Discuss - Metal oxides are alkaline, non-metal oxides are acidic

Moving across the periodic table from left to right will bring about a change in the nature of the element. Generally, there are trends in certain properties in the elements that we can spot, for instance boiling/melting points. Period three elements are from Sodium to Argon inclusive, and possess particular characteristics. All of the period 3 elements except argon and chlorine will combine with oxygen to form oxide molecules (Na2O, MgO, Al2O3, SiO2, P4O10, SO2 and SO3). How does their pH differ, and what is different about Al2O3?

It is not just the properties of the period 3 elements that differ as we move across the periodic table; this movement also affects the properties of the oxides of said elements. These changes are a result of the transition from metal elements to non-metal elements. The oxides of sodium, magnesium and aluminium are all examples of the compounds formed by a metal combined to a non-metal. They all form giant ionic lattices, where the bonding extends throughout the entire compound. The sheer numbers of ionic bonds, in addition to the strength ionic bonds demonstrate mean that the metal oxides of period 3 all have high boiling points.

Other than silicon dioxide, the non-metal oxides have lower boiling points than their metal counterparts. Whilst the non-metals demonstrate covalent bonds, the individual, covalently bonded molecules are held together by weak intermolecular forces (van der Waals and dipole-dipole forces). SO2 and SO3 are gases at 298K (room temp.). The exception to this however is SiO2, as this forms a giant macromolecule held together by only covalent bonds. As these bonds are hard to break, the melting/boiling point of SiO2 is subsequently very high.

Some of the oxides will react with water, whereas some are insoluble and will not. The basic oxide reactions are as follows.

Sodium oxide readily reacts with water to give sodium hydroxide solution (NaOH). This is a strongly alkaline solution.

Na2O (s) + H2O (l) → 2Na+ (aq) + 2OH- (aq) pH ~ 14

Magnesium oxide reacts with water to give magnesium hydroxide, sparingly soluble and produces a somewhat alkaline solution.

MgO (s) + H2O (l) → Mg(OH)2 (s) Mg2+ (aq) + 2OH- (aq) pH ~ 9

As previously mentioned, the oxides of aluminium and silicon are both ...

This is a preview of the whole essay

Some of the oxides will react with water, whereas some are insoluble and will not. The basic oxide reactions are as follows.

Sodium oxide readily reacts with water to give sodium hydroxide solution (NaOH). This is a strongly alkaline solution.

Na2O (s) + H2O (l) → 2Na+ (aq) + 2OH- (aq) pH ~ 14

Magnesium oxide reacts with water to give magnesium hydroxide, sparingly soluble and produces a somewhat alkaline solution.

MgO (s) + H2O (l) → Mg(OH)2 (s) Mg2+ (aq) + 2OH- (aq) pH ~ 9

As previously mentioned, the oxides of aluminium and silicon are both insoluble in water and therefore will not take part in an oxide + water reaction.

Non-metals within period 3 typically produce acidic oxides. For instance, phosphorous pentoxide reacts quite violently with water to produce the acid solution, phosphoric (v) acid. This ionises, so the solution is acidic.

P4O10 (s) + 6H2O (l) → 4H3PO4

The H3PO4 (aq) ionises in stages, the first being:

H3PO4 H+ (aq) + H2PO4- (aq) pH ~0-1

Sulphur dioxide is also fairly soluble in water, and reacts with it to give sulphuric acid. This partially dissociates producing H+ ions. This causes the acidity in the solution.

SO2 (g) + H2O (l) → H2SO3 (aq)

H2SO3 (aq) H+ (aq) + HSO3- (aq) pH ~ 2-3

Sulphur trioxide reacts violently with water to produce sulphuric acid:

SO3 (g) + H2O (l) → H2SO4 (aq) → H+ (aq) + HSO4- (aq) pH ~0-1

This overall pattern in reactivity of the period 3 oxides is that metal oxides will form alkaline solutions win water, whilst non-metal oxides will form acidic ones and those in the middle will not react. This behaviour can be explained by looking at the bonding and structure:

Sodium and magnesium oxides are composed of ions. For instance, sodium oxide contains Na+ and O2- ions. The O2- ion is a very strong base, and readily reacts with water to produce hydroxide ions - a strongly alkaline solution.
Magnesium oxide contains oxide ions as well. However, magnesium oxide is less soluble than sodium oxide and will therefore produce a less alkaline solution.
Aluminium oxide is ionic, but the bonds are too strong to be broken. In addition to this, its covalent bonds also add to its strength.
Silicon dioxide is a giant macromolecule, so will not react with water as is insoluble.
Phosphorous oxides and sulphur oxides are covalent molecules, but will react with water to form acidic solutions.

The general trend is therefore oxides become more acidic as we go across the period.

Aluminium Oxide possesses a special property. As I mentioned above, it contains both ionic and covalent bonds. The reason for this happening is due to the nature of the aluminium ion. It forms a very small ion, but with a high negative charge. This allows it to be able to closely approach the O2- ion and distort its electron cloud. This therefore gives the bond some covalent characteristics. These properties lead to aluminium oxide being labelled as an amphoteric oxide. This means aluminium oxide react both with acids and alkalis.

For example, with HCL, aluminium chloride is formed:

Al2O3 (s) + 6HCl (aq) → 2ACl3 (aq) + 3H2O (l)

However, with hot, concentrated sodium hydroxide, sodium aluminate is formed:

Al2O3 (s) + 2NaOH (aq) → Na2SiO3 (aq) + H2O (l)

Due to the giant covalent lattice, silicon dioxide is very resistant to attack from bases. It will only react with a base given the conditions are right, using hot, concentrated hydroxide solution.

SiO2 (s) + 2 OH – (aq) SiO3 2 – (aq) + H2O (l)

Some of these reactions can be problematic. For instance, sulphur trioxide causes problems when reacting with water as it forms sulphuric acid (see above for equation). Were the SO3 to mix with rain water, it would form acid rain. This provides a number of environmental problems, such as, changing the pH of lakes and harming aquatic life, killing crops by changing pH of soil and ruining limestone buildings.

I would mostly agree with the statement “Metal oxides are alkaline, non-metal oxides are acidic”. Within period 3, the metal oxides react as an alkali whilst the non-metal oxides react as acidic. The only exception to this rule is aluminium oxide, which can act as both an acid and alkali depending on the conditions and the reactant and is referred to as amphoteric. Excluding this exception, the statement proves correct.

Bibliography:

AQA A2 Level Chemistry Textbook - Ted Lister, Janet Renshaw