Electrovoltaic Cells and Batteries

 

 

Electrochemistry is the branch of chemistry that deals with electric currents and how they are formed from oxidation-reduction reactions (Redox reactions). Redox reactions occur when two or more elements react in solution forming different ions. Reduction occurs when one of the elements gains electrons and oxidation occurs when the other looses the electrons. In a redox reaction, both oxidation and reduction take place in two “half reactions” that together form the redox reaction. (Faraday, 2004)(Masterton, 2001)

An acid is a substance that will release H⁺ ions in a solution. There are six strong acids, hydrochloric (HCl), sulfuric (H2SO₄), bromic (HBr), iodic (HI), and chloric (HClO₄). Strong acids dissociate completely in solution forming positive and negative ions as opposed to weak acids, which do not dissociate completely. Because the acids dissociate completely, there will be free electrons that can be used to create an electric current. By using different anodes (-) and cathodes (+) these electrons can be used in a single cell or a multicelled battery. (Masterton, 2001)

The concentration of the acid and the ability of the metals to react determine the voltage of the cell. Concentration is measured in molarity. Molarity (M) is the ratio of moles of acid per liter of water in a solution. The greater the molarity of the acid, the greater the amount of volts produced. For example, a 1 Molar solution of sulfuric acid would not produce the same voltage as a 10 Molar solution of sulfuric acid. (Faraday, 2004)(Masterton, 2001).

Not all cells use acids to produce a current. In a Zinc-Copper cell, the two reactions that make up the redox reaction are

Cu²⁺+2e^-àCu E^o=+0.339V

ZnàZn²⁺+2e^- E^o=+0.762V

When combined, the total redox reaction is

            Cu²⁺+ZnàCu+Zn²⁺ E^o=1.10V

The E^o is the standard voltage of that reaction when using one molar concentrations of solutions. Like in a lead-acid cell, the voltage depends on the concentration of solutions used. (Faraday, 2004)(Masterton, 2001).

The equation used to calculate the voltage produced in a redox reaction that does not use one molar solutions is

            E=E^o-RT/nF x lnQ

This is known as the Nernst Equation after its discoverer. In the Nernst Equation, R is a constant 8.31J/mol K. T is the temperature in Kelvin, F is Faradays constant (9.648x10⁴ J/mol V), and n is the number of moles. Q is concentration. The equation to find Q of a reaction is

            Q=[C]^c+[D]^d/[A]^a+[B]^b

[A] is the concentration of A. Pure solids and liquids are left out of this equation.

 E^o is the standard voltage and is found by using the equation

                        E^o=E^o_red+E^o_ox

E^o_red is the volts produced in the reduction reaction, and E^o_ox is the volts produced in the oxidation reaction. These values are taken from a table of standard values for one molar solutions. (Faraday, 2004)(Masterton, 2001).

An electric current is created when electrons flow to a cathode (+) from an anode (-). The electrons are produced by the oxidation reaction and are consumed in the reduction reaction. For example, electrons will flow from zinc to copper in a zinc-copper cell because zinc will undergo an oxidation reaction where it is oxidized into Zn²⁺ ions and 2 electrons. The electrons flow through an exterior circuit, such as a voltmeter, and then react with Cu²⁺ ions to form copper metal. The ionic solutions that the metals are in provide free ions of the appropriate type in order to facilitate the reaction. A salt bridge consisting of an non-reactive salt solution is necessary for electrons to travel from one side of a cell to the other. This completes the electric circuit. (Carboni, 1998)

 

 

Michael Faraday was a very influential man in the field of electrochemistry. After his humble beginnings as a child, he eventually got a job at the Royal Institute of Great Britain. Starting off as a laboratory assistant, he worked hard and made many great discoveries, that he was eventually hired as the director of chemistry where he made even more great discoveries. Faraday’s experiments at the Royal Institute included the discovery of electrolysis to separate elements and ways to make and calculate the theoretical yield of different batteries. His laws and equations can be used to make better batteries to power our on-the-go lifestyle today. (Unknown, 2005)(Unknown, 2002)(Carboni, 1998)

 

                                                                                                Diagram from (Carboni, 1998)
Hypothesis

 

It is hypothesized that the lower the concentration of the solutions used in an electrovoltaic cell, the lower the voltage will be when compared to the voltage of a one molar cell as predicted by the Nernst equation. Thus, a cell using .1 molar solutions will produce less voltage than a cell using .5 molar solutions and both will produce less that a 1 molar cell.

 

Works Cited

 

Carboni, G. (1998, January). Experiments In Electrochemistry. URL http://www.funsci.com/fun3_en/electro/electro.htm.

 

Faraday, M. (2004). Experimental researches in electricity. London: British Publishing.

 

Masterton, W. (2001). Chemistry: principles and reactions. 4th ed. Philadelphia: Harcourt College Publishing.

 

Unknown. (2002). Electrochemistry. URL http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/electroframe

 

Unknown. (2005). Faraday Page. URL http://www.rigb.org/rimain/heritage/faradaypage.jsp.