Chlorides of the Period 3 Elements – Trends in Chemical Properties of Period 3 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

Trends in Chemical Properties of Period 3 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Period 3 Elements and Their Compounds

4 lessons

Lesson 4 of 4

Chlorides of the Period 3 Elements

25 min

Salt that dissolves, liquid that fumes

Sodium chloride dissolves quietly in water and the solution is neutral. Open a bottle of silicon tetrachloride in moist air and it fumes at once: white clouds of hydrogen chloride and silica pour out of the neck. Drop anhydrous aluminium chloride into water and it hisses and spits. These chlorides are made in the same way, by heating the element in chlorine (Lesson 2), but they behave in completely different ways. The difference lies in their bonding, which changes across the period just as it did for the oxides.

Definition

Hydrolysis: a reaction in which water breaks a compound into new substances. Example: $\mathrm{SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl}$ .

Dative (coordinate) bond: a covalent bond in which both shared electrons come from the same atom. Example: in $\mathrm{Al_2Cl_6}$ , a chlorine atom donates a lone pair into an empty orbital of aluminium.

Polarising power: the ability of a cation to distort the electron cloud of a nearby anion or molecule. It is high for small, highly charged ions such as $\mathrm{Al^{3+}}$ .

Bonding, structure and melting points

Table 14.3: The chlorides of the Period 3 elements

Chloride	Oxidation state	Bonding and structure	m.p. / °C	With water	pH of solution
NaCl	+1	ionic, giant lattice	800.7	dissolves; ions are simply hydrated	7
$\mathrm{MgCl_2}$	+2	ionic, giant lattice	714	dissolves; very slight hydrolysis	about 6.5
$\mathrm{AlCl_3}$	+3	ionic–covalent layer lattice; $\mathrm{Al_2Cl_6}$ molecules in the vapour	sublimes at 180	anhydrous solid reacts vigorously, giving off HCl; forms acidic $\mathrm{[Al(H_2O)_6]^{3+}}$	about 3
$\mathrm{SiCl_4}$	+4	covalent, simple molecular	−68.7	hydrolysed completely: $\mathrm{SiO_2}$ + HCl, white fumes	about 0–1
$\mathrm{PCl_3}$	+3	covalent, simple molecular	−93.6	hydrolysed: $\mathrm{H_3PO_3}$ + HCl	about 0–1
$\mathrm{PCl_5}$	+5	covalent molecules (ionic $\mathrm{[PCl_4]^+[PCl_6]^-}$ in the solid)	160.5	hydrolysed: $\mathrm{H_3PO_4}$ + HCl	about 0–1
$\mathrm{S_2Cl_2}$	+1	covalent, simple molecular (orange liquid)	−80	slowly hydrolysed: S, $\mathrm{SO_2}$ , HCl	acidic

Figure 14.6: Melting points of the Period 3 chlorides. Aluminium chloride sits between the ionic and the molecular chlorides.

As in the oxides, the bonding follows the electronegativity of the element. Sodium and magnesium chlorides are ionic. The small, highly charged $\mathrm{Al^{3+}}$ ion has such high polarising power that it pulls the electron clouds of the chloride ions towards itself: the bonds become largely covalent. Solid aluminium chloride has a layer lattice; at 180 °C it turns straight into a vapour of $\mathrm{Al_2Cl_6}$ molecules (Figure 14.7). In $\mathrm{AlCl_3}$ the aluminium atom has only six outer electrons; in the dimer each Al gains an octet by accepting a lone pair from a chlorine atom of the other unit. From silicon onwards the chlorides are covalent molecules that are liquids or low-melting solids.

Figure 14.7: The $\mathrm{Al_2Cl_6}$ molecule. Two chlorine atoms bridge the aluminium atoms using dative bonds, so each aluminium atom has four bonds and a full octet.

Chlorides with water

Ionic chlorides dissolve. NaCl simply separates into hydrated ions, so the solution is neutral: $\mathrm{NaCl(s) \xrightarrow{water} Na^+(aq) + Cl^-(aq)}$ . $\mathrm{Mg^{2+}}$ is a little more polarising, and the solution is very slightly acidic.

Aluminium chloride is acidic. In water the $\mathrm{Al^{3+}}$ ion is surrounded by six water molecules. Its high charge density pulls electrons from the O–H bonds of these water molecules, so one of them releases a proton: $\mathrm{[Al(H_2O)_6]^{3+}(aq) \rightleftharpoons [Al(H_2O)_5(OH)]^{2+}(aq) + H^+(aq)}$ Anhydrous aluminium chloride added to a little water reacts violently and gives off fumes of hydrogen chloride.

Covalent chlorides are hydrolysed. Water molecules attack the central atom through a lone pair on oxygen, and Si–Cl or P–Cl bonds are replaced by Si–O or P–O bonds: $\mathrm{SiCl_4(l) + 2H_2O(l) \rightarrow SiO_2(s) + 4HCl(aq)}$ $\mathrm{PCl_3(l) + 3H_2O(l) \rightarrow H_3PO_3(aq) + 3HCl(aq)}$ $\mathrm{PCl_5(s) + 4H_2O(l) \rightarrow H_3PO_4(aq) + 5HCl(aq)}$ With only a little water, $\mathrm{PCl_5}$ gives phosphorus oxychloride: $\mathrm{PCl_5 + H_2O \rightarrow POCl_3 + 2HCl}$ .

Why does $\mathrm{SiCl_4}$ hydrolyse while carbon tetrachloride, $\mathrm{CCl_4}$ , does not? Silicon is larger than carbon and (in the model used at this level) has empty 3d orbitals, so a water molecule can approach and attach to it before a Cl leaves. The small carbon atom is crowded by four chlorine atoms and has no low-energy empty orbital, so water cannot attack it.

Aluminium chloride hexahydrate, AlCl₃·6H₂O, a white crystalline powder. In this hydrated form the Al³⁺ ions are already surrounded by water molecules; its solution in water is acidic. Photo: Walkerma, public domain, via Wikimedia Commons

Worked example 1: the pH after hydrolysing silicon tetrachloride

1.70 g of $\mathrm{SiCl_4}$ is added to excess water and the mixture made up to 250 cm³. Calculate the concentration of HCl and the pH, assuming HCl is fully ionised. (Si = 28.09, Cl = 35.45)

Solution. $M(\mathrm{SiCl_4}) = 28.09 + 4(35.45) = 169.89\ \mathrm{g\,mol^{-1}}$ $n(\mathrm{SiCl_4}) = \frac{1.70}{169.89} = 1.00 \times 10^{-2}\ \mathrm{mol} \qquad n(\mathrm{HCl}) = 4 \times 1.00 \times 10^{-2} = 4.00 \times 10^{-2}\ \mathrm{mol}$ $[\mathrm{H^+}] = \frac{4.00 \times 10^{-2}}{0.250} = 0.160\ \mathrm{mol\,dm^{-3}} \qquad \mathrm{pH} = -\log_{10}(0.160) = 0.80$

Worked example 2: evidence for $\mathrm{Al_2Cl_6}$

At 200 °C (473 K) and 101 325 Pa, 1.00 g of aluminium chloride vapour occupies 146 cm³. Calculate its molar mass and decide whether the vapour is $\mathrm{AlCl_3}$ or $\mathrm{Al_2Cl_6}$ . ( $R = 8.314\ \mathrm{J\,K^{-1}\,mol^{-1}}$ ; Al = 26.98, Cl = 35.45)

Solution. From $p V = n R T$ and $n = \frac{m}{M}$ : $M = \frac{mRT}{pV} = \frac{1.00 \times 8.314 \times 473}{101\,325 \times 146 \times 10^{-6}} = 266\ \mathrm{g\,mol^{-1}}$ $M(\mathrm{AlCl_3}) = 133.33$ and $M(\mathrm{Al_2Cl_6}) = 266.66$ . The measured value matches the dimer, so the vapour at 200 °C consists of $\mathrm{Al_2Cl_6}$ molecules.

Warning

Common mistakes. (1) Saying NaCl "reacts" with water: it only dissolves. (2) Giving $\mathrm{AlCl_3}$ as ionic because aluminium is a metal: its bonding is largely covalent. (3) Writing $\mathrm{SiCl_4 + H_2O \rightarrow Si(OH)_4}$ and stopping: the product is hydrated silica, usually written $\mathrm{SiO_2}$ , plus four HCl. (4) Forgetting that $\mathrm{PCl_5}$ gives $\mathrm{H_3PO_4}$ (phosphorus +5) but $\mathrm{PCl_3}$ gives $\mathrm{H_3PO_3}$ (phosphorus +3): the oxidation state does not change in hydrolysis.

Safety. $\mathrm{SiCl_4}$ , $\mathrm{PCl_3}$ , $\mathrm{PCl_5}$ , $\mathrm{S_2Cl_2}$ and anhydrous $\mathrm{AlCl_3}$ react with moisture to release hydrogen chloride, which burns the eyes and lungs. They are kept tightly sealed and opened only by a teacher in a fume cupboard, with goggles and gloves.

Tip

Ionic dissolves; covalent hydrolyses. NaCl and $\mathrm{MgCl_2}$ : pH about 7. Everything from $\mathrm{AlCl_3}$ on: fumes of HCl and an acidic solution. The oxidation state of the element in the chloride, +1 to +5, is the same as in the oxoacid or oxide formed by hydrolysis.

Chemistry with Dr Ellis: Period 3 Chlorides (12 min). Watch for the change from ionic to covalent chlorides and the equations for their reactions with water.

✦

Practice

Write the formulas of the chlorides of Na, Mg, Al, Si and P (in excess chlorine), and give the oxidation state of the element in each.
Explain why sodium chloride has a high melting point but silicon tetrachloride is a liquid at room temperature.
Explain why a solution of aluminium chloride is acidic, with an equation.
Write equations for the hydrolysis of $\mathrm{SiCl_4}$ and of $\mathrm{PCl_5}$ in excess water.
Describe what you would see if you added a few drops of water to (a) solid NaCl and (b) liquid $\mathrm{SiCl_4}$ .
Draw the structure of $\mathrm{Al_2Cl_6}$ , showing the dative bonds, and explain why the dimer forms.
2.085 g of $\mathrm{PCl_5}$ is hydrolysed completely. Calculate the amount (in mol) of HCl formed (P = 30.97, Cl = 35.45).
A white solid chloride of a Period 3 element sublimes at 180 °C. Its aqueous solution has pH 3 and gives a white precipitate with silver nitrate. Identify it and explain each observation.

Show answersHide answers

NaCl (+1); $\mathrm{MgCl_2}$ (+2); $\mathrm{AlCl_3}$ (+3); $\mathrm{SiCl_4}$ (+4); $\mathrm{PCl_5}$ (+5).
NaCl is a giant ionic lattice with strong attractions between ions throughout; $\mathrm{SiCl_4}$ is made of separate molecules held by weak intermolecular forces.
The small, highly charged $\mathrm{Al^{3+}}$ polarises its surrounding water molecules so that one loses a proton: $\mathrm{[Al(H_2O)_6]^{3+} \rightleftharpoons [Al(H_2O)_5(OH)]^{2+} + H^+}$ .
$\mathrm{SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl}$ ; $\mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}$ .
(a) It dissolves; no fumes, no heat to speak of, neutral solution. (b) A vigorous reaction with steamy white fumes of HCl and a white solid (silica); the liquid becomes strongly acidic.
As in Figure 14.7: two Al atoms bridged by two Cl atoms, each bridging Cl forming one normal bond and one dative bond. In $\mathrm{AlCl_3}$ Al has only 6 outer electrons; accepting a lone pair gives each Al an octet.
$M(\mathrm{PCl_5}) = 208.22$ ; $n = \frac{2.085}{208.22} = 0.0100\ \mathrm{mol}$ ; $n(\mathrm{HCl}) = 5 \times 0.0100 = 0.0500\ \mathrm{mol}$ .
Aluminium chloride: it sublimes at 180 °C because its bonding is largely covalent; $\mathrm{[Al(H_2O)_6]^{3+}}$ releases $\mathrm{H^+}$ , so pH is about 3; chloride ions give white AgCl with silver nitrate.

Quick check

What do you see when water is added to liquid silicon tetrachloride?

Key points

NaCl and $\mathrm{MgCl_2}$ are ionic, high-melting and dissolve to give neutral (or almost neutral) solutions.
$\mathrm{AlCl_3}$ is largely covalent: it sublimes at 180 °C, exists as $\mathrm{Al_2Cl_6}$ (dative bonds) in the vapour, and gives acidic $\mathrm{[Al(H_2O)_6]^{3+}}$ solutions.
$\mathrm{SiCl_4}$ , $\mathrm{PCl_3}$ , $\mathrm{PCl_5}$ and $\mathrm{S_2Cl_2}$ are covalent molecules, hydrolysed by water to an oxide or oxoacid plus HCl (pH about 0–1).
Across the period chlorides change from ionic to covalent and from neutral to acidic in water, just like the oxides.

1Salt that dissolves, liquid that fumes

2Bonding, structure and melting points

3Chlorides with water

Salt that dissolves, liquid that fumes

Bonding, structure and melting points

Chlorides with water