Exothermic dissolution reaction

The example of MgO represents most of the candidate oxides because they have negative ASq and exhibit an exothermic dissolution reaction. Exceptions include oxides of the monovalent alkaline metals such as Na, K, and Cs, and a few higher valent metals. Since the hydroxyl ion also has a negative ASq, the dissolution reactions of the MgO-like oxides satisfy AS < 0 and conform to the third condition and hence, these oxides will exhibit nonspontaneous dissolution at low temperatures but may be spontaneous at high temperatures. 

Eq. 6.27 implies that K will increase or decrease with temperature depending on the sign of AH, i.e., if AH > 0 (endothermic dissolution reaction), K will increase, while for AH < 0 (exothermic dissolution reaction), K will decrease. 

Mercuric Nitrate. Mercuric nitrate [10045-94-0] Hg(N02)2, is a colorless dehquescent crystalline compound prepared by the exothermic dissolution of mercury in hot, concentrated nitric acid. The reaction is complete when a cloud of mercurous chloride is not formed when the solution is treated with sodium chloride solution. The product crystallizes upon cooling. Mercuric nitrate is used in organic synthesis as the starting material and for the formulation of a great many other mercuric products. 

The chloride was to be purified by dissolution in dry acetone, but in error, acetone containing 40% of water was used. The acid-catalysed exothermic hydrolysis reaction of the chloride accelerated to runaway, and gas and vapour evolution ruptured the vessel, leading to fire and explosion. 

For divalent oxides, the dissolution reaction is exothermic, and hence K decreases as the temperature increases. As we shall see later, oxides such as MgO conform to this behavior in the acidic region. For AI2O3, however, K initially increases and then decreases thus there is a Aimaximum at a certain temperature. That temperatme, T, may be calculated as follows ... 

Bulk aluminum may undergo the following dangerous interactions exothermic reaction with butanol, methanol, 2-propanol, or other alcohols, sodium hydroxide to release explosive hydrogen gas. Reaction with diborane forms pyrophoric product. Ignition on contact with niobium oxide + sulfur. Explosive reaction with molten metal oxides, oxosalts (nitrates, sulfates), sulfides, and sodium carbonate. Reaction with arsenic trioxide + sodium arsenate + sodium hydroxide produces the toxic arsine gas. Violent reaction with chlorine trifluoride, Incandescent reaction with formic acid. Potentially violent alloy formation with palladium, platinum at mp of Al, 600°C. Vigorous dissolution reaction in... 

Solubility is temperature dependent. By Le Chatelier s principle it can either increase or decrease with increasing temperature depending on whether the dissolution reaction is endothermic or exothermic. 

The intensification of the heat transfer through stirring is seldom the primary stirring task. It mostly occurs in connection with the intensification of mass transfer by stirring (dispersion of gases or hquids with a simultaneous exothermic chemical reaction, dissolution of solids with liberation of the latent heat of solution etc.). Occasionally the stirring heat has to be removed, so that the process can proceed under isothermal conditions. 

Cyclopropancs. A soln. of n-butyllithium in hexane added slowly at -105° during 45 min. to a soln. of benzotrichloride in tetrahydrofuran, then tetra-methylethylene added, the resulting slurry allowed to warm slowly whereupon at ca. -65° complete exothermic dissolution occurs 1-chloro-l-phenyltetra-methylcyclopropane. Y 72%. F. e. and reactions s. D. F. Hoeg, D. I. Lusk, and A. L. Grumbliss, Am. Soc. 87, 4147 (1965) O. Kobrich and W. Drischel, Tetrahedron 22, 2621 (1966). 

The appropriate reaction enthalpy is the solution enthalpy. In case of an endother-mal solution enthalpy, the equilibrium shifts with higher temperature to higher solubilities, and thus results in a positive slope of the solubility curve. On the other hand, an exothermal dissolution process leads to a retrograde solubility behavior. In most cases, the heat of solution is positive, that is, most substances dissolve with absorption of heat. This means that the (positive) lattice enthalpy exceeds the (negative) solvation enthalpy or more energy has to be spent to break the lattice than is evolved by solvation. 

Metals in higher oxidation states form halides which are essentially covalent, for example AICI3, SnCl, FeClj when these compounds dissolve in water they do so by a strongly exothermic process. Indeed it is perhaps incorrect to think of this only as a dissolution process, since it is more like a chemical reaction—but to differentiate for a particular substance is not easy, as we shall see. The steps involved in the case of aluminium chloride can be represented as... 

Potassium siUcates are manufactured in a manner similar to sodium siUcates by the reaction of K CO and sand. However, crystalline products are not manufactured and the glass is suppHed as a flake. A 3.90 mole ratio potassium siUcate flake glass dissolves readily in water at ca 88°C without pressure by incremental addition of glass to water. The exothermic heat of dissolution causes the temperature of the solution to rise to the boiling point. Lithium sihcate solutions are usually prepared by dissolving siUca gel in a LiOH solution or mixing colloidal siUca with LiOH. 

Pulverized maleic anhydride (6 g, 0.061 mole) is dissolved in 20 ml of ethyl acetate by gentle heating on a steam bath. Petroleum ether (20 ml) is added slowly to the solution, which is then cooled in an ice bath. To the cold solution is added 4.8 g (6 ml, 0.073 mole) of cyclopentadiene, and the resulting solution is swirled until the exothermic reaction subsides and the product separates. Recrystallization may be carried out in the reaction solvent by heating until dissolution occurs (steam bath) followed by slow cooling. The product has mp 164-165°, yield about 80%. 

Dimethy/su/famy/thioxanthene To a slurry of dry sodium thioxanthene-2-sulfonate (33.3 grams, 0.111 mol) in 50 ml of N,N-dimethylformamide was added thionyl chloride (14.3 grams, 0.122 mol) in divided portions. An exothermic reaction ensued with complete dissolution being effected in minutes. Treatment of the reaction mixture with crushed ice precipitated a gum which crystallized after a short period of stirring. The sulfonyl chloride was filtered, washed with water, and stirred with 100 ml of liquid dimethylamine. 

A process that releases heat into the surroundings is called an exothermic process. Most common chemical reactions—and all combustions, such as those that power transport and heating—are exothermic (Fig. 6.8). Less familiar are chemical reactions that absorb heat from the surroundings. A process that absorbs heat is called an endothermic process (Fig. 6.9). A number of common physical processes are endothermic. For instance, vaporization is endothermic, because heat must be supplied to drive molecules of a liquid apart from one another. The dissolution of ammonium nitrate in water is endothermic in fact, this process is used in instant cold packs for sports injuries. 

Autoclave reactions involving ethylene oxide with alkanethiols or an (unspecified) alcohol went out of control and exploded violently. Similar previous reactions had been uneventful [1], An arenethiol was being reacted with ethylene oxide under catalysis by a fraction of a percent of sodium hydroxide (solid) dissolved in the thiol to which the oxirane was slowly charged. After an initial exotherm a white solid precipitated, the exotherm died away and later resumed, with dissolution of the solid, the reaction then running out of control from the backlog of charged oxirane [2],... 

Subsequent DTA investigation showed that an exothermic reaction set in above 75°C after an induction period depending on the initial temperature and concentration of reactants, which attained nearly 300°C, well above the decomposition point of the cyclic ester component (170°C). The reaction conditions used could have permitted local over-concentration and overheating effects to occur, owing to slow dissolution of the clumped solid ester and aniline in the nitrobenzene solvent [ ] Crude carbyl sulfate contains excess sulfur trioxide [2]. 

A solution of 64.9 g. (I mole) of 86.5% potassium hydroxide (Note I) in 28 ml. of water is cooled in an ice bath, saturated with hydrogen sulfide, and flushed with nitrogen to ensure complete removal of excess hydrogen sulfide (Notes 2 and 3). The freshly prepared potassium hydrosulfide solution is diluted with 117 ml. of water and stirred under nitrogen at 55-60°. Then 95.3 g. (0.5 mole) of finely ground tosyl chloride (Note 3) is introduced in small portions at a uniform rate so that the reaction temperature is maintained at 55-60° (Note 2). A mildly exothermic reaction ensues, and the solution becomes intensely yellow. After about 90 g. of tosyl chloride has been introduced, the yellow color disappears, and the dissolution of the chloride ceases. The reaction mixture is rapidly filtered with suction through a warmed funnel, and the filtrate is cooled several hours at 0-5°. The crystals of potassium... 

Prior to starting the reaction, 1.5 equivalents (relative to the amount of alcohol 2 to be used) each of carbon tetrabromide and triphenylphosphine were weighed and set aside. An additional 0.5 equivalents of each was also weighed and set aside. Alcohol 2 (37.93 g, 0.118 mol) dissolved in 50 mL of THF was placed in a 250 mL round bottom flask containing a magnetic stirrer and the solution was stirred while carbon tetrabromide (1.5 equiv.) (58.91 g, 0.178 mol) was added. Once dissolution was complete, triphenylphosphine (1.5 equiv.) (42.95 g, 0.178 mol) was added quickly but portionwise (i.e. - a large, full spatula at a time) as the reaction is very exothermic. The solution turned to a yellowish color, which usually occurs when the reaction is near completion. Monitoring by TLC (1% methanol / dichloromethane) confirmed that the reaction was indeed complete. 

Because dissolution of the salts is a highly exothermic process, the water should be added slowly. A stream of nitrogen may be passed through the reaction during the addition of the water to ensure that no fire is started by bits of sodium that may be adhering to the upper walls of the flask. 

Caution. The dissolution of sodium metal in mercury is an exothermic reaction therefore sodium must be added in small pieces. 

A mixture of triphenyl phosphite (0.1 mol), aldehyde (0.15 mol), ZNH2 (0.1 mol), and glacial AcOH (15 mL) was stirred for ca. 1 h until the exothermic reaction subsided. The mixture was heated at 80-85 °C for 1 h and volatile products were removed in vacuo. The oily residue was dissolved in MeOH (180 mL) and left for crystallization at —10 °C. After 1-3 h, the crystalline ester was collected by filtration and recrystallized by dissolution in the minimum amount of hot CHC13 (30-40 mL) and addition of a fourfold volume of MeOH. 

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