chapter 6 electrochemistry
TRANSCRIPT
Electrolytes
Electrolytes are compounds which when molten or dissolved in water conduct electric current and are
decomposed in the process .
Non-electrolyte
A non-electrolyte is a liquid which does not allow the passage of electricity.
Molten Solution
This is composed of lead(II) ions, Pb2 + , and bromide ions, Br-. Its chemical formula is therefore
PbBr2.
A suitable apparatus which could be used to carry out this electrolysis is shown in Figure above.
The bulb helps to show when electricity is flowing in the circuit, and until the lead(II) bromide is
completely molten, the bulb does not light up . This confirms that electrolytes have to be molten
for the ions to start to move to the electrodes and thereby conduct electricity.
At the Cathode At the Anode
Observation
When electricity is flowing, a silvery
deposit of lead metal forms on the
cathode. In fact, as it is molten, it is more
likely to drip off in a molten blob.
Observation
When electricity is flowing, brown
fumes of bromine gas are seen at
the anode.
Half equation
Pb2+ + 2e ---> Pb
Half equation
2Br- ---> Br2 + e
Explanation
The lead(II) ions, as they are positive,
move to the negative cathode, where each
ion gains two electrons to form a lead
atom.
Any reaction at a cathode involved is
again in electrons. This is called
reduction or more exactly, cathodic
reduction .
Explanation
The bromide ions, as they are
negative, move to the positive
anode, where each loses an
electron to form a bromine atom.
Then two of these newly formed
atoms combine to form bromine
gas.
Any reaction at an anode involves
a loss of electrons.
In summary, the lead(II) bromide is split into its component elements :
PbBr2 ---> Pb + Br2
Electrolysis of Aqueous Sulphuric Acid
As sulphuric acid is aqueous, it is composed not only of hydrogen ions (H+ ) and sulphate ions
(SO42-), but also of hydroxide ions (OH-) from the water.
H2SO4 + H2O --> 2H+ + SO42- + H+ + OH-
The apparatus used to carry out this electrolysis and collect the gases given off is shown in Figure
9 .8 .
When we have more than one type of ion moving to an electrode, selective discharge (or
preferential discharge) takes place.
This means that the ion which can lose or gain electrons with the greatest ease is discharged,
and the other ions, which are harder to discharge, remain in solution .
With the electrolyte aqueous sulphuric acid, migration of ions to the electrodes also occurs.
At the Cathode At the Anode
Here we have only one ion, the
hydrogen, H+ (aq), and each ion gains
an electron to become a hydrogen
atom.
Two of these newly formed atoms
then combine to form a hydrogen gas
molecule .
Here we have a choice of either
sulphate, SO42-(aq), or hydroxide
OH- (aq) ions.
Hydroxide is easier to discharge, so
oxygen gas is given off at the anode.
Equation:
2H+ + 2e ---> H2
Equation:
OH- + 4e ---> O2 + H2O
Notes
With electrolysis of aqueous solutions of dilute acids or alkalis, the volume of
hydrogen given off at the cathode is roughly twice that of the oxygen gas at the
anode.
Accordingly, the elements of water are lost and as the electrolysis continues, the
concentration of the acid or alkali increases .
Essentially, the electrolysis of aqueous sulphuric acid is the electrolysis of water,
with hydrogen and oxygen gas being given off in a ratio of 2 : 1 .
Extraction of Metal
The extraction of metals from their ores, in particular aluminium and sodium, is important
industrial uses of electrolysis.
The diagram below shows the methods of extraction for different metals.
We can see that those metals which are less reactive than carbon in reactivity series are
extracted from their ore by displacement reaction using carbon. This will be discussed in detail in
chapter 3, form 5, Oxidation and Reduction.
Copper and mercury can be extracted from their ore by burning directly in air.
Silver (Ag) and gold (Au) need no extraction because they exist as element in nature.
Those metals which are more reactive than carbon are extracted by electrolysis.
Extraction of Aluminium
Aluminium is the most abundant metal found in the earth's crust. It makes up about 8% by weight
of the Earth’s solid surface.
It is also a very useful metal due to its low density and ability to resist corrosion.
The main source of aluminium is bauxite ore (Aluminium Oxide).
In industry, aluminium is extracted by electrolysis from bauxite ore.
Adding Cryolite
In electrolysis, molten aluminium oxide must be used to extract aluminium. Aluminium oxide
decompose to form aluminium and oxide ions when melted.
Al2O3 ---> 2Al3+ + 3O2-
However, the melting point of aluminium oxide is very high (over 2 000°C), so another
aluminium compound called cryolite (Na3AIF6) is added to lower down the melting point
(about 980oC).
The diagram above shows how aluminium is extracted from molten aluminium oxide by
electrolysis.
Graphite is used as the anode and cathode.
During electrolysis, the aluminium ions are attracted towards the graphite cathode.
The ions is discharged and become molten aluminium metal.
The partial equation of this reaction is as follow:
Al3+ + 3e ---> Al
At the anode, oxygen gas which also has commercial value is collected. The partial equation
of this reaction is as follow:
2O2- ---> O2 + 4e
At the temperature of 980 °C, the oxygen burns the carbon anode. Therefore the anode has
to be replaced periodically.
Also, this cell uses large quantities of electricity, and therefore needs cheap sources of power.
Extraction of sodium chloride
In industry, sodium is extracted from molten sodium chloride. Molten sodium chloride is put
into the apparatus as showing in the diagram above.
When sodium chloride is melted, the sodium and chloride ions disassociate to become freely
move ions, as shown in the chemical equation below.
NaCl ---> Na+ + Cl-
In this electrolytic cell, graphite was used as anode while iron is used as cathode.
The negative chloride ions are attracted to the anode and then discharged to form chlorine
gas.
2Cl- ---> Cl2 + 2e
Since chlorine gas is also significant in industry, it is collected and stored.
In cathode, the sodium ions are discharged to form sodium atom.
Na+ + e ---> Na
Due to high temperature, the sodium metal formed is in molten form.
Metal sodium have lower density. Therefore it moves upward and been collected.
Purification Of Copper
In the refining or purification of copper, the impure copper is made the anode and a thin, pure
copper plate is used as a cathode.
The electrolyte is usually acidified copper(II) sulphate solution.
When electricity flows, the copper dissolves from the impure anode and goes into solution as
copper ions.
Impurities in the copper do not dissolve, and instead fall off the anode as anode sludge. At the
cathode, the copper ions are deposited as pure copper metal.
Reaction in anode (impure copper)
In anode, the copper atoms from the electrode are ionised to form copper(II) ions.
Cu ---> Cu2+ + 2e
Reaction in cathode (pure copper)
Cu2+Cu ---> Cu + 2e
Electroplating
Electroplating: Coating with a Thin Protective Layer of Metal
A very common use of electrolysis is to form a thin protective coating of a metal on the
surface of another which is likely to corrode.
The diagram above illustrate the electroplating of a key with copper.
In this process, we need to make the cathode the object for plating (the key.
The anode is then made of the metal we wish to plate with (copper), and the electrolyte needs
to be a solution of a salt of this metal (copper(II) sulphate).
Anode
In anode, the copper atoms from the electrode are ionised to form copper(II) ions.
Cu ---> Cu2+ + 2e
Cathode
In cathode, the copper ions are discharged to form copper atom and then deposit on the
surface of the key
Cu2+ ---> Cu + 2e
Cells and Batteries
A device which converts chemical energy into electrical energy is called a cell or battery. Battery
is a collection of cells.
A cell consists of a pair of dissimilar metals in an electrolyte.
Figure above shows an example of a simple voltaic cell consist of a magnesium electrode and a
copper electrode immerse in magnesium sulphate solution.
When chemical reaction happens, the more reactive metal, magnesium, dissolves in the
magnesium sulphate solution and become magnesium ions, thereby producing electrons, as
shown in the half equation below:
Mg ---> Mg2+ + 2e
As electrons are produced, the magnesium acts as the negative electrode.
These electrons then travel to the copper electrode.
The hydrogen ions around the copper electrode receive the electrons and are discharged to
produce bubbles of hydrogen gas:
2H+ + 2e ---> H2
As electrons are taken in, the copper is the positive electrode.
This production and movement of electrons is electricity, so electrical energy has been
generated and the galvanometer is deflected. *Overall, the chemical reaction can be
represented by the ionic equation:
Mg + 2H+ ---> Mg2+ + H2
In voltaic cell, the negative electrode is the anode whereas the positive electrode
is the cathode, which is the opposite of the electrolytic cell.