SCES2252/ SCES2433 : BASIC ELECTROCHEMISTRY/ ELECTROCHEMISTRY ELEKTROKIMIA ASAS/ ELEKTROKIMIA

Transcription

1 UNIVERSITY OF MALAYA UNIVERSITI MALAYA EXAMINATION FOR THE DEGREE OF BACHELOR OF SCIENCE PEPERIKSAAN IJAZAH SARJANA MUDA SAINS EXAMINATION FOR THE DEGREE OF BACHELOR OF SCIENCE WITH EDUCATION PEPERIKSAAN IJAZAH SARJANA MUDA SAINS DENGAN PENDIDIKAN ACADEMIC SESSION 2012/2013 : SEMESTER 1 SESI AKADEMIK 2012/2013 : SEMESTER 1 SCES2252/ SCES2433 : BASIC ELECTROCHEMISTRY/ ELECTROCHEMISTRY ELEKTROKIMIA ASAS/ ELEKTROKIMIA January 2013 Januari 2013 TIME : 2 hours MASA : 2 jam INSTRUCTIONS TO CANDIDATES : ARAHAN KEPADA CALON : This paper consists of 5 questions. Answer 4 questions only. Kertas ini mengandungi 5 soalan. Jawab 4 soalan sahaja (This question paper consists of 5 questions on 6 printed pages) (Kertas soalan ini mengandungi 5 soalan dalam 6 halaman yang dicetak)

2 SCES2252/SCES2433/02 1. (a) A voltaic cell is constructed with two hydrogen electrodes. Electrode 1 has P H2 =1.00 atm and an unknown concentration of H + (aq). Electrode 2 is a standard hydrogen electrode ([H + ] = 1.00 M, P H2 =1.00 atm ). At 298 K the measured cell voltage is V, and the electrical current is observed to flow from electrode 1 through the external circuit to electrode 2. Calculate [H + ] for the solution at electrode 1. Calculate its ph. Satu sel dibina dengan dua elektrod hidrogen. Elektrod 1 mempunyai P H2 =1.00 dan kepekatan tidak diketahui H + (aq). Elektrod 2 adalah elektrod hidrogen standard ([([H + ] = 1.00 M, P H2 =1.00 atm). Pada 298 K voltan sel yang diukur adalah V, dan arus elektrik diperhatikan mengalir daripada 1 elektrod melalui litar luar ke elektrod 2. Kira [H + ] untuk penyelesaian pada elektrod 1. Kirakan phnya. (5 marks/markah) Describe with the diagram and the cell reactions how a concentration cell works. Terangkan dengan gambarajah dan tindak balas sel bagaimana kepekatan sel berfungsi. (9 marks/markah) Show what products will form during the electrolysis of aqueous solutions of the following salts taking 0.4 V to 0.6 V overvoltage into consideration. Mengambil kira 0.4 V hinga 0.6 V overvoltage, tunjukkan apa yang produk akan terbentuk semasa elektrolisis larutan akuas garam berikut: (i) KBr (E o bromine = V, E o potassium = V and E o water = V) (ii) MgSO 4 (E o magnesium = V) (d) In an aqueous electrolytic cell, nitrate ions never react at the anode but nitrite ions do. Explain. Dalam satu sel elektrolisis berair, ion nitrat tidak pernah bertindak balas di anod tetapi sebaliknya di ion nitrit. Terangkan. (3 marks/markah)

3 SCES2252/SCES2433/03 2. (a) By definition, electroplating requires that an electric current flow between the part being plated and the chemical solution. If the part is non-conductive, then an electric current cannot flow. Being an electrochemist, explain in details how a spoon made of plastic can be given a layer of silver coating on it using the electrochemical process. Show the reactions involved with it. Secara definisi, penyaduran memerlukan arus elektrik mengalir antara bahagian yang bersalut dan cecair kimia. Jika sesuatu bahagian bukan konduktif maka arus elektrik tidak dapat mengalir. Sebagai seorang elektrokimia, terangkan secara terperinci bagaimana sudu diperbuat daripada plastik boleh diberikan lapisan salutan perak di atasnya menggunakan proses elektrokimia. Tunjukkan tindak balas yang terlibat dengannya. Show the cell potential for the following reaction when the pressure of the oxygen gas is 2.50 atm, the hydrogen ion concentration is 0.10 M, and the Br - ion concentration is 0.25 M. Menunjukkan potensi sel bagi tindak balas berikut apabila tekanan gas oksigen adalah 2.50 atm, kepekatan ion hidrogen adalah 0.10 M, dan kepekatan ion Br - ialah 0.25 M. (E o bromine = V and E o water = V). O 2 (g) + 4 H + (aq) + 4 Br - (aq) 2 H 2 O (l) + 2 Br 2 (l) (5 marks/markah) A voltaic cell consists of a metal A/A + electrode and a metal B/B + electrode with A/A+ electrode negative. The initial [A + ]/[B + ] is such that E cell E 0 cell. Satu sel terdiri daripada logam A / A + elektrod dan logam B / B + elektrod dengan A / A + elektrod negatif. Permulaan [A + ]/[B + ] adalah seperti yang E cell E 0 cell. (i) (ii) (iii) How do [A + ] and [B + ] change as the cell operate? Bagaimana [A + ] dan [B + ] berubah semasa sel beroperasi? When will E cell become equal to E 0 cell? Explain from Nernst equation. Bilakah E cell akan menjadi sama dengan E 0 cell? Terangkan dengan menggunakan persamaan Nernst. Is it possible for E cell to be less than E 0 cell? Explain. Adakah kemungkinan untuk E cell kurang daripada E 0 cell cell. Terangkan. (6 marks/markah)

4 SCES2252/SCES2433/04 (d) What happens when an aqueous solution of potassium iodide is electrolyzed? Show the half-cell reactions. Apa yang berlaku apabila larutan akueus kalium iodida di elektrolisis? Papar reaksi setengah-sel. (6 marks/markah) 3. (a) Seawater is a very harsh environment for iron objects. They will corrode 5 times faster in the sea than in freshwater. Explain with the iron corrosion reaction. Air laut adalah persekitaran yang sangat keras untuk objek besi. Mereka akan menghakis 5 kali lebih cepat berbanding dengan air tawar. Terangkan dengan persamaan kakisan besi. (6 marks/markah) Describe the following terms: Terangkan istilah berikut: (i) (ii) (iii) Diffusion current Resapan arus aliran elektrik Migration current Migrasi arus aliran elektrik Capacitance current Muatan arus aliran elektrik (4.5 marks/markah) Describe a silver/silver chloride reference electrode. Huraikan elektrod perak/perak klorida rujukan. (2 marks/markah) (d) Discuss the differences between potentiometry and voltammetry. Bincang perbezaan di antara potensiometri dan voltametri. (12.5 marks/markah) 4. (a) Explain why stripping voltammetry is more sensitive compared with polarographic techniques. Terang kenapa teknik stripping voltammetry adalah lebih sensitif berbanding dengan teknik polarografi.

5 SCES2252/SCES2433/05 Discuss excitation (potential vs. time) and response (current vs. time) waveforms for the potential step method. Bincang gelombang excitation (keupayaan vs. masa) dan respon (arus vs. masa) untuk kaedah potential step. Sketch a cyclic voltammogram in which the electrode product is consumed in a competing chemical reaction: Lakar cyclic voltammogram dimana hasil elektrod adalah digunakan dalam persaingan tindak balas kimia: A + e = P P => D Assume that the product, D, is not observed in any further electrochemical reaction. Anggap hasil, D, tidak diperhatikan dalam tindak balas elektrokimia seterusnya. (9 marks/markah) 5. (a) Discuss the potential-time waveform and resulting voltammograms in an Anodic Stripping Voltammetry experiment. Bincang gelombang keupayaan-masa dan keputusan voltamogram dalam eksperimen Anodic Stripping Voltammetry. (12 marks/markah) Chloroform is an internal standard in the determination of DDT in a polarographic analysis in which each compound is reduced at an electrode surface. A mixture containing mm chloroform and mm DDT gave signals of 15.3 A for chloroform and 10.1 A for DDT ml of an unknown solution containing DDT was placed in a ml volumetric flask and 10.2 L of chloroform (M = g/mol, density = g/ml) were added. After diluting to the mark with solvent, polarographic signals of 29.4 and 8.7 A were observed for the chloroform and DDT, respectively. Calculate the concentration of DDT in the unknown. Klorofom adalah internal standard untuk menentukan DDT dalam analisis polarografi dimana setiap sebatian diturunkan pada permukaan elektrod. Satu campuran mengandungi mm klorofom dan mm DDT menghasilkan isyarat 15.3 A untuk klorofom dan 10.1 A untuk DDT ml larutan tidak dikenali mengandungi DDT telah diletakkan dalam ml kelalang volumetrik dan ditambah dengan 10.2 L klorofom (M = g/mol, density = g/ml). Selepas pencairan dengan pelarut pada tanda, isyarat polarografi yang dikesan adalah 29.4 and 8.7 A untuk klorofom dan DDT, berturut-turut. Kira kepekatan DDT dalam larutan tidak dikenali. (13 marks/markah)

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