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• Due to polarity of H-bond

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) Most polar Least polar

Electronegativity

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

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

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

Oxyacids and Strength Relationship• Acid strength

increases as number of oxygens bonded to central atom increases.

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

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

CaO + H2O Ca(OH)2K2O + H2O 2 KOH