Daniell cell electrochemical reaction, 245

An electrochemical reaction is said to be polarized or retarded when it is limited by various physical and chemical factors. In other words, the reduction in potential difference in volts due to net current flow between the two electrodes of the corrosion cell is termed polarization. Thus, the corrosion cell is in a state of nonequilibrium due to this polarization. Figure 4-415 is a schematic illustration of a Daniel cell. The potential difference (emf) between zinc and copper electrodes is about one volt. Upon allowing current to flow through the external resistance, the potential difference falls below one volt. As the current is increased, the voltage continues to drop and upon completely short circuiting (R = 0, therefore maximum flow of current) the potential difference falls toward about zero. This phenomenon can be plotted as a polarization diagram shown in Figure 4-416. 

During the operation of the cell (or during the direct interaction of zinc metal and cupric ions in a beaker) the zinc is oxidised to Zn and corrodes, and the Daniell cell has been widely used to illustrate the electrochemical mechanism of corrosion. This analogy between the Daniell cell and a corrosion cell is perhaps unfortunate, since it tends to create the impression that corrosion occurs only when two dissimilar metals are placed in contact and that the electrodes are always physically separable. Furthermore, although reduction of Cu (aq.) does occur in certain corrosion reactions it is of less importance than reduction of HjO ions or dissolved oxygen. 

Primary cells are non rechargeable cells, in which the electrochemical reaction is irreversible. They contain only a fixed amount of the reacting compounds and are discharged only once. If the educts are consumed by discharging, the cell cannot, or should not, be used again. A well-known example of a primary cell is the Daniell element, consisting of zinc and copper. 

We will illustrate the above point with the following example. Consider the cell Zn I ZnS04(aq) 11 CuS04(aq) Cu, which is commonly called the Daniell cell. The actual process of cell discharge involves an electrochemical reaction at both electrodes. Since the zinc is the more negative of the two half cells, oxidation would occur on the zinc side of the cell, as follows ... 

The Daniell cell illustrates the basic features of an electrochemical cell. Electrochemical cells always involve a redox reaction. Oxidation occurs at the cathode of the cell and reduction takes place at the anode. Electrons always flow from the anode to the cathode. Electrochemical cells come in many arrangements. To gain an appreciation for the variety of electrochemical cells, consider all the types of batteries available. 

Reaction (4.2) may be carried out by adding metallic zinc to a solution of copper sulfate or by using an electrochemical cell in which the reactants are not in contact. One such cell (a modern form of a Daniell cell) is shown in Figure 4.1. 

In some cases, as in reactions in electrochemical cells or other reactions involving oxidation-reduction, the half reactions of the ions are useful. Consider the Daniell cell, which consists of a zinc electrode in a zinc sulfate solution, and a copper electrode in a copper solution, the two solutions being separated by a porous partition. The half reactions are... 

An electrochemical cell is an experimental apparatus for generating electricity by using a redox reaction. The Daniel cell for the system Zn(s) + C oi) Zn(aq) + Cu(s) is shown in Figure 1.2. 

The Daniell cell is an example of a galvanic cell, in this type of electrochemical cell, electrical work is done by the system. The potential difference, between the two half-cells can be measured (in volts, V) on a voltmeter in the circuit (Figure 7.1) and the value of is related to the change in Gibbs energy for the cell reaction. Equation 7.9 gives this relationship under standard conditions, where is°ceu is the standard cell potential. 

Daniell cell shown in Fig. 17.1, a zinc electrode is immersed in a solution of zinc sulfate and a copper electrode is immersed in a solution of copper sulfate the solutions are in electrical contact through a porous partition that prevents the solutions from mixing. The Daniell cell can produce electrical work which is related to the decrease in Gibbs energy, —AG, of the chemical reaction by relation (10.14). If the cell operates reversibly, then the electrical work produced is equal to the decrease in Gibbs energy. The performance of the electrochemical cell is discussed in detail in Chapter 17. 

In many secondary school textbooks, the topic of electrochemical cells is introduced after redox reactions by demonstrating a galvanic cell, very often the Daniell cell (zinc-copper cell). But for students, this experiment is overwhelming because of this cell does not look like a battery. Moreover, the experiment is confusing for students because of the large number of new phenomena and the complexity of the explanations. We would propose to start with a more simple experiment that clarifies some basic characteristics of cells, especially the concepts of electrolyte, electrode and electrode reaction. 

Dioxygen, like the copper electrode in Daniell cell, receives the two electrons and is the right end of the electrochemical cell we re-write its reductive half-reaction as follows ... 

The theoretical voltage of an electrochemical cell is equal to the difference between the reversible potentials of the two electrode reactions, as calculated by the Nernst equation. For the Daniell cell (Figure 2.10), the theoretical voltage is given by ... 

An electrochemical cell, such as the Daniell cell, operates by the oxidation reaction producing electrons in the zinc anode, which are then pulled round the external circuit (wires, bulbs, voltmeter, etc.) by the reduction reaction at the copper cathode. As long as the overall reaction is not at equilibrium, the oxidation reaction pushes electrons into the external circuit, and the reduction reaction pulls them out. The cell is described as doing work since it produces a force that moves electrons around the external circuit. This work can light a bulb, drive an electric motor, etc. 

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) ... 

It is usually not easy to illustrate Equation 4.1 in a thermodynamics course, so some abstract approaches with a heat engine are used. However, in electrochemical science and engineering, all values of Equation 4.1 are well defined, and two of them, AG and AS, can experimentally be estimated using only electrochemical measurements. If the potential difference of the Daniell cell in the equilibrium mode, is measured, this value can immediately be used to calculate, A,G, the Gibbs energy of the total reaction taking place in the Daniell cell. If the temperature dependence of AjG can experimentally be estimated, the entropy of the electrochemical reaction, A,S, can be calculated as follows ... 

For the Daniel cell, there is no need using the new type of diagram because (1) only three species participate in both half-reactions, (2) two of these three, metal atoms and electrons, in each of the half-reactions are in the same phase, and (3) there are only two-phase boundaries in this electrochemical cell. The double vertical line in the diagram shows that the potential difference between ZnS04 (aq, 0.001 mol kg" ) and CUSO4 (aq, 0.005 mol kg" ), called the diffusion potential (see Chapter 5), is somehow eliminated. 

If 50 mg of Zn is electrochemically deposited in the Daniell cell, how much charge should be passed through the cell assuming that there are not any parasitic reactions in the electrolysis ... 

Electrochemical energy storage and conversion systems described in this chapter comprise batteries and fuel cells [6-11], In both systems, the energy-supplying processes occur at the phase boundary of the electrode-electrolyte interface moreover, the electron and ion transports are separate [6,8], Figures 8.1 and 8.2 schematically illustrate the electron and ion conductions in both the electrodes and the electrolyte in Daniel and fuel cells. The production of electrical energy by the conversion of chemical energy by means of an oxidation reaction at the anode and a reduction reaction at the cathode is also described. 

Homogeneous ET reactions take place in a homogeneous medium from a donor (D) to an acceptor (A). Electrons are exchanged between molecules or ions, for example, one electron may transfer from Fe + to Co +. As always, a metal complex (metal + ligands) is the smallest unity for our attention, not just the metal ions. Heterogeneous ET reactions take place between a solid material and ions in solution. One example is the well-known Daniell electrochemical cell, where Cu + ions in a solution receive electrons from a copper electrode and are deposited there as copper metal. 

Electrochemical Cells and Redox Reactions Example of Daniell s Galvanic Cell... 

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