Determining the Effect of the Cathode and the Concentration of the Electrolyte on the Voltage of a Galvanic Cell
Introduction:
Many of the devices and gadgets we use in our everyday lives are powered by electric cells and batteries. All the cells and batteries are based on the galvanic cell. A galvanic cell is made of two half cells (an electrolyte and an electrode) that produce electricity. This production of electricity is a result of a spontaneous redox reaction that occurs in the two connected half cells.
A redox reaction is when a reduction and an oxidation reaction occur simultaneously. A reduction reaction is a chemical reaction that involves the gain of electrons, while the oxidation
reaction involves the loss of electrons. For example:
2 H+(aq) + 2 e- -> H2(g) (reduction – H+ is reduced)
Cu(s) -> Cu2+(aq) + 2 e- (oxidation – Cu is oxidized)
If the two reactions are combined, it is a redox reaction:
Cu(s) + 2 H+(aq) -> H2(g) + Cu2+(aq)
Some elements and compounds are more easily reduced than others. They are called strong oxidizing agents. In general, non-metals are more easily reduced than metal ions. Elements that are harder to reduce are easier to oxidize. They are strong reducing agents. Therefore metals are generally easier to oxidize than non-metal ions. A redox reaction is spontaneous when the stronger oxidizing agent is being reduced and the stronger reducing agent is being oxidized. Consequently the above example is a spontaneous redox reaction.
In a galvanic cell, one of the half cells has the negative electrode, which is called the anode. Oxidation reactions take place at the anode. The metal electrode loses electrons, and forms ions, which dissolve in the electrolyte. The electrons travel along an external wire to the positive electrode. The positive electrode is called the cathode. Reduction reactions take place at the cathode. Ions in the electrolyte surrounding the cathode gain the electrons and form solids. The two electrolytes surrounding the cathode and the anode are connected with a salt bridge to allow negative ions to flow to the anode and positive ions to flow to the cathode. This prevents the anode from becoming too positive and the cathode from becoming too negative. Thus the current continues to flow along the external wire. A device can be connected along the external wire, which will then operate because of the flow of electricity.
The voltage across the two half cells can be determined by the reduction and oxidation reactions taking place. Each reduction or oxidation reaction produces or absorbs a certain amount of energy per electron (voltage). By using different combinations of cathodes and anodes, different voltages can be obtained across the two half cells.
The most common application of the galvanic cell is the electric cell. Commonly, the electric cell uses only one electrolyte. The electrolyte can be a solution or a paste of jelly-like substance. There are different kinds of electric cells: primary cells, secondary cells and fuel cells.
A primary cell is the kind we most commonly see. The AA and 9 volt batteries we usually use are primary cells. A primary cell cannot be recharged. Once the cell is used, it has to be discarded. The diagram above is a zinc-chloride dry cell. (Zinc-chloride, the alkaline dry cell and the mercury cell are all primary cells.)
A secondary cell is a rechargeable cell. Electricity is used to reverse the reactions that
occur for the cell to produce electricity. (Nickel-cadmium and lead-acid cells are secondary cells.) Secondary cells can therefore be used repeatedly.
A fuel cell can also be used continuously. “Fuel cells produce electricity by the reaction of a fuel.” (Hans Van Kessel, p.691) As long as the fuel is supplied, the fuel cell continues to produce electricity. The hydrogen-oxygen fuel cell is an example of a fuel cell. It uses hydrogen
and oxygen as fuels and only produces water, making it completely environmentally friendly.
This experiment uses an electric cell similar in design to the zinc-chloride cell. The aluminum can is one electrode and a metal rod placed in the can is the second electrode. The electrolyte is a solution. By measuring the voltage of the cell with different metal electrodes and with different concentrations of the electrolyte, this experiment attempts to determine which has a greater effect on the voltage—the type of electrode or the concentration of the electrolyte.
Conclusion:
It was seen from the experiment that changing the cathode of the galvanic cell had a greater effect on the voltage than changing the concentration of the electrolyte.
Bibliography:
Van Kessel, Hans. Chemistry 12. Ontario, Toronto: Nelson, 2003