Ines

Abstract

Introduction

Background Information

Methods

Abstract

Abstract.mp4

Introduction

The history of Galvanic cells goes back to 1780 when Luigi Galvani discovered that when he put two different metals in contact and both are touched at the same time by two different parts of a frog's leg muscle it contracts. He called it “animal electricity”. In fact, the frog here is used as an electrolyte (a substance that dissociates into ions when dissolved in water and has the ability to conduct electricity) to close the circuit. A year after the publication of Galvani’s research, Alessandro Volta proved Galvani wrong by using a brine-soaked paper as the electrolyte thus proving that the animal was not necessary. From there, batteries kept on evolving throughout the years to the batteries we now use everyday. Yet, the basic idea never changed: two electrodes (a cathode and an anode), an electrolyte, and electrons flowing through.

My project aims to study the change in voltage in a galvanic cell when varying the metal used for the electrodes and salts used as electrolytes. I decided to take on this project because I wanted to study electrochemistry and I wanted to look at how much the electrodes and their properties affected the voltage produced by a galvanic cell. We use batteries everyday and going back to their most basic form is fascinating. Additionally, galvanic cells are not usually studied in detail; most of the resources and textbooks point students to standard values and oversimplified sketches instead of really explaining the chemistry behind them.

Background Information

The goal of this experiment was to study the impact of the electrodes’ metal on the voltage yield of galvanic cells. A galvanic cell is a cell that converts the chemical energy of spontaneous redox reactions into electrical energy. It is often seen as two half cells: an anode that undergoes oxidation and a cathode that gets reduced. In the case of the infographic, Copper is the anode and Silver is the cathode. Copper gets oxidized following reaction 1 which releases copper ions into the copper nitrate solution and electrons that flow towards the cathode. Silver ions from the silver nitrate solution get reduced by the electrons resulting from the oxidation of the cathode. It follows reaction 2. The solid silver is then deposited on the cathode. The overall reaction is reaction 3. If the reaction is observed over a long period of time, it becomes noticeable that the anode gets eroded and the cathode gets corroded. The salt bridge here serves two purposes: it closes the circuit and keeps the nitrate solutions’ charge neutral by allowing the flow of nitrate and sodium ions.

The relationship between the electrodes’ metal and the voltage yield of a galvanic cell could seem pretty straightforward : the lower the ionization energy of the anode and the higher the electron affinity of the cathode the higher the voltage. In other words, the more willing the anode is to give an electron away and the more willing the cathode is to take an electron the higher the voltage yield.

Yet, this assumption is wrong because it assumes that the voltage is the result of a simple transfer of electrons which is an oversimplification of reality. In fact, three main processes occur in galvanic cells:

The atom transfer out of the bulk metal into solution or into an oxide
The electron transfer from the atom to the bulk metal, leaving behind an ion (and from metal to cation at the other electrode)
The hydration of the ion by the solvent molecules

These processes add different factors– like the lattice cohesive energies and hydration energies of the metals– to consider when predicting the voltage that two metal electrodes will yield in a galvanic cell.

Methods

For this project, 5 metals were chosen (Zinc, Aluminium, Lead, Nickel, and Copper), two of them were chosen as anodes (Zinc and Aluminium) and three as cathodes (Lead, Nickel, and Copper). These metals were chosen for their availability, ease of use, and difference in electronegativities. Each anode was paired with each cathode which yielded a total of six runs (Aluminium-Lead, Aluminium-Nickel, Aluminium-Copper, Zinc-Lead, Zinc-Nickel, and Zinc-Copper) that were repeated three times each. The galvanic cell was made of two half cells, one for the anode and one for the cathode. Each half cell consisted of a strip of the chosen metal immersed in a 0.2 molar solution of its nitrate. 100mL of the cathode’s salt was poured in a 250mL beaker, 40 mL of the anode’s salt was then poured inside the salt bridge tube which was then immersed in the 250mL beaker. The salt bridge tube was immersed in a solution of nitrate and sodium ions at least until it was saturated before usage. The volumes of salts were chosen because they needed to be at the same level to keep the surface area of the electrodes constant. The anode and cathode were then immersed in their respective salts. The electrodes were then connected to a multimeter set to measure DC voltage using the 2000m setting. The electrodes were then stabilized and a measurement was taken as soon as the values stabilized which happened after five minutes of observation.

Results

Discussion

The goal of this experiment was to study the impact of the electrodes’ metal on the voltage yield of galvanic cells. Experimental results from this experiment cannot be explained by the simplification of a galvanic cell that reduces it to a simple transfer of electrons. To understand the results the factors that should be considered are: the hydration energies of the metals’ ions, the lattice cohesive energies of the metals (which can be measured by the bonding energies), the ionization energies of the metals, and their electron affinity.

Results from this experiment showed that all batteries with Zinc as an anode, with the same cathodes, had a significantly higher voltage than those with Aluminium. Yet, Zinc has a higher ionization energy and lower hydration energy than Aluminium. This is explained by the fact that Zinc has a full d orbital which prevents it from stabilizing through d orbital bonding and, therefore, it will be less stable and have a higher cohesive energy than aluminium since aluminium has “room” in its p orbital. In the context of this experiment, we can infer that the difference in the lattice cohesive energies of the electrodes was the dominant energetic driver of the reactions (since it was such an important factor here).

Out of the three metals used as cathodes, Copper led to the highest voltage produced on average. This is because Copper has the highest electron affinity among the cathodes.

There were also unexpected results. In the Aluminium-Lead battery, the Lead acted as the anode and the Aluminium as the cathode. It is hypothesized that it is because Aluminium has a higher electron affinity than Lead. Lead also led to the lowest voltage produced on average. This can be explained by its very low electron affinity (the lowest among the cathodes) and its high hydration energy.

The main uncertainty in this experiment is the surface area of the electrodes since it greatly affects the voltage. Any small movement of the electrodes would lead to a drastic change in voltage. Additionally, some electrodes were a bit wider or narrower than others–yet the change never exceeded one millimeter– which could have affected the results by changing the surface area. To minimize this uncertainty the electrodes should be cut in order for their shapes to be exactly the same and strips of a nonconductor should be used to stabilize the electrodes. Additionally, as the reactions run, the anodes get eroded and the cathodes get corroded which affects their surface area. Some of these electrodes were reused for different runs or trials which added to the uncertainty. This uncertainty could be reduced by strictly limiting the use of an electrode to one trial.

Further experiments with different metals could help determine the relative importance of each factor more broadly. Additionally, measuring the intensity of the current instead of the voltage and studying the relative impact of each factor on the intensity of the battery could give further insight into what makes a good battery rather than maximizing the voltage alone.

Conclusion

conclusion.mp4

Sources

“How Batteries Store and Release Energy: Explaining Basic Electrochemistry.” Journal of Chemical Education, 2018. https://pubs.acs.org/doi/10.1021/acs.jchemed.8b00479.

“17.2: Galvanic Cells.” Chemistry LibreTexts, September 28, 2015. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_(OpenSTAX)/17%3A_Electrochemistry/17.2%3A_Galvanic_Cells.

“PROPERTIES of ATOMS, RADICALS, and BONDS,” 2004. https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf.

‌Wikipedia Contributors. “Molar Ionization Energies of the Elements.” Wikipedia. Wikimedia Foundation, April 23, 2021. https://en.wikipedia.org/wiki/Molar_ionization_energies_of_the_elements.

‌“5.2: Galvanic Cells.” Chemistry LibreTexts, March 14, 2020. https://chem.libretexts.org/Courses/Nassau_Community_College/General_Chemistry_II/05%3A_Electrochemistry/5.02%3A_Galvanic_Cells.

“The Cell Potential.” Chemistry LibreTexts, October 2, 2013. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Voltaic_Cells/The_Cell_Potential.

‌Splung.com, 2021. http://www.splung.com/fields/batteries.htm.

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