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Investigating the Nernst Equation Investigating the Nernst Equation HOW DOES THE EMF OF A GALVANIC CELL CHANGE WITH THE CHANGE IN CONCENTRATION OF COPPER SULPHATE ELECTROLYTE? RAAHISH KALARIA Contents 1 Introduction .................................................................................................................................... 2 2 Research Question .......................................................................................................................... 5 3 Hypothesis ....................................................................................................................................... 5 4 Variables.......................................................................................................................................... 5 4.1 Independent Variable ............................................................................................................. 5 4.2 Dependent Variable ................................................................................................................ 6 4.3 Controlled Variables ................................................................................................................ 6 5 Apparatus ........................................................................................................................................ 8 6 Pre-Lab Preparation ........................................................................................................................ 9 6.1 Preparing the Salt Bridge ........................................................................................................ 9 6.2 Zinc sulphate (1 mol dm-3, 250 cm3) ..................................................................................... 10 6.3 Copper sulphate (1 mol dm3, 250 cm3) ................................................................................. 11 6.4 Copper sulphate (0.1 mol dm-3, 250 cm3) ............................................................................. 12 6.5 Preparing the Electrodes ....................................................................................................... 14 7 Procedure ...................................................................................................................................... 15 7.1 Setting up the Apparatus ...................................................................................................... 15 7.2 Investigation Procedure ........................................................................................................ 16 7.3 Safety Considerations ........................................................................................................... 17 8 Data Collection and Processing ..................................................................................................... 18 8.1 Qualitative Data .................................................................................................................... 18 8.2 Raw Data Collection .............................................................................................................. 18 8.3 Data Processing ..................................................................................................................... 21 9 Conclusion and Evaluation ............................................................................................................ 29 9.1 Conclusion ............................................................................................................................. 29 9.2 Evaluation ............................................................................................................................. 30 10 Further Investigation................................................................................................................. 31 11 Bibliography .............................................................................................................................. 32 1 1 Introduction This investigation aims to study redox chemical reactions occurring in the Galvanic cell. A Galvanic cell typically consists of Zinc and Copper electrodes, in different containers (half cells), immersed in an aqueous solution containing the salt of the corresponding metal1. The two containers are connected with a salt bridge, which acts like a pathway for the cations of one solution and the anions of the other solution, completing the circuit. The cell, due to the redox reaction taking place, generates a difference in electric potential between the electrodes, thus generating an Electromotive Force (EMF). This investigation studies the effect of the concentration of one electrolyte on the overall EMF produced. In theory, the Nernst Equation is used to calculate the potential of a cell. This investigation also aims to verify this equation. Figure 1.1 – Diagram of a Galvanic Cell Photo taken from: http://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/Galvanic_cell_labeled.svg/2000px- Galvanic_cell_labeled.svg.png Redox reactions are reactions in which there in a transfer of electrons from one reactant to another2. In these reactions, two half-reactions occur, in which one reactant loses electrons (oxidizes), and the other reactant gains electrons (reduces). In this experiment, the two metals used are Zinc, which is the anode, and Copper, which is the cathode. The respective electrolytes used were Zinc Sulphate and Copper Sulphate, of which the concentration of Zinc sulphate was kept constant and the concentration of Copper sulphate was varied. The salt bridge consisted of Potassium Nitrate in aqueous solution which is acts as a good conductor of the ions in the cell. 1http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Electrochemistry_2%3A_Galvanic_cells_ and_Electrodes 2http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Redox_Chemistry/Oxidation- Reduction_Reactions 2 Thus, the reaction occurring at the anode is: 푍푛(푠) → 푍푛2+(푎푞) + 2푒− The reaction occurring at the cathode is: 퐶푢2+(푎푞) + 2푒− → 퐶푢(푠) Due to the positive charge on the Zinc half-cell, the negative sulphate ions are attracted to that side of the cell, and move through the salt bridge, while the positive Zinc ions are attracted to the copper side of the cell, and also move through the salt bridge to the copper half-cell. For any electrode, at standard conditions of 25 °C and concentrations of 1.0 M for the aqueous ions, the measured voltage of the reduction half-reaction is defined as the standard reduction potential, E°. The reduction potentials of Zinc and Copper are given in the table below3: Table 1.1 – Standard Reduction potentials (E0) of Zinc and Copper. Electrode Standard Reduction potential (V) Zinc -0.76 Copper 0.34 The greater the reduction potential, the greater is the tendency for the reduction to occur. So Cu2+ has a greater tendency to be reduced than Zn2+. The cell potential, Ecell, which is a measure of the EMF that the battery can provide, is calculated from the standard half-cell reduction potentials4: 0 0 0 퐸푐푒푙푙 = 퐸푐푎푡ℎ표푑푒 − 퐸푎푛표푑푒 0 Thus, for the reaction taking place in the cell, the E cell is: 0 퐸푐푒푙푙 = (0.34) − (−0.76) 0 퐸푐푒푙푙 = 1.10 푉 As this is positive, we can deduce that the reaction in the cell is spontaneous, from the Gibb’s 0 5 0 equation, ΔG = -nFE cell , as when the E cell is positive, the value of ΔG is negative, which means the reaction is spontaneous. Therefore, there would be a current produced in the cell. The Nernst Equation is used to calculate the potential of a cell. The equation is as follows6: 푅푇 퐸 = 퐸0 − ln (푄) 푐푒푙푙 푐푒푙푙 푛퐹 3 https://www.chem.umn.edu/services/lecturedemo/info/Cu-Zncell.html 4 Pearson Baccalaureate: Higher Level Chemistry for the IB Diploma (Print) 5http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Electrochemistry_and_Thermodynamics 6 https://www.chem.tamu.edu/class/majors/tutorialnotefiles/nernst.htm 3 Where: 0 E cell is the Standard reduction potential of the cell; R is the Universal gas constant, which is 8.314 J K-1 mol-1; 7 T is the temperature of the cell, in Kelvin; n is the number of electrons transferred in the redox reaction, which is 2 in this case; F is the Faraday Constant, which is 96485.3 J V-1 mol-1; 8 Q is the ratio between the concentration of the anode metal ions and the cathode metal ions in the respective electrolytes. [푍푛2+] 푄 = [퐶푢2+] Using the Nernst Equation, a Nernst plot can be obtained on a graph, which would express the relation between the concentration of one of the electrolyte and the Ecell. To do this, we need to get the Nernst Equation in y = mx + c form. The equation can be written as: 푅푇 [푍푛2+] 퐸 = 퐸0 − ln ( ) 푐푒푙푙 푐푒푙푙 푛퐹 [퐶푢2+] 푅푇 퐸 = 퐸0 − (ln([푍푛2+]) − ln ([퐶푢2+])) 푐푒푙푙 푐푒푙푙 푛퐹 푅푇 푅푇 퐸 = 퐸0 − ln([푍푛2+]) + ln ([퐶푢2+]) 푐푒푙푙 푐푒푙푙 푛퐹 푛퐹 Since we are keeping the concentration of Zinc sulphate electrolyte constant, and varying the 푅푇 concentration of Copper sulphate, the 퐸0 − ln([푍푛2+]) part becomes constant. Thus becomes 푐푒푙푙 푛퐹 푅푇 the ‘y-intercept’, or ‘c’ in the equation y = mx + c. Our ‘x’ is ln ([퐶푢2+]), and ‘m’ or the slope is , a 푛퐹 constant. 푦 = 푚푥 + 푐 푅푇 푅푇 퐸 = ln([퐶푢2+]) + (퐸0 − ln([푍푛2+])) 푐푒푙푙 푛퐹 푐푒푙푙 푛퐹 Thus, we can deduce that the Nernst plot is a straight line, which increases with the increase in the value of ‘x’, as the slope is positive. The value of ln([Cu2+]) increases if the value of [Cu2+] increases. 7 http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html 8 http://chemistry.about.com/od/chemistryglossary/g/Faraday-Constant-Definition.htm 4 2 Research Question How does the EMF of a Galvanic
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