1030 Experiment No. 12, Classification of Chemical Substances

Transcription

1030 Experiment No. 12, Classification of Chemical Substances
General Chemistry I
CHEM-1030
Laboratory Experiment No. 12
Classification of Chemical Substances
(revised 05/29/2015)
Introduction
Depending on the kind of bonding present, a chemical substance may be classified, among other ways, as
ionic, molecular, macromolecular or metallic. In an ionic compound, the large electrostatic forces
between positively and negatively charged ions hold the particles strongly together. In a molecular
substance, molecules form by sharing electrons between atoms. However, the forces between molecules
may be relatively weak. When the molecules resulting from covalent bonding contain relatively small
numbers of atoms, the substances are molecular. If the particles are very large and include essentially all
the atoms in a macroscopic particle, the substance is called macromolecular. Metals are characterized by
bonding in which the electrons are much freer to move than in other kinds of substances. The metallic
bond is strong and stable but less localized than other types of bonds.
The terms ionic, molecular, macromolecular and metallic are somewhat arbitrary. Some substances have
borderline properties that place them between two groups. A common borderline designation is
molecular-ionic. Most of the time you should look for the characteristics of the more common ionic,
molecular, macromolecular and metallic categories. However, you may find a few substances whose
properties fall between those of two categories.
Ionic Substances
Ionic substances are usually solids at room temperature. (Recently, ionic liquids have been prepared by
designing huge cations with a single positive charge in the center. A large cation diameter increases the
distance between the cation and anion centers and reduces their electrostatic attraction for each other.
This gives the ions more mobility and allows the compounds to exist as room temperature liquids.)
Normal ionic solids are typically crystalline, but may exist as fine powders as well as clearly defined
crystals. While many ionic substances are stable up to their melting points, some decompose on heating.
It is common for an ionic crystal to release loosely bound waters of hydration at temperatures below 200
C. Anhydrous (dehydrated) ionic compounds have high melting points, usually between 300 C and
1000 C. They boil only at very high temperatures. They may or may not be soluble in water. If they are
water soluble, the resulting solutions conduct electric current. Solid ionic compounds do not conduct
electricity because their ions are locked in the crystal lattice. When an ionic substance melts or dissolves
in water, the ions become mobile, allowing the solutions to conduct. Examples of ionic solids are all the
common salts such as NaCl, KNO3, Li2SO4, CaI2, etc.
Molecular Substances
All gases and essentially all room temperature liquids are molecular in nature. The diatomic gases, Cl 2,
O2, H2, N2, F2 and most binary compounds composed of two nonmetals, such as SO2, CO2, CCl4 and SF6
are molecular. The molecular substance water can exist as a solid, liquid or vapor under ordinary
conditions.
If the formula mass of a molecular substance exceeds about 100 g/mol, it may be a room temperature
solid. The melting points of molecular substances are usually below 300 C. These substances may also
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be volatile, but some larger molecules such as glucose decompose before they melt or boil. Most
molecular substances do not conduct electric current when solid or molten or in aqueous solution because
they are not composed of ions.
Organic compounds, which primarily contain carbon and hydrogen, often combined with other nonmetals, are usually molecular in nature. If an organic compound decomposes on heating, it usually leaves
a black carbonaceous residue. (Think of charred sugar.) Organic molecular substances are frequently
soluble in some organic solvents, especially if the substance and the solvent have similar molecular
structures. Organic molecular substances include all the hydrocarbons such as hexane (C6H14), benzene
(C6H6), paraffin wax (a typical formula is C31H64); common solvents such as acetone (C3H6O) and ethyl
alcohol (C2H6O); carbohydrates such as glucose (C6H12O6), sucrose and ribose; organic acids such as
acetic acid, citric acid, and stearic acid.
Molecular-Ionic Substances
Some molecular compounds are markedly polar, with increased solubility in water and other polar
solvents. Other substances, for instance acetic acid and many other organic weak acids, may ionize
slightly in water, or in the melt, and become conductors of electricity. Often the conductivity is
considerably lower than that of an ionic material. Most polar molecular compounds in this category are
organic, but a few, including some transition metal salts, are inorganic. These substances, which may
have low melting points yet conduct electricity in solution and in the melt, belong to the molecular-ionic
subcategory.
Macromolecular Substances
Macromolecular, or network covalent, substances are all solids at room temperature. They occur in two
principal forms, inorganic and organic.
a) Inorganic macromolecular substances have very low volatility and very high melting points usually
above 1000 C. They have low volatility and are very resistant to thermal degradation. With the
exception of the graphite form of carbon, they do not conduct electricity and are often good
insulators. They are not soluble in any solvents. They are hard and chemically inert and are often
used as abrasives and for cutting tools. Because of their hardness, pieces of inorganic
macromolecular substances are difficult to crush and usually have sharp, well-defined edges.
Examples are silica (SiO2), carborundum (SiC) and the diamond form of carbon.
b) Organic macromolecules include the common polymeric plastics such as Bakelite, polypropylene,
synthetic rubber, phenol formaldehyde resin, neoprene, nylon, Teflon, polyvinyl chloride (PVC or
vinyl), polystyrene, polyethylene, polyacrylonitrile and silicone resins.
While these
macromolecules are typically nonvolatile and have melting points much higher than their
constituent monomers, they will all degrade at temperatures substantially below 1000 C.
Degradation can take the form of charring, ignition, decomposition to their constituent monomers
and decomposition to volatile breakdown products. (Note: Biologically active macromolecules
such as proteins, polysaccharides and nucleic acids will not be studied in this experiment.) Like
inorganic macromolecules, the common plastics do not conduct electricity and are insoluble in
most solvents. Organic macromolecular substances are usually much softer than inorganic ones.
When they are heated to high temperatures, organic macromolecules may show some melting or
charring. If they do char, you may see some smoke or be able to detect the odors of their
decomposition products.
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Metallic Substances
The properties of metals derive mainly from the mobility of their bonding electrons. Metals are good
electrical conductors both in the solid and liquid form, and have characteristic luster and malleability. All
metals but mercury are solids at room temperature and have melting points that range from 30 C to over
2000 C. They are not soluble in water or organic solvents. Some metals are prepared as black powders,
which may not appear to be electrical conductors; if such powders are heated, the particles coalesce to
solids with good electrical conductivity.
Table 1 lists some properties of 17 representative solids.
Experimental
Refer to the data table (Table 2) on page 6. You will work with six “known” substances (labeled A
through F) and two numbered unknown solids. Your instructor may or may not supply the chemical
names of the known substances. Test each of the eight solids for electrical conductivity in the solid state,
approximate melting range, electrical conductivity in the liquid state (melt), water solubility and electrical
conductivity in aqueous solution as outlined in steps 1-10. From your data and the information given in
this introduction, classify each known solid as either: Ionic, Molecular, Molecular-Ionic, Inorganic
Macromolecular, Organic Macromolecular or Metallic. Your instructor will check your conclusions for
the known substances before you leave the laboratory. Your report will be graded on how you classify
your unknowns and on the answers to the report questions.
1. Choose one of the six known solids. Weigh about 1.0 g onto a clean, dry watch glass. If single large
pieces of the solid are available, take one piece instead of weighing a sample.
2. Test the conductivity of the solid on the apparatus demonstrated by the instructor. If the solid is a
powder, make sure both “feet” of the conductivity apparatus contact the powder. If solid is in large
pieces, clamp or squeeze a piece between the electrodes to make good electrical contact. Record the
conductivity in volt or milliamp units as determined by the meter and specified by the instructor.
3. Place the watch glass with the same solid on a hot plate set to low heat. Let it sit for 5 minutes. If the
solid melts, its melting point is below about 300 C. If the solid does not melt in 5 minutes, transfer the
watch glass to a hot plate set to medium heat. (To move a hot watch glass safely, grasp it with tongs and
slide a wire gauze square under the glass for support. If the solid melts after 10 min at medium heat, its
approximate melting range is 300 – 600 C. If it does not melt, assume its melting point is above 600 C.
4. If a solid melts on either hotplate, test the melt conductivity. Make sure the substance is still a liquid.
If it starts to solidify, melt it again and quickly measure its conductivity. If the solid did not melt in step 3,
do not attempt to test its conductivity again. Simply put a large X in the Conductivity as Melt box in the
data table to indicate that you could not do the procedure.
5. Discard the solid as directed by your instructor, either as heavy metal waste or organic waste.
6. Weigh about 0.2 g of a fresh sample of the same solid into a 50 mL beaker. Add 10 mL of distilled or
deionized water and stir. Does the solid dissolve or not?
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7. If the solid dissolves, test the solution conductivity. (Note: To evaluate the electrical conductivity of
an aqueous solution, do not consider just its conductivity reading. Compare the solution conductivity to
that of distilled water, which is not necessarily zero. It is the conductivity difference between distilled
water and a solution that is significant. If the solid does not dissolve in step 6, do not attempt to test its
solution conductivity. Put a large X in the Solution Conductivity data box to show you did not do the
procedure. Discard the solutions and solid-water mixtures as heavy metal waste or as otherwise directed
by your instructor.
8. Repeat steps 1 through 7 for the remaining five known solids.
9. Classify each of the six known substances and ask the instructor to approve your data sheet.
10. Obtain two unknown solids and perform steps 1 through 7 with each unknown. When you have
finished testing your unknowns, check with your instructor for the correct disposal procedure.
Safety
Chemical splash goggles and a waterproof apron must be worn at all times during this and all chemistry
experiments, from the very beginning to the very end of the time you spend in the laboratory.
Waste Disposal
Your instructor will provide disposal information for each substance used in this experiment. Enter each
disposal method in the far right column of Table 2, your data sheet. Remember: few chemicals can be
legally or safely thrown in the trash or poured down a sink drain. Toxic metal compounds, concentrated
acids and concentrated bases must be disposed of in the laboratory heavy metal hazardous waste
collection container. Organic compounds, many of which are toxic or flammable, are properly disposed
of as organic hazardous waste.
Cleanup
Before you leave the lab, wipe down all your work surfaces with a damp sponge.
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Table 1 ─ Physical Properties and Classification of Some Representative Substances
Substance
Appearance
NaCl
White cubic
crystals
MgO
White Powder
Melting
Pt (C)
Boiling
Pt (C)
Solubility
in H2O
801
1413
soluble
2800
3600
slightly
soluble
CoCl2
Blue Crystals
735
1049
soluble
Polyvinyl
chloride
(C2H3Cl)n
White solid
100 to
260
dec.
insoluble
70
255
113
445
327
dec.
0
100
—
122
249
slightly
soluble
17
119
soluble
306
dec.
soluble
C10H8
Sulfur
(S8)
Teflon
(CF2)n
H2O
C6H5COOH
CH3COOH
FeCl3
C
(diamond)
C
(graphite)
SiO2
B4C
Fe
Al
White Crystals
Yellow Crystals
Yellow Powder
White Solid
Clear Crystals
Clear Liquid
White Crystals
Clear liquid
Red or Green
Solid
Clear to Brown
Solid
Black Solid
Clear Glassy
Crystals
Gray to Black
Solid
Silvery solid or
Gray Powder
Silvery Solid
insoluble
insoluble
insoluble
insoluble
>3500
insoluble
>4200
1600
2590
2350
>3500
1535
3000
933
2470
insoluble
insoluble
insoluble
insoluble
Electrical
Conductivity
zero in solid
high in melt
high in soln
zero in solid
low in soln
zero in solid
high in melt
high in soln
zero in solid
zero in melt
zero in solid
zero in melt
zero in solid
zero in melt
zero in solid
zero in melt
zero in solid
low in melt
zero in solid
low in soln.
zero in solid
high in soln.
zero in solid
high in soln.
zero in solid
zero in melt
high in solid
zero in solid
zero in melt
zero in solid
zero in melt
high in solid
high in melt
high in solid
high in melt
Classification
ionic
ionic
ionic
organic
macromolecular
molecular
molecular
organic
macromolecular
molecular
molecular-ionic
molecular-ionic
molecular-ionic
inorganic
macromolecular
inorganic
macromolecular
inorganic
macromolecular
inorganic
macromolecular
metallic
metallic
Symbols and Abbreviations: soluble (at least 0.1 mol/L), slightly soluble (appreciable solubility but
<0.1 mol/L), soln – solution, dec – decomposes
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Table 2 ─ Classification of Chemical Substances
Data Page
Measured Conductivity of Distilled Water alone: __________
Substance
A
B
C
D
Visual
Appearance
Conductivity as
Solid
Melted on
Low
Hotplate?
Melted on
Medium
Hotplate?
Approx.
Melting Range
Conductivity as
Melt
Solubility in
water
(yes or no)
Solution
Conductivity
Reading
Classification
Disposal
Method
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E
F
Unknown
Number
Unknown
Number
General Chemistry I
CHEM-1030
Laboratory Experiment No. 12
Classification of Chemical Substances
Report
1. What is the classification of your first unknown?
Unknown Number____________
For full credit, clearly explain all the reasons for your conclusion.
(8 points)
2. Using your chemistry text or other suitable print or on-line reference, give the names and chemical
formulas of two substances not listed or mentioned in this laboratory writeup that share the same
classification and general properties (melting point, etc.) as your first unknown.
(2 points)
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3. What is the classification of your second unknown?
Unknown Number____________
For full credit, clearly explain all the reasons for your conclusion.
(8 points)
4. Using your chemistry text or other suitable print or on-line reference, give the names and chemical
formulas of two substances not listed or mentioned in this laboratory writeup that share the same
classification and general properties (melting point, etc.) as your second unknown.
(2 points)
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1.
General Chemistry I
CHEM-1030
Laboratory Experiment No. 12
Classification of Chemical Substances
Prestudy
Describe two experiments you could use to distinguish between an ionic solid and a
macromolecular solid.
(2 points)
2.
A certain solid does not conduct electricity in either the solid or the molten state. It is insoluble
in water and melts around 101C. How would you classify the substance? Explain the reasons
for your answer.
(2 points)
3.
A solid melts at 29.7 C. It conducts electricity in both the solid and molten states. It is
insoluble in water. How would you classify the substance? Explain the reasons for your
answer.
(2 points)
4.
A solid does not conduct an electric current. It melts at 449C and the melt conducts electricity.
The solid is soluble in water. How would you classify the substance? Explain the reasons for
your answer.
(2 points)
5.
A solid does not conduct electricity in the solid state. When melted at a temperature greater
than 1870C, the melt also does not conduct electricity. The solid is insoluble in water. How
would you classify the substance? Explain the reasons for your answer.
(2 points)
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