Auto-discharge of a cell: experimental proposal from a continuous professional development courses about applied electrochemistry

Continuous professional development (CPD) courses for high school teachers have shown that they have difficulties in explaining basic concepts of electrochemistry1,2,3. In an applied Electrochemistry course (40 hours in five days) the movement of ions, the function of the saline bridge and the galvanic cell auto-discharge were approached from experimentation. Two galvanic cells were constructed: 1) Zn(s)|H2SO4 0,5 mol.L-1| C(grafite) e 2) Zn(s)|NaCl 0,5 mol.L-1||CuSO4 0,5 mol.L-1| C(grafite)4 and questions were proposed followed by discussion. The twenty-one participating teachers were evaluated before and after the course in order to verify their progress. Figure 1 shows an improvement of correct answers. However, the wrong answers number is still high. An experiment is proposed in this paper to better approach these concepts. In addition to cell 1, it is suggested the construction of the cell Zn(s)|ZnSO4 0,5 mol.L-1||H2SO4 0,5mol.L-1 |H2|C(grafite) in which a 1 mol.L-1 KCl salt bridge is placed between the half-cells. It is suggested also the use of Daniell cell with and without saline bridge. The experiment allows to compare the two galvanic cells, identical when considering its electromotive force (fem), and to verify the role of the saline bridge in eliminating the auto-discharge. It is possible to discuss the transport of ions and electrons, fem, current and power in an electrochemical circuit. From the applied Electrochemistry course it was possible to conclude that these concepts need to be emphasized in CPD courses for high school teachers.