Learning Goal 15
Explain how an acid's strength is related to its polarity and the strength of the H-X bond.

How easily will a molecule containing an H-X bond break, resulting in proton donation?

1. A substance will only donate a proton if the H-X bond is polar.

2. The strength of the H-X bond affects the molecule's ability to donate its protons.

Generally, the stronger the acids, the weaker and more polar their H-X bonds. Hence, strong acids give up protons more easily!


Relating Acid-base Properties to the Periodic Table

A hydride is a compound formed between hydrogen and any other element.

In each horizontal row of the periodic table, the most basic hydrides are on the left and the most acidic hydrides are on the right. Note that the left forms metal hydrides and the right forms non-metal hydrides. Metal hydrides are usually basic in water while non-metal hydrides are often acidic in water.

EXAMPLE

NaH = basic hydride:

Na+ (aq) + H- (aq) + H2O (l) H2 (g) + Na+ (aq) + OH- (aq)

HCl = acidic hydride:

H+ (aq) + Cl- (aq) + H2O (l) H3O+ (aq) + Cl- (aq)

In each vertical row of the periodic table, acidity usually increases with increasing atomic number because the valence (bonding) electrons are farther and farther away from the nucleus and less strongly attracted to the positive nucleus. Thus, the bonds formed by these electrons decrease in strength, making the bond easier to break and release protons.



Learning Goal 16
Understand the factors affecting acidity of species and use the factors to predict the relative strengths of acids.

Electronegativity

  • Electronegativity is a measure of an element's ability to attract electrons on an arbitrary scale between 0 and 4, with 4 being the most electronegative. The metals tend to be the least electronegative, while the non-metals are mostly highly electronegative.

    Electron Drift

  • Electron drift occurs when electrons are attracted by highly electronegative atoms away from bonded OH groups. The electron density between the oxygen and hydrogen atom is distorted by the electrons's attraction to the electronegative atom, and this further polarizes and weakens the O-H bond, making a stronger acid.

    Acid strength increases with :

    1. increasing electronegativity of the central atom: The more electronegative the central atom, the more electron drift occurs, polarizing the O-H bond which makes it easier for the molecule to dissociate.

    2. increasing oxidation number of the central atom: The higher the oxidation state of the central atom, the greater the number of oxygens attached to the central atom. Oxygen is a highly electronegative element, contributing to greater electron drift away from the O-H bond.


    Oxyacids

    An oxyacid is an acid in which (OH) groups (and sometimes additional oxygen atoms) are bound to a central atom. Oxyacids are a special subset of species which have a central atom "Y" which is bonded to an hydroxyl (OH) group and may or may not be bonded to other additional atoms.

    If "Y" has a low electronegativity (usually a metal) it will NOT attract the electron pair it shares with the O.

    Therefore the O will be negatively charged: it will not attract the electron pair it shares with the H, and the O-H bond will NOT be strongly polarized. However, the Y-O bond will be highly polar and easily broken, releasing OH- ions. (animation)

    The compound will behave as a base.

    If "Y" has an intermediate electronegativity (around 2.0), it bonds to O more covalently and loses OH- less easily.

    Such species behave as acids in water, if "Y" attracts the electron pair it shares with O strongly enough. These are known as oxyacids. (animation)

    EXAMPLE

    H2SO4 sulfuric acid:

    Carboxylic Acids

    POINT OF EMPHASIS: Carboxylic acids are organic compounds with the general formula: R-COOH, where "R" is either hydrogen or a carbon-based chain.

    EXAMPLE

    Acetic Acid :

    Like oxyacids, the polar carbonyl (C=O) group causes electron drift away from the O-H bond, resulting in dissociation of the molecule.