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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Review Question 15.7

Explain how the strengths of

(a) binary acids are affected by bond energies and the ionic radii of the anions they produce

(b) oxoacids are affected by the electronegativity of the central nonmetal atom and the number of terminal O atoms





Answer:

(a) The bond energy of the H-X bond is directly related to the acid strength. If the H-X bond is weak, then the bond is easily broken and gives a stronger acid. If the bond is strong, then it is difficult to break the bond, that is difficult to donate H+, so is a weaker acid. Anions with large ionic radii are the conjugate base of strong acids. If an anion has a large ionic radius, then the HX bond formed will be relatively long. Long bonds give small bond energies, which leads to strong acids. Small anionic radii lead to short and strong bonds, which is the description of a weak acid.


(b) The central atom (if it is more electronegative than H) and terminal oxygen atoms draw electron density away from the hydrogen atoms; this makes the H atoms more H+ - like, thus a stronger acid.