Oxides

O₂ combines most of the other elements. The chemistry of oxygen is therefore usually classed as that of the oxides.

The enthalpy of formation of the O^2- ion is ~1000 kJmol-1, and so unless this is balanced by a large lattice enthalpy, oxides are often unstable with respect to the formation of covalent bonds. In fact, the O^2- ion is unstable in the gas phase with respect to loss of an electron, and it is only stabilized in the condensed phase.

Oxides are classified as ionic, polymeric or covalent;

• across a period, Z_eff increases, and so the bonding to O becomes more covalent and localized. There is therefore a change from ionic to polymeric to covalent across a period.

• down a group, the ionization energy decreases and the ionic size increases, so the elements tend to become more metallic and the bonding to O becomes more ionic.

• The oxides of elements in higher oxidation states are more covalent, and contain localized E=O bonds; O forms strong multiple bonds with small highly charged metals. Therefore, O stabilizes high oxidation states, and only F is better at doing this.

Structures of Ionic Oxides

M₂O: antifluorite structure (Cs₂O has the anti-CdCl₂ structure).

MO: rock salt (NaCl) structure (eg. Group 2 and Transition Metal Oxides).

M₂O₃: corundum structure (eg. Al₂O₃ and Transition Metal Oxides).

MO₂: fluorite (CaF₂) and rutile (TiO₂) structures.

M₃O₄: spinel structure (eg. Fe^IIFe^III₂O₄).

ABO₃: perovskite structure (eg. CaTiO₃).

The Acid/Base classification of Oxides:

Highly ionic oxides are basic.

Highly covalent oxides are acidic.

The amphoteric line moves further to the right of the periodic table as the groups are descended. (An amphoteric oxide is one which reacts with both acids and bases.)

For a given element, the acidity of the oxide increases with the oxidation state. This is because the element becomes more polarizing with increasing charge/size ratio. This means that as the ion Eⁿ⁺ gets smaller, it has an increasing preference for O^2- over OH^- over OH₂.

Reaction of basic oxide:

Hydroxides

The hydroxide ion, OH^-, only exists in the hydroxides of electropositive metals.

Non-Metal hydroxides are really acids, eg. Boric acid, B(OH)₃.

Peroxides

Species containing the O₂^2- ion are known as peroxides. The peroxide ion itself is unstable with respect to disproportionation into O⁰ and O^II, but in fact it is not a very labile ion, and so is relatively long lasting in the absence of catalysts for the disproportionation.

Peroxides and Peroxo-salts are widely occurring.

Hydrogen Peroxide

Structure

It has the open book structure; this is not the one predicted by VSEPR.

VSEPR structure (lone pairs on O arrange themselves so they are as far apart as possible).
Actual structure: the O-H bonds are in a gauche arrangement, and this is known as the open book structure.

Reactivity

• It undergoes a slight amount of auto-ionization.

• It can act as an oxidizing agent or as a reducing agent.

  As oxidizing agent:		E = 1.77 V
  As reducing agent:		E = 0.68 V

• It undergoes disproportionation (in the presence of a catalyst).

  ΔG = -105 kJmol-1