Chemistry Experiment no 7 (STPM BAHARU)



Topic :         Electrochemistry – Faraday’s Laws



Purpose :       To determine the value of Avogadro’s constant by the electrolysis

procedure.



Theory :       Faraday’s First law states that the mass of a substance produced at an

electrode during electrolysis is directly proportional to the quantity of

electricity (in Coulombs) passed.

Faraday’s Second law states that the number of moles of a substance

produced at an electrode during electrolysis is inversely proportional to

the charge on the ion. The number of Faradays required to discharge one

mole of an ion at an electrode equals to the charge on the ion.



Apparatus :  (a) One 250cm³ beaker.

                        (b) One set of battery.

                        (c) One ammeter.                   

(d) One rheostat.

                        (e) Some connecting wires with crocodile clips.

                        (f) One stop watch.

                        (g) One hair dryer.

                        (h) One piece of sandpaper.

                        (f) One wash bottle filled with distilled water.

           

Materials :    200cm³ aqueous copper(II) sulphate solution.

2 pieces of copper plates.

Propanone solution.





Procedure :   1. Two copper plates are cleaned with sandpaper. Then, it is washed with

    distilled water and propanone. It is dried by using hair dryer.

                        2. The mass of copper plates is measured and recorded.

                        3. The apparatus is arranged as shown as figure 1. The ammeter reading

    (± 1.0 A) is fixed.

                        4. The copper plates are dipped into aqueous copper(II) sulphate solution

    and switched on.

                        5. After 20 minutes, the copper plates are removed. It is washed with distilled water and
                            dried it. Its mass is measured and recorded
Results :        (b) The readings are recorded and the table below is completed.
 
Initial mass/g
Final mass/g
Mass /g ( Final- Initial)
Copper at anode
1.58
1.76
0.18
Copper at cathode
1.52
1.32
-0.20

Calculation   :

                      Current , I                                                       = 0.5A
                        Time, s                                                 = 20 x 60 = 1200s
                        Quantity of electricity passed through, Q = It
                                                                                                = 0.5 x 1200
                                                                                                = 600C
                       
Mass of copper                                                = ( 0.18 + 0.20 )
                                                                                                               2
                                                                                                = 0.19g
                        Relative atomic mass of copper, Cu   = 63.5g
                        Number of mole of copper, Cu                       = 0.19
                                                                                                   63.5
                                                                                                = 2.99 x 10־³ mol
                       
                        Cu (s) → Cu² (aq) + 2e־
                       
                        1 mol of copper                                   = 2 mol of electrons
                       
Number of Faraday required = 2
                                                                              2F  =          600
                                                                                       2.99x10־³


                                                                                F = 1.00 x 10 C mol

                                                                                F = Le

                                                                                    = 1.00 x 10

                                                                                       1.60 x 10־

                                                                                    = 6.23 x 10²³ mol־¹



Conclusion    : The real Avogadro’s constant is 6.02 x 10²³ but the experimental

                          Avogadro’s constant is 6.23 x 10²³.



Comment      : The reading is slightly different from the value on theory. There are

                           some parallax error when the experiment is carried on.
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