Polar Covalent Bonds Acids and Bases

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The Reaction of Acid with Base

Hydronium ion, product when base H2O gains a proton

HCl donates a proton to water molecule, yielding hydronium ion (H3O+) [conjugate acid] and Cl [conjugate base]

The reverse is also a Brønsted acid–base reaction of the conjugate acid and conjugate base

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Acid and Base Strength

The equilibrium constant (Keq) for the reaction of an acid (HA) with water to form hydronium ion and the conjugate base (A-) is a measure related to the strength of the acid

Stronger acids have larger Keq

Note that brackets [ ] indicate concentration, moles per liter, M.

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Ka – the Acidity Constant

The concentration of water as a solvent does not change significantly when it is protonated

The molecular weight of H2O is 18 and one liter weighs 1000 grams, so the concentration is ~ 55.4 M at 25°

The acidity constant, Ka for HA Keq times 55.6 M (leaving [water] out of the expression)

Ka ranges from 1015 for the strongest acids to very small values (10-60) for the weakest

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pKa – the Acid Strength Scale

pKa = -log Ka

The free energy in an equilibrium is related to –log of Keq (DG = -RT log Keq)

A smaller value of pKa indicates a stronger acid and is proportional to the energy difference between products and reactants

The pKa of water is 15.74

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Predicting Acid–Base Reactions from pKa Values

pKa values are related as logariths to equilibrium constants

Useful for predicting whether a given acid-base reaction will take place

The difference in two pKa values is the log of the ratio of equilibrium constants, and can be used to calculate the extent of transfer

The stronger base holds the proton more tightly

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Organic Acids and Organic Bases

Organic Acids:

characterized by the presence of positively polarized hydrogen atom

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Organic Acids

Those that lose a proton from O–H, such as methanol and acetic acid

Those that lose a proton from C–H, usually from a carbon atom next to a C=O double bond (O=C–C–H)

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