Exothermic Precipitation Reactions

Problem If, in addition to the iron nail experiment, the reaction of iron wool or magnesium powder with the blue copper sulfate solution is shown, the heat of reaction of the exothermic reaction can be determined. On the other hand, the blue solution discolors in this reaction so that, with the disappearance of Cu2+(aq) ions, the discussion of the formation of Cu atoms from the appropriate ions could be expanded. 

Material Test tubes, thermometer iron wool, magnesium powder, copper sulfate solution. 

Procedure Fill two test tubes to one-third with copper sulfate solution. Add iron wool to the first test tube and magnesium powder to the second one. Determine the temperature before and after both reactions by hand or using a thermometer. 

Observation The solutions get quite warm the metals are covered with a red coating copper. The color of the solution changes from blue to colorless mixture of copper sulfate and silver sulfate solution. 

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Because qrxn is a negative quantity, the precipitation reaction is exothermic. 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

The deposited material in VINITI (No. 6103-73) contains a table of the enthalpies of the precipitation reaction but no primary experimental data. These enthalpies carry a positive sign, which contradicts the findings in [69SUP/MA1] for neodymium that is common to both investigations. Since the primary experimental data there show that the precipitation reaction is exothermic, the review has changed the sign of A //... 

A SOLUTION of 250 g. (1.51 moles) of chloral hydrate (Note i) in 450 cc. of warm water (50-60°) is placed in a 3-I. round-bottomed flask bearing a reflux condenser and thermometer (Note 2). The condenser is temporarily removed and 152.5 g. (1.52 moles) of precipitated calcium carbonate added this is followed by 2 cc. of amyl alcohol (Note 3) and a solution of 10 g. of sodium cyanide (Note 4) in 25 cc. of water. Althou the reaction is exothermic, the reaction mixture is heated with a low flame so that it reaches 75° in about ten minutes at this point heating is discontinued. The temperature continues to rise to 80-85° during five to ten minutes and then drops. As soon as the temperature begins to fall the solution is heated to boiling and refluxed for twenty minutes. The mixture is then cooled to 0-5° in an ice bath, acidified with 215 cc. of concentrated hydrochloric acid (sp. gr. 1.18) and extracted with five loo-cc. portions of ether (Note 5). The combined ether extracts are dried with 20 g. of anhydrous sodium sulfate, the ether is removed by distillation from a steam bath, and the residue distilled in vacuum from a Claisen flask with a fractionating side... 

In this experiment, you will carry out three simple aqueous chemical reactions a precipitation reaction, an oxidation-reduction reaction, and a compiexation reaction. For the precipitation reaction the product is easily separated as a solid, and the mass of the solid is determined after it is filtered and dried. For the exothermic oxidation-reduction reaction the change of temperature provides a measure of the heat evolved and thus of the quantity of product formed. For the compiexation reaction the absorbance of visible light by the product provides a measure of the quantity of product formed. 

Fit a 1500 ml. bolt-head flask with a reflux condenser and a thermometer. Place a solution of 125 g. of chloral hydrate in 225 ml. of warm water (50-60°) in the flask, add successively 77 g. of precipitated calcium carbonate, 1 ml. of amyl alcohol (to decrease the amount of frothing), and a solution of 5 g. of commercial sodium cyanide in 12 ml. of water. An exothermic reaction occurs. Heat the warm reaction mixture with a small flame so that it reaches 75° in about 10 minutes and then remove the flame. The temperature will continue to rise to 80-85° during 5-10 minutes and then falls at this point heat the mixture to boiling and reflux for 20 minutes. Cool the mixture in ice to 0-5°, acidify with 107-5 ml. of concentrated hydrochloric acid. Extract the acid with five 50 ml. portions of ether. Dry the combined ethereal extracts with 10 g. of anhydrous sodium or magnesium sulphate, remove the ether on a water bath, and distil the residue under reduced pressure using a Claiseii flask with fractionating side arm. Collect the dichloroacetic acid at 105-107°/26 mm. The yield is 85 g. 

A sodium stannite solution was prepared by addition of aqueous sodium hydroxide (2.5 mol, lOOg) to aqueous stannous chloride (0.25 mol, 56g). The initially formed precipitate redissolved to form a clear solution. This solution was gradually added to a solution of 16.3g (0.1 mol) phenyl-2-nitropropene in THF at room temperature. A slightly exothermic reaction ensued, and the reaction mixture was stirred for 30 min, a saturated sodium chloride solution was added, and the solution was extracted with ether and the pooled extracts were evaporated under vacuum to give essentially pure P2P oxime in 80% yield. 

To a solution of 0.5D mol of KO-tert.-C Hg (uncomplexed base, commercially available) and 180 ml of dry THF was added at room temperature 0.25 mol of the Mannich product. A weakly exothermic reaction took place. The mixture was heated in a bath at 50°C and the THF started to reflux (occasional cooling in an ice-water bath may be necessary). When the refluxing had ceased, the mixture was heated for 30 min in a bath at 70°C, then cooled to room temperature and 300 ml of redistilled, dry pentane were added. The precipitate of potassium raethoxide and... 

Trimethyl chi orosilane (0.35 mol) was added in 20 min with vigorous stirring. A thick precipitate was formed and refluxing of the diethyl ether started after a few minutes. When the exothermic reaction had subsided, the mixture was heated under reflux for 1 h. Ice-water (400 ml) was then added with vigorous stirring. 

In production, anhydrous formaldehyde is continuously fed to a reactor containing well-agitated inert solvent, especially a hydrocarbon, in which monomer is sparingly soluble. Initiator, especially amine, and chain-transfer agent are also fed to the reactor (5,16,17). The reaction is quite exothermic and polymerisation temperature is maintained below 75°C (typically near 40°C) by evaporation of the solvent. Polymer is not soluble in the solvent and precipitates early in the reaction. 

Concentrated sulfuric acid (97 wt %) at 300—400°C has been used to solubili2e niobium from columbite and pyrochlore (18,19). The exothermic reaction is performed in iron or siUcon-iron cmcibles to yield a stable sulfato complex. The complex is filtered free of residue and is hydroly2ed by dilution with water and boiling to yield niobic acid which is removed by filtration as a white coUoidal precipitate. 

Care is required ia miming these reactions because the decomposition of the intermediate sulfoxide and of dimsyl sodium during the heating in the strongly alkaline system is exothermic and also produces a precipitate which can interfere with heat removal. Explosions have occurred (51). 

The reaction of the hydriodic acid with w-nitrobenzene-sulfonyl chloride is mildly exothermic, and iodine crystals precipitate as the reaction proceeds. 

Trimethyl borate (113 ml) is added dropwise to 66 ml of deuterium oxide (exothermic reaction). The mixture is allowed to stand for 1/2 hr and the precipitated boric acid-d3 is removed by filtration. Sodium methoxide is... 

I. 2,5-Dicarbethoxy-1,4-cyclohexanedione (7) A 500-ml, three-necked, round-bottom flask is fitted with a condenser (drying tube) and arranged for magnetic stirring. A solution of sodium ethoxide is prepared in the flask by the addition of sodium (9.2 g, 0.4 g-atom) in small pieces to 90 ml of absolute ethanol. The mixture is heated at reflux for 3 hours (oil bath) to complete the reaction. Diethyl succinate (34.8 g, 0.2 mole) is added to the hot solution in one portion exothermic ) and the mixture is refluxed for 24 hours. (A pink precipitate forms and persists during the reflux.)... 

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