INTRODUCTON
A redox reaction is a type of chemical reaction “that involves a transfer of electrons between two species” (Spohrer, Breitenbuecher & Brar, 2019). For a reaction to be classified as redox, there must be one species that is reduced and another that is oxidised. This means that both reduction (the gain of electrons) and oxidation (the loss of electrons) occur simultaneously in a redox reaction. One such example of a redox reaction is that of electrolysis.
Electrolysis defined broadly is the process of using a “DC (direct current) to drive a non-spontaneous reaction” (Raveendran, 2008). This occurs when a current is conducted through a bath (electrolyte) and two electrodes to decompose a compound. When current is passed through this system, one electrode becomes negatively charged whilst the other becomes positively charged. Thus, as opposite charges attract, the negative electrode (cathode) attract cations and the positive electrode (anode) attract anions. At the anode, the anions lose electrons (oxidation), whilst at the cathode, the cations gain electrons (reduction), therefore making this a redox reaction. In the case of this experiment between copper (anode) and an iron nail (cathode), a redox reaction occurs at the electrodes as seen in the half equations below:
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In certain electrolysis reactions, the aim is not to break apart substances but rather to plate a metal onto one of the electrodes (Madhusa, 2015) This is known as electroplating. Commonly used commercially for protection and aesthetic coats, electroplating is similar to electrolysis in that it uses a current between two electrodes and an electrolyte (which contains the metal ions to be deposited). In the case of electroplating, the cathode becomes plated whilst the anode is either a “sacrificial anode (dissolvable anode) [or a] permanent anode” (Lou & Huang, 2006). On this occasion, a sacrificial anode, made of copper (metal to be deposited on the cathode), was used. As it was a sacrificial anode, the copper oxidises into the electrolyte bath and then deposits as a solid onto the cathode (Mittal, 2019). This continuously supplies the electrolyte with ions to coat the cathode until the sacrificial anode runs out.
The plated mass onto an electrode in electroplating can be manipulated by changing many different aspects of an experiment such as voltage, concentration or surface area. In the case of this investigation, the voltage was manipulated. According to Faraday’s first law of electrolysis, the current is directly proportional to the amount of deposited substance at the electrodes (Marsden, 2019). Moreover, Ohms law states that current is directly correlated to voltage as long as resistance is constant, (V=IR) (Dave, 2019). Thus, an increase in voltage will correspondingly raise the mass of copper plated onto the iron nail. Therefore, it is believed that manipulating the voltage would result in a proportional change in mass as the two should have linear relationship.
ANALYSIS OF DATA
As seen in the collected data, there was the general trend of increasing mass and increasing voltage. However, the data from the voltage 4 trials are clear outliers, which is thought to be the result of abnormal chemical processes caused by human error. Thus, the results from voltage 4 were completely disregarded as to not skew the graphical representation of the data.
In the graph, the correlation of the collated data is very high, at 0.9931 indicating the data’s strong positive correlation. Thus, it can be said that as the independent variable (voltage) increases, the dependent variable (average change in mass) also increases. However, the two variables shared a logarithmic relationship, rather than a linear/proportional one. Thus, it is thought that there is a limit to the amount of mass change which would plateau at a later voltage amount.
Additionally, the graph should pass through the origin as at 0 volts, there should be no change in mass. Instead, it passes through the y-axis at a negative value which is incorrect as at 0 volts, there cannot be a negative change in mass.
DISCUSSION AND EVALUATION
From the collected data, it is clear that as the voltage increases, the mass change does as well. This supports the hypothesis and thus, the reason behind this would be because voltage is closely linked to current. However, the hypothesis was not fully supported as it was expected for the voltage and mass change to have a linear relationship, but the collected data has a shown that the two have a logarithmic relationship. This type of relation is not supported by any of the background research and thus it is believed that this is the result of errors and undetected reactions that may have been simultaneously occurring.
Though the change in mass was fairly linear, the later values (at higher voltages) did not increase proportionally. This is believed to be due to a combination of both experimental errors and ‘burning’. As higher voltages result in more electrons, there will be a point where there is a quick depletion in the number of ions resulting in an insufficient number of ions for the high number of electrons. This results in the electrons reacting with the electrolyte, creating a gas as exemplified by voltage 10 trial B where there was an observation of ‘bubbling’ (Mooney, 2001). In addition to this, there were some major experimental errors which may have also contributed to the logarithmic relationship.
A major weakness of this experiment would be the number of extraneous variables, i.e. the lack of controlled variables. For the example of a random error, the starting weight of the nail was not controlled at all which made it more difficult to compare each trial. The nails may have built up substances over time (e.g. rust) and as each one was cleaned differently to another; it may have contributed to the inaccuracy of the results. This can be amended by specifying the mass of each starting nail and ensuring they are all the same before the experiment is conducted. Another substantial limitation would be in the procedure where, after electrolysis, each nail would be wiped with a paper towel to remove excess solution. However, due to random errors, each nail was not cleaned the same and some may have had leftover solution or had copper taken off when weighing. This issue can be rectified by using scientific equipment such as a drying oven, or simply waiting for the moisture to evaporate so that each nail is able to dry evenly.
Though the experiment was substantially flawed, there were some strengths to it. One of these would be the use of an accurate scale which had very small uncertainty values of ±0.001. This lessened the likelihood of inaccurate results and allowed for the change in mass to be very clear, as some trials had very small mass changes. Moreover, the fact that all the groups used this scale is also a strength as the results were fair and to the same significant figures. Additionally, the replicability of the experiment is thought of as a strength since, in future, reproductions can be easily conducted, allowing for these results to be either further validated or disproved.
An extension of this experiment would be to conduct more trials over a wider range of voltages. This would help form a more dependable trend and thus a more reliable conclusion can be determined. Another way in which the experiment can be furthered is the testing of other factors which are thought to affect the change in mass. These factors could include changes in temperature, time, concentration of the electrolyte or surface area of the electrodes. It would be interesting to see which factor has the biggest impact on mass change and how it could be implemented in real world/industrial situations.
CONCLUSION
To conclude, it is clear that increasing voltage results in an increase in the mass change. However, this relationship should be proportional to each other (linear), rather than the polynomial relationship that was discovered in this experiment. Thus, it is evident that this was a considerably flawed investigation and the results discovered should be taken very lightly.
REFERENCING
- Spohrer, C., Breitenbuecher, C., & Brar, L. (2019). Oxidation-Reduction Reactions - Chemistry LibreTexts. Retrieved 28 October 2019, from https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation-Reduction_Reactions
- Raveendran, B. (2019). Electrolysis and Electroplating - Definition, Working Principle, Application. Retrieved 31 October 2019, from https://byjus.com/physics/electrolysis-and-electroplating/
- Madhusha. (2019). Difference Between Electrolysis and Electroplating | Definition, Mechanism, Examples. Retrieved 1 November 2019, from https://pediaa.com/difference-between-electrolysis-and-electroplating/
- Lou, H., & Huang, Y. (2019). Electroplating [Ebook] (pp. 1-2). Taylor and Francis. Retrieved from https://static1.squarespace.com/static/52fe73f6e4b05f81d6be283d/t/549f2fa2e4b037c197100985/1419718562607/Electroplating.pdf
- Mittal, V. (2019). Electroplating. Retrieved 2 November 2019, from https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Electrolytic_Cells/Electroplating
- Dave. (2019). Relationship and Difference Between Voltage, Current and Resistance -. Retrieved 2 November 2019, from https://www.watelectrical.com/relationship-and-difference-between-voltage-current-and-resistance/
- Mooney, T. (2019). What part does voltage play in electroplating?. Retrieved 2 November 2019, from https://www.finishing.com/80/07.shtml
- Marsden, S. (2019). Faraday's Law. Retrieved 3 November 2019, from https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Faraday's_Law
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