For the alkaline Leclanch cell. (a) write the overall cell reaction. (b) Determine E cell for that

For the alkaline Leclanché cell.

(a) write the overall cell reaction.

(b) Determine E°_cell for that cell reaction.

Figure 19.14

Transcribed Image Text:

The Leclanché (Dry) Cell The most common form of voltaic cell is the Leclanché cell, invented by the French chemist Georges Leclanché (1839-1882) in the 1860s. Popularly called a dry cell, because no free liquid is present, or flashlight battery, the Leclanché cell is diagrammed in Figure 19-14. In this cell, oxidation occurs at a zinc anode and reduction at an inert carbon (graphite) cathode. The electrolyte is a moist paste of MnO2, ZnCl2, NH4Cl, and carbon black (soot). The maximum cell voltage is 1.55 V. The anode (oxidation) half-cell reaction is simple. Oxidation: Zn(s) →→→ Zn²+ (aq) + 2 e The reduction is more complex. Essentially, it involves the reduction of MnO2 to compounds having Mn in a +3 oxidation state, for example, Reduction: 2 MnO₂ (s) + H₂O(1) + 2 e¯ Mn₂O3(s) + 2OH(aq) An acid-base reaction occurs between NH4+ (from NH4Cl) and OH¯. NH4+ (aq) + OH(aq) NH3(g) + H₂O(1) A buildup of NH3(g) around the cathode would disrupt the current because the NH3(g) adheres to the cathode. That buildup is prevented by a reaction between Zn²+ and NH3(g) to form the complex ion [Zn(NH3)2]²+, which crys- tallizes as the chloride salt. → Zn²+ (aq) + 2NH3(g) + 2Cl(aq) [Zn(NH3)2]Cl2(s) The Leclanché cell is a primary cell; it cannot be recharged. This cell is cheap to make, but it has some drawbacks. When current is drawn rapidly from the cell, products, such as NH3, build up on the electrodes, causing the voltage to drop. Also, because the electrolyte medium is acidic, zinc metal slowly dissolves. A superior form of the Leclanché cell is the alkaline cell, which uses NaOH or KOH in place of NH4Cl as the electrolyte. The reduction half-cell reaction is the same as that shown above, but the oxidation half-cell reaction involves the formation of Zn(OH)2(s), which can be thought of as occurring in two steps. Zn(s) Zn²+ (aq) + 2 e Zn²+ (aq) + 2OH(aq) Zn(OH)2(s) Zn(s) + 2OH(aq) → Zn(OH)2(s) + 2 e The advantages of the alkaline cell are that zinc does not dissolve as readily in a basic (alkaline) medium as in an acidic medium, and the cell does a better job of maintaining its voltage as current is drawn from it.