Reactions of Period 3 Oxides with Water and Acid Formation Revision Notes | A Level AQA Chemistry

1. The Big Picture: Why Bonding Determines Behaviour

Across Period 3, there is a clear shift from metallic to non-metallic character as you move left to right. That shift is not cosmetic — it fundamentally changes how each element's oxide behaves when it meets water.

The key principle is this: ionic oxides tend to be basic; covalent oxides tend to be acidic. The bonding type dictates whether the oxide donates hydroxide ions or produces an oxoacid when dissolved in water. Everything in this lesson flows from that single idea.

2. Ionic Oxides and Their Reactions with Water

Sodium oxide and magnesium oxide are both ionically bonded — they consist of a lattice of metal cations and oxide ions ($O^{2-}$). When water is added, the oxide ion acts as a base and accepts a proton from water, generating hydroxide ions.

For sodium oxide, this reaction is vigorous and produces a strongly alkaline solution of sodium hydroxide. The high charge density of $Na^+$ is relatively low, so $NaOH$ is fully soluble and fully dissociates.

Magnesium oxide reacts much more slowly and only slightly dissolves. The charge density of Na+ is relatively low, so NaOH is fully soluble, whereas Mg(OH)2 is only sparingly soluble because Mg2+ has higher charge density. The mixture is therefore only weakly alkaline.

The trend here is clear: as you move from $Na_2O$ to $MgO$, the alkalinity decreases. The oxide ion is still acting as a base in both cases, but the solubility of the resulting hydroxide falls as the metal ion charge density increases.

3. Covalent Oxides: Why They Form Acids

Moving to the right of Period 3, the oxides become covalent. The non-metal atoms form strong covalent bonds with oxygen, and the resulting molecules are polar. When these molecules meet water, they react to form oxoacids — acids where the $H^+$ ions are attached to oxygen atoms bonded to the central non-metal.

The reason covalent oxides form acids rather than bases comes down to the high electronegativity and oxidation state of the central atom. The central non-metal atom pulls electron density strongly towards itself through the oxygen atoms, making the $O-H$ bonds in the resulting acid highly polarised and straightforward to break — releasing $H^+$ into solution.

Silicon dioxide is the exception among the covalent oxides: its giant covalent lattice means there are no discrete molecules to react with water at room temperature. It is acidic in character but only reacts with hot concentrated alkali, not with water itself.

4. Phosphorus(V) Oxide Reacting with Water: Structure of Phosphoric Acid

Phosphorus(V) oxide, $P_4O_{10}$, is a white solid that reacts vigorously and exothermically with water. The product is phosphoric(V) acid, $H_3PO_4$, also called orthophosphoric acid.

The structure of $H_3PO_4$ is tetrahedral around the phosphorus atom. The phosphorus atom sits at the centre, double-bonded to one oxygen atom and single-bonded to three $-OH$ groups. It is the three $O-H$ bonds that are acidic — each one can donate a proton, making $H_3PO_4$ a triprotic (tribasic) acid.

When $H_3PO_4$ dissociates in water, it does so in three successive steps, each releasing one $H^+$ ion. The anions formed are the dihydrogenphosphate ion $H_2PO_4^-$, the hydrogenphosphate ion $HPO_4^{2-}$, and finally the phosphate ion $PO_4^{3-}$.

• First dissociation: $H_3PO_4(aq) \rightleftharpoons H^+(aq) + H_2PO_4^-(aq)$
• Second dissociation: $H_2PO_4^-(aq) \rightleftharpoons H^+(aq) + HPO_4^{2-}(aq)$
• Third dissociation: $HPO_4^{2-}(aq) \rightleftharpoons H^+(aq) + PO_4^{3-}(aq)$

Each successive dissociation is less favourable because removing a proton from an increasingly negative ion requires more energy. Under typical conditions, the dominant anion in dilute solution is $H_2PO_4^-$.

5. Sulphur Oxides Reacting with Water: Structures of Sulphurous and Sulphuric Acid

Sulphur forms two important oxides in Period 3: sulphur dioxide ($SO_2$, sulphur in oxidation state +4) and sulphur trioxide ($SO_3$, sulphur in oxidation state +6). Both react with water to form acids, but the products — and their strengths — are very different.

$SO_2$ dissolves in water to form sulphurous acid, $H_2SO_3$. This is a weak diprotic acid. The reaction is reversible and the equilibrium lies to the left, meaning only a small proportion of $SO_2$ molecules actually react with water to form the acid.

The structure of $H_2SO_3$ has sulphur at the centre bonded to two $-OH$ groups and one $=O$ group, giving a trigonal pyramidal arrangement. Both $O-H$ bonds are acidic. Dissociation produces the hydrogensulphite ion $HSO_3^-$ and then the sulphite ion $SO_3^{2-}$.

• First dissociation: $H_2SO_3(aq) \rightleftharpoons H^+(aq) + HSO_3^-(aq)$
• Second dissociation: $HSO_3^-(aq) \rightleftharpoons H^+(aq) + SO_3^{2-}(aq)$

$SO_3$ reacts much more vigorously with water than $SO_2$ does. The product is sulphuric acid, $H_2SO_4$ — one of the strongest acids known. The reaction is highly exothermic and essentially irreversible under standard conditions.

The structure of $H_2SO_4$ is tetrahedral around the sulphur atom. Sulphur is bonded to two $=O$ groups (non-acidic) and two $-OH$ groups (acidic). Both $O-H$ bonds donate protons readily — $H_2SO_4$ is a strong acid and fully dissociates in its first step, and partially in its second.

• First dissociation (complete): $H_2SO_4(aq) \rightarrow H^+(aq) + HSO_4^-(aq)$
• Second dissociation (partial): $HSO_4^-(aq) \rightleftharpoons H^+(aq) + SO_4^{2-}(aq)$

6. Comparing the Anions: A Structural Summary

Now that you have seen all three covalent oxide reactions, it is worth placing the anions side by side. The AQA specification requires you to know the structures of the acids and the anions formed — so this comparison is directly examinable.

Notice the pattern: the higher the oxidation state of the central non-metal, the stronger the acid produced. Sulphur(VI) in $SO_3$ gives the strong acid $H_2SO_4$, while sulphur(IV) in $SO_2$ gives the weak acid $H_2SO_3$. The same logic applies to phosphorus: $P_4O_{10}$ contains phosphorus in oxidation state +5 and gives a weak acid because phosphorus is less electronegative than sulphur.

7. Worked Example: Writing the Full Equation and Identifying the Anion

Exam questions on this topic often ask you to write a balanced equation for the reaction of a named oxide with water, then state the formula of the anion produced. Here is a model walkthrough for the reaction of $SO_3$ with water, showing the level of detail that earns full marks.

Worked example: Writing the Equation and Anion for $SO_3$ Reacting with Water

Write a balanced equation, including state symbols, for the reaction of sulphur trioxide with water. State the formula of the anion produced when the acid fully dissociates in its first step.

1. $SO_3$ is a covalent oxide of a non-metal. Non-metal oxides react with water to form oxoacids. $SO_3$ contains sulphur in oxidation state +6, so the product is sulphuric acid: $H_2SO_4$. — Knowing the oxidation state of sulphur in $SO_3$ (+6) tells you the acid will be $H_2SO_4$, not $H_2SO_3$. This is the most common point of confusion.
2. $SO_3 + H_2O \rightarrow H_2SO_4$ — Start with the correct formulae for all three species before balancing.
3. Left: S=1, O=3+1=4, H=2. Right: S=1, O=4, H=2. Already balanced. — Count each element on both sides. Here the equation is balanced as written — no coefficients need changing.
4. $$SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$$ — $SO_3$ is a gas at room temperature; water is liquid; $H_2SO_4$ dissolves fully in water so it is aqueous. State symbols are required for full marks in AQA equations.
5. $$H_2SO_4(aq) \rightarrow H^+(aq) + HSO_4^-(aq)$$ — The first dissociation of $H_2SO_4$ is complete. The anion produced is the hydrogensulphate ion, $HSO_4^-$. The sulphate ion $SO_4^{2-}$ is only produced in the second (partial) dissociation.

Answer: Balanced equation: $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$. Anion from first dissociation: $HSO_4^-$ (hydrogensulphate ion).

8. Exam Technique and Common Mistakes

This topic rewards precision. The equations are not complex, but the marks are lost through careless errors in formulae, missing state symbols, and muddled anion names. The guidance below targets the mistakes that appear most frequently in AQA mark schemes.