Exercise 8.1 Hydration Reactions

The computed value for AH is in excellent agreement with the experimental value. We ll look at two similar hydration reactions in Exercise 8.1. 

Students are learning about equilibrium in a chemistry laboratory exercise. The relevant materials include an aqueous solution of C0CI2, concentrated HCI, water, a hot water bath, an ice-water bath, and all necessary glassware. CoCI2 dissociates in water into Cl- ions and Co2+ ions which form the hydrated complex [Co(H20)6]2+. This ionic complex turns the solution pink. When HCI is added, the additional Cl reacts with [Co(H20)6]2+ in a mildly endothermic reversible reaction to form [C0CI4]2-. This ion turns the solution blue. 

Steric factors in the ion-exchanger phase present a complexation depressing effect by restricting rotational and translational motion. This reduces the number of collisions leading to association. The rate of the reverse reaction, hydrolytic dissociation of associated species is, however, not necessarily decreased because of lowered or less ordered hydration of the species in the ion-exchanger phase. In any case, the kinetic aspects seem to exercise a net negative effect on the complexation tendencies. The increase in ion interaction due to the lower medium dielectric constant more than compensates for the above. As a consequence, the lowering in effective collision frequency may become important only when very weak complexes are involved. 

Preparation of Crude Tris(ethylenediamine)platinum(IV) Chloride. Ten and one-tenths grams (0.019 mol, assuming the 6-hydrate) of hexachloroplatinic(IV) acid is dissolved in 80 ml. of absolute ethanol. After cooling this solution in an ice bath, 6.6 ml. of 98% ethylenediamine [5 mols of ethyl-enediamine per mol of hexachloroplatinic(IV) acid] is slowly added with constant stirring. A pale yellow precipitate forms immediately. The mixture is heated to 65° and is maintained at 65 1° in a water bath for 2 hours. Care must be exercised not to heat the reaction mixture above 70° inasmuch as a brown gum occasionally forms at higher temperatures. The mixture is stirred constantly during the digestion period. Absolute ethanol is added as required to keep the volume of the reaction mixture constant. The mixture is allowed to cool to room temperature, and the tris(ethylenediamine)platinum(IV) chloride is separated from the ethanol solution by suction filtration. The crude yellow product is washed twice with absolute ethanol. The yield of the crude wet product is about 17 g. 

CAUTION All reactions involving hydrazine, its hydrates, or its salts are extremely hazardous. Hydrazine, its salts, and its hydrates are toxic and considered cancer suspect agents. Extreme care should be exercised in working with these and related compounds and their derivatives. 

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