DETERMINATION OF THE WATER OF CRYSTALLISATION
April 4, 2012
Chemical compounds that contain discrete water molecules as part of their crystalline structure are called hydrates. Hydrates occur quite commonly among chemical substances, especially among ionic substances. More often than not, such compounds are either prepared in, or are recrystallized from, aqueous solutions. Hydrates exist for ionic compounds most commonly, but hydrates of polar and non polar covalent molecules are also known. In this experiment, you will study some of the properties and characteristics of several ionic hydrates, and then you will determine the percentage by weight of water in an unknown hydrate as well as its mole ratio of water to anhydrous salt. CHARACTERISTICS AND PROPERTIES OF THE HYDRATES
Hydrates are most commonly encountered in the study of metal salts, especially the salts of the transition metals. Water is bound in most hydrates in definite, stoichiometric proportions, and the number of water molecules bound per metal ion is often characteristic of a particular metal ion. A very common hydrate often encountered in the general chemistry laboratory is copper (II) sulfate pentahydrate, CuSO4.5H20. The word “pentahydrate” in the name of this substance indicates that five water molecules are bound in this substance per copper sulfate formula unit. Hydrated water molecules are generally indicated in formulas as shown above for the case of the copper sulfate, using a dot to separate the water molecules from the formula of the salt itself. Although not usually shown as such, you can think of this as CuSO4 (H2O)5. Many hydrated salts can be transformed to the anhydrous (without water) compound by heat. For example, if a sample of copper sulfate pentahydrate is heated, the bright blue crystals of the hydrate are converted to the white, powdery, anhydrous salt. CuSO4.5H2O(s) CuSO4(s) + 5H2O(g)
During the heating of copper sulfate pentahydrate, the water of crystallization is clearly seen escaping as steam from the crystals. It is also possible to reconstitute the hydrate of copper sulfate: if water is added to the white anhydrous salt, the solid will reassume the blue color of the hydrated salt. Anhydrous salts are sometimes used as chemical drying agents, or desiccants, because of their ability to combine with and remove water from their surroundings. For example, most electronic equipment (and even at least one brand of ground coffee) comes packed with small envelopes of a desiccant to protect the equipment from moisture. Substances that absorb moisture and are able to be used as desiccants are said to be hygroscopic. Not all hydrated salts are converted simply into the anhydrous compound when heated, however. Some hydrated metal salts will decompose upon losing the water of crystallization; subsequently they are usually converted to the metal oxide if the heating is carried out in air. Most covalent hydrates decompose rather than simply lose water when heated. The water molecules contained within the crystals of a hydrate may be bound by several different means. For the case in which water molecules are bound with a metal salt, generally a nonbonding pair of electrons on the oxygen atom of the water molecule forms a coordinate covalent bond with empty, relatively low energy d-orbitals of the metal ion. In the case of copper sulfate pentahydrate, for example, four of the five water molecules form such coordinate bonds with the copper (II) ion. In other situations, the water molecules of the hydrate may be hydrogen-bonded to one or more species of the salt. This is especially common for covalently bonded hydrates. SAFETY PRECAUTIONS
•Wear safety glasses and apron at all times while in the laboratory. •Copper, cobalt, nickel, chromium, and barium compounds are all highly toxic. Wash hands after use. •When you are heating the hydrated metal salts, they may spatter...