Academic
Chemistry
B is for Bond
INTRODUCTION
What holds the atoms of compounds together? A
bond holds the atoms together. A bond is really just a
force of attraction between atoms. This attraction is
in the form of energy.
Bonds are classified into two main types. The type of bond in any particular
compound depends primarily on the number of electrons in the valence shell of
the elements involved. The two main types of bonds are ionic and covalent.
In an ionic bond, one or more
electrons from the valence shell
of one atom are transferred to
the valence shell of another
atom.
Lithium loses one electron.
Lithium ends up with the same
number of electrons as the inert
gas, He (2 e-). By losing one
electron, lithium becomes stable.
Chlorine gains one electron from
lithium. Chlorine ends up with the same number of electrons as the inert gas,
Ar (18e-). By gaining one electron, chlorine becomes stable. In covalent
bonding, no electrons are transferred. Instead, the electrons are shared.
Compounds with ionic bonding have different properties from compounds
with covalent bonding. Let's look at these different properties in the
laboratory and then discuss what is happening.
PROCEDURE
1. Copy Table 1.
Compound
Formul
a
Table 1
Ionic or
Covalen
Volatility
Solubility
Electrical
Conductiv
Melting
Time
t?
ity
potassium
chloride
sodium
bromide
starch
benzoic acid
(C6H10O5
)n
C7H6O
2
2. Volatility refers to the ability of a substance to evaporate easily
to form a gas. The easiest way to tell if a compound is volatile is
to smell it. If you can detect an odor, you can assume that the
compound is volatile. If there is no odor, then it is non-volatile.
Report your four substances as volatile or non-volatile.
3. Now test the compounds for solubility in water. Use your spot
plate. You will need 4 clean wells. Add several drops of water to
each well. Add several crystals of each compound to one of the
wells. Use a stirring rod to mix each. Rinse the rod in between
uses. Record the results in Table 1 as soluble or insoluble.
4. Next, test the compounds for electrical conductivity. Again use
the spot plates. You will need 4 clean wells. Add several drops of
water to each well. Add several crystals of each compound to
one of the wells. Use a stirring rod to mix each. Rinse the rod in
between uses. You must NOT cross-contaminate the solutions!
Use your conductivity device to determine if the solutions
conduct an electrical current. Be sure to rinse the electrodes
off with clean deionized water in between tests. Record the
results as "electrolyte" for a conducting solution and "nonelectrolyte" for non-conducting solution.
5. Now test for the melting time. Get several crystals of the NaCl
and place these on a can lid that is supported by a ring clamp on a
ring stand. Heat the lid with a hot flame. Caution: do not get
too close. Measure the time for the sample to melt and record
this in Table 1. If the sample has not melted in 2 minutes, write,
"did not melt" in your table. Allow the can lid to cool, clean it
thoroughly, and test the other compounds in the same way. If
your sample burns before melting, remove it from the heat
immediately, and record the time to burn.
Lab Report Format:
I.
Purpose
II.
Procedure
III.
Data Table
IV.
Conclusions
Wafting Correctly
Step 5: Heat it with a hot flame.
Caution: do not get too close!
CONCLUSIONS
1. Copy Table 2. Using the results of your investigation, PREDICT
the properties of the two compounds listed. For the melting time,
just indicate if you think it will be a long or short time to melt.
You will not be testing these.
Table 2
Compound
Formul
a
sugar
C12H22O1
Ionic or
Covalen
t?
Volatility
Solubility
Electrical
Conductiv
ity
Melting
Time
1
magnesium
sulfate
2. Which compounds tend to be more volatile, ionic or covalent?
3. Which of the two types of compounds tends to be more soluble in
water?
4. Which of the two types of compounds tends to melt at a lower
temperature, i.e., has a faster melting time?
5. In ionic bonds, the electrons are actually transferred from one
atom to another. Let's look at this with sodium fluoride. How
many electrons does fluorine (F) need to fill its valence shell?
6. How many electrons does sodium (Na) have in its valence shell?
7. Explain how the type of bond could determine the volatility of a
substance. (Remember, volatility means that molecules or units
can break off from the solid block to be a gas to travel amongst
air molecules. Why does the type of bond affect how easily it
breaks off from the block?)
8. Explain how the strength of the bond affects the melting point
of a substance.
9. Which do you think would be more dangerous near an open flame,
an ionic or a covalent compound? Explain.
10. Suppose you had a sample of two compounds mixed together.
Both compounds consist of fine, white crystals. You know that
one of the compounds is ionic and the other is covalent. How
might you separate the two compounds? Write a detailed
procedure. Do not just say melt them – this does NOT separate
the compounds.