Chemical compounds that
contain discrete water molecules as part of their crystalline structure are
called hydrates. Hydrates occur quite commonly among chemical
substances. More often than no, some 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. In this experiment, you will study some of the properties and characteristics
of several ionic hydrates.
Hydrates are most commonly
encountered in the study of metal salts, especially those salts of the transition
metals. Water is bound in most hydrated in definite, stochiometric
proportions, and the number of water molecules bond per metal ion is often characteristic
of a particular metal ion.
A very commonly encountered in the general chemistry
laboratory is copper(II) sulfate pentahydrate,
CuSO4.5H2O. 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 water molecules from the formula of the salt
itself.
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 white powdery, anhydrous salt.
CuSO4.5H2O(s)ŕ CuSO4(s) + 5H2O(g)
Blue
white
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 with
and remove water from their surroundings. For example, most electric 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 simple into the anhydrous compound when heated,
however. Some hydrated metal salts will decompose upon loosing the water
of crystallization; subsequently they are usually converted into the metal
oxide if the heating is carried out in air. Most covalent hydrates
decompose rather than simply loose 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 or five of the water molecules form such coordinate bonds
with the copper (II) ion. In other situations, 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.
Wear
safety glasses 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 splatter if heated
too strongly. To avoid this, Heat the solids with as small as flame as possible
at first, and do not heat strongly with the full heat of the burner until most
of the water has been driven from the hydrate. Make certain that the mouth of
the test tube used to heat the hydrate is not pointed at yourself or anyone
else.
Dispose
of the metal salts as directed by the instructor. Do not wash the salts down
the drain, and do not place them in the wastebasket.
Nickel
(II) chloride hexahydrate, cobalt (II) chloride hexahydrate, copper (II) sulfate pentahydrate,
chromium (III) chloride hexahydrate and anhydrous
chloride.
Part 1
Record all data and observations directly in your
notebook in ink.
Your
grade will in part be influenced by how accurate your data is…be painstakingly
careful with measurements (lose up to 4 points).
Determine
the mass of a clean, dry casserole or small evaporating dish to the nearest
milligram (0.001 g). Add to the casserole or evaporating dish a spatula
tip-full of copper (II) sulfate pentahydrate, CuSO4.5H2O,
and reweigh. Calculate the mass of CuSO4.5H2O taken.
Record the appearance of the crystals.
Based on the mass of CuSO4.5H2O
taken and its formula, calculate the theoretical mass of water that
should be lost from the crystals if all the water were driven off.
Set up a wire gauze on a metal
ring, are prepare to heat the casserole/evaporating dish in the burner flame.
Begin the heating with a very small flame.
If there is any evidence that the material is about to spatter, remove
the heat immediately. Record any changes in color/appearance as the hydrate is
heated.
When it is apparent that most of the water has been driven
from the sample, increase the size of the flame. Stir the salt with a clean
stirring rod until the sample is uniform in texture and appearance.
Remove the heat and allow the casserole to cool
completely to room temperature. When the casserole has cooled completely,
reweigh and calculate the mass of water driven off from the crystals. Using
theoretical mass loss calculated above, along with the experimentally
determined mass loss, determine the percent error in
your experiment.
After all mass determinations for the CuSO4.5H2O
sample have been completed, add water drop wise to the sample. Record any
changes in appearance/color.
For the hydrates listed below, first record the
appearance of the crystals. Then transfer tiny amounts of the hydrates to each
separate, clean borosilicate test tubes.
Part 2
Repeat this procedure for a
Nickel or a Cobalt Chloride hydrate compound. Determine the % composition for
the hydrate. Take note that the grade will reflect how close you are to the
correct percent.
1. Use a handbook of
chemical data to find the number of water molecules bound per formula unit in
the common hydrates of the following salts:
Strontium Chloride, SrCI2
Sodium Chromate, Na2CrO4
Nickel (II) Nitrate, Ni(NO3)2
Iron (II) Ammonium Sulfate, Fe(NH4)2(SO4)2
2. As described in the
introduction to this experiment, when copper (II) sulfate pentahydrate
is heated, the deep blue color of the hydrate changes to the white color
of the anhydrous salt. Use the sections of your textbook discussing the
chemistry of the transition elements to determine why such a vivid change in
color is common when such elements’ hydrated compounds are heated.
3. Suppose 2.3754 g of copper(II) sulfate pentahydrate is
heated to drive off the water crystallization. Calculate what weight of
anhydrous salt will remain.
4. CuSO4.5H2O,
it was mentioned that four of the five water molecules held per formula unit of
the salt were attached by coordinate covalent bonds to the copper ion. The
fifth water molecule is attached to the sulfate ion, but by a different
mechanism. Use your textbook or a chemical encyclopedia to determine how a
water molecule might be bonded to a sulfate ion.
Data Part 1
Mass of empty
casserole/evaporating dish, g
Mass of
casserole/evaporating dish plus CuSO4.5H2O, g
Mass of taken CuSO4.5H2O,
g
Appearance of CuSO4.5H2O
Theoretical mass loss
expected on heating CuSO4.5H2O
Mass of
casserole/evaporating dish after heating, g
Mass of water lost from CuSO4.5H2O
crystals, g
% error in mass water lost
from CuSO4.5H2O, g
Appearance of CuSO4
after heating
Appearance of CuSO4
on adding water
Data Part 2
Analysis Questions part 2
5. Use a chemical dictionary
or your book to distinguish between the terms desiccant, hygroscopic, and
deliquescent.
6. Sugars and starches
belong to a class of biological compounds called carbohydrates, indicating
that the general formula for such compounds is of the sort (CH2O)n.
Use your book to find out why such compounds are not really hydrates of carbon
as the family name suggests and record you findings here.
7. Generalize the colors of
the above solutions and use any other reference source to cite generalized
solution colors.
Chance of Vindication
(add up to 4 points)
Repeat the procedure of above with the other chloride • unknown hydrate sample. Take mass readings before and after heating. Determine the mass driven off from the sample and hence the amount of water. Work backwards as before to determine the amount of water in the hydrate molecule. Label and record your data below and show any/all calculations.