Loose compounds

When solutions containing only simple hydrated ions are required, salts of weakly coordinating cations (e.g. K ) or anions (e g. C104, BF4 ) must be used. 

Anhydrous hydrogen chloride is a gas. However, it dissolves in water to give solutions that are very like sodium chloride solutions. That is, they conduct electricity and undergo electrolysis, giving hydrogen at the cathode and chlorine at the anode. Their freezing points are also approximately twice those expected for the presence of HCl molecules. The solutions evidently contain hydrated and Cl ions. They will be discussed further in Chapter 17. 

Sodium dissolves in liquid ammonia. Dilute solutions are blue, concentrated ones are bronze-coloured. The solutions conduct electricity. The other alkali metals and the alkaline earth metals also dissolve in liquid ammonia, giving solutions of the same colour. 

Since sodium metal comprises Na ions and electrons (Chap. 5), the solutions may contain solvated Na ions and electrons. This is supported by the fact that other metals give solutions of the same colour, the colour being due to solvated electrons. The formation of the latter seems to be due to the slowness of the expected reaction  

The solutions are indeed metastable, and catalysts soon give NH2 + /4H2. 

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Berthollet thought that the gas now known as chlorine was a loose compound of hydrochloric acid and oxygen (112), or, to use his own words, that ... 

Some gases present in waste gases are recovered by scrubbing with absorbent chemicals that form loose compounds the absorbent then may be recovered for reuse by elevating the temperature or lowering the pressure in a regenerator. Such loose compounds may exert appreciable back pressure in the absorber, which must be taken into account when that equipment is to be sized. 

The mechanism is complex, but seems to involve the formation of a loose compound between methane and nitrogen oxide which decomposes, as the temperature rises, to yield formaldehyde. The formaldehyde partly decomposes to hydrogen and carbon monoxide and the nitrogen oxide reacts further until completely reduced to nitrogen. The hydrogen formed finally appears as water. 

There are two disc flow tests. Disc Flow I is a very simple test in which a measured amount of room-temperature loose compound is compressed between two heated die plates at a specific pressure and temperature. The resultant molded disc is measured for thickness to determine the flow the thicker the disc, the stiffer the flow. The Disc Flow II procedure is similar to I except that the mold cavity has five concentric rings. The diameter of the molded disc is measured the larger the disc, the softer the flow. 

Coordination compounds are considered in detail in Chapter 13 and loose compounds in Chapter 14. 

Not all salt hydrates are, of course, loose compounds. For many of them the association of at least some of the water molecules is much stronger. An instructive example is provided by the compound CrCl3-6H20, which exists in three forms. 

Loose compounds of the salt-molecule type seem to depend for their formation on the polarity of the molecular component, enabling it to be attracted electrostatically to the ions of the salt (positive poles to anions, negative to cations). This attraction can vary in strength, leading to the spectrum of types from loose compounds to coordination compounds indicated above. 

As discussed for salt-molecule solutions, the forces of attraction between the ions and the molecules cannot be trivial since the ions have to move away from each other to make room for the molecules, and this entails a loss of Coulombic energy. The formation of salt hydrates must be due to the high polarity of the water molecule, producing strong forces of attraction between ions and molecules. A loose compound is not so much one in which the bonding between components is weak, but one in which the net bonding is weak (i.e. the difference in energy between reactants and products is low). 

The distillation procedure described in connection t 4th the isolation of jormaldeh de for purposes of detection may also be employed for the quantitative isolation, of formaldehyde for analysis. Formaldehyde can be quantitath eh remo ed iroru aqueous solutions containing less than 10 per cent CHaO distillation or preferably by steam distillation. This treatment also h> drolyzes loose compounds of formaldehyde in many instances so that the combined formaldehyde can be isolated quantitativel3 ... 

The accuracy of these methods of separation is questionable in many instances and niust be tested in specific cases. Loose compounds are often converted to irre ersible reaction products under the influence of heat and long standing, or heating in the presence of alkalies results in the conversion of formaldehyde to formic acid and methanol by the Cannizaai o reaction. Method, emplo> ed for the an-ah sis of foimaldehyde in the presence of other compounds must be carefully chosen with reference to thek utility in the presence of the impurities in question. 

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