Nonmetal galvanic cells (Item No.: P )

Transcription

1 Teacher's/Lecturer's Sheet Nonmetal galvanic cells (Item No.: P ) Curricular Relevance Area of Expertise: Chemie Education Level: Klasse Topic: Physikalische Chemie Subtopic: Elektrochemie - Potenziale, Leitfähigkeit, Elektrolyse Experiment: Galvanische Zellen aus Nichtmetallen Difficulty Preparation Time Execution Time Recommended Group Size Intermediate 10 Minutes 30 Minutes 2 Students Additional Requirements: Experiment Variations: Filter paper strips Keywords: galvanic cells, non-metals Information for teachers Introduction Principle In exactly the same way as metals, nonmetals also develop different solution pressures and so different potentials, as soon as they can form redox systems in appropriate solvents. A direct electric voltage is measurable, for example, when a standard hydrogen electrode is connected to a half-cell containing the redox system A potential difference is also generated between a hydrogen half-cell and an oxygen half-cell, and this is utilized for generating current in so-called fuel cells. As it is not possible to make solid electrodes out of non- metals such as oxygen or chlorine, the same trick must be used to construct corresponding electrodes as that used for a hydrogen electrode. A material such as graphitic carbon must be coated with the nonmetal. This is achieved in the following experiment by electrolysis (prior to the measurement). During the electrolysis of sulphuric acid, hydrogen is formed at the negative pole from the reduction of hydrogen or hydronium ions. This hydrogen covers the platinum electrode in a thin, invisible and closed layer, so that a simplified hydrogen electrode is practically produced. At the positive pole, water molecules are oxidized to hydronium ions and oxygen by loss of electrons. Oxygen covers the carbon electrode here, so that an oxygen electrode is practically formed. After electrolysis, we have a galvanic hydrogen/oxygen cell, a so-called fuel cell. On connecting the electrodes of this cell with a wire, the following processes would occur: Hydrogen electrode Oxidation of the hydrogen again forms hydrogen ions, or hydronium ions. The liberated electrons flow through the wire to the other half-cell. As a result of the oxidation process, the hydrogen half-cell is the anode. It forms the negative pole of the galvanic cell. Oxygen electrode

2 Teacher's/Lecturer's Sheet Hydronium ions and oxygen are reduced here to water by the supply of electrons. The oxygen electrode is therefore the cathode. Explanation The same processes occur at the hydrogen electrode during electrolysis and also during the output of current as are described above. At the carbon electrodes of cells 3, 4 and 5, chloride, bromide and iodide ions are oxidized to the corresponding halogens during electrolysis. The halogens partly dissolve in water, and partly deposit on the surface of the carbon electrode, which they coat with a thin film. In this way, chlorine, bromine and iodine electrodes are practically formed. These generate the volt- ages measured against the hydrogen electrode. During the output of current, the reverse processes occur to those during electrolysis. The halogens again pass into the ionic state (= reduction) and diffuse into solution. Electric voltages are measurable between the redox system and the redox systems They decrease from chlorine to iodine. The literature values for the standard potentials of the halogens in the succession given above are: V, V and V. Educational objectives So far, the students revised the conventional galvanic cells. In this experiment although, the students will get to know galvanic cells that are set up with nonmetals such as halogens. Preparation of solutions required in the experiment Sulphuric acid (0.5 mol/l): Add 100 ml of distilled water to a beaker. Pipette 13.8 ml of sulphuric acid (96 %) to the beaker and fill up to 500 ml with distilled water. Potassium chloride solution (1 mol/l): Add 37.3 g of potassium chloride to 250 ml distilled water. Stir well and fill up to 500 ml with distilled water. Potassium bromide solution (1 mol/l): Add 59.5 g of potassium bromide to 250 ml distilled water. Stir well and fill up to 500 ml with distilled water. Potassium iodide solution (1 mol/l): Add 83 g of potassium iodide to 250 ml distilled water. Stir well and fill up to 500 ml with distilled water. Potassium nitrate solution (1 mol/l): Add 55.5 g of potassium nitrate to 250 ml distilled water. Stir well and fill up to 500 ml with distilled water.

3 Teacher's/Lecturer's Sheet Fig. 1: Experimental set-up Equipment Position No. Material Order No. Quantity 1 Digital multimeter Connecting cord, 2 mm-plug, 5A, 500 mm, red Connecting cord, 2 mm-plug, 5A, 500 mm, blue Reducing plug 4mm/2mm socket, Alligator clip, insulated, 2 mm socket, 2 pcs Block with 8 holes, d = 40 mm Coverage f.cell-meas.bloc,8 piec Graphite electrode,d=5,l=150,6pc Electrode platinum,short Glass beaker DURAN, tall, 50 ml Dropping bottle,plastic,50ml Flat battery, 4.5 V Additionally needed: Sulphuric acid (c = 0.5 mol/l) Potassium chloride solution (c = 1 mol/l) Potassium bromide solution (c = 1 mol/l) Potassium iodide solution (c = 1 mol/l) Potassium nitrate solution (c = 1 mol/l) Filter paper strips

4 Teacher's/Lecturer's Sheet Safety information The general information for safe experimenting in natural science classes shall be applied in this experiment. Potassium bromide and potassium chloride solutions of concentration c = 1.0 mol/l, and also sulphuric acid solutions of concentration c = 0.5 mol/l, act as irritants. Potassium iodide solutions of concentration c = 1.0 mol/l are harmful if swallowed and may cause sensitization on skin contact. Protect eyes and skin. Avoid contact of the chemicals with eyes and skin. Wear protective gloves and protective glasses!

5 Nonmetal galvanic cells (Item No.: P ) Introduction Application and task Application The discovery and further development of galvanic elements, better known as batteries, is of great importance for humankind. It has enables mobile power supply of various electrical devices, which is a big part of our today's living standard. In exactly the same way as metals, nonmetals also develop different solution pressures and so different potentials, as soon as they can form redox systems in appropriate solvents. A potential difference is also generated between a hydrogen half-cell and an oxygen halfcell, and this is utilized for generating current in so-called fuel cells. Task To prepare a simplified standard hydrogen electrode and also oxygen, chlorine, bromine and iodine half-cells. These half-cells are then to be combined to galvanic cells and their standard potentials measured. Fig. 1: Experimental set-up

6 Equipment Position No. Material Order No. Quantity 1 Digital multimeter Connecting cord, 2 mm-plug, 5A, 500 mm, red Connecting cord, 2 mm-plug, 5A, 500 mm, blue Reducing plug 4mm/2mm socket, Alligator clip, insulated, 2 mm socket, 2 pcs Block with 8 holes, d = 40 mm Coverage f.cell-meas.bloc,8 piec Graphite electrode,d=5,l=150,6pc Electrode platinum,short Glass beaker DURAN, tall, 50 ml Dropping bottle,plastic,50ml Flat battery, 4.5 V Additionally needed: Sulphuric acid (c = 0.5 mol/l) Potassium chloride solution (c = 1 mol/l) Potassium bromide solution (c = 1 mol/l) Potassium iodide solution (c = 1 mol/l) Potassium nitrate solution (c = 1 mol/l) Filter paper strips

7 Set-up and procedure Task 1 As shown in Fig. 2, fill measuring cells 1 and 2 with sulphuric acid (c = 0.5 mol/l), measuring cell 3 with potassium chloride solution, measuring cell 4 with potassium bromide solution and measuring cell 5 with potassium iodide solution. Connect all 5 filled measuring cells with keys made from filter paper strips wetted with potassium nitrate solution as shown in Fig. 2 and place covers on them. Insert a platinum electrode in cell 1 and a carbon electrode in each of the other 4 cells. Connect the platinum electrode in cell 1 to the negative pole and the carbon electrode in cell 2 to the positive pole of a source of direct voltage (4.5 V battery or transformer with rectifier) and electrolyze the sulphuric acid between these two electrodes for about 3 to 5 minutes (connection as in Fig. 1). At the end of this time, break connection to the source of voltage and quickly connect the measuring instrument (set at 2 V-) to the galvanic cell as shown in Fig. 3 (connect the platinum electrode to the earthed socket and the carbon electrode to the voltage socket). Read off the displayed voltage! Measure the voltage and note it down in the report. Fig. 2 Fig. 3 Task 2 Connect half-cell 1 (= the hydrogen electrode) to the negative pole and half-cell 3 to the positive pole of a source of direct voltage, and again electrolyze for 3 to 5 minutes with a voltage of from 4 to 5 V-. Subsequently measure the volt- age between the two half-cells. After having used half-cell 3 for a measurement, carry out the same procedure with half-cell 4 and with halfcell 5 (in each case, first hydrolyze the hydrogen electrode for 3 to 5 minutes and then measure the voltage). Measure the voltage and note it down in the report.

8 Protokoll: Galvanische Zellen aus Nichtmetallen Ergebnis - Frage 1 (5 Punkte) Notiere deine Beobachtungen zur Aufgabe 1. Ergebnis - Frage 2 (5 Punkte) Notiere deine Beobachtungen zur Aufgabe 2.

9 Auswertung - Frage 1 (10 Punkte) Formuliere die ReaBtionsgleichung der Schwefelsäure am Minuspol. Auswertung - Frage 2 (15 Punkte) ErBläre weiterhin, wodurch die Wasserstoff- bzw. SauerstoffeleBrode besteht.

10 Auswertung - Frage 3 (10 Punkte) Zwischen welchen Redoxsystemen ist eine elebtrische Spannung messbar?