Voltaic Cells

Oxidation and reduction, which are driven by electron transfers, are two of the most common chemistry reactions and the fundamental building blocks of electrochemistry1. The study of voltaic cells (chemical cells) is critical in electrochemistry, a broad area of study, owing to their large range of applications in the most advanced energy-hungry technologies. The experiment’s aim is to create voltaic cells and observe their properties. Furthermore, the experimental determination of action sequence for many elements will be carried out. Through the construction of several voltaic cells and by investing electron transfer reactions i.e. oxidation and reduction reactions occurring between the two metal plates of these cells, relative reactivity of metals can be examined. This reactivity information will be used to construct an electromotive series for the metals under experiment. Five different metals namely hydrogen, copper, zinc, sodium and magnesium are taken to construct different voltaic cells and to examine their reactivity in the electrochemical reactions.

The values of voltage in different voltaic cells as found from the experiment will be compared to standard voltage3 for corresponding cathode-anode set. Finally, cell potentials of non-standard cells will be measured and compared to the computed values of these non-standard cells to calculate the percentage difference in voltages. Non-standard cells are prepared through by deviating the concentration of the solution in the voltaic cells2.

Experimental Methods

Following equipment are used in the experiment:

Beakers

Voltmeter

Salt Bridge

1 M Sulfuric Acid (H2SO4)

1 M Magnesium Sulfate (MgSO4)

1 M Copper (II) Sulfate (CuSO4)

1 M Sodium Sulfate (Na2SO4)

1 M Zinc Sulfate (ZnSO4)

Small (4”) Strips of Metals (Zinc, Copper, Magnesium) for electrodes

24-Well Plates

A. Determining the Electromotive Series:

In order to observe relative reactivity of different metals, zinc, copper and magnesium metal strips are alternatively placed in aqueous solutions of different metals and reactions are observed for two 15-minutes periods–one after placing the metal strip into the solutions and one after removing it from the solutions. The solutions are poured in different wells of a 24-well test plate and the metal strips are placed in the wells in a way that the solution cover them partially. If the metal strips are found to be capable of replacing ionic metals from the solutions in the specified 15-minute periods, the equations of the reactions that take place are reported. Finally, based on the results obtained, the electromotive series among the metals taken for the experiment are constructed.

B. Voltaic Cells

Three voltaic cells are constructed using the three different types of metal strips (Mg, Cu, Zn) as electrodes and aqueous solutions of their sulfates. The three sets of voltaic cells will use following metallic electrode sets:

Copper/Zinc Voltaic Cell

Copper/Magnesium Voltaic Cell

Zinc/Magnesium Voltaic Cell

For each set of voltaic cells, two 25-ml of aqueous solutions of corresponding metal are poured into two 100-ml beakers and the 4” metal strips are placed inside it. For example, for a Copper/Zinc voltaic cell, 25-ml of 1M CuSO4 is poured inside the 100-ml beaker and the copper strip is placed inside as electrode. Similarly, 25-ml of 1M ZnSO4 is poured inside the 100-ml beaker and the zinc strip is placed inside as electrode. Salt bridge is placed in such way that the solution in each beaker submerge one leg of the bridge.

Cell potentials of these voltaic cells are measured by connecting two leads of the voltmeter to the two metallic strips. The highest stable voltage is recorded as the reading each time. The measured voltages are compared with the standard cell potentials calculated from the reduction potentials of the participating metals and % differences are reported.

C. Effects of Concentrations Cell Potentials

In this part of the experiment, the concentrations of each the two solutions used in each of the voltaic cells are replaced one at a time by less concentrated solutions. The cell potentials are recorded for each replacement made. The percentage deviation in cell potential from the calculated potentials are reported along with possible explanation.

Results

A. Determining the Electromotive Series:

Solution

Metal

Reaction

H2SO4

Zn

No visible reaction

Cu

No visible reaction

Mg

Dissolved Immediately

MgSO4

Zn

No visible reaction

Cu

No visible reaction

Mg

Turned black

CuSO4

Zn

Turned black

Cu

No visible reaction

Mg

Oxidized, turned black

Na2SO4

Zn

No visible reaction

Cu

Finger print visible/ slight oxidized

Mg

No visible reaction

ZnSO4

Zn

No visible reaction

Cu

No visible reaction

Mg

Oxidized

From the above reactivity of metals in solutions, the activity series can be constructed as: Na, Mg, Zn, H, Cu

B. Voltaic Cells

The cell voltages recorded for each set of voltaic cells are recorded in the following table:

Cell Type

Measured Cell Voltage

Copper/Zinc Voltaic Cell

1.080 Volts

Copper/Magnesium Voltaic Cell

1.9 Volts

Zinc/Magnesium Voltaic Cell

0.995 Volts

C. Effects of Concentrations Cell Potentials

The non-standard cell voltage recorded for Copper/Zinc Voltaic Cell is shown in the table below:

Cell Type

Measured Cell Voltage

0.1 M Copper/ 1M Zinc Voltaic Cell

1.056 Volts

1 M Copper/ 0.1 M Zinc Voltaic Cell

1.078 Volts

Zinc/Magnesium Voltaic Cell

0.995 Volts

Discussion

The activity series is in direct alignment of electromotive series.

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References

(1) Rieger, P. Electrochemistry; Chapman & Hall: New York, 1994.

(2) Electrochemical Cells under Nonstandard Conditions

https://chem.libretexts.org/Core/Analytical_Chemistry/Electrochemistry/Voltaic_Cells/Electrochemical_Cells_under_Nonstandard_Conditions (accessed Aug 7, 2017).

(3) Kotz, J.; Treichel, P.; Weaver, G. Chemistry & chemical reactivity; Thomson Brooks/Cole: Belmont, CA, 2006.