Trends in Chemical Properties of Period 3 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Period 3 Elements and Their Compounds

4 lessons

Lesson 3 of 4

Oxides of the Period 3 Elements

25 min

From antacid to acid rain

Magnesium oxide and magnesium hydroxide are swallowed to neutralise excess stomach acid. Sulfur dioxide from burning coal and smelting sulfide ores dissolves in rain and makes it acidic. Sand, which is mostly silicon dioxide, ignores both water and most acids, yet glassmakers dissolve it by melting it with sodium carbonate. Three oxides of the same period: one a base, one an acid, one that needs a hot alkali. This lesson shows how the bonding of the Period 3 oxides changes across the period, and how their acid–base behaviour follows.

Definition

Basic oxide: an oxide that reacts with acids to form a salt and water (and, if soluble, gives an alkaline solution). Example: $\mathrm{MgO + 2HCl \rightarrow MgCl_2 + H_2O}$ .

Acidic oxide: an oxide that reacts with bases to form a salt and water (and, if soluble, gives an acidic solution). Example: $\mathrm{SO_3 + 2NaOH \rightarrow Na_2SO_4 + H_2O}$ .

Amphoteric oxide: an oxide that reacts with both acids and bases. Example: aluminium oxide, $\mathrm{Al_2O_3}$ .

Bonding and structure

Table 14.2: The oxides of the Period 3 elements

Oxide	Bonding	Structure	m.p. / °C	With water	pH of solution	Character
$\mathrm{Na_2O}$	ionic	giant ionic	1132	reacts: NaOH	about 14	basic
MgO	ionic	giant ionic	2852	slight reaction: $\mathrm{Mg(OH)_2}$	about 9–10	basic
$\mathrm{Al_2O_3}$	ionic, with some covalent character	giant ionic	2054	insoluble	7	amphoteric
$\mathrm{SiO_2}$	covalent	giant covalent	1713	insoluble	7	acidic
$\mathrm{P_4O_{10}}$	covalent	simple molecular	562 (sublimes)	reacts violently: $\mathrm{H_3PO_4}$	about 0–1	acidic
$\mathrm{SO_2}$	covalent	simple molecular	−72	reacts: $\mathrm{H_2SO_3}$	about 2–3	acidic
$\mathrm{SO_3}$	covalent	simple molecular	16.9	reacts violently: $\mathrm{H_2SO_4}$	about 0–1	acidic
$\mathrm{Cl_2O_7}$	covalent	simple molecular	−91.5	reacts: $\mathrm{HClO_4}$	about 0	acidic

The pH values are typical for concentrated solutions made in the laboratory.

Figure 14.5: Melting points of the Period 3 oxides. Giant ionic and giant covalent oxides melt above 1000 °C; molecular oxides melt low because only intermolecular forces are overcome.

The bonding follows the difference in electronegativity between the element and oxygen (3.44). Sodium (0.93) and magnesium (1.31) differ from oxygen by more than 2, so they form ionic oxides with $\mathrm{O^{2-}}$ ions in giant lattices. MgO melts higher than $\mathrm{Na_2O}$ because $\mathrm{Mg^{2+}}$ is smaller and doubly charged, so the lattice is held more strongly. Aluminium oxide is mainly ionic, but the small, highly charged $\mathrm{Al^{3+}}$ ion pulls on the oxide ions' electrons, giving the bonds some covalent character. Silicon dioxide is a giant covalent lattice in which every Si atom is bonded to four O atoms. From phosphorus onwards the oxides are small covalent molecules.

Acid–base behaviour

Basic oxides. The oxide ion is a strong base: it takes a proton from water, $\mathrm{O^{2-} + H_2O \rightarrow 2OH^-}$ . $\mathrm{Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)} \qquad \mathrm{MgO(s) + H_2O(l) \rightarrow Mg(OH)_2(s)}$ Magnesium hydroxide is only slightly soluble, so its solution is only weakly alkaline. Both oxides neutralise acids: $\mathrm{Na_2O + 2HCl \rightarrow 2NaCl + H_2O} \qquad \mathrm{MgO + 2HCl \rightarrow MgCl_2 + H_2O}$

Amphoteric aluminium oxide. It is insoluble in water, but reacts with both acids and bases: $\mathrm{Al_2O_3(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2O(l)}$ $\mathrm{Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2NaAl(OH)_4(aq)}$ The product of the second reaction is sodium tetrahydroxoaluminate (sodium aluminate).

Acidic oxides. Silicon dioxide is insoluble in water, but it reacts with hot, concentrated sodium hydroxide (or molten sodium carbonate, in glassmaking): $\mathrm{SiO_2(s) + 2NaOH(aq) \rightarrow Na_2SiO_3(aq) + H_2O(l)}$ The molecular oxides react with water to form oxoacids: $\mathrm{P_4O_{10}(s) + 6H_2O(l) \rightarrow 4H_3PO_4(aq)} \qquad \mathrm{SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)}$ $\mathrm{SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)} \qquad \mathrm{Cl_2O_7(l) + H_2O(l) \rightarrow 2HClO_4(aq)}$ and with bases to form salts, for example $\mathrm{P_4O_{10} + 12NaOH \rightarrow 4Na_3PO_4 + 6H_2O}$ and $\mathrm{SO_2 + 2NaOH \rightarrow Na_2SO_3 + H_2O}$ .

Why acidic? In a non-metal oxide the central atom is in a high oxidation state and is quite electronegative. In the oxoacid it forms, it pulls electron density away from the O–H bonds, so $\mathrm{H^+}$ is released easily. The more oxygen atoms and the higher the oxidation state, the stronger the acid: $\mathrm{H_2SO_4}$ is stronger than $\mathrm{H_2SO_3}$ .

Magnesium oxide, a white powder, in a glass dish. It is a basic oxide with a giant ionic lattice; it melts at 2852 °C, so it is used to line furnaces, and it neutralises acids. Photo: Walkerma, public domain, via Wikimedia Commons

Worked example 1: neutralising phosphorus(V) oxide

0.142 g of $\mathrm{P_4O_{10}}$ is added to water and the solution made up to 100 cm³. Calculate the volume of 0.100 mol dm⁻³ sodium hydroxide needed to neutralise all of the phosphoric acid to $\mathrm{Na_3PO_4}$ . (P = 30.97, O = 16.00)

Solution. $M(\mathrm{P_4O_{10}}) = 4(30.97) + 10(16.00) = 283.88\ \mathrm{g\,mol^{-1}}$ $n(\mathrm{P_4O_{10}}) = \frac{0.142}{283.88} = 5.00 \times 10^{-4}\ \mathrm{mol}$ $\mathrm{P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4}$ , so $n(\mathrm{H_3PO_4}) = 2.00 \times 10^{-3}\ \mathrm{mol}$ . Each $\mathrm{H_3PO_4}$ needs 3 NaOH: $n(\mathrm{NaOH}) = 6.00 \times 10^{-3}\ \mathrm{mol}$ . $V = \frac{6.00 \times 10^{-3}}{0.100} = 0.0600\ \mathrm{dm^3} = 60.0\ \mathrm{cm^3}$

Worked example 2: telling three white oxides apart

Three unlabelled white powders are MgO, $\mathrm{Al_2O_3}$ and $\mathrm{SiO_2}$ . Use their acid–base behaviour to identify them.

Solution. Add a little of each to dilute hydrochloric acid, and separately to warm sodium hydroxide solution.

Oxide	Dilute HCl	Warm NaOH(aq)	Conclusion
MgO	dissolves	no reaction	basic
$\mathrm{Al_2O_3}$	dissolves	dissolves	amphoteric
$\mathrm{SiO_2}$	no reaction	no visible reaction in dilute alkali; dissolves only in hot, concentrated or molten alkali	acidic (but unreactive)

The powder that dissolves in both is $\mathrm{Al_2O_3}$ ; the one that dissolves only in acid is MgO; the one that dissolves in neither is $\mathrm{SiO_2}$ . (Aluminium oxide that has been strongly heated dissolves only slowly, so the test works best with fresh, fine powder.)

Warning

Common mistakes. (1) Saying $\mathrm{SiO_2}$ is not acidic because it does not dissolve in water: it is an acidic oxide, shown by its reaction with hot concentrated alkali. (2) Writing that $\mathrm{Al_2O_3}$ dissolves in water: it is insoluble; it is amphoteric because it reacts with acids and bases. (3) Writing $\mathrm{H_2SO_4}$ as the product of $\mathrm{SO_2}$ and water: $\mathrm{SO_2}$ gives sulfurous acid, $\mathrm{H_2SO_3}$ . (4) Explaining the high m.p. of $\mathrm{SiO_2}$ by ionic bonding: it is giant covalent.

Safety. $\mathrm{P_4O_{10}}$ and $\mathrm{SO_3}$ react violently with water and are corrosive; only a teacher handles them, with gloves and goggles, in a fume cupboard. Sulfur dioxide is toxic. Hot concentrated sodium hydroxide causes severe burns to skin and eyes: if any splashes, rinse with running water for at least 15 minutes and get medical help.

Tip

Basic → amphoteric → acidic, ionic → covalent. Read the period like a pH scale turned round: $\mathrm{Na_2O}$ (most basic) … $\mathrm{Al_2O_3}$ (both) … $\mathrm{Cl_2O_7}$ (most acidic). The switch follows the electronegativity of the element.

MSJChem: Bonding of the Period 3 Oxides and Their Acid–Base Character (4 min). Watch for how the change from ionic to covalent bonding matches the change from basic to acidic oxides.

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Practice

Classify each oxide as basic, amphoteric or acidic: $\mathrm{Na_2O}$ , $\mathrm{Al_2O_3}$ , $\mathrm{SiO_2}$ , $\mathrm{SO_3}$ , MgO.
Write equations for the reactions of (a) MgO with dilute nitric acid and (b) $\mathrm{SO_2}$ with water.
Write two equations to show that aluminium oxide is amphoteric.
Explain why MgO has a higher melting point than $\mathrm{Na_2O}$ .
Explain why $\mathrm{SiO_2}$ melts at 1713 °C but $\mathrm{SO_2}$ at −72 °C.
0.620 g of sodium oxide is dissolved in water to make 250 cm³ of solution. Calculate the concentration of hydroxide ions (Na = 22.99, O = 16.00).
Calculate the mass of $\mathrm{SO_3}$ that dissolves in water to give 1.00 dm³ of 0.500 mol dm⁻³ sulfuric acid (S = 32.06).
A Period 3 oxide is a gas at room temperature and dissolves in water to give a solution of pH 2–3, which turns acidified potassium dichromate(VI) from orange to green. Identify the oxide and write the equation for its reaction with water.

Show answersHide answers

Basic: $\mathrm{Na_2O}$ , MgO; amphoteric: $\mathrm{Al_2O_3}$ ; acidic: $\mathrm{SiO_2}$ , $\mathrm{SO_3}$ .
(a) $\mathrm{MgO + 2HNO_3 \rightarrow Mg(NO_3)_2 + H_2O}$ ; (b) $\mathrm{SO_2 + H_2O \rightarrow H_2SO_3}$ .
$\mathrm{Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O}$ ; $\mathrm{Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2NaAl(OH)_4}$ .
Both are giant ionic, but $\mathrm{Mg^{2+}}$ has twice the charge of $\mathrm{Na^+}$ and is smaller, so the attraction between ions in the lattice is much stronger.
$\mathrm{SiO_2}$ is giant covalent: melting breaks strong Si–O bonds throughout the lattice. $\mathrm{SO_2}$ is simple molecular: melting only overcomes weak forces between molecules.
$M(\mathrm{Na_2O}) = 61.98$ ; $n = \frac{0.620}{61.98} = 0.0100\ \mathrm{mol}$ ; $n(\mathrm{OH^-}) = 0.0200\ \mathrm{mol}$ ; $[\mathrm{OH^-}] = \frac{0.0200}{0.250} = 0.0800\ \mathrm{mol\,dm^{-3}}$ .
$n(\mathrm{H_2SO_4}) = 0.500\ \mathrm{mol} = n(\mathrm{SO_3})$ ; $M(\mathrm{SO_3}) = 80.06$ ; mass $= 0.500 \times 80.06 = 40.0\ \mathrm{g}$ .
Sulfur dioxide: $\mathrm{SO_2 + H_2O \rightarrow H_2SO_3}$ . The sulfite formed is a reducing agent, so it reduces orange dichromate(VI) to green chromium(III).

Quick check

An oxide of Period 3 is insoluble in water but dissolves in both dilute hydrochloric acid and sodium hydroxide solution. Which is it?

Key points

Across Period 3 the oxides change from giant ionic ( $\mathrm{Na_2O}$ , MgO, $\mathrm{Al_2O_3}$ ) through giant covalent ( $\mathrm{SiO_2}$ ) to simple molecular ( $\mathrm{P_4O_{10}}$ , $\mathrm{SO_2}$ , $\mathrm{SO_3}$ , $\mathrm{Cl_2O_7}$ ), following the electronegativity of the element.
They change from basic ( $\mathrm{Na_2O}$ , MgO) to amphoteric ( $\mathrm{Al_2O_3}$ ) to acidic ( $\mathrm{SiO_2}$ onwards).
Ionic oxides contain $\mathrm{O^{2-}}$ , a strong base; molecular oxides react with water to give oxoacids.
Giant-structure oxides melt above 1000 °C; molecular oxides melt low.

Sources and credits

Image: Quartz, Tibet.jpg by JJ Harrison, via Wikimedia Commons, licensed under CC BY-SA 2.5.
Image: Magnesium oxide.jpg by Walkerma, via Wikimedia Commons, public domain.
Image: Schwefeltrioxid Ampulle.jpg by H. Hiller, via Wikimedia Commons, public domain.
Melting points from the Wikipedia articles Sodium oxide, Magnesium oxide, Aluminium oxide, Silicon dioxide, Phosphorus pentoxide, Sulfur trioxide, Sulfur dioxide and Dichlorine heptoxide, Wikipedia contributors, licensed under CC BY-SA 4.0.
Video: 3.2 Bonding of the period 3 oxides, acid/base character, metallic character (SL), Mike Sugiyama Jones (MSJ Chem), embedded from YouTube.
The text, table, chart, worked examples and Practice items were written and drawn by Ganzaa.
This lesson uses CC BY-SA material and is shared under CC BY-SA 4.0.

1From antacid to acid rain

2Bonding and structure

3Acid–base behaviour

From antacid to acid rain

Bonding and structure

Acid–base behaviour