Comparing Properties of Ionic and Covalent Compounds
Prelab:
Background:
Chemical compounds can be separated into two groups: ionic and molecular compounds. Members of the first
group, known as the ionic compounds, occur as hard solids and have a distinctly crystalline appearance. Ionic
compounds are never found as liquids or gases at room temperature, and often result from the reaction of a metal and a
nonmetal. Ionic compounds melt only at very high temperature, often up to 1000 °C. Their boiling temperatures are
even higher, and not commonly observed. Although most ionic compounds are quite soluble (dissolvable), a substantial
number do not dissolve in water. Examined as solids, most ionic compounds do not conduct electricity very well and
may be described as electrical insulators (non-conductors). When heated above their melting points, however, liquid
ionic compounds are found to be good conductors of electricity. In addition, ionic compounds form electrically
conducting solutions when dissolved in a suitable liquid solvent, such as water. Although neutral overall, the ionic
compounds are composed of individual positively and negatively charged atoms or groups of atoms known as ions.
Chemists often refer to ionic compounds as “salts.”
Covalent compounds, also known as molecular compounds, generally melt and boil at much lower temperatures
than ionic compounds. Covalent compounds are composed of individual electrically neutral molecules, and are often
found as liquids or gases even at room temperature. Covalent compounds occurring as solids are relatively soft and may
have either a crystalline or amorphous appearance. Covalent compounds are electrical insulators in the solid, liquid or
gaseous state. Most covalent compounds are only sparingly soluble in water, although a significant number are quite
soluble.
The solubility of compounds in water depends on upon the ability of the polar water molecule to interact with the
solute. This interaction with ionic compounds is due to strong ion-dipole attractive forces. With covalent compounds,
solubility results from strong dipole-dipole attraction between solute and water molecules.
In this investigation you will investigate the electrical conductance of various aqueous (water based) solutions to
help you decide if a compound is ionically or covalently bonded. Substances that are electrical conductors contain
electrically charged particles that are free to move throughout the substance . Such particles might be negatively charged
electrons, negatively or positively charged ions, or negatively or positively charged polyatomic molecules. This
requirement for mobility prevents most solid materials (including ionic and covalent compounds) from conducting
electricity. Metallic substances are prominent exceptions; they conduct electricity because their atoms pool some of their
valence electrons to generate a mobile “sea” of electrons. Most ionic and covalent compounds do not show any evidence
of metallic behavior and neither group conducts in the solid state. Substances that dissolve in water to form electrically
conducting solutions are called electrolytes. Substances that dissolve to form nonconducting solutions are known as
nonelectrolytes. Water-soluble ionic compounds are all electrolytes. Water molecules are able to pull the positively and
the negatively charged ions out of their lattice positions, completely surround the ions, preventing their reprecipitations
as a solid, and carry the ions along to distribute them uniformly throughout the solution. Most covalent compounds are
nonelectrolytes. Molecules of covalent compounds are separated from each other by molecules without the liberation of
charged particles. Some covalent compounds, however, actually react with water to form new ionic particles. Because
such solutions do conduct electricity, these particular compounds can be called electrolytes. If all the molecules react to
form ionic products, the solutions become strongly conducting and the solutes are referred to as strong electrolytes. If
only a fraction of the molecules react in this way, the solution becomes weakly conducting and the compound is known
as a weak electrolyte.
Prelab:
Background:
In this lab, you will make observations of several compounds to determine if the compound is ionic (ionically
bonded) or molecular (covalently bonded).
Construct a data table to compare the properties of ionic and molecular compounds. Include: appearance, bond
type, general descriptions of melting and boiling temperatures, conductivity, solubility, and type of electrolyte.
Equipment: What does the conductivity apparatus test for? How does it work?
Safety: What safety precautions should you take when using the conductivity tester?
Problem statement: To classify compounds as ionic or molecular (covalent) by……
Materials:
Procedure: WEAR GOGGLES!
1) Observe the solid compounds. Record your observations.
2) Test the conductivity of each aqueous solution. Follow safety instructions. Record observations of the
relative brightness of the light bulb.
Data Collection:
Record observations in data tables.
Data Processing:
Classify the substances using a data table(s). Classify each substance as strong, weak or nonelectrolyte.
Based on all of your observations, classify each compound as ionic or molecular.
Results:
Conclusion:
Any of these: all 0.010 M (is this too weak?)
HCl
sucrose
NaOH
CuSO4
NH3
Urea H2NCONH2
NaOOCCH3 Sodium acetate
C8H807 citric acid
CH3OH methanol
H3PO4 phosphoric acid
or
KI
benzoic acid C6H5COOH
MgCl2
paradicholorobenzene C6H4Cl2
NaNO3