Period 3 Oxides - Acid-Base Behavior — AQA A-level Chemistry

SPEC 3.2.4 - Period 3 Elements & Oxides

Period 3 Oxides: Structure, Bonding & pH

Understanding how oxide structure determines acid-base character

Introduction to Period 3 Oxides

As you move across Period 3 from left to right, there are important trends in the physical properties of Period 3 compounds. These trends result from the change from metal elements on the left to non-metal elements on the right. The oxides are representative of these trends.

Key Principle

The structure and bonding of Period 3 oxides changes systematically across the period. This determines their physical properties (like melting point) and their acid-base behaviour when they react with water.

The oxides we study are: Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂, and SO₃. Each has distinct bonding that determines its properties.

General Trend

Solutions of the oxides of Period 3 elements go from alkaline to acidic across the Period. Metal oxides form alkaline solutions; non-metal oxides form acidic solutions.

Bonding and Structure of Period 3 Oxides

The oxides of Period 3 elements show a range of different types of bonding and structures:

Na₂O and MgO are ionic metal oxides that form giant ionic lattice structures. The large electronegativity difference between the metal and oxygen creates strong ionic bonds between oppositely charged ions.

Al₂O₃ is an ionic oxide that forms a giant ionic lattice with mixed bonding . The small, highly charged Al³⁺ ion can get close to the O²⁻ ion and distort its electron cloud, resulting in the formation of a partial covalent bond between Al and O.

SiO₂ forms a giant covalent network with a continuous, strong covalent structure extending in all directions. Each silicon atom shares electrons with four oxygen atoms and each oxygen shares electrons with two silicon atoms.

P₄O₁₀ and SO₃ form simple molecular structures where discrete molecules are held together by weak intermolecular forces. Within each molecule there are strong covalent P-O and S-O bonds.

Exam Tip

It is possible to predict the ionic character of a bond by considering the difference in electronegativities between the two atoms. The bigger the difference, the greater the ionic character of the bond.

Electronegativity and Bonding Character

As electronegativity increases across Period 3, the bonding in the oxides changes from ionic to covalent:

ElementNaMgAlSiPS

Electronegativity0.91.21.51.82.12.5

Chemical BondingIonicIonicIonic (with covalent character)CovalentCovalentCovalent

StructureGiant ionicGiant ionicGiant ionicGiant covalentSimple molecularSimple molecular

Key Trends

Covalent character INCREASES across the period (left to right)

Ionic character DECREASES across the period (left to right)

This is because the electronegativity difference between the element and oxygen decreases as you move right. Oxygen has an electronegativity of 3.5.

Electrical Conductivity

The ionic oxides (Na₂O, MgO, Al₂O₃) conduct electricity when molten because the ions are free to move. The covalent oxides (SiO₂, P₄O₁₀, SO₂, SO₃) do not conduct electricity in any state because they have no mobile charged particles.

Melting Points Relate to Bonding and Structure

The strength of bonds in an oxide determines the amount of energy needed to melt it from a solid to a liquid. The stronger the bonds in an oxide, the more energy is required to break those bonds and the higher its melting point.

OxideNa₂OMgOAl₂O₃SiO₂P₄O₁₀SO₂SO₃

M.P. / K1548312523451883853200290

State at 25°CSolidSolidSolidSolidSolidGasLiquid

Explaining the Melting Point Trend

Ionic Oxides (Na₂O, MgO, Al₂O₃) — These have giant lattice structures and thus high melting points. The ionic oxides have strong electrostatic forces between oppositely charged ions, requiring substantial energy to overcome.

MgO has the highest m.p. because the Mg²⁺ ion is smaller and more highly charged than the Na⁺ ion. This creates stronger electrostatic attractions between ions in the MgO lattice structure.

Al₂O₃ has a lower m.p. than MgO because the ionic bonding in Al₂O₃ has some covalent character, meaning the electrostatic forces between ions are weaker.

Giant Covalent Oxide (SiO₂) — Silicon dioxide is a giant covalent molecule (macromolecule). The millions of covalent bonds within this giant structure are extremely strong, giving it a high melting point.

Simple Molecular Oxides (P₄O₁₀, SO₂, SO₃) — These are small molecules with only weak intermolecular forces of attraction between them. They have the lowest melting points as very little energy is needed to overcome these weak forces.

Note

Sulfur trioxide (SO₃) has a slightly higher melting point than sulfur dioxide (SO₂) because SO₃ forms larger molecules with more electrons, creating stronger induced dipole-dipole forces between molecules. Both are gases at room temperature, but SO₃ is a liquid at 25°C.

Reactions of Period 3 Oxides with Water

The oxides show different reactions with water due to differences in their structure and bonding. The table below summarises the bonding, structure and reactions with water:

OxideBondingIons Present After ReactionAcidity/AlkalinitypH

Na₂OIonicNa⁺(aq), OH⁻(aq)Strongly alkaline14

MgOIonicMg²⁺(aq), OH⁻(aq)Weakly alkaline10

Al₂O₃Covalent/ionicInsoluble, no reaction—7

SiO₂CovalentInsoluble, no reaction—7

P₄O₁₀CovalentH⁺(aq), H₂PO₄⁻(aq)Strongly acidic2

SO₂CovalentH⁺(aq), HSO₃⁻(aq)Weakly acidic3

SO₃CovalentH⁺(aq), HSO₄⁻(aq)Strongly acidic1

Sodium and Magnesium Oxides Form Alkaline Solutions

Na₂O and MgO have giant ionic lattice structures. When dissolved in water, these lattices separate into hydrated ions:

Na_{2} O_{(s)} \to 2 Na_{(a q)}^{+} + O_{(a q)}^{2 -}

MgO_{(s)} \to Mg_{(a q)}^{2 +} + O_{(a q)}^{2 -}

Why O²⁻ is a Strong Base

The oxide ion, O²⁻, is a very strong base (it strongly attracts protons) and so readily reacts with water to produce hydroxide ions:

O_{(a q)}^{2 -} + H_{2} O_{(l)} \to 2 OH_{(a q)}^{-}

This makes the solution alkaline. For example:

Na_{2} O_{(s)} + H_{2} O_{(l)} \to 2 NaOH_{(a q)}

(pH 14)

MgO_{(s)} + H_{2} O_{(l)} \to Mg(OH)_{2 (a q)}

(pH 10)

Why the pH Difference?

Magnesium oxide produces a less alkaline solution than sodium oxide because it is less soluble. Only a small amount dissolves, so fewer OH⁻ ions are present in solution.

Aluminium Oxide and Silicon Dioxide are Insoluble

Aluminium oxide (Al₂O₃) is ionic but the bonding is too strong for the ions to be separated, partly because of the additional covalent bonding it has. The small, highly charged Al³⁺ ion can approach closely to the O²⁻ ion and distort its electron cloud, giving the bond some covalent character.

Why Al₂O₃ Doesn't React with Water

Although aluminium oxide is ionic, the oxide ions are held too strongly in the ionic lattice. This means the ions cannot be separated by water molecules.

Silicon dioxide (SiO₂) has a giant covalent (macromolecular) structure. It is the main component of sand. The bonding extends throughout the giant structure, with millions of strong covalent bonds. Water will not affect this type of structure because there are no ions to dissolve and no weak intermolecular forces to overcome.

Key Point

Each silicon atom shares electrons with four oxygen atoms, and each oxygen shares electrons with two silicon atoms. This creates a continuous 3D network.

Important

Neither Al₂O₃ nor SiO₂ reacts with water — the pH remains at 7. However, both DO react with acids or bases, which is how we classify their acid-base character.

Phosphorus Pentoxide and Sulfur Oxides Form Acidic Solutions

P₄O₁₀, SO₂ and SO₃ have simple molecular structures. When water is added, they react vigorously to form acidic solutions containing hydrogen ions (H⁺) that are donated as the acidic oxides dissolve.

Phosphorus pentoxide:

P_{4} O_{10 (s)} + 6 H_{2} O_{(l)} \to 4 H_{3} PO_{4 (a q)}

Phosphoric(V) acid — pH 1-2

H₃PO₄ ionises in stages, the first being:

H_{3} PO_{4 (a q)} ⇌ H_{(a q)}^{+} + H_{2} PO_{4 (a q)}^{-}

Sulfur dioxide:

SO_{2 (g)} + H_{2} O_{(l)} \to H_{2} SO_{3 (a q)}

Sulfurous acid (weak acid) — pH 2-3

H₂SO₃ partially dissociates:

H_{2} SO_{3 (a q)} ⇌ H_{(a q)}^{+} + HSO_{3 (a q)}^{-}

Sulfur trioxide:

SO_{3 (g)} + H_{2} O_{(l)} \to H_{2} SO_{4 (a q)}

Sulfuric acid (strong acid) — pH 0-1

Trend from Alkaline to Acidic Oxides

As you move left to right across Period 3, the oxides transition from alkaline to acidic:

Alkaline oxides — Sodium oxide, magnesium oxide

Amphoteric oxide — Aluminium oxide (reacts with both acids AND alkalis)

Acidic oxides — Silicon dioxide, phosphorus pentoxide, sulfur dioxide, sulfur trioxide

Key Principle

The alkaline oxides on the left react with acids. The acidic oxides on the right react with bases. Aluminium oxide reacts with both acids and alkalis, so it is classed as an amphoteric oxide.

01Worked Example

Predict the approximate pH when each of the following oxides is added to water, and explain your reasoning:

(a) Na₂O (b) SO₃ (c) Al₂O₃

(a)

Na₂O: pH 14 — Na₂O is a basic (alkaline) oxide. It reacts with water: Na₂O + H₂O → 2NaOH. NaOH is a strong alkali that fully dissociates, giving a high concentration of OH⁻ ions.

(b)

SO₃: pH 1 — SO₃ is an acidic oxide. It reacts vigorously with water: SO₃ + H₂O → H₂SO₄. Sulfuric acid is a strong acid that fully dissociates, giving a very low pH.

(c)

Al₂O₃: pH 7 (no change) — Al₂O₃ is insoluble in water and does not react with it. The ionic bonding has covalent character, making the lattice too strong to be broken by water. The pH remains neutral.

Reactions of Alkaline Oxides with Acids

The alkaline metal oxides Na₂O and MgO neutralise acids to form salts and water.

Sodium oxide + hydrochloric acid:

Na_{2} O_{(s)} + 2 HCl_{(a q)} \to 2 NaCl_{(a q)} + H_{2} O_{(l)}

Magnesium oxide + hydrochloric acid:

MgO_{(s)} + 2 HCl_{(a q)} \to MgCl_{2 (a q)} + H_{2} O_{(l)}

Sodium oxide + sulfuric acid:

Na_{2} O_{(s)} + H_{2} SO_{4 (a q)} \to Na_{2} SO_{4 (a q)} + H_{2} O_{(l)}

Real World Application

Magnesium oxide is used in indigestion remedies (antacids) because it neutralises excess hydrochloric acid in the stomach. The reaction produces magnesium chloride and water, relieving symptoms of acid indigestion.

General Pattern

Basic oxide + Acid → Salt + Water

This is a neutralisation reaction. The oxide ion (O²⁻) from the basic oxide accepts H⁺ ions from the acid to form water.

Reaction of Aluminium Oxide with Acids and Alkalis

The amphoteric aluminium oxide neutralises both acids and alkalis. This behaviour proves that the chemical bonding in aluminium oxide is not purely ionic nor covalent — therefore it exhibits amphoteric character.

With Hydrochloric Acid

The reaction with acids produces a salt and water:

Al_{2} O_{3 (s)} + 6 HCl_{(a q)} \to 2 AlCl_{3 (a q)} + 3 H_{2} O_{(l)}

With sulfuric acid:

Al_{2} O_{3 (s)} + 3 H_{2} SO_{4 (a q)} \to Al_{2} (SO_{4})_{3 (a q)} + 3 H_{2} O_{(l)}

With Hot, Concentrated Sodium Hydroxide

The reaction with alkalis produces an aluminate salt:

Al_{2} O_{3 (s)} + 2 NaOH_{(a q)} + 3 H_{2} O_{(l)} \to 2 NaAl(OH)_{4 (a q)}

The product is sodium tetrahydroxoaluminate, containing the [Al(OH)₄]⁻ ion.

Remember

Al₂O₃ does NOT react with water because although it is ionic, the oxide ions are held too strongly in the ionic lattice. Its amphoteric nature is only revealed when it reacts with acids or bases.

Reaction of Acidic Oxides with Alkalis

The acidic oxides SiO₂, P₄O₁₀, SO₂, and SO₃ neutralise alkalis to produce salt solutions.

Silicon dioxide reacts as a weak acid with strong bases:

SiO_{2 (s)} + 2 NaOH_{(a q)} \to Na_{2} SiO_{3 (a q)} + H_{2} O_{(l)}

(Requires hot, concentrated NaOH)

Phosphorus pentoxide reacts directly with sodium hydroxide:

P_{4} O_{10 (s)} + 12 NaOH_{(a q)} \to 4 Na_{3} PO_{4 (a q)} + 6 H_{2} O_{(l)}

Phosphoric Acid Neutralisation (Three Stages)

When P₄O₁₀ first reacts with water to form H₃PO₄, the acid has three acidic -OH groups. Each hydrogen is replaced by sodium in stages:

H_{3} PO_{4 (a q)} + NaOH_{(a q)} \to NaH_{2} PO_{4 (a q)} + H_{2} O_{(l)}

NaH_{2} PO_{4 (a q)} + NaOH_{(a q)} \to Na_{2} HPO_{4 (a q)} + H_{2} O_{(l)}

Na_{2} HPO_{4 (a q)} + NaOH_{(a q)} \to Na_{3} PO_{4 (a q)} + H_{2} O_{(l)}

Reactions of Sulfur Oxides with Alkalis

Sulfur dioxide and sulfur trioxide neutralise alkalis to form salts and water.

Sulfur Dioxide (Two Stages)

First, sodium hydrogensulfite(IV) is formed:

SO_{2 (a q)} + NaOH_{(a q)} \to NaHSO_{3 (a q)}

Then with more NaOH, sodium sulfite(IV) is formed:

NaHSO_{3 (a q)} + NaOH_{(a q)} \to Na_{2} SO_{3 (a q)} + H_{2} O_{(l)}

Overall for SO₂:

SO_{2 (g)} + 2 NaOH_{(a q)} \to Na_{2} SO_{3 (a q)} + H_{2} O_{(l)}

(Sodium sulfite formed — note: sulfITE not sulfATE)

Sulfur trioxide with sodium hydroxide:

SO_{3 (g)} + 2 NaOH_{(a q)} \to Na_{2} SO_{4 (a q)} + H_{2} O_{(l)}

(Sodium sulfate formed)

Remember

SO₂ produces sulfITE salts (S is +4). SO₃ produces sulfATE salts (S is +6). The oxidation state of sulfur is preserved in the product.

02Worked Example

Write balanced equations for the following reactions:

(a) Magnesium oxide with dilute sulfuric acid

(b) Sulfur dioxide with sodium hydroxide solution

(c) Aluminium oxide with hydrochloric acid

(a)

MgO(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂O(l) — Basic oxide + acid → salt + water. The oxide ion accepts 2H⁺ to form water.

(b)

SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l) — Acidic oxide + base → salt + water. Note: the product is sodium sulfITE (not sulfATE) because sulfur is in the +4 state in SO₂.

(c)

Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l) — The amphoteric oxide acts as a base here, accepting H⁺ ions from HCl.

Practice

Question 1: Explain why MgO has a higher melting point than Na₂O, even though both have giant ionic structures.

MgO has a higher melting point because:

•

Mg²⁺ has a higher charge than Na⁺ (2+ vs 1+)

•

Mg²⁺ is smaller than Na⁺, giving it a higher charge density

•

This creates stronger electrostatic attractions between the Mg²⁺ and O²⁻ ions in the MgO lattice

•

More energy is required to overcome these stronger forces, hence the higher melting point

Practice

Question 2: Write equations to show that aluminium oxide is amphoteric, giving its reactions with (a) hydrochloric acid and (b) sodium hydroxide.

(a) With hydrochloric acid (acting as a base):

Al_{2} O_{3 (s)} + 6 HCl_{(a q)} \to 2 AlCl_{3 (a q)} + 3 H_{2} O_{(l)}

(b) With sodium hydroxide (acting as an acid):

Al_{2} O_{3 (s)} + 2 NaOH_{(a q)} + 3 H_{2} O_{(l)} \to 2 NaAl(OH)_{4 (a q)}

The fact that Al₂O₃ reacts with both an acid and a base proves it is amphoteric.

Practice

Question 3: Sulfur dioxide and sulfur trioxide both react with water. Write equations for these reactions and explain why the products have different acid strengths.

Equations:

SO₂(g) + H₂O(l) → H₂SO₃(aq) (sulfurous acid, weak)

SO₃(g) + H₂O(l) → H₂SO₄(aq) (sulfuric acid, strong)

Explanation of acid strength difference:

•

H₂SO₄ is a strong acid (fully dissociates); H₂SO₃ is a weak acid (partially dissociates)

•

In H₂SO₄, sulfur is in the +6 oxidation state with more oxygen atoms bonded

•

The additional electronegative oxygen atoms withdraw electron density from the O-H bond

•

This weakens the O-H bond, making H⁺ easier to release

•

In H₂SO₃ (S is +4), fewer oxygen atoms means the O-H bond is stronger, so H⁺ is released less readily

Summary

Key Concepts

ConceptDescription

Structure TrendGiant ionic → Giant covalent → Simple molecular across Period 3

Melting Point TrendMgO highest (strong ionic), then Al₂O₃, SiO₂ (covalent), then P₄O₁₀, SO₃ lowest (weak IMF)

Acid-Base TrendAlkaline (Na₂O, MgO) → Amphoteric (Al₂O₃) → Acidic (SiO₂, P₄O₁₀, SO₂, SO₃)

Insoluble OxidesAl₂O₃ and SiO₂ do not react with water (pH stays at 7)

Amphoteric Al₂O₃Reacts with both acids (→ AlCl₃) and bases (→ NaAl(OH)₄)

Key Exam Skills

Write equations for oxide + water, oxide + acid, oxide + base

Predict pH of solutions from oxide type

Explain melting point trends using bonding and structure

Explain amphoteric behaviour of Al₂O₃

Explain why Al₂O₃ has covalent character (polarisation)