CHEM 212 Fall 2011
Dr. Miertschin
Hydrates
Introduction
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 and,
particularly, those of transition metals. 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 nonpolar covalent molecules are also known. 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 A5H2 O. The name and formula indicate that there are five water molecules per copper
sulfate formula unit. Hydrated water molecules are generally indicated in formulas as shown using a dot to
separate the water molecules from the formula of the salt.
Many hydrated salts can be transformed to the anhydrous (without water) compound by application of
heat. For example, if a sample of copper(II) sulfate pentahydrate is heated, the bright blue crystals of the hydrate
are converted to a white, powdery, anhydrous salt.
CuSO4 A5H2 O(s) 6 CuSO4 (s) + 5 H2 O(g)
During the heating of copper(II) sulfate pentahydrate, the waters of crystallization can be 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 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.
In this experiment, you will study some of the properties, both qualitative and quantitative, of several
ionic hydrates.
Safety and Waste
Goggles must be worn at all times. Solid CuSO4 A5H2 O(s) should be placed in a waster beaker for proper
disposal. The other salts can be disposed of in the trash or down the drain provided only very small amounts are
used. Confirm this by consulting the MSDS for each of the four salts used.
You will be using Bunsen burners. Take proper precautions when dealing with an open flame.
Exp. 5 Hydrates
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CHEM 212 Fall 2011
Dr. Miertschin
Materials
CuSO4A5H2O(s)
crucibles
NiCl2A6H2O(s)
test tubes
CoCl2A6H2O(s)
Bunsen burners
CrCl3A6H2O(s)
Procedure Notes
For the quantitative analysis, approximately 0.2 g CuSO4A5H2O(s) will be heated in a crucible. No lid is
necessary, but be sure to heat gently. The goal is to prove that there are five moles of water per anhydrous
copper sulfate formula unit.
For the qualitative analysis, all salts (except CuSO4A5H2O) will be heated in small test tubes utilizing very
small amounts, just enough to observe the color and appearance changes.
For all salts, attempt to reconstitute the hydrate by adding water drop wise.
Report Notes
1.
Record qualitative observations for all salts.
2.
Calculate a theoretical yield of water based on your mass of CuSO4A5H2O.
3.
Calculate a % error in mass of water lost from the CuSO4A5H2O.
Exp. 5 Hydrates
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