Reactions of Period 3 Chlorides with Water

The reactions of Period 3 chlorides with water show a clear transition from ionic behavior (simple dissolution) on the left side of the periodic table to covalent behavior (complete hydrolysis) on the right. This behavior is dictated by the bonding type and the presence of accessible d-orbitals.

Summary Table of Period 3 Chlorides

Chloride	Structure & Bonding	Reaction with Water	Approx. pH
$\text{NaCl}$	Giant Ionic Lattice	Dissolves completely (Hydration)	$\text{pH } 7$ (Neutral)
$\text{MgCl}_2$	Giant Ionic Lattice	Slight hydrolysis (Weakly acidic)	$\text{pH } 6.5$
$\text{AlCl}_3$	Covalent Dimer ($\text{Al}_2\text{Cl}_6$)	Extensive hydrolysis (Strongly acidic)	$\text{pH } 3$
$\text{SiCl}_4$	Simple Molecular Covalent	Complete, violent hydrolysis	$\text{pH } 2$
$\text{PCl}_5$	Simple Molecular Covalent	Complete, violent hydrolysis	$\text{pH } 2$

1. Sodium Chloride ($\text{NaCl}$)

Sodium chloride is a giant ionic compound. When added to water, no chemical reaction or hydrolysis occurs. The water molecules merely break down the ionic lattice by hydrating the ions.

$$\text{NaCl}(s) \xrightarrow{\text{H}_2\text{O}} \text{Na}^+(aq) + \text{Cl}^-(aq)$$

Observation: Colorless solution forms. Because $\text{Na}^+$ has a low charge density, it cannot polarize water molecules to release $\text{H}^+$ ions, leaving the solution neutral at $\text{pH } 7$.

2. Magnesium Chloride ($\text{MgCl}_2$)

Magnesium chloride is also ionic, but the $\text{Mg}^{2+}$ ion has a smaller ionic radius and a higher charge than $\text{Na}^+$, giving it a higher charge density.

$$\text{MgCl}_2(s) + 6\text{H}_2\text{O}(l) \rightarrow [\text{Mg}(\text{H}_2\text{O})_6]^{2+}(aq) + 2\text{Cl}^-(aq)$$ $$\text{[Mg}(\text{H}_2\text{O})_6]^{2+}(aq) \rightleftharpoons [\text{Mg}(\text{H}_2\text{O})_5(\text{OH})]^{+}(aq) + \text{H}^+(aq)$$

Observation: Colorless solution forms. The $\text{Mg}^{2+}$ ion slightly polarizes the coordinated water molecules, releasing a trace amount of $\text{H}^+$ ions. This results in a weakly acidic solution of $\text{pH } 6.5$.

3. Aluminium Chloride ($\text{AlCl}_3$)

Aluminium chloride exists as a covalent dimer ($\text{Al}_2\text{Cl}_6$) in the vapor phase but undergoes extensive, highly exothermic hydrolysis when added to water due to the incredibly high charge density of the $\text{Al}^{3+}$ ion.

$$\text{AlCl}_3(s) + 6\text{H}_2\text{O}(l) \rightarrow [\text{Al}(\text{H}_2\text{O})_6]^{3+}(aq) + 3\text{Cl}^-(aq)$$ $$[\text{Al}(\text{H}_2\text{O})_6]^{3+}(aq) \rightleftharpoons [\text{Al}(\text{H}_2\text{O})_5(\text{OH})]^{2+}(aq) + \text{H}^+(aq)$$

Observation: Hissing sound, colorless solution, and white fumes of $\text{HCl}$ gas may be observed if water is limited. The small, highly charged $\text{Al}^{3+}$ ion strongly polarizes the $\text{O}-\text{H}$ bonds of the attached water ligands, causing significant dissociation of $\text{H}^+$ ions. The solution is strongly acidic at $\text{pH } 3$.

4. Silicon Tetrachloride ($\text{SiCl}_4$)

Silicon tetrachloride is a simple covalent molecular liquid. It reacts violently with water because the central silicon atom has vacant, accessible $3\text{d}$-orbitals that allow lone pairs from water molecules to attack it readily.

$$\text{SiCl}_4(l) + 2\text{H}_2\text{O}(l) \rightarrow \text{SiO}_2(s) + 4\text{HCl}(g)$$

Observation: Violent reaction with thick, white suffocating fumes of $\text{HCl}$ gas and a white precipitate of silicon dioxide ($\text{SiO}_2$). Dissolved $\text{HCl}$ drops the solution to $\text{pH } 2$.

5. Phosphorus Pentachloride ($\text{PCl}_5$)

Phosphorus pentachloride is a covalent solid that undergoes a violent, multi-stage hydrolysis reaction with water.

With limited water (Partial Hydrolysis):

$$\text{PCl}_5(s) + \text{H}_2\text{O}(l) \rightarrow \text{POCl}_3(l) + 2\text{HCl}(g)$$

With excess water (Complete Hydrolysis):

$$\text{PCl}_5(s) + 4\text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{PO}_4(aq) + 5\text{HCl}(g)$$

Observation: Energetic reaction with dense white fumes of $\text{HCl}$. The final solution contains two strong acids—phosphoric acid ($\text{H}_3\text{PO}_4$) and hydrochloric acid ($\text{HCl}$)—yielding a highly acidic $\text{pH } 2$.

💡 Singapore H2 / IB Exam Tip: Examiners frequently ask why Carbon Tetrachloride ($\text{CCl}_4$) does not react with water, whereas $\text{SiCl}_4$ reacts violently. Always answer using these two specific points:

1. The carbon atom in $\text{CCl}_4$ does not have accessible vacant 3d-orbitals to coordinate with the lone pair of electrons from a water molecule.
2. The carbon atom is kinetically protected because it is sterically hindered by the four bulky chlorine atoms crowding around it.