THERMODYNAMICS OF LEAD STORAGE CELL . HEAT CAPACITY AND ENTROPY OF LEAD DIOXIDE FROM 15 TO 318 DEGREES K

The heat capacity of an electrolytic sample of lead dioxide has been measured from 15 to 318°K. The composition was PbO2·1.519 × 10-2PbO·2.558 × 10-2H2O. After correction, the entropy of PbO2 was found to be 17.16 gibbs/mol at 298.15°K. The entropy change in the cell reaction, H2 + PbO2 + H2SO4 (x M) = PbSO4 + 2H2O (in x M H2SO4), calculated from the third law of thermodynamics, is in excellent agreement with dE/dT, as determined by Beck, Singh, and Wynne-Jones. This agreement supports the use of third-law data on PbO2, Pb, PbSO4, and H2SO4 (x M) to calculate the temperature coefficient of the lead storage cell. The table of Giauque, Hornung, Kunzler, and Rubin, on the thermodynamic properties of aqueous sulfuric acid, has been extended from 1 to 0.1 M. A table giving the change of voltage of the lead storage cell over the range 0-60° and from 0.1 to 14 M H2SO4 has been based on the third law of thermodynamics. For the reaction Pb + PbO2 + 2H2SO4 (pure) = 2PbSO4 + 2H2O (pure), ΔF° = -120,200 cal/mol and ΔH° = -121,160 cal/mol at 298.15°K. Numerous unsuccessful attempts to prepare stoichiometric PbO2 are described.