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Building on the reactions from the previous lesson, we now examine the behaviour of Period 3 oxides and chlorides in more detail -- specifically their reactions with water and their acid-base properties. These reactions are a staple of Edexcel A-Level examination questions.
Na2O dissolves readily in water to form a strongly alkaline solution:
Na2O(s) + H2O(l) -> 2NaOH(aq)
The oxide ion (O2-) is a strong base that abstracts a proton from water. NaOH is a strong alkali -- it fully dissociates, giving pH approximately 13-14.
Ionic equation: O2-(s) + H2O(l) -> 2OH-(aq)
MgO reacts slowly with water (it is sparingly soluble):
MgO(s) + H2O(l) -> Mg(OH)2(aq)
Mg(OH)2 is only slightly soluble, so the resulting solution is weakly alkaline (pH approximately 9-10). Despite its limited solubility, MgO is classified as a basic oxide.
Why is MgO less soluble than Na2O? MgO has a much higher lattice energy than Na2O because Mg2+ is smaller and more highly charged than Na+. The energy released by hydration of the ions is not sufficient to overcome this stronger lattice energy completely, so dissolution is limited.
Al2O3 does not dissolve in water. It is insoluble because the strong ionic/covalent bonding in the lattice requires too much energy to break.
However, Al2O3 is amphoteric -- it reacts with both acids and bases:
With acid: Al2O3(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2O(l)
With base: Al2O3(s) + 2NaOH(aq) + 3H2O(l) -> 2NaAl(OH)4(aq)
(or simplified: Al2O3 + 2NaOH -> 2NaAlO2 + H2O)
This amphoteric character places Al2O3 at the transition between basic and acidic oxides. The Al3+ ion has a high charge density that gives the oxide significant covalent character alongside its ionic character.
SiO2 does not react with water -- its giant covalent structure is too stable for water molecules to break apart at room temperature. However, it is classified as an acidic oxide because it reacts with hot, concentrated alkalis and with molten alkalis:
SiO2(s) + 2NaOH(l) -> Na2SiO3(l) + H2O(l) (at high temperature)
SiO2 also reacts with basic oxides at high temperatures. Its inability to react with water is a kinetic barrier, not a thermodynamic one.
P4O10 reacts vigorously (and exothermically) with water to form phosphoric acid:
P4O10(s) + 6H2O(l) -> 4H3PO4(aq)
The solution is acidic (pH approximately 1-2). P4O10 is such a powerful dehydrating agent that it can even remove water from other acids (e.g., converting HNO3 to N2O5).
SO2 dissolves in water to form sulfurous acid:
SO2(g) + H2O(l) -> H2SO3(aq)
H2SO3 is a weak diprotic acid (pH approximately 2-3). It can act as both a reducing agent and an oxidising agent.
SO3 reacts very vigorously (and exothermically) with water to form sulfuric acid:
SO3(g) + H2O(l) -> H2SO4(aq)
H2SO4 is a strong diprotic acid (pH approximately 0-1). The reaction is highly exothermic and can be dangerous if not controlled. In the industrial Contact process, SO3 is dissolved in concentrated H2SO4 (not water) to form oleum (H2S2O7), which is then safely diluted.
| Oxide | Reaction with Water | pH of Solution | Character | Bonding in Oxide |
|---|---|---|---|---|
| Na2O | Dissolves -> NaOH | 13-14 | Strongly basic | Ionic |
| MgO | Slightly soluble -> Mg(OH)2 | 9-10 | Basic | Ionic |
| Al2O3 | Insoluble | -- | Amphoteric | Ionic/covalent |
| SiO2 | No reaction | -- | Weakly acidic | Giant covalent |
| P4O10 | Vigorous -> H3PO4 | 1-2 | Acidic | Covalent molecular |
| SO2 | Dissolves -> H2SO3 | 2-3 | Acidic | Covalent molecular |
| SO3 | Very vigorous -> H2SO4 | 0-1 | Strongly acidic | Covalent molecular |
graph LR
A["Na2O<br/>pH 13-14<br/>Strongly basic"] --> B["MgO<br/>pH 9-10<br/>Basic"]
B --> C["Al2O3<br/>Insoluble<br/>Amphoteric"]
C --> D["SiO2<br/>No rxn<br/>Weakly acidic"]
D --> E["P4O10<br/>pH 1-2<br/>Acidic"]
E --> F["SO2/SO3<br/>pH 0-3<br/>Strongly acidic"]
The chlorides of Period 3 elements also show a clear trend from ionic to covalent, which determines their reactions with water.
Sodium chloride (NaCl): Ionic, dissolves in water to give a neutral solution (pH = 7). The ions simply dissociate:
NaCl(s) -> Na+(aq) + Cl-(aq)
No hydrolysis occurs because neither Na+ nor Cl- reacts significantly with water.
Magnesium chloride (MgCl2): Ionic, dissolves in water. The solution is very slightly acidic (pH approximately 6.5) because the small, highly charged Mg2+ ion polarises water molecules:
MgCl2(s) -> Mg2+(aq) + 2Cl-(aq)
The Mg2+ ion attracts water molecules, and the O-H bonds in coordinated water are weakened, allowing slight hydrolysis:
[Mg(H2O)6]2+(aq) is in equilibrium with [Mg(OH)(H2O)5]+(aq) + H+(aq)
Aluminium chloride (AlCl3): Has significant covalent character. In the solid state, AlCl3 is ionic, but when heated it sublimes to form Al2Cl6 dimers (covalent). AlCl3 reacts vigorously with water in a hydrolysis reaction:
AlCl3(s) + 3H2O(l) -> Al(OH)3(s) + 3HCl(aq)
The solution is strongly acidic (pH approximately 3) due to the HCl produced. White fumes of HCl may be observed.
Silicon tetrachloride (SiCl4): Covalent liquid that reacts vigorously with water, producing white fumes of HCl:
SiCl4(l) + 2H2O(l) -> SiO2(s) + 4HCl(aq)
The solution is strongly acidic. The reaction is a hydrolysis -- the Si-Cl bonds are broken and replaced by Si-O bonds.
Phosphorus pentachloride (PCl5): Covalent solid that reacts violently with water, producing dense white fumes:
PCl5(s) + 4H2O(l) -> H3PO4(aq) + 5HCl(aq)
The solution is strongly acidic. PCl5 is useful as a test for the presence of -OH groups in organic compounds: it reacts with alcohols and carboxylic acids to produce steamy white fumes of HCl.
The covalent chlorides (AlCl3, SiCl4, PCl5) all react with water because:
NaCl and MgCl2 do not hydrolyse because their bonding is primarily ionic -- the ions simply dissociate in water rather than undergoing covalent bond-breaking and reforming. The Na+ ion is too large and too low in charge to significantly polarise water molecules.
Misconception: "All chlorides produce acidic solutions when added to water."
Correction: NaCl gives a neutral solution (pH 7). MgCl2 gives a very slightly acidic solution (pH approximately 6.5). Only the covalent chlorides (AlCl3, SiCl4, PCl5) undergo hydrolysis to produce strongly acidic solutions. The difference is in the type of bonding: ionic chlorides dissociate; covalent chlorides hydrolyse.
| Chloride | Type | Reaction with Water | pH | Products | Observation |
|---|---|---|---|---|---|
| NaCl | Ionic | Dissolves, no hydrolysis | 7 | Na+(aq) + Cl-(aq) | Dissolves quietly |
| MgCl2 | Ionic | Dissolves, very slight hydrolysis | ~6.5 | Mg2+(aq) + 2Cl-(aq) | Dissolves quietly |
| AlCl3 | Ionic/covalent | Hydrolyses vigorously | ~3 | Al(OH)3 + HCl | White fumes, white precipitate |
| SiCl4 | Covalent | Hydrolyses vigorously | ~1 | SiO2 + HCl | White fumes, white solid |
| PCl5 | Covalent | Hydrolyses violently | ~1 | H3PO4 + HCl | Dense white fumes |
Both the oxides and chlorides of Period 3 elements show the same underlying trend:
| Property | Left (Na, Mg) | Middle (Al) | Right (Si, P, S, Cl) |
|---|---|---|---|
| Bonding in oxide | Ionic | Ionic/covalent | Covalent |
| Oxide character | Basic | Amphoteric | Acidic |
| Bonding in chloride | Ionic | Ionic/covalent | Covalent |
| Chloride + water | Dissolves (neutral/slightly acidic) | Hydrolyses (acidic) | Hydrolyses (strongly acidic) |
This consistent pattern arises from the increasing electronegativity and non-metallic character of the elements as you move across the period.
Edexcel 9CH0 specification Topic 1, sub-topic 1.5 (and Topic 4 for inorganic chemistry of group 13/14) covers the structure and bonding of period 3 oxides (Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂/SO₃) and chlorides (NaCl, MgCl₂, AlCl₃ / Al₂Cl₆, SiCl₄, PCl₃/PCl₅); the pH of their aqueous solutions (basic for Na₂O/MgO; amphoteric for Al₂O₃; insoluble for SiO₂; acidic for P/S oxides; neutral or acidic for chlorides via hydrolysis); and the change in bonding type from ionic (left) to covalent (right) (refer to the official Pearson Edexcel specification document for exact wording). Examined in Paper 1 (Advanced Inorganic and Physical Chemistry) with synoptic application in Topic 4 and Topic 12 (Acid-base equilibria).
Question (8 marks):
(a) Predict, with reasoning, the bonding type and physical state at room temperature of (i) Na₂O, (ii) Al₂O₃, (iii) SiO₂, (iv) SO₃. (4) (b) Write equations for the reactions of (i) Na₂O with water, and (ii) SiCl₄ with water. State the pH of the resulting solutions. (4)
Solution with mark scheme:
(a) B1 — (i) Na₂O: ionic, ΔEN(Na–O) = 3.5 − 0.9 = 2.6, well above the ionic threshold. White solid at room temperature with high m.p. (1132 °C). Giant ionic lattice.
B1 — (ii) Al₂O₃: ΔEN(Al–O) = 3.5 − 1.5 = 2.0; described as ionic with significant covalent character ("intermediate"). White solid, very high m.p. (2072 °C). Strong giant ionic-covalent lattice.
B1 — (iii) SiO₂: ΔEN(Si–O) = 3.5 − 1.8 = 1.7; covalent, but in a giant covalent network where each Si is bonded to four O and each O is bonded to two Si. White solid, very high m.p. (1713 °C).
B1 — (iv) SO₃: ΔEN(S–O) = 3.5 − 2.5 = 1.0; covalent, in a simple molecular form (trigonal planar SO₃ molecules, with weak London forces between molecules). Liquid at room temperature (m.p. 17 °C, b.p. 45 °C); under cool conditions can polymerise to (SO₃)ₓ.
Common error: candidates conflate "covalent" with "low m.p." — SiO₂ is covalent but very high m.p. because of the giant network.
(b) B1 — Na₂O + H₂O: Na₂O(s) + H₂O(l) → 2NaOH(aq). Strongly basic solution, pH ~13–14.
B1 — pH 13–14 (highly alkaline).
B1 — SiCl₄ + H₂O: SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq). The reaction is hydrolysis; the SiO₂ precipitates as a white solid; HCl produced is strongly acidic.
B1 — pH ~1–2 (highly acidic, because of the four moles of HCl per mole of SiCl₄).
Common error: writing SiCl₄ + H₂O → Si(OH)₄ + HCl — this can occur in some conditions but the spec accepts the SiO₂ + HCl form. Always check the products are physically reasonable.
Total: 8 marks (B4 + B4).
Question (6 marks): Aluminium oxide is described as amphoteric.
(a) Define the term amphoteric in the context of metal oxides. (1) (b) Write balanced equations for the reaction of Al₂O₃ with (i) hot dilute HCl, and (ii) hot concentrated NaOH. (2) (c) Explain, with reference to electronegativity and bonding, why Al₂O₃ shows amphoteric behaviour while MgO is purely basic. (3)
Mark scheme decomposition by AO:
| Part | AO1 | AO2 | AO3 | Marks |
|---|---|---|---|---|
| (a) | 1 | 0 | 0 | 1 |
| (b) | 2 | 0 | 0 | 2 |
| (c) | 0 | 2 | 1 | 3 |
| Total | 3 | 2 | 1 | 6 |
(a) B1 (AO1.1) — an oxide that can react both as an acid (with bases/alkalis) and as a base (with acids).
(b) B1 (AO1.1) — Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l). B1 (AO1.1) — Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq) (sodium aluminate).
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