Daniell’s galvanic cell

Brief Description of an Electrochemical Cell Daniell s Galvanic Cell... 

In Daniell s galvanic cell, electrons spontaneously go from the zinc strip to the copper one. This indicates that, spontaneously, the copper strip is more positive than the zinc strip. The electrode signs indicated in Fig. 2.4 follow this conclusion. 

The second point to emphasize is that the electromotive forces mentioned above are measured at null current. They are the zero cell potentials. This is a reversibility condition. In Daniell s galvanic cell experiment, electrical current can drive an electrical motor, for example. There is then an electrical energy recovery from the free enthalpy initially contained in the chemical system. Due to the current s occurrence, there are thus a release of heat due to the Joule effect as well as an entropy creation. The process is irreversible. However, the current can be very weak with the use of an opposition montage. In this condition, by avoiding any other irreversibility source, the free enthalpy decrease can be fully converted into electrical energy. Then the process becomes reversible. (The opposition montage even permits the reaction course to be inverted in some cases. The cell is then called a reversible one see Chap. 13.)... 

It is just the opposite of that occurring in Daniell s galvanic cell. Reaction (13.6) cannot be achieved spontaneously in a pure chemical manner without receiving energy (heat, for example) from the surroundings. An electrochemical cell whose reaction cell is not spontaneous is called an electrolytic cell or a substance-producing device (Fig. 13.3). 

Hence, two kinds of electrical current exist in an electrochemical cell the electronic current and the ionic current. According to Kirchhoffs law, no current can accumulate at any point of the circuit. As a result, the question arises concerning the nature of the phenomenon permitting the continuity of the current. It is clear that the junction between the two kinds of current cannot be located anywhere else than at the electrode-solution interface since each member of the interface possesses its own kind of current. The answer to the question is that the current s continuity is achieved by the reactions taking place at the interfaces, that is, by the electrochemical reactions. Indeed, electron transfer between two phases is the fundamental act of electrochemistry. In the case of Daniell s galvanic cell, reactions (13.2) and (13.3) take place. During the course of reaction (13.2) ... 

In Daniell s galvanic cell, the cathode is the copper wire and the anode is the zinc wire. In the corresponding electrolytic cell, it is the reverse The zinc wire is the cathode and the copper wire the anode. Hence, the same electrode may be either an anode or a cathode depending on the experimental conditions. 

According to this definition, Daniell s galvanic cell and the corresponding electrolytic cell constitute a reversible cell. An example of an irreversible cell is provided... 

Chemists use a special notation to specify the structure of electrode compartments in a galvanic cell. The two electrodes in the Daniell cell, for instance, are denoted Zn(s) Zn2+(aq) and Cu2+(aq) Cu(s). Each vertical line represents an interface between phases—in this case, between solid metal and ions in solution in the order reactant product. 

Rather than describing a galvanic cell in words, it s convenient to use a shorthand notation for representing the cell. For the Daniell cell in Figure 18.2, which uses the reaction... 

From an operational standpoint, electrochemical cells in which faradaic currents are flowing are classified as either galvanic or electrolytic cells (a faradaic current occurs if charges, e.g., electrons, are transferred across the metal-solution interface— see electrochemistry). The example of Daniell s cell permits us to clarify these points. 

A Daniell cell is a type of galvanic cell that uses the Zn/Cu reaction to produce electricity. In the Daniell cell, a piece of zinc metal is placed in a solution of zinc sulfate in one container, and a piece of copper metal is placed in a solution of copper(ll) sulfate in another container. These strips of metal are called the cell s electrodes. They act as a terminal, or a holding place, for electrons. 

Let s now consider an electrochemical system analogous to the galvanic Daniell cell but that differs from it by the presence of a power supply instead of the electrical... 

Note that running the Daniell cell in the galvanic mode, Faraday s law of electrolysis can be applied with an experimental possibility to better nnderstand and verify the law. 

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