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Effect of Structure on Acid-Base Properties

Effect of Structure on Acid-Base Properties. What structural properties of a molecule cause it to behave as an acid or base?. Presence of H atoms. Any molecule containing H is potentially an acid Many molecules do not exhibit acidic properties WHY???? Due to the strength of H-bond

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Effect of Structure on Acid-Base Properties

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  1. Effect of Structure on Acid-Base Properties What structural properties of a molecule cause it to behave as an acid or base?

  2. Presence of H atoms • Any molecule containing H is potentially an acid • Many molecules do not exhibit acidic properties • WHY???? • Due to the strength of H-bond • Due to polarity of H-bond

  3. Strength of Bonds • In general, the stronger the H-bond, the less likely the bond is to break to form H+ ions • Therefore, the less acidic the substance Example with Halide Acids: Acid Strength HF < HCl < HBr < HI Bond Energy 570 432 366 298 (kJ/mol) Least polar Most polar

  4. Electronegativity • H-F is very strong, difficult to break • H are reluctant to dissociate in water

  5. Atomic Size and H bonds • The larger the atom bonded to H, the weaker the bond (due to H orbital overlaps) • Ex. bond between H and a large atom (I or Te) is weaker than the bond between H and a small atom (F or O) • TREND = acid strengths of binary hydrides increase as we go down a column

  6. Investigate Oxyacids • Contain one or more O-H bonds • OH groups bonded to central atom

  7. Oxyacids and Strength Relationship • Acid strength increases as number of oxygens bonded to central atom increases.

  8. Why this behavior? • Electronegative oxygens draw electrons away from less electronegative atoms • Result: Weakens O-H bond • Result: Polarizes O-H bond • H+ Proton is more easily produced by the molecule with the largest number of attached oxygens

  9. Investigate Oxides • Covalent oxides dissolved in water yield acidic solutions called acidic oxides • Ionic oxides dissolved in water yield basic solutions called basic oxides SO3 + H2O   H2SO4 CO2 + H2O   H2CO3 K2O + H2O   2 KOH CaO + H2O   Ca(OH)2

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