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ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION:

http://www.ucc.ie/academic/chem/dolchem/html/dict/electrol.html Electrolysis of Aqueous Solutions In the electrolysis of aqueous solutions only one ion is involved in the selective discharge of ions at each electrode during the electrolysis. The ion which is selected for discharge at an electrode depends on a number of factors, including Position of the ions in the electrochemical series, For positive ions, the facility of discharge decrease in going from those least electropositive to those most electropositive. For example, if both copper and hydrogen ions are present in solution, it will be the copper ions which take electrons from the cathode to become copper atoms. For negative ions, the ease of discharge decrease in going from those least electronegative to those most electronegative. Concentration of the Ions in Solution Irrespective of the position of the ions in the electrochemical series, there is a tendency to promote the discharge of the most concentrated ion present. For example, in concentrated sodium chloride solution (i.e. brine) , the two cations present are the chlorine ion and the hydroxyl ion. Although the hydroxyl ion is more easily oxidised than the chlorine ion, it is the chlorine ion which will be discharged because its concentration is much greater than that of the hydroxyl ion. Nature of the Electrode This is not as important as either of the other two factors, except in certain cases. For example in the electrolysis of molten sodium chloride using a mercury cathode, sodium ions are discharged in preference to hydrogen ions which are lower in the series.

Electrolysis of an Aqueous Copper Sulphate Solution using Copper Electrodes The electrolysis of an aqueous solution of copper sulphate using copper electrodes (i.e. using active electrodes) results in transfer of copper metal from the anode to the cathode during electrolysis. The copper sulphate is ionised in aqueous solution. CuSO4 ==> Cu(++) + SO4(-.-) The positively charged copper ions migrate to the cathode, where each gains two electrons to become copper atoms that are deposited on the cathode. Cu(++) + 2e(-) ==> Cu At the anode, each copper atom loses two electrons to become copper ions, which go into solution.

ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION: Cu ==> Cu(++) + 2e(-) The sulphate ion does not take part in the reaction and the concentration of the copper sulphate in solution does not change. The reaction is completed when the anode is completely eaten away. This process is used in electroplating.

Electrolysis of an Aqueous Solution of Sodium Sulphate The electrolysis of an aqueous solution of sodium sulphate using inert electrodes produces hydrogen at the cathode and oxygen at the anode and a neutral solution of sodium sulphate remains unaltered by the electrolysis. Cathode Reaction : 4 H2O + 4 e(-) ==> 2 H2 + 4 OH(-) Anode Reaction : 2 H2O ==> O2 + 4 H(+) + 4 e(-) The overall cell reaction is : 6 H2O ==> 2 H2 + O2 +4 H(+) +4 OH(-) If the reaction is carried out in a Hofmann Voltammeter, with some universal indicator in the solution, it will be noticed that around the cathode the solution becomes alkaline and around that anode the solution becomes acidic. This is explained as follows : At the cathode : Hydrogen ions are being removed from solution, thereby leaving an excess of hydroxyl ions which makes the solution alkaline, and At the anode : Hydroxyl ions are being removed, so leaving an excess of hydrogen ions which makes the solution acidic.

Electrolysis of a solution of dilute Sulphuric Acid The electrolysis of an aqueous solution of dilute sulphuric acid is often carried out in a Hofmann Voltammeter, an apparatus in which the gases evolved at the anode and cathode can be collected in separate graduated tubes. When the solution is electrolyzed hydrogen is produced at the cathode and oxygen at the anode. These gases can be shown to be present in a 2 to 1 ratio and result from the electrolysis of water under acidic conditions. Sulphuric acid is a strong electrolyte is fully dissociated in aqueous solution. H2SO4 ==> 2 H(+) + SO4(2 -) Water is a weak electrolyte and is only slightly dissociated H2O ==> H(+) + OH(-) During electrolysis, the hydrogen ions migrates towards the cathode, and are discharged there (i.e. they gain an electron and are converted to hydrogen gas).

ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION: 2 H(+) + 2 e(-) ==> H2 At the anode the concentration of hydroxyl ions is too low to maintain a reaction and the sulphate ions are not oxidized but remain on in solution at the end. Water molecules must be the species reacting at the anode. 2 H2O ==> O2 + 4 H(+) + 4 e(-) The overall reaction is

Cathode Reaction : 2 H(+) + 2e(-) ==> H2 4 H(+) + 4e(-) ==> 2H2 Anode Reaction : 2 H2O ==> O2 + 4 H(+) + 4 e(-) Overall Cell Reaction: 4 H(+) + 2 H2O ==> 2 H2 + O2 + 4 H(+) For every hydrogen ions discharged at the anode, another hydrogen ion is formed at the cathode. The net result is that the concentration of the sulphuric acid remains constant and this electrolysis consists of the decomposition of water with the overall reaction 2H2O ==> 2H2 + O2

http://wikieducator.org/images/4/46/Electrolysis.pdf 1.Electrolysis : It is a chemical process where a substance in its molten state or in an aqueous solution is decomposed by the passage of electric current. 2.Electrolyte : A compound that allows electric current to pass through itself, when either in a molten state or in an aqueous

ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION: solution, is called an electrolyte. Solutions of sodium chloride, copper sulphate, dilute sulphuric acid are examples of strong electrolytes. Weak electrolytes are those compounds which are poor conductors of electricity when they are in a molten state or in an aqueous solution. 3. Nonelectrolyte : A compound which does not allow electric current to pass through itself in any state, molten or aqueous, is called a non-electrolyte. 4.Electrodes : The strips of metals inserted in the electrolytes for conduction of electricity are called electrodes. The metal electrode connected to the positive terminal of the battery is called the anode (+). The metal electrode connected to the negative terminal of the battery is called the cathode (-). 5.Electrolytic cell : The complete set-up for electrolysis is called the electrolytic cell. This consists of the vessel containing the electrolyte, anode, cathode, battery and wires. Electrolytic cell is also known as a voltameter, since it generates voltage (or current) at its two terminals. To summarize the process of electrolysis, we can say the following Electrolytes dissociate to form negatively charged anions and positively charged cations. The ions conduct electricity through the electrolyte. Cations are attracted towards the negative electrode. They take the excess electrons from the electrode and neutralize themselves. Anions are attracted towards the positive electrode. They give up the excess electrons from the electrode and neutralize themselves. The electrolyte dissociates and the constituent elements of the salt are liberated at the electrodes. Electrolysis of Molten Lead(II) bromide with carbon electrodes Carbon (graphite) electrodes dipped into molten salt which has been strongly heated in a crucible. It is difficult to collect the gases at the electrodes! The salts may be very high melting, so sometimes a small amount of another salt impurity is added to lower the melting point. 4 Reaction at anode: (Oxidation of Bromide ions to bromine gas) 2Br- Br2 + 2eReaction at cathode: (Reduction of lead(II) ions to lead atoms) Pb2+ +2ePb Electrolysis of water ( Electrolysis of dilute sulphuric acid) The apparatus or the electrolytic cell, required for performing electrolysis of water is shown in the figure below. The cell is called Hoffman’s voltameter. Since water is a covalent compound, pure or distilled water is a very weak electrolyte. A few drops of ionic compound like dilute sulphuric acid are enough to make the water become an electrolyte. The anode collects oxygen and the cathode arm collects hydrogen gas. The overall reaction that takes place is: H2SO4 2H+ + SO4 2- 5 Water also is capable of dissociation. H2O H+ + OH- Reaction at the cathode 2H+ + 2 e - H2 Reaction at the anode 4OH - H2O + O2 + 4e - At the anode OH- ions are released in preference to SO4 2ions. This is because it is easier for an OH- ion to give up an electron quickly than for the SO4 2- ion to do so. Example: Electrolysis of sodium chloride solution The ions present in the solution are: sodium ions chloride ions hydrogen ions hydroxide ions Na+ Cl- H+ OHAt the cathode The positive ions are attracted to the negative cathode. There is competition between the sodium ions and the hydrogen ions. As the hydrogen ion | hydrogen redox equilibrium appears lower in the electrochemical series than the sodium ion | sodium equilibrium, then the hydrogen ions are preferentially reduced and hydrogen gas is produced at the electrode (bubbles are seen) 2H+ + 2e H2 At the anode There is competition between the negative ions at the positive anode. The chloride ions compete with the hydroxide ions to release their electrons to the anode. When the solution is fairly concentrated the chloride ions preferentially lose electrons to become chlorine atoms (and then molecules) 2Cl - - 2e Cl2 Ions remaining in solution 6 The ions that are removed from the solution, then, are the hydrogen ions and the chloride ions. This means that the sodium ions and the hydroxide ions remain in the solution i.e sodium hydroxide is also produced. Note: When the solution of chloride ions is dilute then

ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION: OH. ions are preferentially released at the anode. Example: Electrolysis of copper (II) sulphate solution The ions present in the solution are: copper ions sulphate ions hydrogen ions hydroxide ions Cu2+ SO4 2- H+ OHAt the cathode The positive ions are attracted to the negative cathode. There is competition between the copper ions and the hydrogen ions. As the hydrogen ion | hydrogen redox equilibrium appears higher in the electrochemical series than the copper ion | copper equilibrium, then the copper ions are preferentially reduced and copper metal is deposited at the electrode (a pink layer is observed) Cu2+ + 2e Cu At the anode There is competition between the negative ions at the positive anode. The sulphate ions compete with the hydroxide ions to release their electrons to the anode. The hydroxide ions are much better reducing agents and are preferentially released AS OXYGEN GAS and water 4OH- - 4e 2H2O + O2 Ions remaining in solution The ions that are removed from the solution, then, are the copper ions and the hydroxide ions. This means that the hydrogen ions and the sulphate ions remain in the solution - i.e sulphuric acid is also produced

Electrolysis of copper(II) sulfate Copper(II) sulfate solution can be electrolysed using carbon electrodes. During electrolysis: ●

the cathode gets coated with copper

●

bubbles of oxygen are given off at the anode

ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION: Copper(II) sulfate solution is blue because of the copper(II) ions it contains. As these ions are discharged as copper atoms at the cathode, the blue colour of the solution gradually fades.

Factors affecting the amount of product made The amount of product made during electrolysis depends upon the time and theelectric current. The amount can be the mass of solid produced (such as copper) or the volume of gas produced (such as oxygen). The amount isdirectly proportional to the time taken and to the current flowing. For example, if the time taken is doubled or the current flowing is doubled, the amount of product is doubled. If both the time and the current are doubled, the amount of product increases four times.

Half equations and calculations – Higher tier The following half equations describe what happens at the two electrodes. At the cathode: Cu2+ + 2e- → Cu At the anode: 4OH- - 4e- → O2 + 2H2O The amount of electric charge transferred during electrolysis is equal to the current multiplied by the time. This allows you to calculate the amount of product made if you know the current flowing and the time taken.

http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/chemistry_out_there/elec trolysis/revision/3/

ELECTROLYSIS OF DILUTE COPPER SULPHATE SOLUTION:

In this experiment we have made many mistakes that could affect the results leading some of the results to be outliers, the first thing that we have messed up on is the power pack being overloaded. the contact with the wires because the negative...