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AS & A-Level Chemistry 06 — Periodic patterns and inorganic reactions

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Independent Deckloop AS Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 6 of 18: Periodic patterns and inorganic reactions. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.

Chemistry EN A-Level
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Periodicity of chemical properties of the elements in Period 3\text{Period}\,3

The Period 3\text{Period}\,3 elements exhibit systematic variations in their chemical reactivity, reflecting their valence electron counts and transition from metallic to non-metallic character. The metals (Na\mathrm{Na}, Mg\mathrm{Mg}, Al\mathrm{Al}) and non-metals (P\mathrm{P}, S\mathrm{S}) react with oxygen upon heating to form oxides (Na2O\mathrm{Na_2O}, MgO\mathrm{MgO}, Al2O3\mathrm{Al_2O_3}, P4O10\mathrm{P_4O_{10}}, SO2\mathrm{SO_2}). The maximum oxidation state shown in these compounds generally corresponds to the total number of outer shell electrons (e.g., +5+5 in P4O10\mathrm{P_4O_{10}} and +6+6 in SO3\mathrm{SO_3}, although burning sulfur in air only yields SO2\mathrm{SO_2}). Similar behaviour is observed with chlorine, forming chlorides (NaCl\mathrm{NaCl}, MgCl2\mathrm{MgCl_2}, AlCl3\mathrm{AlCl_3}, SiCl4\mathrm{SiCl_4}, PCl5\mathrm{PCl_5}) where the element again utilises its available valence electrons.

For the specified reactions with water, compare sodium and magnesium. Sodium reacts vigorously with cold water to form NaOH\mathrm{NaOH} and hydrogen gas. Magnesium reacts very slowly with cold water to form sparingly soluble Mg(OH)2\mathrm{Mg(OH)_2} but reacts vigorously with steam to form MgO\mathrm{MgO}. The oxides of Period 3\text{Period}\,3 elements show a marked transition in their reaction with water: Na2O\mathrm{Na_2O} reacts readily to form NaOH\mathrm{NaOH}, while MgO\mathrm{MgO} forms sparingly soluble Mg(OH)2\mathrm{Mg(OH)_2}. Their aqueous mixtures are alkaline. Al2O3\mathrm{Al_2O_3} and SiO2\mathrm{SiO_2} are practically insoluble and do not appreciably change the pH of pure water. P4O10\mathrm{P_4O_{10}} and SO3\mathrm{SO_3} react vigorously with water; SO2\mathrm{SO_2} dissolves to give an acidic solution. H3PO4\mathrm{H_3PO_4} is a weak acid despite the low pH possible in a sufficiently concentrated solution. Approximate pH ranges used for classroom comparisons assume 25 ∘C25\ ^\circ\mathrm{C} and appreciable quantities of oxide; numerical pH depends on concentration and solubility, not just the oxide formula.

Key points

  • Oxygen reactions: 4Na+O2→2Na2O4\mathrm{Na} + \mathrm{O_2} \rightarrow 2\mathrm{Na_2O}; 2Mg+O2→2MgO2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}; 4Al+3O2→2Al2O34\mathrm{Al} + 3\mathrm{O_2} \rightarrow 2\mathrm{Al_2O_3}; P4+5O2→P4O10\mathrm{P}_4 + 5\mathrm{O_2} \rightarrow \mathrm{P_4O_{10}}; S+O2→SO2\mathrm{S} + \mathrm{O_2} \rightarrow \mathrm{SO_2}.
  • Chlorine reactions: 2Na+Cl2→2NaCl2\mathrm{Na} + \mathrm{Cl_2} \rightarrow 2\mathrm{NaCl}; Mg+Cl2→MgCl2\mathrm{Mg} + \mathrm{Cl_2} \rightarrow \mathrm{MgCl_2}; 2Al+3Cl2→2AlCl32\mathrm{Al} + 3\mathrm{Cl_2} \rightarrow 2\mathrm{AlCl_3}; Si+2Cl2→SiCl4\mathrm{Si} + 2\mathrm{Cl_2} \rightarrow \mathrm{SiCl_4}; P4+10Cl2→4PCl5\mathrm{P}_4 + 10\mathrm{Cl_2} \rightarrow 4\mathrm{PCl_5}.
  • Water reactions: 2Na(s)+2H2O(l)→2NaOH(aq)+H2(g)2\mathrm{Na(s)} + 2\mathrm{H_2O(l)} \rightarrow 2\mathrm{NaOH(aq)} + \mathrm{H_2(g)}; Mg(s)+H2O(g)→MgO(s)+H2(g)\mathrm{Mg(s)} + \mathrm{H_2O(g)} \rightarrow \mathrm{MgO(s)} + \mathrm{H_2(g)}.
  • The highest oxidation states considered here correspond to the number of outer-shell electrons, for example +4+4 in SiCl4\mathrm{SiCl_4} and +6+6 in SO3\mathrm{SO_3}.
  • Illustrative pH ranges at 25 ∘C25\ ^\circ\mathrm{C}: Na2O\mathrm{Na_2O}/water 13–14, MgO\mathrm{MgO}/water 9–10, pure water with insoluble Al2O3\mathrm{Al_2O_3} or SiO2\mathrm{SiO_2} about 7, and sufficiently concentrated solutions from acidic oxides around 1–3. These are typical comparisons, not fixed pH values for every amount and volume.

Worked example

Question

Write an equation for the reaction of sodium oxide with water and state the likely pH of the resulting solution. Explain the oxidation number of sodium in this oxide. Treat the pH as an approximate classroom comparison at 25 ∘C25\ ^\circ\mathrm{C}, with enough oxide to form an appreciably alkaline solution.

Solution

1. Identify the reactants and products: Solid sodium oxide reacts with water to form aqueous sodium hydroxide.

2. Write the balanced equation: Na2O(s)+H2O(l)→2NaOH(aq)\mathrm{Na_2O(s)} + \mathrm{H_2O(l)} \rightarrow 2\mathrm{NaOH(aq)}.

3. NaOH\mathrm{NaOH} supplies OH−\mathrm{OH^-}. A typical appreciable concentration may give pH 13–14 at 25 ∘C25\ ^\circ\mathrm{C}, but the exact pH requires the concentration.

4. Relate oxidation number to electrons: Sodium is in Group 1, has one outer shell electron, and loses it to form a +1+1 oxidation state.

Na2O(s)+H2O(l)→2NaOH(aq)\mathrm{Na_2O(s)} + \mathrm{H_2O(l)} \rightarrow 2\mathrm{NaOH(aq)}. The solution is alkaline; pH 13–14 is a typical illustrative value, not a universal result. Sodium has oxidation number +1+1 in Na2O\mathrm{Na_2O} and one outer-shell electron in its atom.

Common pitfalls

  • Assuming sulfur reacts with oxygen to directly form SO3\mathrm{SO_3}. Burning sulfur in air or oxygen primarily produces SO2\mathrm{SO_2}; a catalyst and specific conditions are required to form SO3\mathrm{SO_3}.
  • Assuming Al2O3\mathrm{Al_2O_3} or SiO2\mathrm{SiO_2} reacts with pure water. Both are practically insoluble and do not appreciably change the pH; pure water is neutral at approximately pH 7 at 25 ∘C25\ ^\circ\mathrm{C}.

Prerequisites

  • Study Atomic structure and electron arrangement first.
  • Study Bonding, shape and states of matter first.
  • Study Enthalpy, chemical energy and redox first.
  • Study Equilibria, acids and reaction rates first.