Chemistry

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  • Topic: Electrochemistry, Galvanic cell, Electrochemical cell
  • Pages : 10 (1957 words )
  • Download(s) : 47
  • Published : October 1, 2008
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ABSTRACT

The experiment deals with determining the standard reduction potentials of different electrochemical half-cells through pairing it with Cu2+(0.1 M)|Cu half-cell and then comparing it with the theoretical value. Galvanic or voltaic cells contain the anodic and cathodic cell reactions, and in order to get the value of Ecell, we add both half-reactions. The more positive the Ecell,the more negative ΔG would be, thus, giving us a spontaneous reaction. After comparing the cell potentials, formation constant of [Cu(NH3)4]2+ and the Ksp of Cu(OH)2, we’ve seen that their deviation where great, since for Ksp, the value does not only depend on the concentrations of the reactants but also with their respective ionic strengths. There are many different factors affecting the value of the cell potential and the emf reading, namely: concentration, temperature and reaction quotient. Other than human error, these factors affect the calculated values for Ecell.

INTRODUCTION

Electrochemistry is the branch of chemistry that deals with the relationship of electricity and chemical reactions. Batteries, control of corrosion, metallurgy and electrolysis are some of the many applications of electrochemistry in everyday life. Electrochemistry always involves an oxidation-reduction process, wherein electrons are transferred from one substance to the other. This reaction is thermodynamically spontaneous and thus releases energy in the form of heat into their surroundings, and when in a controlled system, it produces electricity. Devices which carry out this process are called electrochemical cells.

There are two types of electrochemical cells: electrolytic and voltaic cells. Electrolytic cells are those in which nonspontaneous chemical reactions are supplied with an electrical charge to drive the process to spontaneity. On the other hand, voltaic or galvanic cells are those in which spontaneous reactions occur to produce electrical energy. Electrochemical cells have two conductive electrodes, the anode (oxidation) and the cathode (reduction). For a voltaic cell, the positively charged electrode is the cathode and the negative one is the anode (see Figure I). As seen in Figure II, while for an electrolytic cell, the cathode is negatively charged electrode, making the anode the positive one. These electrodes are immersed in an electrolyte solution. A salt bridge (for a voltaic cell) is used to maintain electrical neutrality by putting in ions in solution.

Figure I. Voltaic or Galvanic cell.

Figure II. Electrolytic cell.

The potential differences between electrodes, Ecell, also called cell voltage or electromotive force (emf), is the measure of the amount of energy per unit charge. The charge accounted is caused by the oxidation-reduction reaction where the electrons move from one electrode to another, specifically from anode to cathode, through an external circuit. It signifies the spontaneity of a reaction - a positive Ecell accounts to a spontaneous reaction. A standard electrode potential, Eºcell, is a measure of the tendency for a reduction process to occur at an electrode. These reduction potential values are based from the Standard Hydrogen Electrode or SHE which is given a reduction potential value of 0. We can relate the standard and nonstandard value of the electrode potential using the Nernst Equation:

Ecell = E˚cell –

where R is the gas constant given at 8.314 J/mol K, T is the absolute temperature at 298 K, n is the number of moles of electrons transferred, and F is the Faraday’s constant (96485 C/s). With this equation, we can also relate Ecell with pH, equilibrium constant and concentration of the involved species.

Another important relationship of cell potential is with Gibb’s free energy, ΔGº given in the equation

ΔGº= -nFEºcell

RESULTS AND DISCUSSION

Half-cell standard reduction potentials were determined in the...
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