Galvanic Cells: Anodes, Cathodes, Signs and Charges

Electrochemistry is a difficult subject for students at school and beyond and even for their teachers. This article explores the difficult "truth" that, when a current flows from a galvanic cell, positive ions within the cell electrolyte move towards the electrode labelled positive. This seems to contravene the basic rule that like charges repel one another. The author attributes the difficulties he has had with this to the fact that electrolysis is usually learned well before the chemistry of the galvanic cell, and that the terminology and explanation are often complex and sometimes arbitrary (and may seem contradictory). The logic of a complete circuit is that if negative charge flows from the negative terminal to the positive terminal of a cell through the external circuit, then to complete the circuit "within the cell" it must flow from positive to negative. This can be explained once it is realised that the cell is set up to allow the cell reaction to take place only by the transfer of electrons through the external circuit and thus, when no current is flowing from the cell, the electrode compartments are at equilibrium and the emf is balanced by the chemical potential of the cell reaction that "wants" to proceed. The value of labelling the electrodes of a cell is questioned and it is concluded that it is better to maintain the convention of giving signs to the cell electrodes that are consistent with the direction of electron/current flow in the external circuit. It is then necessary to remember that "when current flows" from and through a cell the electrode compartments are no longer at equilibrium and "from an internal perspective" the electrode labelled positive is negative--or less positive than it was at equilibrium and thus it "tries" to gain more positive charge. (Contains 4 figures.)