The Daniell cell

If such a cell is set up, there is a flow of electrons from the zinc to the copper electrode in the outer circuit. This means that a positive current must be moving from left to right in the cell itself. By convention, an emf corresponding to an external flow of electrons from the left-hand electrode to the right-hand electrode is said to be a positive emf. The magnitude of the emf developed (with 1 m solutions) is approximately 1.1 volts (V). This positive value is said to be the standard emf of 

We shall employ the symbol AE for such standard voltages, f The processes which occur when this cell operates are shown in the figure. Since 

Some of these zinc ions pass through the membrane into the right-hand solution, and at the right-hand electrode, cupric ions interact with electrons to form metallic 

Every time a zinc atom dissolves and a copper atom is deposited, two electrons travel around the outer circuit. 

The Daniel cell and many other cells behave in a reversible fashion. It is possible to balance their eraf by an external emf, for example by using a potentiometer wire. If the counter-emf is exactly equal to the emf developed by the cell, no current passes. If the counter-emf is slightly less than the emf of the cell, there is a small electron fiow from left to right in the outer circuit. If the counter-emf is adjusted 

Consider the electrochemical cell, the Daniell cell, shown in Fig. 17.1. It consists of two electrode systems—two /zu/f-ce//s—separated by a salt bridge, which prevents the two solutions f rom mixing but allows the current to flow between the two compartments. Each half-cell consists of a metal, zinc or copper, immersed in a solution of a highly soluble salt of the metal such as ZnS04 or CUSO4. The electrodes are connected to the exterior by 

Assume that the switch in the external circuit is open and that the local electrochemical equilibria are established at the phase boundaries and within the bulk phases. At the Ptj Zn and the Cu Ptn interfaces the equilibrium is established by the free passage of electrons across the interface. The equilibrium conditions at these interfaces are 

The difference 4 w — 0i is measurable since it is a difference of potential between two phases having the same chemical composition (both are platinum). 

Suppose we connect the two platinum wires through an ammeter to a small motor we observe that (1) some zinc dissolves, (2) some copper is deposited on the copper electrode, (3) electrons flow in the external circuit from the zinc to the copper electrode, and (4) the motor runs. The changes in the cell can be summarized as  

At the left electrode In the external circuit At the right electrode 

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

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 ... 

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 ... 

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. 

Ans. No. If the Daniell cell were to be recharged, the Cu2t ions would get into the zinc half-cell through the salt bridge. There, they would react directly with the zinc electrode, and the cell would be destroyed. 

Consider the Daniell cell Zn Zn2+ Cu2+ Cu. The cell emf is about 1.1 V when prepared with clean, pure electrodes and both solutions at unit activity. The emf decreases to about 1.05 V after adding lithium chloride to the copper half-cell. Adding more LiCl, but this time to the zinc solution, increases the emf slightly, to about 1.08 V. 

The zinc ions in the other half of the Daniell cell can similarly interact with ions added to solution, causing the zinc electrode to see fewer Zn2+ species, and the voltmeter again reads a different, smaller value of EZni+Zn. Since the emf represents the separation between the electrode potentials of the two half-cells, any changes in the emf illustrate the changes in the constituent electrode potentials. 

Cell notation is a shorthand notation of representing a galvanic cell. To write the cell notation for the Daniell cell you ... 

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

Figure 11.1 shows one example of a galvanic cell, called the Daniell cell. One half of the cell consists of a piece of zinc placed in a zinc sulfate solution. The other half of the cell consists of a piece of copper placed in a copper(II) sulfate solution. A porous barrier, sometimes called a semi-permeable membrane, separates these two half-cells. It stops the copper(II) ions from coming into direct contact with the zinc electrode. 

The Daniell cell is named after its inventor, the English chemist John Frederic Daniell (1790-1845). In the photograph shown here, the zinc sulfate solution is placed inside a porous cup, which is placed in a larger container of copper sulfate solution. The cup acts as the porous barrier. 

The redox reaction takes place in a galvanic cell when an external circuit, such as a metal wire, connects the electrodes. The oxidation half-reaction occurs in one half-cell, and the reduction half-reaction occurs in the other half-cell. For the Daniell cell ... 

The electrode at which oxidation occurs is named the anode. In this example, zinc atoms undergo oxidation at the zinc electrode. Thus, the zinc electrode is the anode of the Daniell cell. The electrode at which reduction occurs is named the cathode. Here, copper(II) ions undergo reduction at the copper electrode. Thus, the copper electrode is the cathode of the Daniell cell. 

A typical galvanic cell, such as the Daniell cell shown here, includes two electrodes, electrolyte solutions, a porous barrier, and an external circuit. Electrons flow through the external circuit from the negative anode to the positive cathode. 

The Daniell cell is fairly large and full of liquid. Realistically, you could not use this type of cell to power a wristwatch, a remote control, or a flashlight. Galvanic cells have been modified, however, to make them more useful. 

A dry cell is a galvanic cell with the electrolyte contained in a paste thickened with starch. This cell is much more portable than the Daniell cell. The first dry cell, invented by the French chemist Georges Leclanche in 1866, was called the Leclanche cell. 

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