Soft positive, lead oxides

Kargel (2001), Thomson and Delaney (2001), and O Brien et al. (2002) have hypothesized that a Europan ocean could become thermally and com-positionally stratified. Disturbance and overturning of this ocean could lead to exsolution of gases and a massive thermal flux to the surface via cryo-volcanism (Fig. 5.21). Such massive fluxes, if frequent enough, could play an important role in cycling oxidants compared with the presumably slower process of solid-phase convective flow in the soft ice. There may also be hydrothermal vents in a Europan ocean. If massive enough, these vents could... 

Of great importance was his realisation of the significance of oxidation state on the hardness. Increased positive charge on the acceptor atom would lead to increased hardness, as a rule, though there were a few exceptions. A key conclusion was that metal atoms in the zero oxidation state would always be class (b) or soft. This would extend to bulk metals as well and led to an immediate understanding of poisoning in such catalysts. 

Soft acids are electron pair acceptors (also called Lewis acids). The accepting atom has a zero or low positive charge, and are relatively large in size. This leads to low electronegativity and valence electrons with high polarizability, which makes them easy to oxidize. 

The features which bring out class (a) behavior are small size and high positive oxidation state. Class (b) behavior is associated with a low or zero oxidation state and/or with large size. Both metals and nonmetals can be either (a) oder (b) type acids depending on their charge and size. Since the features which promote class (a) behavior are those which lead to low polarizability, and those which create type (b) behavior lead to high polarizibility, it is convenient to call class (a) acids hard acids and class (b) acids "soft acids. We then have the useful generalization that hard acids prefer to associate with hard bases, and soft acids prefer soft bases. 

The electrolyte contains hydrogen ions (H ) and sulfate ions (S04 ). During discharge, H+ ions react with the lead(IV) oxide to give lead(II) oxide and water Pb02 + 2H+ + 2e- PbO + H2O This reaction takes electrons from the lead(IV) oxide plates, causing the positive charge. A further reaction follows, which yields soft lead sulfate ... 

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