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Change of Potential Difference in Voltaic Cells Lab Report

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Introduction

The Change of potential difference in Voltaic Cells I. Introduction Oxidation-Reduction (redox) reaction is a group of reactions that are linked to the transfer of electrons between species. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. Each reaction by itself is called a half-reaction because there must be two halves of reactions to form a whole reaction. When a redox reaction takes place, electrons are transferred from one species to the other. If the reaction is spontaneous, energy is released. A Voltaic Cell (Galvanic Cell) is an electrochemical cell that uses spontaneous redox reactions to generate electricity. It consists of two separate half-cells. A half-cell is composed of an electrode within a solution containing Mn+ ions in which M is the metal and n is the number of charges of the metal. The two half cells are linked together by a wire running from one electrode to the other with a voltmeter to measure the potential difference between the two electrodes. A salt bridge also connects to the half cells to keep solutions neutral and allow free flow of electrons. In the absence of a salt bridge, electrons won't transfer and the voltmeter won't measure any voltage. When an electrode is oxidized in a solution, it is called an anode and when an electrode is reduced in a solution it is called a cathode. The electrons will flow from the more negative half reaction to the more positive half reaction (i.e. from anode to cathode). The readings from the voltmeter present the reaction's cell voltage (potential difference) between its two half-cells. Standard Cell Potential: Eocell = Eo (cathode) - Eo (anode) The Eo values of metals are tabulated with all solutes at 1 M and all gases at 1 atm. These values are called standard reduction potentials. Each half-reaction has a different reduction potential; the difference of two reduction potentials gives the voltage of the cell. ...read more.

Middle

When adding distilled water to a copper sulfate solution it doesn't mix completely with the solution instantaneously. When placing the copper sulfate solution in the volumetric flask, it shows a more accurate reading of the volume of the solution than when placed in a glass beaker. Zinc Sulfate dissolves easily in distilled water. The colour of the Zinc Sulfate solution remains transparent when distilled water is added to zinc sulfate. No reaction occurs when the copper electrode is added to the copper sulfate solution and no reaction occurs when adding zinc electrode to zinc sulfate solution. When placing the insulated connective wires to electrodes, the potential difference remains zero on the voltmeter. Filter paper had to be rolled and completely immersed in potassium chloride solution before placing it between the two beakers (two half cells). Voltmeter shows readings only when the salt bridge is placed to connect the two half cells. The voltaic cell of 1.0 M copper sulfate solution records the highest voltage. Throughout the experiment, there was no change in the color of copper sulfate or zinc sulfate solutions. The color of zinc and copper metals (electrodes) remained the same, gray and golden. Quantitative Data This data table represents the results recorded when conducting the first trial of the experiment for the potential difference using different concentrations of copper sulfate solution. Trial 1 Copper Sulfate Concentrations (M) Experimental Voltage (V) 0.2 0.75 0.4 0.80 0.6 0.83 0.8 0.83 1.0 0.85 As shown from the data table above, the copper sulphate solutions' concentration is directly proportional to the potential difference between two half-cells. Hence, we can see that as the concentration of copper sulphate increases the potential difference between the two half-cells increases. Data Processing The processed data will be represented as a scatter graph for the concentration of the copper sulfate solution (M) versus experimental voltage (potential difference) of the two half-cells (V). ...read more.

Conclusion

Time shortage decreases the accuracy of readings and therefore, increases the percentage error of the experiment. Improvements For more accurate results; 1. Use a Differential Voltage Probe (measured in low and high voltages) and the Lab Quest machine for data collection instead of a voltmeter. The Differential Voltage Probe is more accurate than the regular voltmeter. 2. Use of a 50 ml volumetric pipette instead of a 50 ml graduated cylinder to use 50ml of copper sulfate, zinc sulfate and potassium chloride solutions during the experiment. This will decrease the uncertainty and the percentage error when using a volumetric pipette. 3. Use of U-shaped glass tubing for the salt bridge instead of a filter paper. Using U-shaped glass tubing as a salt bridge can assure that there's enough KCl needed instead of making sure that the filter paper is fully immersed in the solution. This will allow more passage of ions and electrons through the bridge and can give more accurate reading as there is the same amount of KCl used for each different concentration of copper sulfate. . Furthermore, the way of folding the filter paper won't be the same each time. 4. Decrease the total volume for each solution to 25 ml instead of 50 ml. By decreasing the total volume of each solution to 25 ml, this would require fewer amounts of copper sulfate and zinc sulfate for each concentration and would decrease the amount of time taken for the preparations of the solutions and the amount of waste, too. Hence, decrease in percentage error as the solutions won't be stored for a long period of time. 5. Prepare extra 10 ml of copper sulfate and zinc sulfate solutions to avoid shortage of the solutions while running the experiment. A variation in measuring the volume occurs between the graduated cylinder and the volumetric flask which is the most accurate 6. It is important to know the exact amount of solutions needed in order to reduce the waste of copper sulfate and zinc sulfate solutions because they are toxic and harmful to the environment. ...read more.

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