Endothermic process equilibrium positions

The effect of a temperature change on solubility equilibria such as these can be predicted by applying a simple principle. An increase in temperature always shifts the position of an equilibrium to favor an endothermic process. This means that if the solution process absorbs heat (AHsoin. > 0), an increase in temperature increases the solubility. Conversely, if the solution process is exothermic (AH < 0), an increase in temperature decreases the solubility. 

For uptake of solute from solution by porous solids the rate will be endothermic rather than exothermic if intraparticle transport is the rate-limiting mechanism. Because diffusion is an endothermic process while adsorption is exothermic, rate of uptake of solute by porous solids will often increase with increasing temperature while for the same system the equilibrium position of adsorption or adsorption capacity will decrease with increasing temperature. 

When a chemical reaction proceeds, we have established (by reference to experiment) that energy will be conserved. But we have not found a way of predicting in which direction the reaction will go. In other words we have not found a suitable definition of the position of equilibrium. We have discovered that for molecular systems (which may approach equilibrium by endothermic processes) the energy, unlike the potential energy in mechanical systems, does not provide a sufficient criterion for equilibrium. A new factor must be introduced which will enable us to understand why heat always flows from hot to cold bodies and why a perfect gas will expand to fill its container, even though no loss of energy (by the system) accompanies these processes. 

Dissociation of reactant A is an endothermic process in which the entropy change is positive. Consequently, the equilibrium constant increases at higher temperature via Le Chatelier s principle, which shifts the reaction to the right in favor of... 

Strategy At the melting point, liquid and solid benzene are at equilibrium, so AG = 0. From Equation (18.10) we have AG = 0 = AH — TAS or AS = AH/T. To calculate the entropy change for the solid benzene liquid benzene transition, we write A5j,s = AHfaJTf. Here A//f s is positive for an endothermic process, so ASf s is also positive, as expected for a solid to liquid transition. The same procedure applies to the liquid benzene —> vapor benzene transition. What temperature unit should be used ... 

For an exothermic reaction, the difference in enthalpy between products and starting materials is negative and the equilibrium constant (K) is greater than 1. For an endothermic reaction (read the first reaction backward to produce an endothermic process), the enthalpy difference is positive and K is less than 1. 

To understand how the nitrogen dioxide-dinitrogen tetroxide equilibrium is affected by temperature, we need to review endothermic and exothermic reactions. Recall that endothermic reactions absorb energy and have positive Mi values. Exothermic reactions release energy and have negative Mi values. The forward reaction is an exothermic process, as the equation below shows. 

Enzymatic catalysis lowers AG to accelerate the reaction rate, but does not affect the AG° that controls the position of equilibrium. However, cells also have an elegant ability to overcome an unfavorable equilibrium by coupling an endothermic reaction with exothermic ATP hydrolysis. For example, the reaction of acetate with coenzyme A (CoASH) giving acetyl CoA is uphill by about 7 kcal/mol. The hydrolysis of ATP to ADP and phosphate is downhill by about the same amount. By coupling these two reactions shown below for acyl-CoA synthetase, the cell achieves a total AG° of almost 0 for a ATgq of about 1. The phosphorylation of acetate by ATP improves the leaving group for the second reaction with CoASH. There are many enzymes whose catalytic process remains a mystery enzymes still have much to teach us about reaction mechanisms. 

However, if we add solid NaN03 to water we find that heat is absorbed and the system cools down. (The process is endothermic.) In this case we have a system which climbs uphill on the energy scale to reach its position of equilibrium (Fig. 1.8). In other words in this system energy cannot be the sole factor determining the position of equilibrium. 

In your own words, paraphrase Le Chatelier s principle. Give an example (including a balanced chemical equation) of how each of the following changes can affect the position of equilibrium in favor of additional products for a system the concentration of one of the reactants is increased one of the products is selectively removed from the system the reaction system is compressed to a smaller volume the temperature is increased for an endothermic reaction the temperature is decreased for an exothermic process. 

Note that because H is a state variable, AH is perfectly well defined between any two equilibrium states. But when the two states are at the same pressure, AH becomes equal to the total heat flow during the process from one to the other, and in practice enthalpy is little used except in this context. Processes having a negative A,.H (A,.H < 0) are termed exothermic, and those having a positive A,.H are termed endothermic. 

In summary, a temperature increase favors an endothermic reaction, and a temperature decrease favors an exothermic process. Temperature affects the position of an equilibrium by changing the value of the equilibrium constant. Figures 15.10 and 15.11 illustrate the effects of various stresses on systems at equUibrium. 

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