Chemistry Lab Report 1

CHEMISTRY LAB REPORT

OBJECTIVE

To investigate the electrolysis of copper (II) sulphate using copper electrodes.

INTRODUCTION

Electrolysis is the process in which electrical energy is used to make a nonspontaneous chemical reaction occur. It is also a passage of a direct electric current through an ionic substance (electrolyte) that is either molten or dissolved, resulting in chemical reactions at the electrodes and decomposition of  the electrolyte. Electrolysis takes place in an electrolytic cell. The main components required to achieve electrolysis are: 

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An electrolyte - an ionic or electrovalent compound containing free ions which are the carriers of electric current in the electrolyte. However in a solid salt, where the ions are not mobile, electrolysis cannot occur. A direct current (DC) supply- in the form of batteries or dry cells. They provide the energy necessary to induce chemical reaction in the electrolyte. Electric current is carried by electrons in the external circuit. Two electrodes - electrical conductors which provide the physical interface between the electrical circuit providing the energy and the electrolyte.

The components which make contact with the electrolyte are called electrodes. The electrode which is attached to the negative pole of the battery, and which supplies electrons to the electrolyte, is called the CATHODE. Reduction takes place at the cathode. The electrode which is attached to the positive pole of the battery, and which accepts electrons from the electrolyte, is called the ANODE. Oxidation takes place at the anode. Electrodes of metal, graphite and semiconductor material are widely used. Choice of suitable electrode depends on chemical reactivity between the electrode and the electrolyte and the cost of manufacture. For example, graphite or platinum is usually used in laboratory because they are inert.

There are three types of electrolyte: 

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Strong electrolyte: dissociate or ionizes completely or almost completely to form free mobile ions in the aqueous or molten state. Weak electrolyte: ionizes or dissociates only partially to form free mobile ions. Most of the electrolyte remains as un-ionised molecules. Non-electrolyte: does not ionise at all in solution and thus current does not flow through such solution.

In order to ionize, the aqueous electrolyte which contain hydrogen and hydroxide ions as well as the ions of the solute compete with each other at the electrodes. The ions that are successfully released at the electrodes depend on three factors 1. The position of the ion in the electrochemical series 2. The concentration of the ion in the solution 3. The nature of the electrode

MATERIALS           

Batteries (6V) Connecting wires with crocodile clips Ammeter Rheostat Beaker 250 ml 1M copper (II) sulphate solution Copper strips Acetone Distilled water Electronic balance Stop watch

PROCEDURE

1. Two copper strips are obtained and cleaned with acetone. Then, the strips are rinsed with distilled water and they are finally dried completely. 2. The strips are weighed and their mass is recorded. 3. The strips are then dipped into 50 ml of 1M copper (II) sulphate solution. 4. The electrolytic cell is set up using batteries, wires, ammeter, rheostat and two copper strips dipped in the beaker of copper (II) sulphate solution. 5. The rheostat is adjusted to 1 ampere. The timer is started as soon as the circuit is complete for the reaction. 6. The reaction is allowed to run for 30 minutes. 7. After that, the copper strips are removed from the solution and cleaned with acetone. 8. The copper strips are dried completely and the copper strips are weighed. The mass is recorded.

RESULTS

Initial mass of copper strips (g) Final mass of copper strips (g) Mass (g)

Anode 0.85 0.68 0.17

Cathode 0.75 0.91 0.16

DISCUSSION 8V

Copper electrodes Copper(II) sulphate solution

 Electrolysis of copper II sulphate solution using copper electrodes (participating electrodes)

The ions present in the solution are: copper ions 2+

Cu

chloride ions SO4

2-

hydrogen ions

hydroxide ions

H+

OH-

At 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 or hydrogen redox equilibrium appears higher in the electrochemical series than the copper ion or copper equilibrium, the copper ions are preferentially reduced and copper metal is deposited at the electrode. A brown layer deposited at the cathode is observed. Cu

2+

+ 2e

Cu

After the electrolytic cell is switched off, the copper cathode are taken out and weighed. There is significant change in the mass of the cathode before and after the reaction. The mass increased by 0.16g. This increase and the brown colour shows that there is copper metal deposited at the cathode. At the anode, In this case, the electrode is made of copper and it is easier for the copper to dissolve leaving its electrons behind on the anode than for any other ion to be released. Cu

Cu

2+

+ 2e

After the electrolytic cell is switched off, the copper anode is taken out and weighed. There is significant change in the mass of the anode before and after the reaction. The mass decreased by 0.21g. This shows that there is copper metal loss at the anode as it has been oxidized to form copper(II) ions. The ions released are now motile in the copper(II) sulphate solution.

In short, copper is deposited at the cathode and is dissolved at the anode. Cu

2+

+ Cu

2+

Cu + Cu

Consequently the concentration of copper ions in solution remains constant as the rate of ionization of copper atom and the rate of reduction of  copper(II) ion is the same. That is why there is no significant change in the blue colour of copper(II) sulphate solution at the beginning and the end of  experiment.

PRECAUTIONARY STEPS 

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The copper strips must be cleaned thoroughly using acetone to remove any impurities that might interrupt the reaction. The copper strips must also be dried completely to obtain an exact mass of the copper strips. The copper (II) sulphate solution used as electrolyte must be freshly prepared for the experiment to prevent any impurities that might affect the reaction. The same balance must be used for all weighing purposes to obtain an accurate reading. Lab coats and gloves must be worn at all times when conducting the experiment. The electrolytic cell must be connected according to their respective terminals to ensure current flows in the circuit. Acetone must be handled in the fume chamber as it is highly volatile.

CONCLUSION

At anode, copper metal release two electrons to produce copper(II) ion. At cathode, copper(II) ion receive two electrons to produce copper metal. The product of the electrolysis of copper (II) sulphate is copper metal.

QUESTIONS 1. Explain the processes that occurred in the electrolysis of copper (II) sulphate as carried out in the experiment above. Identify the anode and cathode, and write the equations involved.

The anode is the positive terminal while the cathode is the negative terminal. At anode, copper atom releases two electrons to produce copper(II) ion. Oxidation occurs at the anode. 2+ Cu Cu + 2e At cathode, copper(II) ion receive two electrons to produce copper atom. Reduction occurs at the cathode. 2+ Cu + 2e Cu

2. Briefly explain 3 uses of electrolysis in industries. Electrometallurgy: The process of reduction of metals from metallic compounds to obtain the pure form of metal using electrolysis. Electroplating: The process of layering metals to fortify them. Electroplating is used in many industries for functional or decorative purposes, as in vehicle bodies and nickel coins. Electrolytic Etching: This process etch the metal surfaces like tools or knives with a permanent mark or logo. 

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3. State 3 compulsory precautions that should be taken to improve this experiment and the results obtained. Ensure that the copper strips are cleaned with acetone and dried completely at the beginning of the experiment. The copper (II) sulphate solution must be of freshly prepared to prevent the presence of any impurities so that it does not affect the reaction. The same balance must be used when taking measurements so that accurate results can be obtained. 

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REFERENCE   

Raymond chang chemistry tenth edition http://www.physchem.co.za/OB12-che/electrolysis.htm http://en.wikipedia.org/wiki/Electrolysis