Download Determination of electrochemical equivalent of copper and

Warsaw University of Technology
Electrical Department
Laboratory of Materials Technology
KWNiAE
Practice 11
Determination of electrochemical
equivalent of copper
and Faraday's constans.
1. Electrolytes and ionic conduction
Generally said, electrolytes are substances, which are built of atoms and their groups
with charges of counter signs called ions. Coulomb forces are acting among them.
Electrolytes are salts dissolved in water, bases and acids characterized by a big value of
electric permeability. If we dissolve or melt such a substance, coulomb forces diminish
and slight energy (e.g. thermical movement of ions) can break ionic bonds – this process
is called dissociation.
While in metals conduction was connected with the movement of small unitary mass
electrons, in case of electrolytes charge bearers are hard ions. Conduction of electrolytes
must be connected with transport of mass. The dissociation process aims at a complete
dissolution of a substance in solvent. Nevertheless because of the processes of
recombination of ions, by set temperature, given type of the substance and concrete
solvent a permanent state of balance between the process of dissociation and the process
of recombination is established – at the same time the same amounts of particles
dissociate and recombinate.
In order to be able to classify electrolytes we introduce the concept of the degree of
dissociation α:
Nd – the number of dissociated particles
N – overall number of dissolved particles
If 0,8 < α < 1 the electrolyte is called strong and it means that almost all dissolved
particles were dissociated. For middle electrolytes α = 0,5 and the number of particles
dissociated and not dissociated is similar. When α < 0,01 the electrolyte is a weak one. In the
practice we will use copper sulfate solution which is a strong electrolyte.
2. Electrolyte in an electric field.
At the moment when the electric field is applied to the electrolyte, e.g. through dipping
the electrodes connected with the voltage source, electrostatic forces start to act on ions.
These forces cause cations (positive ions) to move to the negative electrode (cathode), and
anions to move to the positive electrode (anode). This phenomenon is an arranged movement
of electric charges, so current is flowing in the electrolyte. Due to the bases of electrotechnics
the direction of current is same as the direction of the movement of cations. The whole of
phenomena occurring in electrolyte during the flow of current is called electrolysis.
In our practice electrolyte is water solution of copper sulfate. When ions reach electrodes
they precipitate on them or react with the solvent. If we use coal of platinum electrodes, the
reactions are as follows:
On the anode a layer of metallic copper will be isolated, and around the anode –
oxygen. If we use copper electrodes then the reaction will follow similarly, only on the anode
a particle CuSO4 will create, as the reaction of secondary SO4 with electrode’s copper. The
concentration of the solution will not change. The only change will be diminished mass of the
anode and increased mass of the cathode. This reaction can be written down as follows :
3. Faraday’s first law and the change of electrodes’ mass.
As the result of experiments with electrolytes Faraday formulated a law defining the
dependence between the quantities of the substances being isolated during electrolysis and the
moved electric charge. The amount of mass isolated on the electrodes is proportional to the
charge which flowed through the electrolyte.
where:
I- current intensity
t- time of electrolysis
k- coefficient defined as electrochemical equivalent
On the drawing below the principle of the movement of ions during electrolysis is
schematically shown.
Faraday found that the coefficient k depends only on the kind of substance being
isolated and completely does not depend on the shape of electrodes or the concentration of the
solution. If the description of the phenomenon is made on a molecule level then we can take
the mass of singular ion as m’, its charge as e’ and the number of ions as N’. So we can write
the following equations:
which finally gives us:
and after transformation:
which is the interpretation of electrochemical equivalent.
4. Second law and Faraday’s constant.
In order to compute the mass of a single ion m’ it’s enough to know that the Avogard’s
number of NA ions weights as much as is ion’s atomic weight.
where M is ion’s molal mass.
Starting with this formula it was experimentally found that univalent ions have electric
charge equal to e, bivalent to 2e, and generally every ion has charge equal to:
where w= (1,2,3, …) is a valency of ion.
It is the main meaning of Faraday’s second law.
From these considerations result basic ions’ features:
• Ion is an atom or a group of them, which lost one or more electrons.
• Ions with a valence w are atoms, which lost precisely w electrons.
If we add both first and second Faraday’s law and transform obtained formulas:
Because in chemistry a quotient M/w= R is called a gram- equivalent and F= NA*e is
called a Faraday’s constant, we can state that m= R/F* Q.
Faraday’s constant is then a charge, which will be isolated on the electrode by one
gram- equivalent of a substance and is equal F= 96484 [C].
Altogether the content of both Faraday’s laws may be presented as follows:
Electric current flowing through the electrolyte is isolating on the electrodes masses of ions
proportional to the transported charge so that for every 96484 [C] of charge it will isolate one
gram- equivalent of each ion.
5. The description of the measurement system and the method of performing the
exercise.
Electrolyzer is made of a transparent container in which there is a water solution of copper
sulfate CuSO4 and two copper electrodes. For increasing the active surface of electrodes one
of them is double and placed on the verges of the container. In the middle there is an electrode
of double thickness. The system is powered from a stabilized power supply adaptor.
Performing the task we act as follows:
1.
2.
3.
4.
5.
6.
We take out the middle electrode and wash it in water, and then dry it.
We weight the dry electrode and note its initial weight m1.
We put the electrode back in the electrolyzer.
We set the amount of current so as not to exceed density 1 [A/dm2].
We join the power supply adaptor with electrodes’ terminals.
We turn on the power supply adaptor and observe the value of current. If the value is
changing we note it down in 1- minute distances.
7. We calculate the time of electrolysis with the watch or a stopper. It cannot be shorter
than one hour.
8. After the electrolysis is over we turn off the power supply adaptor. We take out the
middle electrode, wash and dry it.
9. Again we weight the middle electrode and note its final mass m2.
The scheme shows the measurement system for electrolysis.
Voltmeter and ammeter are instruments built into the stabilized power supply adaptor.
6. Calculations and description of the results.
After the measurement compute:
•
•
•
•
The change of mass Δm (the loss or the increase),
The charge transported by the electrolysis. Knowing current I, the time of electrolysis
when the current was changing its value, the charge is the sum of charges for every
one- minute period of time.
Compute k and F using the formulas from the instruction,
Introduce your own conclusions.