Conductivity of Electrolytes in Solution
Introduction:
Electrical current can be thought of as the movement of electrons
or ionic charges from an area of high potential to an area of low potential.
Materials which conduct current via the movement of electrons are called
primary conductors. This class of conductors contains the metallic
elements and substances like graphite (an allotrope of carbon). Primary
conductors are usually solid but they may also be liquids, such as
mercury or molten metals.
Materials which conduct current via the movement of ions are
called secondary conductors. Anions move toward the area of positive
potential while cations move towards the area of negative potential.
Therefore, it is required that the ions be mobile. There are two general
situations when ions are relatively mobile: (1) when an ionic compound
is molten and (2) when a substance dissolves in a media which supports
the presence of ion pairs (a cation and an anion). Solutions which
conduct electricity are called electrolytic solutions.
During the process of passing current through an electrolytic
solution, chemical changes occur at the interface between the primary
conductor (called the electrode) and the secondary conductor. Cations
move towards the cathode where reduction occurs while anions move
towards the anode where oxidation occurs. The diagram below illustrates
the process:
H+
Cl–
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The electrolytic cell contains an aqueous solution of hydrogen chloride
(hydrochloric acid) that has ionized to form hydrogen ions, H+, and
chloride ions, Cl–. The cathode and anode are primary conductors such
as graphite or a metal which is chemically inert towards HCl. The
electrodes are connected by a metallic wire to the battery which supplies
the electrical potential causing current to flow. The equations below show
the chemical processes occurring at the electrodes:
At the anode:
2 Cl– → Cl2 + 2 e–
At the cathode:
2 H+ + 2 e– → H2
Substances which form secondary conductors are termed
electrolytes. Soluble salts such as NaCl or NH4NO3, and inorganic acids
like H2SO4, HCl, or HNO3 are examples of good electrolytes. Substances
which do not dissolve or do not ionize in solution are termed nonelectrolytes. Common examples of non-electrolytes are sugar and alcohol.
Other strong and weak electrolytes are listed below:
Strong electrolytes
Most salts
HCl
H2SO4
HNO3
NaOH
KOH
Ba(OH)2
Ca(OH)2
Weak electrolytes
HC2H3O2
H2SO3
HNO2
H2CO3
H2S
H2C2O4
H3PO4
NH3
It is important to recognize the roll of the solvent in forming
electrolytic solutions. The solvent must support the formation of the
separation of substances into ion pairs. Good solvents for this process
are polar such as water. Poor solvents would be non-polar such as
benzene or chloroform.
A simple apparatus to determine conductance is shown below:
Small light
Bare Wires
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When the two wire electrodes are connected either by immersion in an
electrolytic solution or by direct contact through a primary conductor,
the light bulb shines brightly. If the connection between the two
electrodes is poor, then the light may shine dimly. And if no connection
between the electrodes is made that will conduct electricity, the bulb
doesn’t shine at all.
Procedure:
Rinse the testing apparatus with distilled water and blot dry with a
small piece of paper. Make sure no metal objects are in direct contact
with the electrodes.
Conductance in Solids. Obtain a beaker containing one of the solids
listed in the data section. Bring the testing apparatus in contact with the
solid the apparatus and record your observations. Test each solid listed
in the data section.
Conductance in Aqueous Solutions. Obtain a bottle containing one
of the solutions listed in the data section. Insert the testing apparatus’
electrodes in the solution. If the light bulb glows at all, then classify the
solution as an electrolyte. If the bulb does not appear to glow, then
classify the solution as a non-electrolyte. Clean the electrodes with a
squirt bottle holding the apparatus over the sink or a large beaker. Dry
the electrodes with a Kimwipe or paper towel. Repeat the procedure for
each of the aqueous solutions listed in the data section. Use the
solutions prepared. Do not mix any.
Conductance in Non-Aqueous Solutions. Repeat the procedure
described above using the non-aqueous chemicals listed in the data
section.
Ionic Reactions. Prepare a solution by mixing 10 mL of dilute NH3
and 10 mL dilute HC2H3O2. Test the conductivity of the mixture. Record
your results in the data section. Pour the mixture down the sink when
finished.
Prepare a solution by mixing 10 mL of dilute H2SO4 and 10 mL
dilute Ba(OH)2. Test the conductivity of this mixture. Record your results
in the data section. Dump this mixture down the sink as well.
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Name
Lab Day
Report for Conductivity Experiment:
Data:
Conductance in Solids:
Material
Test Result
Copper
Rubber
Iron
Sodium Chloride
Sugar
Magnesium
Copper(II) Sulfate
Graphite
Aluminum
Zinc
Conductance in Aqueous Solutions:
Solution
Tap water
NaCl
NH3
HC2H3O2
HCl
H2SO4
C12H22O11 (sugar)
CuSO4
Ba(OH)2
Test Result (classification)
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Conductance in Non-Aqueous Solutions:
Solution
Test Result (classification)
Pure Cyclohexane
Pure Ethyl Alcohol
HCl in Cyclohexane
HCl in Ethyl Alcohol
NaCl in Cyclohexane
NaCl in Ethyl Alcohol
Ionic Reactions:
1.
What was the conductivity of the mixture of dilute NH3 and
dilute HC2H3O2?
2.
What was the appearance of the mixture?
3.
What was the conductivity of the mixture of dilute H2SO4
and dilute Ba(OH)2?
4.
What was the appearance of the mixture?
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Questions:
Conductance in Solids:
1.
List the solids that were conductors.
2.
List the solids that were non-conductors.
3.
How is current conducted through solid conductors (how
does it move)?
4.
How are these conductors classified?
Conductance in Aqueous Solutions:
5.
List the conductors and the non-conductors.
6.
How is current conducted through solutions (what moves)?
7.
Why does sugar form a non-electrolyte solution?
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8.
Why is HCl a good conductor and NH3 or HC2H3O2 are not?
Conductance in Non-Aqueous Solutions:
9.
Why are aqueous solutions of HCl electrolytic while
cyclohexane solutions of HCl are non-electrolytic?
10.
Explain the difference between HCl in cyclohexane and HCl
in ethyl alcohol.
11.
Arrange the solvents: water, cyclohexane, and ethyl alcohol
according to polarity.
Hi Polar
12.
Lo Polar
Explain how polarity of the solvent affects the ionization of
acids.