26.07.2021 Views

General Chemistry Principles, Patterns, and Applications, 2011

General Chemistry Principles, Patterns, and Applications, 2011

General Chemistry Principles, Patterns, and Applications, 2011

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Equation 8.7 may be used as a tool for predicting which ionic compounds are likely to form from<br />

particular elements. As we have noted, ΔH1 (ΔHsub), ΔH2 (I), <strong>and</strong> ΔH3 (D) are always positive numbers, <strong>and</strong><br />

ΔH2 can be quite large. In contrast, ΔH4 (EA) is comparatively small <strong>and</strong> can be positive, negative, or zero.<br />

Thus the first three terms inEquation 8.7 make the formation of an ionic substance energetically<br />

unfavorable, <strong>and</strong> the fourth term contributes little either way. The formation of an ionic compound will be<br />

exothermic (ΔHf < 0) if <strong>and</strong> only if ΔH5 (−U) is a large negative number. This means that lattice energy is<br />

the most important factor in determining the stability of an ionic compound. Another example is the<br />

formation of BaO:<br />

Equation 8.10<br />

Ba(s)+12O2(g)® BaO(s)<br />

DH = DHf<br />

whose Born–Haber cycle is compared with that for the formation of CsF in Figure 8.5 "Comparison of the<br />

Enthalpy Changes Involved in the Formation of Solid CsF <strong>and</strong> BaO from Their Elements".<br />

Figure 8.5 Comparison of the Enthalpy Changes Involved in the Formation of Solid CsF <strong>and</strong> BaO from Their<br />

Elements<br />

The lattice energy of BaO, with a dipositive cation <strong>and</strong> a dinegative anion, dominates the Born–Haber cycle.<br />

Reaction 1<br />

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

Saylor.org<br />

690

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!