The Zinc-Copper Cell

A cell in which all reactants and products are in their thermodynamic standard states (1 M for dissolved species and 1 atm partial pressme for gases) is called a standard cell. 

The mass of the zinc electrode decreases. The concentration of increases in the solution around the zinc electrode as the cell operates. 

The Zn electrode loses mass because some Zn metal is oxidized to Zn ions, which go into solution. Thus the Zn electrode is the anode. At the cathode, CT+ ions are reduced to Cu metal. This plates out on the electrode, so its mass increases. 

Unless otheiwise noted, all content on this page is Cengage Learning. 

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Consider how these standard potentials are used to determine the voltage of an electric cell. In the zinc-copper cell described earlier, the two halfreactions must be added to determine the cell EMF. (See Table 12-2.)... 

In the chart of standard electrode potentials (Table 12-1), reactions are arranged in order of their tendency to occur. Reactions with a positive EMF occur more readily than those with a negative EMF. The zinc-copper cell has an overall EMF of + 1.10 volts, so the solution of zinc and deposition of copper can proceed. 

The activity series shows that zinc is a better loser of electrons than copper. It is this relative difference in the two metals that causes zinc to be the electron loser in the zinc/copper cell. Zinc responds differently when paired with a metal higher that it is in the activity series. 

The Zinc-Copper Cell 21-10 The Copper-Silver Cell... 

Left) A strip of zinc was placed in a blue solution of copper(II) sulfate, CUSO4. The copper has been displaced from solution and has fallen to the bottom of the beaker. The resulting zinc sulfate solution is colorless. This is the same overall reaction as the one that occurs when the two half-reactions are separated in the zinc-copper cell (see Figure 21 -6). (Right) No reaction occurs when copper wire is placed in a colorless zinc sulfate solution. The reaction... 

Voltaic cells can be represented as follows for the zinc-copper cell. 

Recall that in the zinc-copper cell the copper electrode is the cathode now in the copper-silver cell the copper electrode is the anode. 

Now let s return to the zinc-copper cell and use the measured value of Eceii (l.lO V) and the value we just found for °i c to calculate copper-... 

The potential of the zinc-copper cell changes as concentrations change during cell operation. The only concentrations that change are [reactant] = [Cu ] and [product] = [Zn ] ... 

Figure 21.11B summarizes these four key stages in the operation of a voltaic cell. Let s find/f for the zinc-copper cell. At equilibrium. Equation 21.10 becomes... 

Thus, the zinc-copper cell does work until the [Zn" ]/[Cu" ] ratio is very high. 

A, A plot of Eceii vs. Q (on a logarithmic scale) for the zinc-copper cell shows a linear decrease. When Q < 1 (left), [reactant] is relatively high, and the cell can do relatively more work. When 0 = 1, ceii = leh When Q > 1 (right), [product] is relatively high, and the celi can do relatively less work. B, A summary of the changes in Eceii as the cell operates, including the changes in [Zn ], denoted [P] for [product], and [Cu ], denoted [R] for [reactant]. 

There is obviously a slight problem with the scheme outlined above, and that is that if we add the voltages in (18.11) and (18.12) we do not get the cell voltage of the zinc-copper cell (f ceii + 4ii)> because the hydrogen electrode voltage does not cancel but is doubled. That s why we qualified the presentation with if we arrange things properly . 

Now considering the zinc-copper cell, which could be constructed by substituting a copper electrode in a cupric ion solution for the SHE in Figure 18.1, we could write... 

The force of gravity always causes a diver to fall downward to a lower energy state, never upward to a higher energy state. When a diver steps off a diving board, his or her spontaneous motion is always downward. Similarly, in the zinc-copper cell, under standard conditions, copper(II) ions at the cathode accept electrons more readily than the zinc ions at the anode. Thus, the redox reaction occurs spontaneously only when the electrons flow from the zinc to the copper. 

The redox reaction, the movement of electrons in the metallic or external part of the circuit, and the movement of ions in the solution or internal part of the circuit of the zinc-copper cell are very similar to the actions that occur in the electrofytic cell of Figure 17.4. The only important difference is that the reactions of the zinc-copper cell are spontaneous. This spontaneity is the crucial difference between all voltaic and electronic cells. 

Figure 21.7 Measurement of a standard cell potential. The zinc-copper cell, operating at 298 K under standard-state conditions, produces a voltage of 1.10 V.

Note that this result is identical to Equation 21.8, which we obtained from AG°. Solving for K of the zinc-copper cell (Eceii = 110 V),... 

The Zinc-Copper Cell 21-10 The Copper-Silver Cell Standard Electrode Potentials 21-11 The Standard Hydrogen Electrode 21-12 The Zinc-SHE Cell 21-13 The Copper-SHE Cell 21-14 Standard Electrode Potentials 21-15 Uses of Standard Electrode Potentials 21-16 Standard Electrode Potentials for Other Half-Reactions 21-17 Corrosion 21-18 Corrosion Protection... 

This is the zinc-copper cell depicted in Fig. 14.2 on the next page, sometimes called a Daniell cell. The two electrolyte phases are separated by a liquid junction represented in the cell diagram by the dashed vertical bar. If the liquid junction potential can be assumed to be negligible, the liquid junction is instead represented by a pair of dashed vertical bars ... 

For example, it is found that the zinc-copper cell of Fig. 14.2, with typical electrolyte molalities, has its positive terminal at the copper electrode. When we write the cell diagram... 

Now imagine a reaction vessel that has the same temperature and pressure as the galvanic cell, and contains the same reactants and products at the same activities as in the cell. This reacrion vessel, unlike the cell, is not part of an electrical circuit. In it, the reactants and products are in direct contact with one another, so there is no constraint preventing a spontaneous reaction process. This reaction will be called the direct reaction. For example, the reacrion vessel corresponding to the zinc-copper cell of Fig. 14.2 would have zinc and copper strips in contact with a solution of both ZnS04 and CUSO4. Another example is the slow direct reaction in a cell without liquid junction described on page 453. 

Calculating gn from fhalt-cell The overall redox reaction for the zinc-copper cell is the sum of its half-reactions ... 

As with any voltaic cell, the potential of the zinc-copper cell changes during cell operation as the concentrations of the components change. With two of the four components solids, the only variables are [Cu +] and [Zn +l ... 

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