Before developing analytical methods based on electroles-grizzlys-catalans.orgistry, it is worth exploring aspects about electroles-grizzlys-catalans.orgical cells. Concepts needed to comprehend the nature of an electroles-grizzlys-catalans.orgical cell are informative in understanding some of the analytical methods we will develop. From a more practical standpoint, batteries are examples of electroles-grizzlys-catalans.orgical cells.
Describe what you know about an electroles-grizzlys-catalans.orgical cell.
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The components of an electroles-grizzlys-catalans.orgical cell are shown in Figure 7.
Figure 7. Diagram of the components in an electroles-grizzlys-catalans.orgical cell.
The particular cell shown involves a half reaction with zinc and a half reaction with copper.
Based on the two Eo values, the copper ion will be reduced and zinc metal will be oxidized. In an electroles-grizzlys-catalans.orgical cell, the reduction half reaction is referred to as the cathode and the oxidation half reaction is referred to as the anode. By convention, the anode is always put on the left and the cathode on the right in the diagram.
The zinc half-cell consists of a piece of zinc metal in a solution containing zinc ion. The copper half-cell consists of a piece of copper metal in a solution containing copper ion. If a half reaction does not form a solid metallic species (e.g., Fe3+ + e– = Fe2+) an inert metal such as platinum is used in the cell.
The two half-cells need to be connected to complete the circuitry and allow the reaction to proceed. Two connections are needed for a complete circuit. One is a metal wire that connects the two pieces of metal. The other is something known as a salt bridge that connects the two solutions.
What processes are responsible for conduction of electricity in an electroles-grizzlys-catalans.orgical cell?
The processes responsible for the current flow in an electroles-grizzlys-catalans.orgical cell depend on which part of the cell you are in. For the metallic components (zinc, copper, copper connecting wire), electrons are responsible for the current flow. In the solution, conduction of electricity is caused by migration of ions.
The ability of ions to conduct electricity is the reason why someone should never use a hairdryer while sitting in a bathtub full of water. If a hairdryer is dropped into the water, the water conducts electricity because of ions in it with the end result that the person will be electrocuted. Conductivity is a measurement of the ability of a solution to conduct electricity. The conductivity of a solution directly correlates with the ionic strength of the solution. Many science buildings have a device that is designed to generate highly purified water. One of the goals of these purification systems is to deionize the water. With these systems the conductivity is measured to determine the degree to which the water has been deionized (the reading is reported as a resistance and the higher the resistance, the less conductive the solution).
It is also important to consider the portions of the cell where the metal interfaces with the solution. In the cathode where reduction occurs, electrons must “jump” from the metal to a species in solution. In the anode of the cell represented in Figure 7, zinc atoms need to give up two electrons and a zinc ion is released into the solution. For an anodic half-cell with two water-soluble species (e.g., Fe2+ = Fe3+ + e–), an electron would need to “jump” from a species in solution to the platinum electrode.
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What is the purpose of the salt bridge?
In order to understand the purpose of the salt bridge it is necessary to consider the process taking place in each of the half cells in Figure 7. If each half cell started at standard state conditions, the cathode would begin with a 1 M concentration of a copper salt such as copper sulfate (
What would you put inside a salt bridge?
First, it is important to put ionic species into the salt bridge that will not be reduced or oxidized in either of the half cells. Alkali cations and halide anions would be ideal for this purpose. It is also important that the charge balance in each of the half cells facilitated by the ions in the salt bridge occurs at the same rates. That means that the halide anions moving from the salt bridge into the anode to balance out the excess Zn2+ ions do so at the same rate as the alkali cations moving from the salt bridge into the cathode to balance out the depletion of Cu2+ ions. Ions have a property known as mobility and the mobility of an ion depends on its size. Smaller ions have a higher mobility than larger ions. That means that the ideal species for a salt bridge should have a cation and anion of the same size and charge. Potassium chloride is the ideal species for incorporation into a salt bridge, as K+ and Cl– have the same number of electrons and are approximately the same size. Potassium nitrate (K+NO3–) can also be used in a salt bridge. Amazingly, the nitrate ion, which has atoms with second shell electrons, has approximately the same size as a chloride ion, which has atoms with third shell electrons.
Another thing to consider is the concentration of KCl in the salt bridge. It is desirable to have a salt bridge that can overcome the possibility of a large charge buildup. To achieve this and not deplete the ions in the salt bridge over the course of the reaction, the KCl is typically at a high concentration, usually 4 M.