5. Alkaline batteries operate in a potassium hydroxide liquid electrolyte which

Question

5. Alkaline batteries operate in a potassium hydroxide liquid electrolyte which allows current to flow at room temperature. The two half-cell reactions, written as reduction reactions, are given below along with their standard potentials in the alkaline solution. V-1.25 V V" = 0.30 V a) What material is the anode in an AA battery? b) What is the valence of the cathode cation after reduction? c) Write the overall reaction for the battery d) Calculate the value of AG (in kJ/mol) for the overall reaction. e) Calculate the value of the equilibrium constant at room temperature f) Calculate G° for MnO(OH) formation at room temperature. (Hint: Assume that Zn(OH)2 decomposes to ZnO and H:O without changing the voltage (AV) of the overall reaction). Values for AG at room temperature are -237.14 kJ/mol for water, -320.476 k/mol for zinc oxide, and-465. 138 kJ/mol for manganese dioxide.

Explanation / Answer

(f)

Overall reaction is,

Zn + 2 MnO2 + 2 H2O -------------> 2 MnO(OH) + Zn(OH)2

But given that zinc hydroxide will decompose with out changing voltage,

Zn + 2 MnO2 + 2 H2O -------------> 2 MnO(OH) + ZnO + H2O  

deltaG0r = 2 * deltaG0f(MnO(OH)) + deltaG0f(ZnO) + deltaG0f(H2O) - deltaG0f(Zn) - 2 * deltaG0f(MnO2) - 2 * deltaG0f(H2O)

- 299.104 = 2 * deltaG0f(MnO(OH)) - 320.476 - 237.14 - 0 - 2 ( - 465.138) - 2 ( - 237.14 )

2 * deltaG0f(MnO(OH)) = - 1146.044

deltaG0f(MnO(OH)) = - 573.022 kJ

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