IGCSE Chemistry 0620: Oxidation and Reduction in the Extraction of Silicon

A school technician demonstrates the extraction of silicon from sand. A mixture of silicon(IV) oxide (the main compound in sand) and magnesium powder is ignited in a crucible, and the reaction proceeds vigorously:

$\text{SiO}_2 + 2\text{Mg} \rightarrow \text{Si} + 2\text{MgO}$

(a) State, in terms of oxygen, what happens to the silicon(IV) oxide in this reaction. [1]

(b) State, in terms of oxygen, what happens to the magnesium in this reaction. [1]

(c) Identify which substance is the reducing agent in this reaction, and give a reason for your answer in terms of oxygen transfer. [2]

(d) State the term used to describe a reaction, like this one, in which both oxidation and reduction happen at the same time. [1]

Part (a): Silicon(IV) oxide

In the equation, silicon(IV) oxide, $\text{SiO}_2$ , starts with two oxygen atoms per formula unit and ends up as silicon, $\text{Si}$ , with none. Silicon(IV) oxide has lost oxygen, so it is reduced.

Part (b): Magnesium

Magnesium, $\text{Mg}$ , starts with no oxygen and ends up as magnesium oxide, $\text{MgO}$ , which contains oxygen. Magnesium has gained oxygen, so it is oxidised.

Part (c): Identifying the reducing agent

The reducing agent is the substance that causes another substance to be reduced, by supplying the “route” for oxygen to be removed from it, and in doing so, the reducing agent is itself oxidised.

Here, magnesium is the reducing agent: it takes oxygen away from silicon(IV) oxide (reducing it to silicon), while magnesium itself gains that oxygen and becomes magnesium oxide.

Part (d): Naming the reaction type

Because oxidation (of magnesium) and reduction (of silicon(IV) oxide) happen together in the same reaction, this is called a redox reaction.

Final answers

(a) Silicon(IV) oxide loses oxygen. It is reduced.
(b) Magnesium gains oxygen. It is oxidised.
(c) Magnesium is the reducing agent, since it removes oxygen from silicon(IV) oxide.
(d) A redox reaction.