The Effect of a Change in Temperature

Of the three types of disturbances—a change in concentration, in pressure, or in temperature—only temperature changes alter K. To see why, we must take the heat of reaction into account  

The forward reaction is exothermic (releases heat AH° 0), so the reverse reaction is endothermic (absorbs heat AH° 0)  

If we consider heat as a component of the equilibrium system, a rise in temperature means heat is added to the system and a drop in temperature means heat is removed from the system. As with a change in any other component, the system shifts to reduce the effect of the change. Therefore, a temperature increase (adding heat) favors the endothermic (heat-absorbing) direction, and a temperature decrease (removing heat) favors the exothermic (heat-releasing) direction. 

Let s review these ideas with a sample problem. 

SAMPLE PROBLEM 1 7.11 Predicting the Effect of a Change in Temperature 

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Le Chateliers principle can be used to predict the effect of a change in temperature on the position of an equilibrium. In general, an increase in temperature causes Ihe endothermic reaction to occur. This absorbs heat and so tends to reduce the temperature of the system, partially compensating for the original temperature increase. 

To complete our analysis, we must determine the effect of a change in temperature. According to Equation, kinetic energy is proportional to temperature, and according to Equation, kinetic energy is proportional to the square of the molecular speed. Thus, the square of the molecular speed is proportional to temperature. 

Interpretation of temperature effects, however, remained uncertain. With polypeptides in particular the effect of a change in temperature was unpredictable. Certain ones followed rather closely the Stokes-Einstein relationship for free diffusion others deviated strongly. It was... 

The effect of a change in temperature or solvent on chemical shift is a parameter which must be carefully studied. On the basis of several examples it was proposed that evidence for hydrogen bonding (36) could be derived from the failure of an amide-proton chemical shift to be sensitive to a change in temperature. Later observations from this laboratory have shown that the chemical shift of certain amide protons not hydrogen bonded also may not be sensitive to a change in temperature. 

The effect of a change in temperature is a double one, as it affects both equilibrium percentage and reaction rate. While catalyst efficiency... 

Almost always the data from the apparatus above is analyzed by using the time-temperature superposition principle to form a master curve over a wide frequency range at a selected reference temperature. The basis for this procedure is that for thermorheologically simple materials the effect of a change in temperature on... 

A binary liquid solution, having mole fraction x of component 1, is in equilibrium with a vapor that has mole fraction v of that component. Show that for this mixture the.effect of a change in temperature on the equilibrium pressure at fixed liquid composition is approximately... 

The Effect of a Change in Temperature 567 The Lack of Effect of a Catalyst 568 The Industrial Production of Ammonia 570... 

In considering the effect of a change in temperature, we are assuming that the heat is added (or removed) at constant pressnte. 

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