Reversible reaction heat

The reversible reaction heat of the cell is defined as the reaction entropy multiplied by the temperature [Eq. (15)]. For an electrochemical cell it is also called the Peltier effect and can be described as the difference between the reaction enthalpy AH and the reaction free energy AG. If the difference between the reaction free energy AG and the reaction enthalpy AH is below zero, the cell becomes warmer. On the other hand, for a difference larger than zero, it cools down. The reversible heat W of the electrochemical cell is therefore ... 

A solid oxide fuel cell is an electrochemical device which converts the Gibbs free enthalpy of the combustion reaction of a fuel and an oxidant gas (air) as far as possible directly into electricity. Hydrogen and oxygen are used to illustrate the simplest case. This allows the calculation of the reversible work for the reversible reaction. Heat must be transferred reversibly as well to the surrounding environment in this instance. 

A benzene ring can be sulfonated with fuming or concentrated sulfuric acid. Sulfonation is a reversible reaction heating benzenesulfonic acid in dilute acid removes the sulfonic acid group. The principle of microscopic reversibility states that the mechanism of a reaction in the reverse direction must retrace each step of the mechanism in the forward direction in microscopic detail. 

This reaction can be reversed by heat and the potassium carbonate and carbon dioxide recovered. (Other compounds which absorb carbon dioxide and evolve it again at a lower temperature are also in common usage" ). 

This reaction can be reversed by heating and is a convenient method of obtaining anhydrous hydrogen fluoride from an aqueous solution. 

Tripolyphosphates. The most commercially important tripolyphosphate salt is sodium tripolyphosphate (STP), Na P O Q. Three distinct crystalline forms are known two are anhydrous (STP-I and STP-II) the other is the hexahydrate [15091 -98-2] Na P O Q 6H20. Sodium tripolyphosphate anhydrous Form I is the high temperature, thermodynamically stable phase sodium tripolyphosphate anhydrous Form II is the lower temperature form which can be readily converted to STP-I by heating to above 417 8° C, the transition temperature. However, the reverse reaction is extremely slow below 417°C. Both anhydrous forms of sodium tripolyphosphate are therefore stable enough to coexist at room temperature. 

Of the alkaline-earth carbonates, BaCO requires the greatest amount of heat to undergo decomposition to the oxide. Thus carbon in the form of coke, tar, or carbon black, is added to the carbonate to lower reaction temperature from about 1300°C in the absence of carbon to about 1050°C. The potential for the reverse reaction is decreased by removing the CO2 as shown in equation lb. 

Hydrogen chloride or a few drops of hydrochloric acid cataly2e the conversion of //-butyraldehyde iato the trimer, parabutyraldehyde, C22H24O2, (2,4,6-tripropyl-l,3,5-trioxane [56769-26-7] (1). The reaction is reversed by heating the parabutyraldehyde ia the presence of acid. Anhydrous hydrogen chloride at —40°C converts //-butyraldehyde iato l,l -dichlorodibutyl ether, (2) ia 70—75% yield (10). 

CSTBs—minimum volume of battery, maximum yield, optimal temperature for reversible reaction, minimum total cost, reactor volume with recycle, maximum profit for reversible reaction with recycle, and heat loss... 

Methanol synthesis will be used many times as an example to explain some concepts, largely because the stoichiometry of methanol synthesis is simple. The physical properties of all compounds are well known, details of many competing technologies have been published and methanol is an important industrial chemical. In addition to its relative simplicity, methanol synthesis offers an opportunity to show how to handle reversible reactions, the change in mole numbers, removal of reaction heat, and other engineering problems. 

In the case of thermodynamics, the designer can investigate the nature of the reaction heat and whether the reaction is reversible. If these exothermic reactions are irreversible, attention may be focused on the influence of reactor design on conversion and with heat transfer control. An objective of reactor design is to determine the size and type of reactor and mode of operation for the required job. The choice... 

Thiooarbamide.—This is an example of a reversible reaction, in which either ammonium-thiocyanate or thiouiea when heated yields the same equilibiium mi.xttue. It may bo shown by melting a little thiourea for a minute, when the presence of thiocyanate is indicated by the addition of P eCl,. 

The reverse reaction to give the gaseous species AlX(g) at high temperature accounts for the enhanced volatility of AIF3 when heated in the presence of A1 metal, and the ready volatilization of A1 metal in the presence of AICI3. Using calculations of the type outlined on p. 82 the standard heats of formation of the crystalline monohalides AIX and their heats of disproportionation have been estimated as ... 

AH for a reaction is equal in magnitude but opposite in sign to AH for the reverse reaction. Another way to state this rule is to say that the amount of heat evolved in a reaction is exactly equal to the amount of heat absorbed in the reverse reaction. This again is a common-sense rule. If 6.00 kj of heat is absorbed when a mole of ice melts,... 

Now we wish to obtain reaction (5) by combining reactions (6) and (7). Since NO is a reactant in reaction (5), we need the reverse of reaction (6). We obtain the heat of the reverse reaction merely by changing the algebraic sign of AHe. If 21.6 kcal of heat are absorbed when one mole of NO is formed, then 21.6 kcal of heat will be released when one mole of NO is decomposed in the reverse reaction ... 

The relationship between activation energies for the forward and reverse reactions can be expressed mathematically. The activation energy is denoted by the symbol A// (read delta-//-cross ) and the heat of the reaction by AH. Hence we may write ... 

An example of a reversible reaction in the liquid phase is afforded by the esterification reaction between ethanol and acetic (ethanoic) acid forming ethyl acetate and water. Since, however, ethyl acetate undergoes conversion to acetic acid and ethanol when heated with water, the esterification reaction never proceeds to completion. 

The heating rate has only a small effect when a fast reversible reaction is considered. The points of inflexion B and C obtained on the thermogravimetric curve for copper sulphate pentahydrate (Fig. 11.2) may be resolved into a plateau if a slower heating rate is used. Hence the detection of intermediate compounds by thermogravimetry is very dependent upon the heating rate employed. 

Acyl-3.4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes 1, on heating at 250-280 C for a short time without solvent, rearrange to the 1-acyl-1-benzazepines 2 (Method A).23-38 In some cases, rearrangement is accompanied by minor amounts of Ar-aeyl-l-naphthylamine and, at higher temperatures, the acylnaphthylatnine can become the major product (see Section 3.2.2.6.). In the presence of silver(I) tetrafluoroborate (Method B) rearrangement takes place at lower temperatures but the yields of benzazepine are inferior as the silver(I) ion also catalyzes the reverse reaction (see Section 3.2.2.1.). 

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