Daniell cells

Daniell cell A ZnjZn lCu /Cu cell. The e.m.f. of the Daniell cell is MOV and is virtually independent of temperature. 

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. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

One of the most well-known electrochemical cells that is used for the conversion of chemical energy into electrical energy is the Daniell cell... 

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. 

An interesting application of electrode potentials is to the calculation of the e.m.f. of a voltaic cell. One of the simplest of galvanic cells is the Daniell cell. It consists of a rod of zinc dipping into zinc sulphate solution and a strip of copper in copper sulphate solution the two solutions are generally separated by placing one inside a porous pot and the other in the surrounding vessel. The cell may be represented as ... 

Daniell cell 64 d.c. arc source 763, 771 Dead-stop end points 635 Decantation 119 Decomposition potential 504 Degreasing agent 80 Delves cup 788 Demasking agents 312, 334 Densitometers 231, 232 Depolariser anodic, 515 cathodic, 509... 

Steam, gas, petrol, and hot-air engines are heat engines a thermopile coupled with an electromotor also constitutes a heat engine. An electromotor is not a heat engine, since its effect is produced at the cost of electrical energy, which may, it is true, ultimately be derived from a heat engine coupled with a dynamo, but may equally well arise from chemical action in voltaic cells absorbing practically no heat from their environment (e.g., the Daniell cell). 

The copper pole of the Daniell cell will therefore tend to cool itself during action. 

In assembling cells for making thermodynamic measurements, one should try not to combine half-cells in a manner that results in a junction potential. Figure 9.7 is a schematic representation of the Daniell cell, which is one with a junction potential. The half-cell reactions are... 

Figure 9.7 The Daniell cell, an example of a cell with a junction potential.

The electrodes in the Daniell cell are made of the metals involved in the reaction. However, not all electrode reactions include a conducting solid directly. For example, to use the reduction 2 H+(aq) + 2e"- H2(g) at an electrode, a... 

FIGURE 12.3 The Daniell cell consists ot copper and zinc electrodes dipping into solutions of coppertll) sultate and zinc sulfate, respectively. The two solutions make contact through the X>rous barrier, which allows ions to pass through and complete the electric circuit. 

The emf of the Daniell cell for certain concentrations of copper and zinc ions is 1.04 V. What is the reaction Gibbs free energy under those conditions ... 

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. 

We report the structure of a cell in a symbolic cell diagram, by using the conventions specified by IUPAC and used by chemists throughout the world. The diagram for the Daniell cell, for instance, is... 

In this expression, E° is the standard emf of the cell, the emf measured when all the species taking part are in their standard states. In practice, this condition means that all gases are at 1 bar, all participating solutes are at 1 molT-1, and all liquids and solids are pure. For example, to measure the standard emf of the Daniell cell, we use 1 M CuS04(aq) and a pure copper electrode in one electrode compartment and 1 M ZnS04(aq) and a pure zinc electrode in the other. 

The cell diagram is written to correspond to how the chemical equation for the reaction is written, not to the way that the cell is arranged in the laboratory. Thus, the Daniell cell can be described as either... 

SOLUTION The Daniell cell and the corresponding cell reaction are... 

Attention is now confined to one of the electrodes of the Daniell cell, say the copper electrode. The reaction in this is ... 

Electrolysis of hydrochloric add is considered. 2 Daniell cell is considered. 

Fig. 1.1. Daniell cell, (a) at zero current, (b) delivering current-energy, (c) consuming current-...

The three afore-mentioned possibilities are illustrated in Fig. 1.1 for the well known Daniell cell (the arrows of current flow are conventional, i.e., the flow direction of the positive charge which is opposite to the actual flow of electrons as far as metallic conductance and charge transfer at the electrodes are concerned). 

The phases in the scheme are always numbered from the left to the right, but the cell can be depicted graphically in two ways. For example, there are two possibilities for the Daniell cell ... 

Calculate the potential of the Daniell cell in which the copperOD ion concentration and the zinc ion concentrations are both 0.100 M. 

Ans. The Daniell cell has a standard potential of 1.10V, as calculated in Example 14.5. The Nernst equation is used to calculate the actual potential. 

Explain why a Daniell cell cannot be placed in a single container, like the lead storage cell. 

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