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Lab 13 - Redox Reactions - Lab 13 Redox Reactions Contents...

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4/15/13 Lab 13 - Redox Reactions www.webassign.net/ebooks/wsugencheml1/lab_13/manual.html 1/10 Contents > Lab 13 - Redox Reactions Lab 13 - Redox Reactions Purpose To determine relative oxidizing and reducing strengths of a series of metals and ions. Goals 1 To explore the relative oxidizing and reducing strengths of different metals. 2 To gain practice working with electrochemical cells. 3 To use experimentally determined cell potentials to rank reduction half-reactions. Introduction The movement or transfer of electrons is central to our understanding of chemical reactions. The study of the transfer of electrons from one reactant to another is the study of electrochemistry. Electrons can move spontaneously from higher energy levels to lower energy levels within an atom. A similar movement can take place between two different chemical reactants. If there are electrons in one reactant that are at higher energy than unfilled orbitals of the other reactant, the high energy electrons can transfer to the unfilled orbitals at lower energy. This transfer of electrons from one chemical substance to another is known as an oxidation-reduction (redox) or electron transfer reaction. Consider the redox reaction (1) and Figure 1 below: ( 1 ) Zn( s ) + Cu 2+ ( aq ) → Zn 2+ ( aq ) + Cu( s ) Figure 1: Energy Diagram for Reaction between Zinc Metal and Copper(II) Ion One reactant, zinc metal, has a pair of electrons at a much higher energy level than an unfilled orbital in the
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4/15/13 Lab 13 - Redox Reactions www.webassign.net/ebooks/wsugencheml1/lab_13/manual.html 2/10 other reactant, copper(II) ion. The electrons in the higher energy orbital in zinc can spontaneously move to the lower energy orbital in copper(II). This electron transfer is a redox reaction. As the reactant with the high energy electrons "loses" its electrons, its oxidation state increases. In this example, elemental zinc has an oxidation state of 0; loss of two electrons raises its oxidation state to +2. Loss of electrons is an oxidation reaction. Conversely, as the reactant with the low energy orbital "gains" electrons, its oxidation state is reduced. Copper(II) has an oxidation state of +2; the elemental metal has an oxidation state of 0. Gain of electrons is a reduction reaction. In a redox reaction, the reactant that loses electrons (is oxidized) causes a reduction and is called a reducing agent. In the example above, zinc metal is the reducing agent; it loses two electrons (is oxidized) and becomes Zn 2+ ion. The reactant that gains electrons (is reduced) causes an oxidation and is called an oxidizing agent. Cu 2+ ion gains two electrons (is reduced) to form copper metal. In order to have a complete, balanced redox system, there must be at least one reduction and one oxidation; one cannot occur without the other and they will occur simultaneously. For a balanced system, the number of electrons lost in the oxidation reaction must be equal to the number of electrons gained in the reduction step. This is the key to balancing equations for redox reactions. To keep track of electrons, it is convenient to write the oxidation and
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