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Chemistry 142 Lab 9
Electrochemistry
Introduction
Chemical reactions always involved, in one way or another, the movement of
electrons.
Since the flow of electrons through a wire is what we call electricity, it
should not be surprising
to learn that there is an intimate connection between chemistry and electricity.
The study of the
connection between chemistry and electricity is referred to as ELECTROCHEMISTRY.
There are basically two complementary facets to electrochemistry
• Deriving electricity from chemical reactions
• Using electricity to cause a chemical reaction
Batteries
We derive electricity from chemistry by using an electrochemical cell...this is
known
colloquially as a battery. A battery is a source of EMF (electromotive force)
also known as
electrical potential and also known as voltage. his flow of electrons is
referred to as the current.
When a voltage is applied by a battery to a wire, it causes electrons to flow in
the wire. A
battery has two poles, a positive pole and a negative pole. When we attach a
wire to the two
poles of a battery, electrons flow from the negative pole to the positive pole.
The rate of flow of
charge in a wire is called the current.
To summarize: we can use a battery to extract a voltage from a chemical
reaction. This
voltage can be used to push electrons through an external wire. The rate at
which the electrons
flow is called the current.
Electrolysis
When we pass an electrical current through an ionic solution, such as salt
water, or
through an ionic liquid, such as molten salt, we induce one or more chemical
reactions. This
process is known as electrolysis.
For example, if two platinum wires are immersed in water, in which a small
amount of
sodium sulfate is dissolved, and a voltage from a battery or other source of EMF
is applied to the
wires, bubbles of hydrogen gas will be produced at one wire and oxygen will be
produced at the
other. The entire process is referred to as the electrolysis of water, and is a
chemical reaction in
which water molecules are broken up into their constituent parts.
The process is composed of two separate constituent parts, called halfreactions, one of
which takes place at the positively charged wire (positive electrode or anode)
and the other at the
negatively charged wire (the negative electrode or the cathode)
ANODE
2H2O(l) O2(g) + 4H+(aq) + 4e–
CATHODE
4H2O(l) + 4e– Æ 2H2(g) + 2OH–(aq)
The hydroxide ions and hydrogen ions “meet” in the center to re-form 4 molecules
and so there
is a net loss of 2 waters, giving an overall reaction of:
OVERALL REACTION
2H2O(l) Æ O2(g) + 2H2(g)
Electrolysis can also occur in molten ionic compounds. When electrical current
is passed
through molten sodium chloride, for example, sodium metal is formed at the
cathode and
chlorine gas is formed at the anode. This is how free sodium metal is produced
industrially.
Basic Electricity
Electricity usually refers to the flow of electrons through wires and other
electrical
components. Basically, what happens, is that an EMF (or voltage) is applied to a
wire and this
pushes electrons through the wire. The rate at which the electrons flow through
the wire is called
the current. Usually wires resist, to a certain degree, the attempt to push
current through them.
The numerical quantity which represents this resistance is
called...well...er...the resistance.
Although electricity usually is associated with the flow of electrical current
in wires,
electrical current can flow in any substance which can provide some kind of
charged particle to
carry that current. For, example, electricity can flow through solutions which
contain ions. Any
substance which can carry electricity is called a conductor. A substance which
cannot carry
electricity is called an insulator, and a substance whose properties are
intermediate between
those of conductors and those of insulators is called a semiconductor.
A liquid or a solution which can conduct electricity is called an electrolyte. A
liquid or a
solution which cannot conduct electricity is called a non-electrolyte.
Electrolytes can either be
strong or weak: a strong electrolyte is one in which the dissolved substance is
completely
dissociated, giving the maximum number of ions to carry the electricity. A weak
electrolyte is
one that only partially dissociates.
Pure water is a non-electrolyte. Table salt, strong acids and bases are all
strong
electrolytes when dissolved in water. Acetic acid (vinegar) is a weal
electrolyte, because it is a
weak (partially dissociated) acid.
There are many different kinds of electrical components from which we can build
a
circuit. Each has its own special symbol. The components we will use today, and
their symbols,
are shown below:
A wire is perhaps the simplest of electrical components. Wires are generally
made of
conducting material and are fabricated so that they have the least resistance
possible.
A resistor is a circuit element that has a known value of resistance, and which
we can use
to control or limit the flow of current in the circuit.
Every circuit has a source of EMF. A battery is an example of such a source. One
pole on
a battery is always positive and the other is always negative. A source of EMF
which always
maintains the same polarity is referred to as a DC (direct current) source.
Sometimes, the
polarity is alternated--this is the case with the electricity that comes out of
our walls. Such an
EMF source is referred to as an AC (alternating current) source of electricity.
Many circuit components perform very specialized tasks. Often, these more
complicated
components are fabricated from semiconductors. A diode, for example, is a
semiconductor that
functions as a one way valve for electricity, letting current flow in only one
direction. A Light
Emitting Diode is a diode that...well...emits light. It is often referred to as
an LED (you
pronounce the letters when you say it...Elll....EEEE.....DEEE). Today we will
use these
components to fashion a simple circuit which powers and lights up an LED. We
will then use
this circuit to test for various kinds of electrolyte.
Experimental
Basic Electricity
Use the multimeter to measure the voltage of the supplied batteries. Make sure
that the
meter is set to read voltage and that it is set to the appropriate voltage
range. Make sure the leads
are plugged into the proper input sockets. Pay special attention to what happens
when you
reverse the leads.
Now use the battery to light up an LED (light emitting diode) by making the
following
circuit:
Use the alligator clips provided as wires. Check both possible orientations of
the LED.
Note that the resistor is an absolute necessity. Without it, the LED will burn
out.
Now use the multimeter to measure the voltage from the DC power supply. Use the
DC
power supply in place of the battery to power your LED circuit.
Strong and Weak Electrolytes
Using your battery and LED circuit, test distilled water, vinegar, and salt
water. From the
behavior of the LED when the liquid is made part of the circuit, infer whether
each substance is a
strong, weak, or non-electrolyte.
You can use the hookup wire to place the beaker of liquid into your circuit. See
below fro
a schematic diagram of the circuit.
Batteries
You will now construct a series of batteries by using a variety of metals
submerged in a
strong electrolyte. Use a teaspoon of table salt in a 250 ml beaker filled 3/4
of the way with
distilled water.
You have been provided with the following items: 4 pieces of galvanized sheet
iron, two
pieces of copper foil, a large iron nail, a small piece of silver wire, and a
two small pieces of
platinum wire.
Set up a cell and connect it to a voltmeter as shown schematically below, using
first a
piece of sheet metal, and a piece of copper foil as your electrodes.
Hook up the circuit with the galvanized sheet metal hooked to the positive input
of the
voltmeter, and the copper to the negative or “common” input.
Now, in turn, replace the galvanized metal with electrodes made of iron, silver,
copper
and platinum. Note down the voltages.
Now take a clean piece of galvanized metal an place it into 2M HCl and leave it
there
until the bubbling has ceased. Rinse with distilled water and then try this out
as an electrode.
Galvanized iron, is iron with a thin coating of zinc. When you place it into
HCl, the zinc
goes into solution, displacing hydrogen gas. This leaves a bare surface of iron.
Do your numbers
reflect this? If you know the voltage that you get from zinc and copper, and you
know the
voltage you get from iron and copper, should you be able to predict the voltage
you should get
from zinc and iron. Try it and see.
Now build a series of three cells using copper and galvanized steel. Make sure
that you
DO NOT use the piece that you treated with the acid.
Connect this to your LED (you don’t need the resistor). It should be able to
light it
dimly).
Electrolysis
You can electrolyze water forming hydrogen and oxygen gas using a DC power
supply
and platinum electrodes. The equations we wrote down above for the halfreactions at each
electrode predict that hydroxide ions will be formed near the cathode and that
hydrogen ions will
be formed near the anode.
To test this prepare two 100 ml beakers containing distilled water in which is
dissolved a
small amount of sodium sulfate. Place a few drops of phenolphthalein in one of
them and a few
drops of methyl orange in the other. As you recall, phenolphthalein turns bright
pink in the
presence of a base and methyl orange turns red in the presence of acid.
Use about 8 volts from the DC power supply and observe and note the evolution of
gas at
the electrodes in each beaker, taking note of any color changes.
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