The Effect of Temperature on Spontaneity

In an endothermic process heat flows from the surroundings into the system. In an exothermic process heat flows into the surroundings. 

Steam rising from a thermal pool in Yellowstone. 

To explore the interplay of ASsys and ASsurr in determining the sign of ASunj , we will first discuss the change of state for 1 mole of water from liquid to gas, 

What happens to the entropy of water in this process A mole of liquid water (18 g) has a volume of approximately 18 mL. A mole of gaseous water at 1 atm and 100°C occupies a volume of approximately 31 L. Clearly, there are many more positions available to the water molecules in a volume of 31 L than in 18 mL thus the vaporization of water is favored by this increase in positional probability. That is, for this process the entropy of the system increases ASsys has a positive sign. 

Remember it is the sign of ASuniv that tells us whether the vaporization of water is spontaneous. We have seen that ASsys is positive and favors the process and that ASsurr is negative and unfavorable. Thus the components of ASuniv are in opposition. Which one controls the situation The answer depends on the temperature. We know that at a pressure of 1 atm water changes spontaneously from liquid to gas at all temperatures above 100°C. Below 100°C the opposite process (condensation) is spontaneous. 

To predict whether a given process will be spontaneous, we must know the sign of AAuniv If A5univ IS positivo, the entropy of the universe increases, and the process is spontaneous in the direction written. If A5univ is negative, the process is spontaneous in the opposite direction. If A u iv is zero, the process has no tendency to occur, and the system is at equilibrium. To predict whether a process is spontaneous, we must consider the entropy changes that occur both in the system and in the surroundings and then take their sum. 

What if ASuniv was a state function How wouid the world be different  

In a living cell, large molecules are assembled from simple ones. Is this process consistent with the second law of thermodynamics  

To reconcile the operation of an order-producing cell with the second law of thermodynamics, we must remember that A5 univ, not AS ys, must be positive for a process to be spontaneous. A process for which AS ys is negative can be spontaneous if the associated A5su . is both larger and positive. The operation of a cell is snch a process. 

Boiling water to form steam increases its volume and thus its entropy. 

What about the entropy change in the surroundings Although we will not prove it here, entropy changes in the surroundings are determined primarily by the flow of energy 

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In a process occurring at constant temperature, the tendency for the system to lower its energy results from the positive value of ASciirr. 

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The opposite situation holds for reactions that have negative values for A iiT ° and A S °. These reactions are spontaneous at low temperature because their release of heat disperses energy into the surroundings. The favorable AH ° dominates A G ° as long as T does not become too large, and the reaction is enthalpy-driven. At high temperature, however, the unfavorable A S ° dominates A G °, and the reaction is no longer spontaneous. The effects of temperature on spontaneity are summarized in Table 14-3. 

Sharf, R. and Margalith, P. 1983. The effect of temperature on spontaneous wine fermentation. Appl. Microbiol. Biotechnol. 17, 311—313. 

Predict the effect of temperature on spontaneity given A//and A5. (Section 19.6)... 

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