EXPERIMENT 6 : ELECTROCHEMICAL CELLS
Electrochemistry is a field that studies the relationship between electricity and chemistry. Electrochemical processes are redox reactions in which the energy released by a spontaneous reaction is converted to electricity or in which electricity is used to drive a non-spontaneous chemical reaction. In redox reactions, electrons are transferred from one substance to another. The redox reaction between magnesium metal and hydrochloric acid is an example of redox reactions: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g) The loss of electrons by an element during oxidation is marked by the increase in the element’s oxidation number. In reduction, there is a decrease in oxidation number resulting from a gain of electrons by an element. In the preceding reaction, Mg metal is oxidized and H⁺ ions are reduced; the Cl⁻ ions are spectator ions. Generally there are two types of electrochemical cells: the galvanic and the electrolytic cells. Galvanic cell or voltaic cell, after the Italian scientists Luigi Galvani and Alessandro Volta, is an electrochemical cell in which redox reaction occurs spontaneously to generate electricity. It is a device which converts chemical energy into electrical energy. The chemical reactions that take place inside the cell cause the flow of electrons and hence, electricity will be produced. The salt bridge, which is an inverted U-tube containing a potassium chloride, KCl solution, provides an electrically conducting medium between two solutions. Oxidation will occur at the anode, which causes electrons to flow to the cathode where reduction will occur. A complete circuit is obtained. A standard reduction potential is the tendency for a chemical species to be reduced, and is measured in volts at standard conditions where the concentration of solution is 1.0 M, the gas partial pressure is 1 atm and temperature is 25 ˚C. The more positive the potential is, the more likely it will be reduced. The standard reduction potential values are arranged in a certain order and the list is known as the Standard Reduction Potential Table or the e.m.f. Series. The potential difference between the two electrodes of a voltaic cell provides the driving force, that pushes electrons through the external circuits. The potential difference or electromotive force (emf). The emf of a cell is also called the cell potential. The difference in potential energy per electrical charge between the two electrodes is measured in units of volts. The cell potential at the standard condition can be written as: E˚ = E˚ - E˚
The dependence of the cell emf on concentration can be obtained from the dependence of the free energy change on the concentration. The cell potential can be calculated by using the Nernst equation. E = E˚ - 0.0592 log Q n
An electrolytic cell is an electrochemical cell that undergoes a redox reaction when electrical energy is applied. It converts electrical energy to chemical energy. It is most often used to decompose chemical compounds, in a process called electrolysis. When electrical energy is added to the system, the chemical energy is increased. Similarly to a galvanic cell, electrolytic cells usually consist of two half cells. An electrolytic cell has three component parts: an electrolyte and two electrodes (a cathode and an anode). The electrolyte is usually a solution of water or other solvents in which ions are dissolved. Molten salts such as sodium chloride are also electrolytes. When driven by an external voltage applied to the electrodes, the electrolyte provides ions that flow to and from the electrodes, where charge-transferring (also called faradaic or redox) reactions can take place.
(A) Galvanic cell
A galvanic cell, or...
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