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The reactions of Period 3 elements with oxygen and water reveal a clear trend: moving from left to right across the period, the elements become less metallic and their oxides change from strongly basic to amphoteric to acidic. This lesson covers the key reactions and the patterns they reveal.
All Period 3 elements (except argon) react with oxygen, though with varying vigour and producing different types of oxide.
flowchart LR
subgraph "Vigorous Reaction"
Na["Na: bright yellow flame"]
Mg["Mg: intense white flame"]
P["P4: spontaneous ignition"]
end
subgraph "Moderate/Slow"
Al["Al: slow (oxide layer)"]
Si["Si: high temperature only"]
S["S: blue flame"]
end
subgraph "No Reaction"
Cl["Cl: does not react"]
Ar["Ar: inert"]
end
Sodium burns vigorously in air with a bright yellow flame:
4Na(s) + O2(g) -> 2Na2O(s)
Sodium forms the ionic oxide Na2O (sodium oxide), a white solid. In excess oxygen, sodium can also form the peroxide Na2O2.
The reaction is vigorous because sodium is a highly reactive Group 1 metal with a low ionisation energy -- it readily loses its 3s1 electron.
Magnesium burns with an intense white flame:
2Mg(s) + O2(g) -> 2MgO(s)
Magnesium oxide (MgO) is a white ionic solid with a very high melting point (2852 degrees C) due to strong ionic bonding between Mg2+ and O2- ions (high charge density, small ions).
Aluminium reacts with oxygen, but the reaction is often slow because a thin layer of Al2O3 forms on the surface and protects the metal from further reaction (passivation):
4Al(s) + 3O2(g) -> 2Al2O3(s)
Aluminium oxide (Al2O3) is a white solid with a very high melting point (2072 degrees C). Once the oxide layer is breached (e.g., by scraping or using thermite conditions), aluminium reacts vigorously.
Silicon reacts with oxygen at high temperatures:
Si(s) + O2(g) -> SiO2(s)
Silicon dioxide (SiO2) has a giant covalent structure (similar to quartz). It has a very high melting point (1713 degrees C) because strong covalent bonds must be broken.
White phosphorus (P4) ignites spontaneously in air:
In excess oxygen: P4(s) + 5O2(g) -> P4O10(s)
In limited oxygen: P4(s) + 3O2(g) -> P4O6(s)
Phosphorus(V) oxide (P4O10) is a white solid that reacts vigorously with water. Note: P4O10 is sometimes written as P2O5 (the empirical formula), but the molecular formula P4O10 is more accurate.
Sulfur burns with a blue flame:
S(s) + O2(g) -> SO2(g)
Sulfur dioxide (SO2) is a colourless, toxic gas with a choking smell. In the presence of a catalyst (e.g., V2O5) and excess oxygen:
2SO2(g) + O2(g) -> 2SO3(g)
Sulfur trioxide (SO3) is also formed.
Chlorine does not react directly with oxygen under normal conditions. Chlorine oxides exist but are unstable and are not formed by direct combination.
| Element | Oxide | Formula | Type of Bonding | Acid-Base Character | Observation |
|---|---|---|---|---|---|
| Na | Sodium oxide | Na2O | Ionic | Strongly basic | Yellow flame |
| Mg | Magnesium oxide | MgO | Ionic | Basic | White flame |
| Al | Aluminium oxide | Al2O3 | Ionic (with covalent character) | Amphoteric | Slow, oxide layer |
| Si | Silicon dioxide | SiO2 | Giant covalent | Weakly acidic | High temp only |
| P | Phosphorus(V) oxide | P4O10 | Covalent (molecular) | Acidic | Spontaneous |
| S | Sulfur dioxide/trioxide | SO2/SO3 | Covalent (molecular) | Acidic | Blue flame |
| Cl | -- | -- | -- | -- | No reaction |
The trend is clear: oxides become less basic and more acidic from left to right across Period 3.
Sodium reacts vigorously with water, fizzing on the surface and often melting into a ball:
2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)
The solution becomes strongly alkaline (pH approximately 13-14). Sodium hydroxide is a strong base that fully dissociates. If a large piece of sodium is used, the hydrogen gas may ignite, producing a yellow flame.
Magnesium reacts very slowly with cold water:
Mg(s) + 2H2O(l) -> Mg(OH)2(aq) + H2(g)
The reaction is much faster with steam:
Mg(s) + H2O(g) -> MgO(s) + H2(g)
Magnesium hydroxide is slightly soluble, producing a weakly alkaline solution (pH approximately 9-10).
Why faster with steam? The higher temperature provides more kinetic energy to overcome the activation energy barrier. Also, the product with steam is MgO (not Mg(OH)2), which does not coat the metal surface as effectively, allowing the reaction to continue.
Aluminium does not appear to react with water because of its protective oxide layer. If the oxide layer is removed (e.g., by amalgamation with mercury), aluminium does react:
2Al(s) + 6H2O(l) -> 2Al(OH)3(s) + 3H2(g)
In practice, this reaction is rarely observed because the oxide layer reforms instantly.
Silicon does not react with water under normal conditions. Phosphorus and sulfur also do not react with water directly (although their oxides react vigorously -- covered in the next lesson).
Chlorine reacts with water in a special type of reaction:
Cl2(g) + H2O(l) -> HCl(aq) + HOCl(aq)
This is a disproportionation reaction -- chlorine is simultaneously oxidised and reduced:
The solution is slightly acidic. HOCl (hypochlorous acid) is a powerful disinfectant, which is why chlorine is used in water treatment.
| Element | Reaction | Products | pH | Vigour |
|---|---|---|---|---|
| Na | Vigorous | NaOH + H2 | 13-14 | Fast, fizzes |
| Mg | Very slow (cold), fast (steam) | Mg(OH)2 + H2 or MgO + H2 | 9-10 | Slow |
| Al | None (oxide layer) | -- | -- | -- |
| Si | None | -- | -- | -- |
| P | None | -- | -- | -- |
| S | None | -- | -- | -- |
| Cl | Disproportionation | HCl + HOCl | ~4-5 | Moderate |
graph LR
A["Na2O<br/>Strongly basic"] --> B["MgO<br/>Basic"]
B --> C["Al2O3<br/>Amphoteric"]
C --> D["SiO2<br/>Weakly acidic"]
D --> E["P4O10<br/>Acidic"]
E --> F["SO2 / SO3<br/>Strongly acidic"]
The change from basic to acidic oxides across Period 3 reflects the changing nature of bonding:
This trend is a direct consequence of the transition from metallic to non-metallic character across the period, which itself results from the increasing ionisation energy and electronegativity of the elements.
Misconception: "The reactivity of Period 3 elements with oxygen decreases from left to right."
Correction: This is not a simple trend. Sodium and phosphorus both react very vigorously with oxygen (sodium burns readily; P4 ignites spontaneously). Aluminium reacts slowly only because of its protective oxide layer, not because of low inherent reactivity. The pattern of vigour depends on factors including oxide layer formation, not just position in the period.
Misconception: "Chlorine reacts with water to form HCl only."
Correction: Chlorine undergoes disproportionation with water, forming BOTH HCl and HOCl. If only HCl formed, it would not be a disproportionation reaction, and water treatment with chlorine would be less effective (HOCl is the active disinfectant).
Edexcel 9CH0 specification Topic 1, sub-topic 1.5 (and Topic 4 for related reactions) covers the reactions of period 3 elements with oxygen, water and chlorine; the products formed (oxides, chlorides, hydroxides); the trends in vigour of reaction as one moves across the period; oxidation state changes and redox interpretation; and the equations and observations for representative reactions (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 (Inorganic chemistry of group 1, 2, 7 and 13/14) and in Paper 3 (General and Practical) for observation-based questions.
Question (8 marks):
(a) Write balanced equations, including state symbols, for the reactions of Na, Mg and Al with excess oxygen at high temperature. (3) (b) Describe the observations when each of Na, Mg, S burns in oxygen, including any flame colour. (3) (c) Compare the reactivity of Na with cold water and Mg with cold water, giving equations and observations. (2)
Solution with mark scheme:
(a) B1 — 4Na(s) + O₂(g) → 2Na₂O(s). (Note: with limited O₂, the peroxide Na₂O₂ also forms; spec accepts the simple oxide.)
B1 — 2Mg(s) + O₂(g) → 2MgO(s).
B1 — 4Al(s) + 3O₂(g) → 2Al₂O₃(s).
Common error: balancing slips (forgetting Al's +3 oxidation state gives Al₂O₃ not AlO).
(b) B1 — Na: bright yellow/orange flame, melts and skitters on the surface, white solid (Na₂O) formed.
B1 — Mg: brilliant white flame (so bright it can damage eyes; safety glasses required), white powdery ash (MgO) produced.
B1 — S: pale blue flame, choking pungent gas (SO₂) produced. (S burns in oxygen rather than melting/boiling at ordinary heating.)
Common error: candidates write "white flame" for Na (Na's flame test colour is yellow due to the 3p → 3s electronic transition).
(c) B1 — Na with cold water: vigorous, exothermic; Na floats, melts to a sphere, fizzes (H₂ released), moves on the surface, may ignite. Equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Universal indicator turns blue/purple (NaOH is strongly alkaline).
B1 — Mg with cold water: very slow reaction, only a few bubbles. Equation: Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g) (very slow). With steam, Mg burns brightly: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
The sodium reaction is much more vigorous because Na has lower IE1 (496 vs 738 kJ mol⁻¹) and forms Na⁺ + e⁻ much more readily than Mg → Mg²⁺ + 2e⁻ requires.
Total: 8 marks (B3 + B3 + B2).
Question (6 marks): Aluminium and silicon react differently with chlorine.
(a) Write equations, including state symbols, for the reactions of Al and Si with chlorine. (2) (b) Predict and explain the difference in vigour and conditions required. (2) (c) Discuss whether the products AlCl₃ and SiCl₄ have ionic or covalent bonding, citing electronegativity differences. (2)
Mark scheme decomposition by AO:
| Part | AO1 | AO2 | AO3 | Marks |
|---|---|---|---|---|
| (a) | 2 | 0 | 0 | 2 |
| (b) | 0 | 2 | 0 | 2 |
| (c) | 0 | 1 | 1 | 2 |
| Total | 2 | 3 | 1 | 6 |
(a) B1 (AO1.1) — 2Al(s) + 3Cl₂(g) → 2AlCl₃(s) (or Al₂Cl₆(s) for the dimer).
B1 (AO1.1) — Si(s) + 2Cl₂(g) → SiCl₄(l).
(b) M1 (AO2.1) — Al reacts more readily; the formation of Al³⁺ (or polar covalent Al–Cl bonds) is favoured by Al's lower electronegativity and lower IE.
A1 (AO2.1) — Si requires higher temperatures because Si's higher electronegativity (1.8) reduces the ionic character of the Si–Cl bond, and Si has no easily available electron for ion formation. Both react in dry chlorine; AlCl₃ is the more reactive of the two.
(c) M1 (AO2.1) — ΔEN(Al–Cl) = 3.0 − 1.5 = 1.5; ΔEN(Si–Cl) = 3.0 − 1.8 = 1.2. Both are polar covalent by the conventional cutoff (~2.0).
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