Copper sulfate, electrochemistry

Most people associate the word electricity with wires and the motion of electrons through those wires. Makes sense, because that s the kind of electricity we encounter on a day-to-day basis. In the previous section, though, all of your electrical circuits involved liquids—saltwater, copper sulfate dissolved in water, and vinegar. In each case, ions (charged atoms or molecules) in the liquids moved around instead of electrons. An electric current is defined as any movement of charges, not just the movement of electrons. Because we re dealing with ions in addition to electrons, we call the process electrochemistry. 

In his creation of the first electric battery in 1799, Alessandro Volta laid the foundation of electrochemistry. His battery consisted of a zinc and a copper electrode dipped into an aqueous solution of sulfuric acid and connected externally by copper wires. The procedure for setting up and running this experiment is similar to the earlier practical of the electrolysis of aqueous copper sulfate solution (page 265), but without the external power supply. If a voltmeter is connected in parallel to the external circuit, it can be used to measure the potential difference. The half reactions and the overall reaction are as follows ... 

In a Daniell cell, the pieces of metallic zinc and copper act as electrical conductors. The conductors that carry electrons into and out of a cell are named electrodes. The zinc sulfate and copper(II) sulfate act as electrolytes. Electrolytes are substances that conduct electricity when dissolved in water. (The fact that a solution of an electrolyte conducts electricity does not mean that free electrons travel through the solution. An electrolyte solution conducts electricity because of ion movements, and the loss and gain of electrons at the electrodes.) The terms electrode and electrolyte were invented by the leading pioneer of electrochemistry, Michael Faraday (1791-1867). 

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

You will learn more about the importance of half-reactions when you study electrochemistry in Chapter 21. For now, however, you can learn to use halfreactions to balance a redox equation. First, look at an unbalanced equation taken from Table 20-3 to see how to separate a redox equation into half-reactions. For example, the following unbalanced equation represents the reaction that occurs when you put an iron nail into a solution of copper(II) sulfate, as shown in Figure 20-8. Iron atoms are oxidized as they lose electrons to the copper(ll) ions. 

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