INTRODUCTION/BACKGROUND: The Zn(s) is the reducing agent

INTRODUCTION/BACKGROUND: Chemical reactions involving the transfer of electrons from one reactant to another are redox reactions. In redox reactions, two half-reactions occur; one reactant loses electrons (oxidation) and the other reactant gains electrons (reduction). Zinc going into  solution as zinc ions, with each Zn atom giving up 2 electrons, is an oxidation half-reaction. 1) Zn(s) ? Zn2+(aq) + 2e- The oxidation number of Zn(s) is 0 and the oxidation number of the Zn2+ is +2 So, the oxidation number increases, which is another definition of oxidation. For that matter, the reverse is a reduction. 2) Zn2+(aq) + 2e- ? Zn(s) In a reduction there is a decrease (or reduction) in oxidation number. Chemical equation representing half-reactions must be both mass and charge balanced. In the half-reactions above, there is one zinc on both sides of the equation. The charge is balanced because the 2+ charge on the zinc ion is balanced by two electrons, 2e-, giving zero net charge on both sides. Another example of reduction is the formation of solid copper from copper ions in solution.3)  Cu2+(aq) + 2e- ? Cu(s) In this half-reaction the oxidation number of the copper ion is +2, which decreases to 0 for the solid copper, and again charge and mass are balanced.These half-reaction cannot react by themselves. A redox reaction occurs when an oxidation and a reduction half-reaction are combined: 4) Zn(s) + Cu2+(aq) ? Zn2+(aq) + Cu(s) The Zn(s) is the reducing agent as it drives the Cu2+ to be reduced to Cu. The Cu2+ is called the oxidizing agent as it drives the Zn(s) to be oxidized to Zn2+. Any half-reaction can be expressed as a reduction as illustrated in the case where equation 1) can be reversed to equation 2). A measure of the affinity for a reduction to occur is reduction potential, E, measured in volts. At standard conditions, 25 °C and concentrations of 1.0 M for the aqueous ions, the measured voltage of the reduction half- reaction are standard reduction potential, E°. Standard reduction potentials have been measured for many half-reactions and they are listed in tables. For the reduction half-reactions in equations 2) and 3), the standard reduction potentials are –0.76 V for zinc and +0.34 V for copper. The more positive (or less negative) the reduction potential, the greater is the tendency for the reduction to occur. So Cu2+ has a greater tendency to be reduced than Zn2+. Furthermore, Zn has a greater tendency to be oxidized than Cu. The values of E° for the oxidation half- reactions are opposite in sign to the reduction potentials: +0.76 V for Zn and –0.34 V for Cu. A galvanic cell or voltaic cell is a device in which a redox reaction, such as the one in equation 4), spontaneously occurs and produces an electric current. In order for the transfer of electrons in a redox reaction to produce an electric current and be useful, the electrons are made to pass through an external electrically conducting wire instead of being directly transferred between the oxidizing and reducing agents. The design of a galvanic cell (shown in Figure 1.1 for the equation 4) reaction) allows this to occur. In a galvanic cell, two solutions, one containing the ions of the oxidation half-reaction and the other containing the ions of the reduction half-reaction, are placed in separate containers, half-cells. For each half-cell, the metal electrode, is placed in the solution and connected to an external wire. The electrode at which oxidation occurs is called the anode Zn in equation 4) and the electrode at which reduction occurs is called the cathode Cu in equation 4). The two half-cells are connected by a salt-bridge that completes the circuit of electron current. When the two electrodes are connected to an electric load (such as a light bulb or voltmeter) the circuit is completed, the oxidation-reduction reaction occurs, and electrons move from the anode (?) to the cathode (+), producing an electric current. The electric current measured in volts is the movement of electrons. The cell potential, Ecell, which is a measure of the voltage that the battery can provide, is calculated from the half-cell reduction potentials: Ecell = Ecathode – EanodeA Zinc-Copper electrochemical cell has the standard electrode potential of  Ecell = +1.10 . As the cell potential is a positive value this indicates that the electrochemical should have a redox reaction, therefore functioing as a galvanic cell.

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