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General Chemistry Principles, Patterns, and Applications, 2011

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anions are the conjugate bases of species that would lie below water in Table 16.3 "Values of ", are all<br />

strong bases that react essentially completely (<strong>and</strong> often violently) with water, accepting a proton to give a<br />

solution of OH − <strong>and</strong> the corresponding cation:<br />

Equation 16.31<br />

K2O s<br />

( ) + H2O( l) ® 2OH - ( aq) + 2K + ( aq)<br />

Equation 16.32<br />

NaOCH 3 s<br />

Equation 16.33<br />

NaNH2 s<br />

( ) + H2O( l) ® OH - ( aq) + Na + ( aq) + CH 3OH ( aq)<br />

( ) + H2O( l) ® OH -( aq) + Na + ( aq) + NH 3( aq)<br />

Other examples that you may encounter are potassium hydride (KH) <strong>and</strong> organometallic compounds such<br />

as methyl lithium (CH3Li).<br />

Polyprotic Acids <strong>and</strong> Bases<br />

As you learned in Chapter 4 "Reactions in Aqueous Solution", polyprotic acids such as H2SO4, H3PO4, <strong>and</strong><br />

H2CO3 contain more than one ionizable proton, <strong>and</strong> the protons are lost in a stepwise manner. The fully<br />

protonated species is always the strongest acid because it is easier to remove a proton from a neutral<br />

molecule than from a negatively charged ion. Thus acid strength decreases with the loss of subsequent<br />

protons, <strong>and</strong>, correspondingly, the pKa increases. Consider H2SO4, for example:<br />

Figure 16.5<br />

Equation 16.34<br />

HSO4 - aq<br />

( ) SO42 - ( aq) + H + ( aq) pKa = 1.99<br />

The equilibrium in the first reaction lies far to the right, consistent with H2SO4 being a strong acid. In<br />

contrast, in the second reaction, appreciable quantities of both HSO4 − <strong>and</strong> SO4 2− are present at equilibrium.<br />

Note the Pattern<br />

For a polyprotic acid, acid strength decreases <strong>and</strong> the pKa increases with the sequential loss of each<br />

proton.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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