Reaction with Oxygen (AQA A-Level Chemistry): Revision Notes

6.1.2 Reaction with Oxygen

Reactivity with Oxygen

The elements sodium ($Na$), magnesium ($Mg$), aluminium ($Al$), silicon ($Si$), phosphorus ($P$), and sulphur ($S$) react with oxygen to form oxides. The reactivity trends, types of oxides formed, and their resulting pH properties reflect each element's position on the periodic table and type of bonding.

Reactions of Each Element with Oxygen

Sodium ($Na$)

Reaction:

Sodium reacts vigorously with oxygen, producing a bright yellow flame and forming a white solid, sodium oxide.

Equation:

$$4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}$$

pH of $\text{Na}_2\text{O}$ in Water:

Sodium oxide dissolves in water to form a strongly alkaline solution of sodium hydroxide (pH approx 13-14).

Magnesium ($Mg$)

Reaction:

Magnesium reacts vigorously with oxygen when heated, producing a bright white light and forming magnesium oxide, a white solid.

Equation:

$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$

pH of $\text{MgO}$ in Water:

Magnesium oxide is sparingly soluble in water, producing a weakly alkaline solution of magnesium hydroxide pH approx 9-10.

Aluminium ($Al$)

Reaction:

Aluminium reacts slowly with oxygen in powdered form to produce aluminium oxide, a white solid.

Equation:

$$4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3$$

pH of $\text{Al}_2\text{O}_3$ in Water:

Aluminium oxide is amphoteric, meaning it can react with both acids and bases, but it is insoluble in water and does not affect pH directly.

Silicon ($Si$)

Reaction:

Silicon reacts slowly with oxygen when finely divided and heated, forming silicon dioxide, a white solid (the main component of sand).

Equation:

$$\text{Si} + \text{O}_2 \rightarrow \text{SiO}_2$$

pH of $\text{SiO}_2$ in Water:

Silicon dioxide is insoluble in water and does not affect pH, but it behaves as an acidic oxide, reacting with strong bases.

Phosphorus ($P$)

Reaction:

Phosphorus combusts spontaneously in air with a very bright white flame, forming phosphorus(V) oxide (white smoke).

Equation:

$$\text{P}_4 + 5\text{O}_2 \rightarrow \text{P}_4\text{O}_{10}$$

pH of $\text{P}_4\text{O}_{10}$ in Water:

Phosphorus(V) oxide dissolves in water to form phosphoric acid ($\text{H}_3\text{PO}_4$), resulting in a strongly acidic solution pH approx 1-2.

Sulfur ($S$)

Reaction:

Sulphur burns with a blue flame in oxygen, producing misty fumes of sulphur dioxide (a pungent gas).

Equation:

$$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$$

Further Oxidation:

Sulphur dioxide can be further oxidised to sulphur trioxide in the presence of a vanadium(V) oxide catalyst:

$$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$$

pH of $\text{SO}_2$ and $\text{SO}_3$ in Water:

$\text{SO}_2$ dissolves in water to form sulphurous acid ($\text{H}_2\text{SO}_3$), resulting in an acidic solution pH approx 3-4.

$\text{SO}_3$ dissolves in water to form sulfuric acid ($\text{H}_2\text{SO}_4$), creating a highly acidic solution pH approx 1.