Electrochemical cells-report PDF

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Electrochemical cells report...

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Investigating Electrochemical cells Aim: Purpose: To construct an electrochemical series based on the potential differences between two metals. Objectives: 1) To investigate the voltage between different combination of metals. 2) To investigate how electrochemical cells work. 3) To construct an electrochemical series. Introduction: Electrochemical cell is a device that comprised of two half-cells. Electrochemical cells that can generate energy from the chemical reaction occurs are called as galvanic cell or voltaic cell. Electrochemical cell that uses electrical energy to facilitate a reaction is called as electrolytic cell. A half-cell consists of an electrode submerged in an electrolyte with the same metal ions as the electrode. A salt bridge is a solution that contains salt or inert ions that does not produce reaction with the electrolyte. Salt bridge allows the flow of ions in order to complete the circuit (Tan, Loh & Tan, 2016). The two half-cells are connected with a saltbridge which enable ionic contacts between the half-cells without mixing them together. Filter paper dipped in a salt solution such as potassium nitrate and potassium chloride is often used as the salt bridge. Electrochemical cells consist of an anode and a cathode. Oxidation will occur at anode and reduction will occur at cathode. This process will continue until an equilibrium has reached. An example of electrochemical cell is shown in Figure 1 (Byjus, 2019).

Figure 1: Example of electrochemical cell (Byjus, 2019). Potential difference is the difference in the amount of energy of two charge carriers in a circuit. It is measured in voltage, with unit of volts (V) (Seneca Learning, n.d.). The tendency of the electrons to travel from a half-cell to another half-cell. When electrons flow from the anode to cathode through external circuit in a galvanic cell, current is produced due to the difference in the potential energy between two electrodes. When the valence electrons in an

electrode has a higher potential energy than another electrode, electrons will flow from the electrode with higher potential energy to the electrode with lower potential energy (Libre Texts, 2020). Electrochemical series, or e.m.f, series, is the arrangement of a series of half-cells with the order of decreasing standard reduction potential. Standard reduction potential is the tendency of the element to be reduced. The element will lower standard reduction potential will get oxidized easily. In electrochemical series, the elements with higher reduction potential, which has a positive value of standard potential, is placed on top of the electrochemical series. Conversely, the elements will lower reduction potential will be placed lower at the electrochemical series. As move down the series, the electropositivity of the elements increases. Therefore, the bottom element in the electrochemical series would be more reactive. Electrochemical series can be constructed based on the potential difference of two metals in a voltaic cell. A higher voltage will be produced if the metals are further apart in the electrochemical series. Conversely, if the metals are closer in the electrochemical series, the potential difference will be lower (More, 2019). An example of electrochemical series is shown in Figure 2.

Figure 2: Electrochemical series (Toppr, n.d.) In this experiment, the position of each unknown metal A, B, and C in electrochemical series is determined by investigating the potential difference between combination of different metals. The oxide layer on the metals are sanded off by using sandpaper. Each metal acts as an electrode and is placed in their salt solution, respectively. Then, the half-cells are created. Filter paper immersed in potassium chloride solution will be used as a salt bridge to connect two half-cells. Then, the connecting wires is connected to the electrode of each combination of the voltaic cells. When the metal will lower standard reduction potential is connected to the negative terminal of the voltmeter, the needle of voltmeter will deflect. The readings are recorded to calculate the voltage of each half-cell in order to construct an electrochemical series. Results: Table 1: Potential difference of different combination of cells

Anode half-cell (-) B/B2+ C/C2+ C/C2+

Cathode half-cell (+) A/A2+ A/A2+ B/B2+

Voltage (V) 0.05 0.30 0.25

Data Analysis: Equation of the first combination: Anode: B/B2+ Half equation: B(s) → B2+ + 2eCathode: A/A2+ Half equation: A2+(aq) + 2e- → A(s) Overall equation: A2+(aq) + B(s) → A(s) + B2+(aq) For the second combination: Anode: C/C2+ Half equation: C(s) → C2+ + 2eCathode: A/A2+ Half equation: A2+(aq) + 2e- → A(s) Overall equation: A2+(aq) + C(s) → A(s) + C2+(aq) For the third combination: Anode: C/C2+ Half equation: C(s) → C2+ + 2eCathode: B/B2+ Half equation: B2+(aq) + 2e- → B(s) Overall equation: B2+(aq) + C(s) → B(s) + C2+(aq) Potential difference between metal A, metal B and metal C: A 0.25V 0.30V

B C

Calculation of voltage: First combination: VA + VB = 0.05V VA = 0.05 – VB ----------------------(1)

0.05V

Second combination: VA + VC = 0.30V---------------------(2) Third combination: VB + VC = 0.25V VC = 0.25 – VB -----------------------(3) When substitute (1) and (3) into (2): VA + VC = 0.30V (0.05 – VB) + (0.25 - VB) = 0.30V -2VB + 0.30 = 0.30 VB = 0V VA = 0.05 – 0 = 0.05V VC = 0.25 – 0 = 0.25V Electrochemical series based on the potential difference of metal A, metal B and metal C: A B

Reduction potential decreases Reactivity increases

C Discussion: When the two half-cells are connected with a salt bridge, the needle of the voltmeter deflected. This is due to the electrons generated at the anode travelled through the external wire, produced electricity, and registered a reading at the voltmeter. In this experiment, redox reaction has occurred. The half-cell where oxidation reaction occur is anode while the half-cell where reduction reaction occur is cathode. As electron is released at the anode, anode is considered as negative electrode while cathode is the positive electrode. When the electrons are moving from one half-cell to another half-cell, charge imbalance occurred. To deal with it, a salt bridge that contains ionic compound is used to migrate ions to either side of the voltaic cell to maintain the charge imbalance. In the case of the first combination, which is metal strip B as an electrode in the electrolyte of salt solution B2+ and metal strip A as an electrode in the electrolyte of salt solution A 2+, voltage is produced when half-cell B act as the anode and half-cell A act as the cathode. In this case, redox reaction occurred where reduction occurred at the anode and oxidation occurred at the cathode. Metal B has the higher potential to be oxidized and is a stronger reductant than metal A. Therefore, the oxidation number of metal B will increase from 0 to 2+, this can be shown in the equation B(s) → B 2+(aq) + 2e-. Metal A has the higher

potential to be reduced and is a stronger oxidant than metal B. The oxidation number of metal A will decrease from 2+ to 0. This can be shown in the equation A 2+ (aq) + 2e- → A(s). This is because metal B is more electropositive than metal A in the electrochemical series. Therefore, the tendency for metal B to donate electron is higher. The standard reduction potential of metal B is also lower. Thus, electrons would travel from the half-cell B to the half cell A. This produces electricity and causes deflection of the voltmeter with a reading of 0.10V. In the second combination, which is metal strip C as an electrode in the electrolyte of salt solution C2+ and metal strip A as an electrode in the electrolyte of salt solution A 2+, voltage is produced when half-cell C act as the anode and half-cell A act as the cathode. In this case, redox reaction occurred where reduction occurred at the anode and oxidation occurred at the cathode. Metal C has the higher potential to be oxidized and is a stronger reductant than metal A. Therefore, the oxidation number of metal C will increase from 0 to 2+, this can be shown in the equation C(s) → C 2+(aq) + 2e-. Metal A has the higher potential to be reduced and is a stronger oxidant than metal C. The oxidation number of metal A will decrease from 2+ to 0. This can be shown in the equation A 2+ (aq) + 2e- → A(s). This is because metal C is more electropositive than metal A in the electrochemical series. Therefore, the tendency for metal B to donate electron is higher. The standard reduction potential of metal C is also lower Thus, electrons would travel from the half-cell C to the half-cell A. This produces electricity and causes deflection of the voltmeter with a reading of 0.30V. In the third combination, which is metal strip C as an electrode in the electrolyte of salt solution C2+ and metal strip B as an electrode in the electrolyte of salt solution B 2+, voltage is produced when half-cell C act as the anode and half-cell B act as the cathode. In this case, redox reaction occurred where reduction occurred at the anode and oxidation occurred at the cathode. Metal C has the higher potential to be oxidized and is a stronger reductant than metal B. Therefore, the oxidation number of metal C will increase from 0 to 2+, this can be shown in the equation C(s) → C 2+(aq) + 2e-. Metal B has the higher potential to be reduced and is a stronger oxidant than metal C. The oxidation number of metal A will decrease from 2+ to 0. This can be shown in the equation B 2+ (aq) + 2e- → B(s). This is because metal C is more electropositive than metal B in the electrochemical series. Therefore, the tendency for metal C to donate electron is higher. The standard reduction potential of metal B is also lower. Thus, electrons would travel from the half-cell C to the half-cell B. This produces electricity and causes deflection of the voltmeter with a reading of 0.25V. According to the results, an electrochemical series is constructed based on the potential difference obtained from the experiment. When a voltaic cell is conducted with half-cell B and half-cell A, voltage is produced when half-cell B acts as an anode and half-cell A acts as a cathode. This has shown that metal B is a stronger reductant compared to metal A. Therefore, metal B should be placed below A in the electrochemical series and has a lower reduction potential compared to metal A. When a voltaic cell is conducted with half-cell C and half-cell A, voltage is produced when half-cell C acts as an anode and half-cell A acts as a cathode. This has shown that metal C is a stronger reductant compared to metal A. Therefore, metal C should be placed below A in the electrochemical series and has a lower reduction potential compared to metal A. When a voltaic cell is conducted with half-cell B and half-cell C, voltage is produced when half-cell C acts as an anode and half-cell B acts as a cathode. This has shown that metal C is a stronger reductant compared to metal B.

Therefore, metal C should be placed below B in the electrochemical series and has a lower reduction potential compared to metal B. Based on the interpretation, the increasing order of the metal A, B and C in the electrochemical series should be A, B, C. This also shows that C is the most reactive metal followed by B, then A. There are some possible errors that may occur during the experiment, causing an inaccurate reading of the voltmeter. Firstly, the oxide layer on the metal A, metal B and metal C may not be completely sanded off. This will cause the metal to be less reactive and lead to inaccurate reading of voltmeter and the calculation of potential difference. Besides, contamination of the salt solution may occur due to the remaining impurities in the beaker which it not fully cleaned. This will also decrease the efficiency of the reactions. Besides, parallax error may occur while taking readings during the experiment. For example, the voltmeter reading would be slightly different than the actual voltmeter reading because the readings may not be observed through the same angle at all time. This would affect the readings collected and the potential difference calculated. To improve this experiment, the oxide layer on the metal should be sand off multiple times to ensure the oxide layer is mostly removed. Moreover, the beaker should be cleaned again and wiped with tissue papers to avoid impurities being left on the beaker. This will ensure the reaction process is not altered by the impurities. Besides that, the experiment should be conducted for multiple times. Therefore, more data can be collected, and the average data counted will be more reliable. For example, another metal is taken, and the unknown salt solution is taken again to examine the potential difference. Then, average value of the readings is calculated to increase the accuracy and the reliability of the data. Green Chemistry Considerations: The first green chemistry principle used in this experiment is ‘waste prevention’. Exact volume of potassium chloride solution, salt solution A 2+, salt solution B 2+ and salt solution C2+ are measured and taken. The chemicals will only be taken again when the chemicals are not enough. This is to prevent wastage and save the chemicals. The second green chemistry principle ‘real-time pollution prevention’ is applied in this experiment. The salt solution A 2+, potassium chloride solution, salt solution B 2+ and salt solution C2+ are being diluted and disposed in an appropriate way. This is to avoid the chemical from polluting the air and the river, causing harms to human and aquatic life. Conclusion: The aim of the experiment is achieved. The electrochemical series is constructed by investigating the potential differences between different combination of two metals in voltaic cells. The simple electrochemical series constructed based on the decreasing reduction potential is metal A, B and C. The increasing reactivity of metals is also constructed, which is metal A, B and C. References: Byjus. (2019). Electrochemical cell. Retrieved from https://byjus.com/chemistry/electrochemical-cell/

Libre Texts. (2019). Electrochemical Potential. Retrieved from https://chem.libretexts.org/Courses/Bellarmine_University/BU %3A_Chem_104_(Christianson)/Phase_4%3A_Harnessing_Chemical_Power/10%3A _Electrochemistry/10.3%3A_Electrochemical_Potential Libre Texts. (2020). Voltaic cells. Retrieved from https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry__The_Central_Science_(Brown_et_al.)/20%3A_Electrochemistry/20.3%3A_Voltaic_ Cells More, H. (2019). Electrochemical series and its applications. Retrieved from https://thefactfactor.com/facts/pure_science/chemistry/physicalchemistry/electrochemical-series/5877/ Seneca Learning. (n.d.). Potential difference. Retrieved from https://senecalearning.com/enGB/definitions/potential-difference/ Tan, Y.T. & Loh, W.L. & Tan, O.T. (2016). Voltaic cells. SUCCESS Chemistry SPM. (pp. 158162). Shah Alam, Selangor: Oxford Fajar Sdn. Bhd. Toppr. (n.d.). Electrochemical series – definition. Retrieved from https://www.toppr.com/content/concept/electrochemical-series-a-detailed-study203322/...