DETERMINATION OF ELECTRODE POTENTIALS

J.G.K.B. CORTEZ11COLLEGE OF PUBLIC HEALTHUNIVERSITY OF THE PHILIPPINES, ERMITA, CITY OF MANILA 1000, PHILIPPINESDATE SUBMITTED: JULY 7, 2015DATE PERFORMED: JULY 14, 2015

ANSWERS TO QUESTIONS

1. Discuss the differences between galvanic and electrolytic cells.

Galvanic cells (also known as voltaic), is a type of electrochemical cell that stores electrical energy. The most common type of these is batteries. The distinguishing characteristic of galvanic cells is that the reaction occurring in the electrodes does not need any external force to happen. Thus, it is spontaneous (Skoog, West, Holler, & Crouch, 2004). Apart from this, the flow of electrons is observed to be from the anode, where oxidation occurs, to the cathode, where reduction occurs, thru an external conductor (Chang, 2010). This means that the negative electrode (anode), when the cell is discharged, will be the potential source of electrons to the circuit of the system. It can be noted that due to the spontaneous nature of the cell, the net reaction during discharged is called spontaneous cell reaction (Skoog, West, Holler, & Crouch, 2004).On the other hand, there also exists another type of cell called an electrolytic cell. This particular type of cell needs an external source of electrical energy to produce the required reaction between the cells (Skoog, West, Holler, & Crouch, 2004). Apart from this, the electron flow of electrolytic cells is the opposite of voltaic ones. The electrons will start from the cathode, and then transfer to the anodes. This is due to the reversal of reaction in each electrode (Chang, 2010). A good example for this type of cell would be the process of metal plating (Skoog, West, Holler, & Crouch, 2004).

2. What is the measured value of Ecell for each cell and its relevance to the Eored for the iron half-cell and halogen half-cells?

Table1. Ecell values of each cellElectrochemical CellEcell

Zinc-Copper Cell1.08 V

Iron-Copper Cell0.30 V

Bromine Cell0.18 V

Iodine Cell0.007 V

The Ecell will be the used in determining the Ered of the substance being observed through substitution with equation 1 for standard state conditions and 2 for halide group cell calculations. Through substitution and manipulation of data and units, Ered has been calculated for the cells (Chang, 2010). (1) (2) 3. Compare the calculated Eored for each half-cell with their literature value.Table2.Calculated EOred for each half-cell and their theoretical valuesHalf CellExperimental (V)Theoretical (V)

Zn2+|Zn-0.74-0.763

Fe3+|Fe2|C0.64+0.771

Br-|Br2|C-+1.087

I-|I2|C-+0.536

As the researchers have computed for Zinc and Iron, there are certain discrepancies with the experimental and theoretical values of the reduction potential. As seen in table 2, Zinc has a 3.01% error for the experimental value while Iron has 16.99% error. As the researchers have failed to gather accurate data with respect to Bromine and Iodine, it has been decided that the theoretical data shall still be put in the table for reference. It can be seen that Bromine has a high reduction potential. Its half-cell reaction is readily reduced; therefore it is a good oxidizing agent (Chang, 2010). Apart from this, Iodine has also shown a positive reduction potential meaning it also is a good oxidizing agent though not as good as bromine.

4. Based on your observations, what are the most effective reactions which occur at the anode and cathode during electrolysis?The most effective reactions are supposed to be with Bromine and Iodine at the anode which will both show, theoretically, a yellowish color once the current is passed through. These halides are easily reduced and are seen as good oxidizing agents. Once electrolysis is done, Bromine and Iodine are readily reduced and will gain electrons in the process (Blaber, 2010). 5. Based on the calculated values, what are the most effective reducing and oxidizing agents?Based on Table 2 in question number 3, we can see that the biggest reduction potential present in the experiment is from the Br-|Br2|C half cell reaction. Although the value is theoretical, it is believed to have a reduction potential of +1.087 (Skoog, West, Holler, & Crouch, 2004). This means that it is the most powerful oxidizing agent in the group of substances being observed. On the other hand, the table also showed that Zn2+|Zn half cell reaction has the most negative reduction potential. This shows that it has a biggest oxidation potential in the group with an experimental value of 0.74 V. Zinc is the best reducing agent among the substances being observed.6. Are values of standard reduction potentials helpful in determining the spontaneity of a reaction?As Skoog, et al. has stated (2004), a positive electrode potential of an electrochemical cell exhibits a spontaneous reaction with respect to its comparison to the standard hydrogen electrode half-reaction. A positive electrode potential is essentially the value of a reduction potential (Chang, 2010). It has the ability to indicate if the substance has a great tendency to be reduced. As the case of Gibbs free energy, a positive Eo means the redox reaction will favor the formation of products at equilibrium. That is, the reaction will display a negative Go for spontaneity (Blaber, 2010).

7. What are the possible sources of errors and their effect on the calculated parameters? Rationalize.

Apart from personal errors during solution preparation, it can be noted that other factors might affect the calculated parameters of the experiment. One of this is the possible contamination of another metal to the ion being observed. Contamination may come from unclean specimen containers or from reusing spatula while transferring metals. The presence of other metals cans lower or higher the reading depending on the ions identity (Jenkins, 2007). The failure of the multimeter can also influence the results of the experiment. This may stem from the simplest of causes such as the battery shortage. It should be noted that batteries for digital devices used in the laboratory should be monitored and safely stored in temperature-stable environments. Apart from battery issues, product defect may also be considered. Issues regarding knobs, the LED screen, and open wires should always be taken into account when using the multimeter (Jenkins, 2007).Also, the researchers should be wary of the placement of cathode and anodes depending on the cell being used. Electrodes should always be placed in the beaker in such a manner that it will be submerged in the solution without touching the salt bridge. Touching the salt bridge may increase the reading due to the interaction with electrons in the area (Jenkins, 2007).

REFERENCESBlaber, M. (2010). Sponteneity of Redox Reactions. Retrieved July 8, 2015, from mikeblaber.org: http://www.mikeblaber.org/oldwine/chm1046/notes/Electro/Spontan/Spontan.htmChang, R. (2010). Chemistry, 10th Edition. In R. Chang, Chemistry, 10th Edition (p. 842). McGraw-Hill.Jenkins, F. (2007). Nelson Chemistry. Toronto: Thomson Nelson.Skoog, D., West, D., Holler, J., & Crouch, S. (2004). Fundamentals of Analytic Chemistry. In S. Kiselica (Ed.). Thomson Learning Academic Resource Center.

COMPUTATIONS

Zn2+|Zn

Due to the absence of data, the researcher have just decided to put the formula for halide group EOcell computations in this part of the ATQ.

It can be transformed to this form

and