Exothermic reactions spontaneity

Regeneration with air can be done with continuous or periodic addition of small amounts of air. Both must be done carefully because of exothermic reaction. Regeneration is never complete, so the beds must be eventually changed out. This must be done carefully because of the pyrophoric (spontaneously combustible) nature of the iron sulfide. The entire bed is wetted first. 

Decomposition reactions are a special class of propagating flames where a molecule can undergo spontaneous exothermic reaction. The most widely reported incidents in which decomposition reactions occur are for acetylene where decomposition primarily gives carbon and hydrogen, as shown in the following reaction ... 

A suspension of 40 g 3-acetylaminomethyl-5-amino-2,4,6-triodobenzoic acid in 180 ml acetic anhydride were mixed with 0.4 ml concentrated sulfuric acid. An exothermic reaction was thereby initiated. Acetylation was completed by heating to 80°C for three hours. The reaction mixture was then evaporated to dryness in a vacuum at a temperature not exceeding 50°C. The residue was treated with a mixture of 30 ml concentrated aqueous ammonium hydroxide and 40 ml water, whereby the solid material dissolved with spontaneous heating. Within a few minutes, the ammonium salt of the acetylated product started precipitating. The precipitate and residual liquid were cooled externally with ice after about 15 minutes. The salt was separated from the liquid by filtration with suction, and was washed with ice cold saturated ammonium chloride solution. 

A negative value of AH. Exothermic reactions (AH < O) tend to be spontaneous, inasmuch as they contribute to a negative value of AG. On the molecular level, this means that there will be a tendency to form strong bonds at the expense of weak ones. 

It is more common to find that AH° and AS° have the same sign (Table 17.2, III and IV). When this happens, the enthalpy and entropy factors oppose each other. AG° changes sign as temperature increases, and the direction of spontaneity reverses. At low temperatures, AH° predominates, and the exothermic reaction, which may be either the forward or the reverse reaction, occurs. As the temperature rises, the quantity TAS° increases in magnitude and eventually exceeds AH°. At high temperatures, the reaction that leads to an increase in entropy occurs. In most cases, 25°C is a low temperature, at least at a pressure of 1 atm. This explains why exothermic reactions are usually spontaneous at room temperature and atmospheric pressure. 

This situation has some similarities to the chemical change in a spontaneous, exothermic reaction. The reactants of high heat content react spontaneously to form products of lower heat content. As each molecular reaction occurs, the excess heat content becomes kinetic energy. The product molecules separate from each other with high kinetic energy. As they collide with... 

Another difficulty is that spontaneous chemical reactions do not go to completion. Even if a spontaneous reaction is exothermic, it proceeds only till it reaches equilibrium. But in our golf ball analogy, equilibrium is reached when all of the golf balls are on the lower level. Oui analogy would lead us to expect that an exothermic reaction would proceed until all of the reactants are converted to products, not to a dynamic equilibrium. 

In an exothermic reaction, such as the synthesis of ammonia or a combustion reaction, the heat released by the reaction increases the disorder of the surroundings. In some cases, the entropy of the system may decrease, as when a gaseous reactant is converted into a solid or liquid. However, provided that AH is large and negative, the release of energy as heat into the surroundings increases their entropy so much that it dominates the overall change in entropy and the reaction is spontaneous (Fig. 7.18). 

FIGURE 7.18 In an exothermic reaction, (a) thp overall entropy change is certainly positive when the entropy of the system increases. (b)The overall entropy change may also be positive when the entropy of the system decreases. The reaction is spontaneous in both cases. 

For an exothermic reaction (AH0 < 0) with a negative reaction entropy (AS° < 0), — TAS° contributes a positive term to AG°. For such a reaction, AG° is negative (and the pure reactants are poised to form products spontaneously) at low temperatures because AH° dominates — TAS°, but it may become positive (and the reverse reaction, the decomposition of pure products, spontaneous) at higher temperatures when —TAS° dominates AH° (Fig. 7.28a). 

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. 

Exothermic reactions increase disorder by releasing heat into the surroundings. As a result, strongly exothermic reactions are spontaneous even if the change in entropy for the reacting system is itself negative. 

Potentially hazardous reactions. Bretherick (1990) used a few general types of potentially hazardous reactions to classify the majority of exothermic reactions involving two or more components. By far the most common type is that involving an oxidant and an oxidizable material. The most common oxidant is air. Some materials will react so rapidly with air that ignition occurs. spontaneously. Finely divided metals or metal hydrides, or fully alkylated... 

Many exothermic reactions are spontaneous. A critical question facing chemists in the late eighteenth century was how to tell spontaneous reactions from nonspontaneous ones without performing an experiment. What characteristics must the reactants have to proceed without the prod of added energy In other words, what drives chemical reactions ... 

Boron reacts with sulfur at 600°C becoming incandescent [1]. Mixtures of sulfur with lamp black or freshly calcined charcoal ignite spontaneously, probably owing to adsorbed oxygen on the catalytic surface [2], Mixtures of yellow phosphorus and sulfur ignite and/or explode on heating [3], Ignition of an intimate mixture of red phosphorus and sulfur causes a violent exothermic reaction [4],... 

All of these reagents also undergo reaction spontaneously with acyl halides to generate the corresponding acyl-phosphoryl products (Figure 3.4). Reaction is easily accomplished by the slow addition of the acyl halide to the stirred phosphorus reagent, upon which an exothermic reaction ensues. This approach has been extensively exploited for the preparation of a wide range of a-ketophospho-nates.31-52... 

B We expect the value of the equilibrium constant to increase as temperature decreases since this is an exothermic reaction and it should become more spontaneous (shift right) at lower temperatures.Thus, we expect to be larger than 1000, which is its value at 4.3 x 102 K. 

L (a) An exothermic reaction (one that gives off heat) may not occur spontaneously if, at the same time, the system becomes more ordered, that is, AS0 < 0. This is particularly true at a high temperature, where the TAS term dominates the AG expression. An example of such a process is freezing water (clearly exothermic because the reverse process, melting ice, is endothermic) at temperatures above 0 °C. 

A process is described [224] in which an exothermic reaction takes place in a semi-batch reactor at elevated temperatures and under pressure. The solid and liquid raw materials are both toxic and flammable. Spontaneous ignition is possible when the reaction mass is exposed to air. Therefore, the system must be totally enclosed and confined in order to contain safely any emissions arising from the loss of reactor control, and to prevent secondary combustion reactions upon discharge of the materials to the atmosphere. Further, procedures and equipment are necessary for the safe collection and disposal of solid, liquid, and gaseous emission products. 

The total energy of a system involves both enthalpy and entropy. Thus, whichever causes the greater change in overall energy during the reaction will be the one controlling the reaction and determining whether it is exothermic, endothermic, spontaneous, or not spontaneous [11]. 

The combustion of gasoline is a fast and highly exothermic reaction. Gasoline is stored in a car s fuel tank, where it is exposed to oxygen in the air. Why does gasoline in a fuel tank not burst into flames spontaneously ... 

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