The Electrolysis Of Copper (ii) Sulphate Solution Using Copper Electrodes

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20l COURSEWORK2000/2001                       SAM MOED

CHEMISTRY: 

THE ELECTROLYSIS OF COPPER (II) SULPHATE SOLUTION USING COPPER ELECTRODES

1.PLANNING:

A. POSSIBLE FACTORS WHICH COULD AFFECT    THE MASS OF COPPER DEPOSITED OR LOST:

There are in fact many factors that could affect the mass of Copper lost or deposited on the electrodes:

  • The current intensity or strength
  • Time (for how long the current is passed)
  • Concentration of the CuSO4 solution
  • Size and surface area of the plates
  • Depth of immersion of plates
  • Temperature of solution
  • Distance between plates
  • Voltage

The factor that is to be investigated in this experiment is one of these: current intensity and so all the others will have to be kept constant to be sure that any correlation between the increase in current and the increase in mass is due to the former only.

B. Electrolysis in general

Electrolysis is the chemical decomposition of an ionic compound by the passing through it of an electric current. The ionic compound must either be in the molten state or in aqueous solution; this is because the ions need to be mobile since it is the movement of ions that constitutes the conductivity.

Electrodes are rods or in this case plates of a conducting material by which current enters or leaves the ionic compound called the electrolyte. The negative electrode is called the cathode by which the current enters the electrolyte and the positive electrode is called the anode by which the current leaves the electrolyte.

The ions become attracted to their oppositely charged electrodes; so the anions go the anode and the Cations to the Cathode. When these ions reach the electrodes lose their charge by the processes of oxidation at the anode and reduction at the cathode, the term for this is discharging.

When the anode is made of an unreactive metal, such as in this experiment Copper, no anion actually discharges at it. Instead the anode itself loses electrons which change into cations and pass into the electrolyte mixture. The reason why this change occurs rather than an anion discharging is because it requires less energy and hence occurs more easily.

The discharged ions are the products of electrolysis and if this is solid like Copper it deposits at one electrode.

In the electrolysis of Copper(II) Sulphate solution using Copper electrodes the ions present are CU2+ and SO42- from CuSO4 and H+ and OH- from H2O:

                                                                                               

The ionic half equations in this experiment are:

At the anode no ions will discharge since it is easier for the anode to lose electrons which become Copper ions in the solution (no discharge of SO42+ or OH-):                         Cu(s) – 2electrons  Cu2+ (aq)

At the cathode Cu2+ discharges in preference to H+ since it is lower in the reactivity series:      Cu2+ + 2electrons  Cu   

It can be seen from these ionic half equations that for every one Copper ione produced in the solution from the anode, one Copper ion is discharged at the cathode as Copper which means that the loss in mass at the anode equals the gain in mass at the cathode. This means that the concentration of Copper(II) Sulphate doesn’t change. The overall effect of this electrolysis is the transference of Cu from the anode to the cathode.

The current in the wires is constituted by electron flow whereas the current through the electrolyte is constituted of ion flow. The process of electrolysis in general is shown in the diagram below:

C. QUANTITATIVE ELECTROLYSIS:                                 

There is a method of calculating the quantities and hence masses of products  produced by a certain electrolysis. The calculation involves current and time used.

During all electrolysis the number of electrons lost at the anode must equal the number of electrons gained at the cathode.

When the current is increased more electrons flow, that is more electrons are being passed so more ions are discharged. Doubling the current doubles the number of electrons flowing in a given time and so doubles the number of ions discharged in a given time. So the current is directly proportional to the number of ions discharged.

The number of ions discharged determines the mass of product which is produced. Doubling the former doubles the mass of product formed so the number of ions discharged is directly proportional to the mass of product formed.

The same applies for time:

Doubling the time for which a current is passed doubles the number of ions discharged and so the mass. So time is directly proportional to mass deposited at the electrode:

As a result of these statements it can be derived that Quantity of charge is related to the current and time for which the current is passed by the following formula:           Q = I x t

                 

However charge can be measured in Faradays or Coulombs. When one mole of charge is passed a quantity of charge equal to one Faraday or 96500 Coulombs is represented by this.

Substituted into the formula with Q in Coulombs (C), I in Amps (A) and t in seconds (s):                          C = A x s

This equation is used, in conjunction with the ionic half equation to deduce the mass of product obtained when a certain amount of charge is passed. I will use this method to verify my results.

A current of Y Amps was passed for five minutes through Copper(II) Sulphate solution using copper electrodes knowing the RMM of Copper to be 64:

Q = I x t

    = Y x 5 x 60

    = 300Y

From ionic half equation;  Cu(s) – 2electrons  Cu    (aq)

                                                          193000C       64g

So; 193000C             deposits   64g

        300Y Coulombs deposits   64 x 300Y / 193000 =19200Y / 193000    

                                                                                         = Y x 19200 / 193000

                                                   

D. Collision theory and electrolysis 

For a reaction to occur reactant molecules must collide with each other. However not all collisions between reactant molecules produce a reaction. For this to take place two conditions must be fulfilled:

  1. The reactant molecules must collide head on and not glance each other
  2. The molecules must contain a minimum amount of energy called activation energy. Molecules with less than this don’t react even on head on collision. Only those molecules called activated molecules with Ea or above will react on collision.

The rate of a reaction is therefore dependant on the number of activated molecules and the frequency of collisions them. It is true that the rate of the reaction is directly proportional to the number of activated molecules present.

Therefore the mass of Copper deposited will be dependant upon the rate at which the ions discharge which is dependant on the number of effective collisions of the ions with the electrodes (effective collisions are those where the ions possess activation energy or above for the reaction on collision with the electrodes).

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Increasing current intensity or voltage

Increasing the current or voltage causes the ions to move through the electrolyte faster which increases the frequency of effective collisions. Doubling the current or voltage doubles the frequency of energetic collisions between the ions and electrodes. This means that the mass of product will also double. So mass is directly proportional to the current intensity or voltage.

Increasing the temperature

At any temperature the molecules possess a range of different energies. Those with more energy move faster than those with low energy and some molecules have activation energy. This ...

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