Temperature change equilibrium effect

Changing the column temperature can produce a variety of additional effects. Temperature changes the balance between enthalpy and entropy effects on retention mechanisms. Changing the temperature changes the equilibrium constants of both solvent and solutes, and it changes the... 

Ealy, Jr., "Effect of Temperature Change on Equilibrium Cobalt Complex" Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), p. 60-61. Concentrated hydrochloric acid is added to pink [Co(H20)5]2+ until blue [C0CI4]2- is formed. When heated the solution turns darker blue when cooled the solution turns pink, indicating that the reaction is endothermic. Students are asked to examine the equilibrium reaction and predict how the system will shift upon the addition of water. 

The composition of an equilibrium mixture can be altered by changes in concentration, pressure (volume), or temperature. The qualitative effect of these changes is predicted by Le Chatelier s principle, which says that if a stress is applied to a reaction mixture at equilibrium, net reaction occurs in the direction that relieves the stress. Temperature changes affect equilibrium concentrations because Kc is temperature-dependent. As the temperature increases, Kc for an exothermic reaction decreases, and Kc for an endothermic reaction increases. 

Temperature dependence equations for pK and pK values show the importance of good temperature control during pH meter calibration as well as in measurements. To a greater or lesser extent, all acid-base reactions vary with temperature. The effect of temperature changes for equilibrium reactions are closely described by the Valentiner-Lannung equation [80-82] ... 

Lee R. Summerlin and James L. Ealy, jr., "Effect of Temperature Change on Equilibrium Cobalt Complex," Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988) pp. 79-80. 

Effect of a Temperature Change on Equilibrium Again, the effect of temperature on a reaction allows chemists to choose conditions that will favor desired reactions. Higher temperatures favor endothermic reactions, while lower temperatures favor exothermic reactions. Most reactions will occur faster at higher temperature, so the effect of temperature on the rate, not just on the equilibrium constant, must be considered. 

SAMPLE PROBLEM 13.8 Effect of Temperature Change on Equilibrium... 

Determining the Effect of a Temperature Change on Equilibrium (14.9) Add heat... 

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. 

Le Chatelier s principle A relation stating that when a system at equilibrium is disturbed it responds in such a way as to partially counteract that change, 337-338 buffers and, 385 compression effects, 339-340 expansion effects, 339-340 precipitation equilibrium, 442 reaction conditions, 348q temperature changes, 340 Lead, 2,501 Leclanch cell, 500 Leucine, 622t... 

The Van t Hoff isotherm establishes the relationship between the standard free energy change and the equilibrium constant. It is of interest to know how the equilibrium constant of a reaction varies with temperature. The Varft Hoff isochore allows one to calculate the effect of temperature on the equilibrium constant. It can be readily obtained by combining the Gibbs-Helmholtz equation with the Varft Hoffisotherm. The relationship that is obtained is... 

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. 

Determine the effectiveness factor for the ion exchange resin at 85 °C, assuming that the reaction is reversible even though the authors presumed the reaction to be irreversible in reporting their data. They note that at 100 °C the equilibrium for the reaction corresponds to a conversion greater than 94%. If the equilibrium constant for the reaction is expressed as the ratio of the t-butanol concentration to the isobutylene concentration and corrected for the temperature change in going from 100 °C to 85 °C, a value of 16.6 may be considered appropriate for use. 

Temperature changes could also have another effect. Steroidal ketones have been demonstrated to have nearly degenerate zn-n and states, and the possibility of a thermal equilibrium between those states has been pointed out. 146> In fact, testosterone acetate and 10-methyl-A -oct alone, both very similar molecules to the reactant in Eq. 55, undergo photoreactions from two different triplet states. 144> So we ought to consider what the reactivity of the n-n state would be, and whether product distribution would differ from the two states. If so, temperature variation could change product distribution due to a change in the relative proportions of triplet states. 

All partitioning properties change with temperature. The partition coefficients, vapor pressure, KAW and KqA, are more sensitive to temperature variation because of the large enthalpy change associated with transfer to the vapor phase. The simplest general expression theoretically based temperature dependence correlation is derived from the integrated Clausius-Clapeyron equation, or van t Hoff form expressing the effect of temperature on an equilibrium constant Kp,... 

The important quantities AH and AS are assumed to be temperature independent. This is often quite a good approximation, but the vibrational component of the entropy, which has been neglected altogether, will become increasingly important at high temperatures. The effects of these factors can cause the major defect type present to change as the temperature increases. Near to the transition temperature a complex equilibrium between both defect types will be present. 

Changing the temperature changes the value of the equilibrium constant. It also changes the amount of heat in the system and can be treated as a concentration effect. To treat it this way, one must know which reaction, forward or reverse, is exothermic (releasing heat). One last time, let s consider the Haber reaction ... 

---