Endothermic reactions spontaneity

There is still another basic objection to using the sign of AH as a general criterion fir spontaneity. Endothermic reactions that are nonspontaneous at room temperature often become spontaneous when the temperature is raised. Consider far example, the decomposition of limestone ... 

At 25°C and 1 atm, this reaction does not occur. Witness the existence of the white cliffs of Dover and other limestone deposits over eons of time. However if the temperature is raised to about 1100 K, limestone decomposes to give off carbon dioxide gas at 1 atm. In other words, this endothermic reaction becomes spontaneous at high temperatures. This is true despite the fact that AH remains about 178 kj, nearly independent of temperature. 

Some endothermic reactions proceed spontaneously. One example is the evaporation of... 

FIGURE 7.19 An endothermic reaction is spontaneous only if the entropy of the system increases enough to overcome the decrease in entropy of the surroundings. 

For an endothermic reaction (AH° > 0) with a positive reaction entropy (AS° > 0 ), the reverse is true (Fig. 7.28b). In this case, AG° is positive at low temperatures but may become negative when the temperature is raised to the point that TAS° becomes larger than AH°. The formation of products from pure reactants becomes spontaneous when the temperature is high enough. 

FIGURE 7.28 The effect of an increase in temperature on the spontaneity of a reaction under standard conditions. In each case, "spontaneous" is taken to mean AC0 < 0 and "nonspontaneous" is taken to mean AC° > 0. (a) An exothermic reaction with negative reaction entropy becomes spontaneous below the temperature marked by the dotted line, (b) An endothermic reaction with a positive reaction entropy becomes spontaneous above the temperature marked by the dotted line, (c) An endothermic reaction with negative reaction entropy is not spontaneous at any temperature, (d) An exothermic reaction with positive reaction entropy is spontaneous at all temperatures. 

EXAMPLE 7.16 Calculating the temperature at which an endothermic reaction becomes spontaneous... 

C. At that temperature some endothermic reactions take place that at lower temperatures are not spontaneous under standard conditions (pure substances, each at I bar). Estimate the temperatures at which the following stratospheric reactions become spontaneous under standard conditions. Which ones could occur during re-entry of the space shuttle (a) The formation of nitrogen monoxide from nitrogen and oxygen ... 

Rain unaffected by human activity contains mostly weak acids and has a pH of 5.7. The primary acid present is carbonic acid, H2C03, a weak acid that results when atmospheric carbon dioxide dissolves in water. The major pollutants in acid rain are strong acids that arise from human activities. Atmospheric nitrogen and oxygen can react to form NO, but the endothermic reaction is spontaneous only at the high temperatures of automobile internal combustion engines and electrical power stations ... 

Heat energy must be supplied continuously to keep the temperature from falling while the endothermic reaction proceeds. Otherwise, the temperature would quickly fall below the minimum value at which H2 synthesis is spontaneous. For this reason, the production of H2 (and ultimately of fertilizer) requires considerable amounts of... 

In either event, the products are ZnCl 2, CO, and A12O 3 The zinc oxide cools and whitens the smoke by consuming atomic carbon in an endothermic reaction that occurs spontaneously above 1000°C (equation 8.6). The reaction with aluminum (equation 8.4 or 8.8) is quite exothermic, and this heat evolution controls the burning rate of the smoke mixture. A minimum amount of aluminum metal will yield the best white smoke. Several "HC" smoke compositions are listed in Table 8.4. 

Strategy The reaction becomes spontaneous when the reaction free, energy, AG° = AH° — TASr°, becomes negative. An endothermic reaction l therefore becomes spontaneous at temperatures above which AG° = 0 that " is, the reaction is spontaneous for temperatures above which l AH° — TAS° = 0. The reaction free energy changes sign at... 

Comment on the following statement Exothermic reactions are spontaneous, but endothermic reactions are nonspontaneous. 

However, there are numerous endothermic reactions that occur spontaneously. Thus energy alone then is not sufficient. 

B. An endothermic reaction that increases the order of the system cannot be spontaneous. 

Although the majority of spontaneous reactions are exothermic and do lead to a decrease in the enthalpy there are a minority of endothermic reactions which are spontaneous (see Note 13.1). 

Chemical reactions, like the metathesis reaction that produces lead (II) iodide, can occur spontaneously, just like physical processes. It was once believed that only exothermic processes occurred spontaneously however, it has been shown that many endothermic reactions can occur spontaneously as well. Another factor that must be considered when determining the spontaneity of a reaction is entropy. 

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