AS Chemistry 9701: Reactions of Period 3 Elements with Oxygen and Chlorine

A technician burns small samples of sodium, magnesium and silicon, each in an excess of dry oxygen gas. She then reacts fresh samples of sodium and magnesium separately with an excess of dry chlorine gas.

(a) Construct balanced symbol equations, including state symbols, for the reactions of sodium and of magnesium with oxygen. [2]

(b) Silicon reacts with oxygen only when heated strongly, forming solid silicon(IV) oxide. Construct the balanced symbol equation, including state symbols, for this reaction. [1]

(c) Suggest, in terms of structure and bonding, why sodium and magnesium react with oxygen readily at room temperature, whereas silicon must be heated strongly before it reacts. [2]

(d) Construct balanced symbol equations, including state symbols, for the reactions of sodium and of magnesium with excess chlorine gas. [2]

(e) State one observation the technician would make as magnesium burns in chlorine gas. [1]

Part (a): Sodium and magnesium with oxygen

Sodium only forms a $1+$ ion, so its oxide is $\text{Na}_2\text{O}$ : $4\text{Na(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Na}_2\text{O(s)}$

Magnesium forms a $2+$ ion, so its oxide is MgO in a $1:1$ ratio: $2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$

Part (b): Silicon with oxygen

Silicon forms silicon(IV) oxide, $\text{SiO}_2$ , in which each Si atom is bonded to two oxygen atoms: $\text{Si(s)} + \text{O}_2\text{(g)} \rightarrow \text{SiO}_2\text{(s)}$

Part (c): Why silicon needs strong heating but Na and Mg do not

Sodium and magnesium are reactive metals with relatively low ionisation energies: their outer electron(s) transfer readily to oxygen atoms to form ionic bonds, so both metals react with oxygen spontaneously (Na very vigorously, Mg readily once ignited) without needing much activation energy.

Silicon, however, exists as a giant covalent lattice in which every atom is held to its neighbours by strong, directional Si–Si covalent bonds. A significant amount of energy is needed to disrupt enough of this strongly-bonded structure to reach the activation energy for new Si–O bonds to start forming, so silicon only reacts with oxygen once it is heated strongly.

Part (d): Sodium and magnesium with chlorine

$2\text{Na(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{NaCl(s)}$

$\text{Mg(s)} + \text{Cl}_2\text{(g)} \rightarrow \text{MgCl}_2\text{(s)}$

Part (e): Observation as magnesium burns in chlorine

Magnesium burns in chlorine gas with a bright white flame/light, producing a white solid product ( $\text{MgCl}_2$ ).

Final answers

(a) $4\text{Na(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Na}_2\text{O(s)}$ ; $2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$
(b) $\text{Si(s)} + \text{O}_2\text{(g)} \rightarrow \text{SiO}_2\text{(s)}$
(c) Na/Mg have low ionisation energies and react readily by forming ionic bonds; Si’s giant covalent lattice of strong Si–Si bonds needs much more energy (strong heating) to break before reaction can occur.
(d) $2\text{Na(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{NaCl(s)}$ ; $\text{Mg(s)} + \text{Cl}_2\text{(g)} \rightarrow \text{MgCl}_2\text{(s)}$
(e) A bright white flame/light, producing a white solid.