UNDERSTANDING THE GALVANIC CELL
Editor's Note: The following submission by Belle F. ('15) explains how a galvanic cell works based on the galvanic cell that she constructed with Jordan K. ('15) as part of their Independent Study work.
Oxidation-reduction (redox) reactions are chemical reactions that involve a transfer of electrons. The electrons are transfered from a "reducing agent" to an "oxidizing agent." Subsequently, one element in the reaction is "reduced" (it gains electrons) and the other element is "oxidized" (it loses electron). The images above, a diagram and a photo, show a galvanic cell. A galvanic cell harnesses electricity by separating the oxidizing agent from the reducing agent in a redox reaction, thus forcing the electron transfer to happen through a wire. As shown in this specific case, one beaker contains a copper electrode in 1M copper sulfate solution; the other beaker contains a zinc electrode in 1M zinc sulfate solution. The two half reactions that represent this cell are:
In our cell, Zn is oxidized and Cu+2 is reduced. This means that electrons will be transferred through the wire from Zn to Cu+2, making Zn the reducing agent, and Cu+2 the oxidizing agent. But how and why does this happen? The zinc atoms in the zinc electrode in the anode compartment will lose electrons, becoming zinc ions in the zinc sulfate solution. This will be observable through a loss of mass in the zinc electrode. The electrons that leave the zinc electrode will travel through the wire to the cathode (the compartment containing the copper electrode). The positive copper ions in the copper sulfate solution will be attracted to these extra electrons flowing from the anode, causing a build up of solid copper on the electrode. If this were the full extent of our cell, we would notice a build up of negative charge in the cathode and positive charge in the anode. It is necessary to also allow a connection between the solutions in each compartment. This is the purpose of the salt bridge. The decrease in positive copper ions in the copper sulfate solution will cause the negative sulfate ions to be attracted to the positive zinc ions in the anode. Those sulfate ions will flow through the salt bridge from the cathode into the anode, as positive zinc ions simultaneously flow from the anode to the cathode. Without the salt bridge (in our case a U-tube stuffed with cotton balls), the "circuit" would not be completed. It is important to note that in this explanation it may sound like one step is happening distinctly before the next--this is not the case, as the process is happening all at once.
Zumdahl, S. S., S. A. (2003). Electrochemistry. In S. S. Zumdahl and S. A. Zumdahl. Chemistry (pp. 828-832). Boston, MA: Houghton Mifflin.