, the exothermic heat of reaction for polymerization is 70 kj/gmol, and the ceiling temperature is 310 °C. Ceiling temperatures for select polymers are shown in Table 2.5. 

This equilibrium constant in partition equilibria Is known as the partition coefficient and, like all equilibrium constants, it is dependent on temperature. It is not affected by the volume of the solvents or the amount of solute added. However, it does depend on the type of solute and solvent used. 

Solvent-separated ion pairs, in which the first solvation shells of both ions remain intact on pairing may be distingnished from solvent-shared ion pairs, where only one solvent molecule separates the cation and the anion, and contact ion pairs, where no solvent separates them (Fig. 2.6). The parameter a reflects the minimum distance by which the oppositely charged ions can approach each other. This eqnals the sum of the radii of the bare cation and anion pins 2, 1, and 0 diameters of the solvent, respectively, for the three categories of ion pairs. Since a appears in Eq. (2.49), and hence, also in Q(b), it affects the value of the equilibrium constant, K s- The other important variable that affects K ss is the product T and, at a given temperature, the value of the relative permittivity, e. The lower it is, the larger b is and, hence, also K s-... 

If the three gases in the reaction were at equilibrium and you then increased the carbon monoxide concentration, some Bt2 would combine with added CO to produce COBr2 and thereby minimize the increase in CO. Alternatively, if you decrease the CO concentration, some COBtj would decompose to produce CO and Br2 and thereby minimize any decrease in CO. Notice how the concentrations of all constituents shift to counteract the imposed change in a single substance. Of course, this shift does not affect the value of the equilibrium constant. Only a change in temperature can do that. 

Because the temperature was kept constant, as was the size of the flask, and because some of the original 250.0 grams of CutNOsij was left as solid in the flask at equilibrium, any extra CutNOsij introduced into the flask would remain as solid—there would be no substantial change in the pressure. The amount of pure solid does not affect equilibrium (as long as some solid remains at equilibrium). 

D) Choices 3 and 4 will all cause the equilibrium to shift either left or right which causes a change in reactant and product concentrations which changes the value of the reaction quotient. Only a change in the temperature will cause a change in the value of the equihbrium constant. A catalyst or adding an inert gas will not affect either the equilibrium constant or reaction quotient. 

From Eq. 13 one can immediately recognize that, when the temperature is equal to P, any changes in AH never affect the reaction rate or equilibrium and the original AG value is conserved under any reaction conditions as far as no change in mechanism occurs. The critical temperature p, which gives the same rate or equilibrium constant irrespective of the reaction conditions, is called the isokinetic... 

Not only is the extent of the equilibrium reaction, that is, the ratio of the concentrations of the products to those of the reactants, governed by the initial composition and the equilibrium constant, affected by the solvent, so also is the temperature dependence, that is, the enthalpy and entropy of the reaction. There is again a complicated interplay in the solvation of the two species between polarity and hydrogen-bonding abilities of the solvents. The negative entropies of this reaction, that become more negative along the same sequence, are due more to the solute properties than the solvent ones. 

The above equilibrium reaction is unaffected by volume change since the total number of coefficients of gases on each side of the reaction equation are equal. Pressure of a gas is inversely proportional to volume. When the volume of a gas increases, the pressure of the gas decreases. When the volume of a gas decreases, the pressure of the gas increases. Thus, change in pressure at constant temperature affects the equilibrium reaction conversely in respect to volume change. 

However, if the valences of the exchanging cations are equal, the selectivity coefficient or pseudo-equilibrium constant is not affected by concentration. As already mentioned, one isotherm corresponds to a specific temperature in the case of adsorption or ion exchange of equal valence ions, whereas additionally, the same normality is required for the existence of only one isotherm in the case of ion exchange of different valence ions, due to the concentration-valence effect (Helfferich, 1962). The determination of the true equilibrium constant should be based on the thermodynamic activities (activity coefficients) of the species rather than concentrations. It is clear that the difficulties in the determination of activity coefficients also complicate the determination of the true equilibrium constant (Culfaz and Yagiz, 2004). 

The concept of apparent values is very useful, and it appears in other phenomena, such as in pKa values (section L3). Quite often, a pKa value does not represent the microscopic ionization of a particular group but is a combination of this value and various equilibrium constants between different conformational states of the molecule. The result is an apparent pKa which may be handled titri-metrically as a simple pKa. This simple-minded approach must not be taken too far, and, when one is considering the effects of temperature, pH, etc., on an apparent Km, one must realize that the rate-constant components of this term are also affected. The same applies to kcat values. The literature contains examples in which breaks in the temperature dependence of kcat have been interpreted as indicative of conformational changes in the enzyme, when, in fact, they are due to a different temperature dependence of the individual rate constants in cal, e.g., k2 and k2 in equation 3.22. 

This was first demonstrated in 1862 by Berthclot and Saint-Gilles. who found that when equivalent quantities of ethyl alcohol and acetic acid were allowed to react, the esterification stopped when two-thirds of the acid had reacted. Similarly, when equal molar proportions of ethyl acetate and water were heated together, hydrolysis of the ester slopped when about one-third of the ester was hydrolyzed. By varying the molar ratios of alcohol to acid, yields of ester ft(V were obtained hy displacement of the equilibrium. The results of these tests were in accordance with the mass action law- shown. K = letter- lwuter f acid uU-ohrd]. However, in many eases the equilibrium constant is affected by the proportion of reactants. The temperature as well as the presence of salts may also affect the value or the equilibrium constant. 

Explain why the following statements about elementary reactions are wrong, (a) The equilibrium constant for a reaction equals the ratio of the forward and reverse rates, (b) For an exothermic process, the rates of the forward and reverse reactions are affected in the same way by a rise in temperature. 

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