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Unformatted text preview: E LECTROCHEMICAL C ELLS Objective: To demonstrate how to construct a spontaneous electrochemical cell and determine the voltage generated. Theory: Oxidation-reduction reactions are among the most common and important chemical reactions, found in many aspects of everyday life. The process of oxidation refers to the loss of electrons, whereas the process of reduction refers to the gaining of electrons. Thus, oxidation and reduction reactions are merely two ways in which electrons are transferred. The flow of electrons brings about what is known as electricity. Electrochemistry is the study of the relationship between electricity and chemical reactions. A reaction can be classified as an oxidation-reduction reaction, or redox reaction, by keeping track of the oxidation numbers of all the elements involved in the reaction. The oxidation number is a positive or negative whole number assigned to an element in a molecule or ion, or the charge. For any given reaction to be considered a redox reaction, both oxidation and reduction must occur. The only way for a substance to gain electrons, is for another substance to lose electrons, and vice versa. The substance that makes it possible for the other substance to undergo oxidation is known as the oxidizing agent, or oxidant. This also is the substance that undergoes reduction. The substance that makes it possible for the other substance to undergo reduction is known as the reducing agent, or reductant. This is the substance that undergoes oxidation. The redox reaction requires much more than just a change in the chemical make- up of a substance, and thus in addition to the law of conservation of mass which requires the balancing of atoms, the balancing of charges, or flow of electrons, also must occur. Redox reactions, although occurring simultaneously, are often written as two separate processes, the reduction reaction, and the oxidation reaction. These separate reactions are known as half reactions. Redox reactions can occur in different environments, acidic, basic, and neutral. In an acidic environment, each half reaction is first balanced based upon all elements other than H and O. The O is then balanced through the addition of H 2 O. The H is then balanced through the addition of H + . Finally, the addition of electrons is used to finally balance the charge. A basic environment is not very different, aside from the fact that for every H + added in the half reaction, an OH- is added to turn it to water. Thus while acidic reactions contain H 2 O and H + , basic reactions contain H 2 O and OH- . Neutral redox reactions are simply balanced based upon charges. A spontaneous reaction is defined as an irreversible reaction that, although obeying the first law of thermodynamics, occurs without any ongoing outside intervention. Redox reactions that occur spontaneously release energy through the flow of electrons. This energy released can be used to perform electrical work. In order for this energy to be harnessed, a voltaic cell (also known as galvanic cell) is used. This device energy to be harnessed, a voltaic cell (also known as galvanic cell) is used....
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This note was uploaded on 02/25/2012 for the course CHEM 2142 taught by Professor Roberts during the Fall '10 term at Texas State.
- Fall '10
- Oxidation-Reduction Reactions