Experiment 12

Experiment 12

Topic :  Electrochemistry – Electromotive force of a cell.

Purpose :              To investigate the effect of concentration on the electromotive force of a

                                cell.

Theory :           Zn²+ (aq) + 2e ® Zn (s)

                                If the concentration of  Zn²+(aq) ion decreases, the equilibrium position will

                                shift to left and EZn will become more positive. Thus, the electromotive

                                force of the electrochemical cell increases.

Apparatus :          -    5 strips of filter paper                                 

-          1 zinc plate and 1 copper plate

-          Galvanometer                                                             

-          Potentiometer / wheatstone bridge

-          Jockey

-          Five 30cm³ connecting wires with crocodile clips

-          2V accumulator or 2 dry cell (each 1.5V)

-          Two measuring cylinders

-          Six 100cm³ beakers

-    One piece of sandpaper

-    Onewash bottle filled with distilled water

               

Materials :          KA 1  is 1.0 mol dmˉ³ aqueous copper (II) sulphate

                                KA 2  is 2.0 mol dmˉ³ aqueous zinc sulphate

                                KA 3  is 1.0 mol dmˉ³ aqueous zinc sulphate

                                KA 4  is 0.1 mol dmˉ³ aqueous zinc sulphate

                                KA 5  is 0.01 mol dmˉ³ aqueous zinc sulphate

                                KA 6  is 0.001 mol dmˉ³ aqueous zinc sulphate

                                A saturated solution of potassium chloride.

Procedure :         (a) By means of measuring cylinders, fill beaker A with 50.0cm³ of

                                      KA 1  and beaker B with 50.0cm³ of   KA 2 . Immerse a strip of

                                      filter paper into the saturated solution of potassium chloride;

                                      then dip the ends of the filter paper into the 2 beakers A and B

                                      to build a salt bridge. Clean the copper and zinc plate with

                                      sandpaper, wash with distilled water, and dry. Clip the copper

                                      plate with the crocodile clip and connect it to the end of the

                                      potentiometer to which the positive terminal of the accumulator

                                      is connected. The zinc metal is connected to a galvanometer to

                                       which a jockey is attached. The negative terminal of the

                                       accumulator is then connected to the other end of the

                                       potentiometer as shown in the diagram below.                     

                                   Dip the two metal plates into their respective solutions. After

                                          one minute has elapsed, touch the jockey along the length of

                                          the potentiometer wire until the galvanometer shows a zero

                                          reading indicating no current flow. Do not slide the jockey

                                          along the length of the potentiometer wire to avoid a build up

                                          of resistance. Note down the length of the potentiometer wire

                                          as x cm and record your readings in the table on the next page.

                               

                                          Repeat the experiment above by replacing solution  KA 2  with

                                          solutions as shown in the table below. (New filter paper are to

                                          be used for every new solution.)

                                          Calculate Ecell  =      __________x_____________  ×  2.0V

                                                                                                Length of potentiometer wire

Results :               (b) Length of potentiometer wire = 100.0cm

Experiment	Beaker A	Beaker B	Length of Wire x/cm	Ecell  /V	[ Zn²+  ] / mol dmˉ³	lg [ Zn²+ ]
1	50.0cm³ KA1	50.0cm³ KA2	40.3	0.806	2.000	0.301
2	50.0cm³ KA1	50.0cm³ KA3	44.5	0.890	1.000	0.0
3	50.0cm³ KA1	50.0cm³ KA4	50.5	1.010	0.100	-1.000
4	50.0cm³ KA1	50.0cm³ KA5	64.0	1.280	0.010	-2.000
5	50.0cm³ KA1	50.0cm³ KA6	71.1	1.422	0.001	-3.000

Question :          

(c)  Write a cell diagram for the electrochemical system.

                 Zn (s) / Zn²+   //  Cu²+  (aq) / Cu (s)

\(d)  Write half-cell equations for the reactions taking place in beaker A and B and hence

       write an equation for the overall cell reaction.

                                                Beaker A:  Zn (s) → Zn²+ (aq) +  2eˉ

                                                Beaker B:  Cu²+ (aq)  +  2eˉ  →  Cu (s)________

                                                Overall   : Zn (s) + Cu²+ (aq) →  Zn²+(aq) + Cu (s)

                                  

(e)  Plot a graph of Ecell against lg [ Zn²+  ] in beaker B and explain  how the e.m.f of

      the cell varies with the concentration of Zn²+  in the beaker B.

           (f)  What will happen to the Ecell value if

(i) The solution in beaker A is replaced with 0.1 mol dmˉ³ aqueous copper (II)

      sulphate?

                     The value of Ecell will decrease because the concentration of Cu²+ ions have

                 decreased.

                (ii) The solution in beaker B is replaced with 0.1 mol dmˉ³ aqueous aluminium

      chloride, and the plate of zinc metal is replaced with aluminium metal?

      The electrode potential of the aqueous aluminium chloride and copper metal.

      Therefore, the e.m.f becomes higher.

Conclusion :        The electromotive force of a cell is inversely proportional to the

                                concentration of Zn²+  in beaker B.

Comment :           1. The resistance of copper wires are different because the wires used are

    not the same.

                                2. The concentration of potentiometer which is not tight enough will cause

    the reading recorded not accurately.

                                3. The contact between the jockey and the copper wire is not perfect and it

    causes the reading not accurately.

                                4. Current provided in the dry cell will decreased during the experiment

    and causes the reading not accurately.