4241LNOT08_S11_PP - Environmental Geochemistry GLY...

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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 1 8-1 8-2 LECTURE 8 - Acids and Bases Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Acids The classical definition of an acid is a substance that dissociates to yield free protons: Later it was realized that existing in water was impossible for a “bare” proton (no electrons). Instead the "hydronium" ion is formed: In reality the hydronium ion is not only H 3 O + , but rather, some hydrated form with the general formula H 2n+1 O n + . Nitric acid is a stronger acid than water. Therefore nitric acid acts as an acid in the presence of water by donating a proton to water and forming the hydronium ion. Brønsted defined an acid to be a "proton donor." Lewis suggested an alternative definition of an acid. The Lewis definition of an acid is any substance that accepts a lone pair of electrons (see figure 8-1). For most purposes, this definition is not important in geology; only in the study of catalysts does it apply.
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 2 Figure 8-1 8-3 In geology the term "acidic rock" has long been used. The term means any rock containing an excess of nonmetallic oxide (principally silica) over metallic oxides. Many nonmetallic oxides (CO 2 , SO 2 , SO 3 , NO 2 , etc.) will dissolve in water to yield acids (H 2 CO 3 , H 2 SO 4 , etc.) and this is the reason that nonmetallic oxides were originally called acidic. However, SiO 2 is almost insoluble in water and therefore the term, acidic, applied to silica is very misleading. An effort to replace the term acidic by the word felsic occurred. The older literature still contains references to acidic rocks, however. Many geologists continued to use the term acidic. So the word has now been redefined as weight percent silica content, not referring to the pH of any solution in contact with the rock. Bases
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4241LNOT08_S11_PP - Environmental Geochemistry GLY...

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