Reactions of the Group 13 Elements – Trends in Chemical Properties of Group 13 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

Trends in Chemical Properties of Group 13 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Group 13 Elements and Their Compounds

4 lessons

Lesson 2 of 4

Reactions of the Group 13 Elements

25 min

A reactive metal that does not corrode

Aluminium sits high in the reactivity series, above zinc and iron, yet aluminium window frames, saucepans and roofing sheets stay bright for years while iron sheets rust. A freshly cut aluminium surface reacts with air in a fraction of a second, but the product is a tough, transparent film of aluminium oxide only a few nanometres thick that seals the metal underneath. Remove that film and aluminium shows its true reactivity: it reacts with water, burns brightly as a powder, and can pull oxygen out of iron oxide so violently that the iron produced is molten. This lesson works through the reactions of the Group 13 elements with oxygen, water, acids, alkalis and the halogens, and asks why boron behaves like a non-metal in each of them.

Definition

Passivation: the protection of a metal by a thin, unreactive surface layer, usually an oxide, that stops further reaction. Example: aluminium is passivated by its $\mathrm{Al_2O_3}$ film, and concentrated nitric acid thickens this film, which is why concentrated nitric acid can be carried in aluminium tanks.

Electron-deficient compound: a covalent compound in which an atom has fewer than eight outer electrons. Example: in $\mathrm{BCl_3}$ and monomeric $\mathrm{AlCl_3}$ the central atom has only six outer electrons and an empty p orbital, so it accepts a lone pair from another molecule. Such an electron-pair acceptor is a Lewis acid.

Reaction with oxygen

All the Group 13 elements form oxides when heated in oxygen. Boron burns when strongly heated, aluminium powder burns with a dazzling white flame, and gallium and indium form their oxides when heated:

\mathrm{4B(s) + 3O_2(g) \rightarrow 2B_2O_3(s)} \qquad \mathrm{4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)}

Thallium shows the +1 state that becomes important at the bottom of the group: in air it forms mainly thallium(I) oxide, $\mathrm{Tl_2O}$ . The oxides change from acidic ( $\mathrm{B_2O_3}$ ) through amphoteric ( $\mathrm{Al_2O_3}$ , $\mathrm{Ga_2O_3}$ ) to basic ( $\mathrm{In_2O_3}$ , $\mathrm{Tl_2O}$ ), the topic of Lesson 3.

Reaction with water

Boron does not react with water or steam. Aluminium would react with water, but its oxide film stops it. If the film is broken, for example by rubbing the metal with a little mercury(II) chloride solution, the aluminium reacts steadily with cold water:

\mathrm{2Al(s) + 6H_2O(l) \rightarrow 2Al(OH)_3(s) + 3H_2(g)}

Gallium and indium are also protected by oxide films and do not react with water at room temperature.

Reaction with acids

Aluminium, gallium and indium dissolve in dilute hydrochloric and sulphuric acids and give hydrogen. With aluminium the reaction starts slowly while the acid removes the oxide film, then speeds up and gets hot:

\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)}

\mathrm{2Al(s) + 3H_2SO_4(aq) \rightarrow Al_2(SO_4)_3(aq) + 3H_2(g)}

Thallium dissolves to give $\mathrm{Tl^{+}}$ , not $\mathrm{Tl^{3+}}$ . Boron, like a non-metal, is not attacked by non-oxidising acids; hot concentrated nitric acid oxidises it to boric acid, $\mathrm{H_3BO_3}$ . Concentrated nitric acid makes aluminium passive instead of dissolving it.

Reaction with alkalis

Here aluminium behaves like a non-metal. It dissolves in hot sodium hydroxide solution, giving hydrogen and the aluminate ion, $\mathrm{[Al(OH)_4]^-}$ (tetrahydroxoaluminate). Gallium does the same; indium and thallium do not.

\mathrm{2Al(s) + 2NaOH(aq) + 6H_2O(l) \rightarrow 2Na[Al(OH)_4](aq) + 3H_2(g)}

Boron reacts only with fused (molten) alkali, forming a borate and hydrogen. Any Group 13 metal that dissolves in both acids and alkalis is behaving amphoterically.

Reaction with the halogens

All the elements combine directly with the halogens. Heated aluminium burns in a stream of dry chlorine:

\mathrm{2Al(s) + 3Cl_2(g) \rightarrow Al_2Cl_6(s)}

Anhydrous aluminium chloride is not a typical ionic chloride. It sublimes (turns straight from solid to vapour) at 180 °C, and in the vapour and the melt it exists as $\mathrm{Al_2Cl_6}$ dimers (Figure 8.3). Aluminium fluoride, in contrast, is ionic and does not sublime until about 1290 °C. The reason is the size and charge of $\mathrm{Al^{3+}}$ : its charge density, $\frac{3}{53.5\ \mathrm{pm}} = 0.056$ , is nearly six times that of $\mathrm{Na^+}$ , $\frac{1}{102\ \mathrm{pm}} = 0.0098$ . The small, highly charged ion polarises the large, soft chloride ion, pulling its electrons back into the space between the nuclei, so the Al–Cl bond is largely covalent. The small fluoride ion is hard to polarise, so AlF₃ stays ionic.

In a single $\mathrm{AlCl_3}$ molecule, aluminium has only six outer electrons. Two molecules complete each other's octets: a lone pair on a chlorine atom of one molecule forms a dative (coordinate) bond to the aluminium of the other.

Figure 8.3: The $\mathrm{Al_2Cl_6}$ dimer. Arrows show the dative bonds. At high temperature the dimer splits into trigonal planar $\mathrm{AlCl_3}$ molecules, which have the same shape as $\mathrm{BCl_3}$ . Boron trichloride stays a monomer (a gas boiling at 12.6 °C), because the boron atom is too small to fit four large chlorine atoms around it.

Thallium again prefers +1: it forms the poorly soluble white thallium(I) chloride, TlCl.

Reaction with nitrogen and with metal oxides

At high temperature boron and aluminium combine with nitrogen to form nitrides, $\mathrm{2B + N_2 \rightarrow 2BN}$ and $\mathrm{2Al + N_2 \rightarrow 2AlN}$ . Aluminium also removes oxygen from the oxides of less reactive metals. The thermite reaction is so exothermic that the iron formed is molten; it is used to weld railway lines:

\mathrm{2Al(s) + Fe_2O_3(s) \rightarrow Al_2O_3(s) + 2Fe(l)}

Table 8.3: Reactions of the Group 13 elements

Reagent	Boron	Aluminium	Gallium and indium	Thallium
Oxygen, heated	$\mathrm{B_2O_3}$ , acidic	$\mathrm{Al_2O_3}$ , amphoteric	$\mathrm{M_2O_3}$	mainly $\mathrm{Tl_2O}$ , basic
Water	no reaction	none (oxide film); $\mathrm{Al(OH)_3 + H_2}$ if the film is broken	no reaction	reacts slowly, forming TlOH
Dilute HCl	no reaction	$\mathrm{AlCl_3 + H_2}$	$\mathrm{MCl_3 + H_2}$	TlCl + $\mathrm{H_2}$ (slow; TlCl coats the metal)
NaOH(aq)	only when fused	$\mathrm{[Al(OH)_4]^- + H_2}$	Ga: $\mathrm{[Ga(OH)_4]^- + H_2}$ ; In: no reaction	no reaction
Chlorine, heated	$\mathrm{BCl_3}$ , covalent gas	$\mathrm{Al_2Cl_6}$ , covalent dimer	$\mathrm{Ga_2Cl_6}$ ; $\mathrm{InCl_3}$	TlCl

Worked example 1: hydrogen from aluminium and alkali

0.540 g of aluminium foil reacts completely with excess hot sodium hydroxide solution. Calculate the volume of hydrogen formed at room temperature and pressure (molar volume $24.0\ \mathrm{dm^3\,mol^{-1}}$ ). $A_r(\mathrm{Al}) = 26.98$ .

Solution. From the equation, 2 mol Al give 3 mol $\mathrm{H_2}$ . $n(\mathrm{Al}) = \frac{0.540\ \mathrm{g}}{26.98\ \mathrm{g\,mol^{-1}}} = 0.02001\ \mathrm{mol}$ $n(\mathrm{H_2}) = 0.02001 \times \frac{3}{2} = 0.03002\ \mathrm{mol}$ $V = 0.03002\ \mathrm{mol} \times 24.0\ \mathrm{dm^3\,mol^{-1}} = 0.721\ \mathrm{dm^3} = 721\ \mathrm{cm^3}$ The same mass of aluminium would give the same volume with excess hydrochloric acid, since 2 mol Al again give 3 mol $\mathrm{H_2}$ .

Worked example 2: the energy of the thermite reaction

Use the standard enthalpies of formation $\Delta_f H^\circ(\mathrm{Al_2O_3, s}) = -1675.7\ \mathrm{kJ\,mol^{-1}}$ and $\Delta_f H^\circ(\mathrm{Fe_2O_3, s}) = -824.2\ \mathrm{kJ\,mol^{-1}}$ to find $\Delta H$ for $\mathrm{2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe}$ . Then calculate the heat released, and the mass of iron and of aluminium involved, when 1.00 kg of iron(III) oxide reacts. $M(\mathrm{Fe_2O_3}) = 159.7\ \mathrm{g\,mol^{-1}}$ ; $A_{r}$ : Fe 55.85, Al 26.98.

Solution. Elements have $\Delta_f H^\circ = 0$ , so $\Delta H = \Delta_f H^\circ(\mathrm{Al_2O_3}) - \Delta_f H^\circ(\mathrm{Fe_2O_3}) = -1675.7 - (-824.2) = -851.5\ \mathrm{kJ\,mol^{-1}}$ $n(\mathrm{Fe_2O_3}) = \frac{1000\ \mathrm{g}}{159.7\ \mathrm{g\,mol^{-1}}} = 6.262\ \mathrm{mol}$ Heat released $= 6.262 \times 851.5 = 5332\ \mathrm{kJ} \approx 5.33\ \mathrm{MJ}$ .
Iron formed: $2 \times 6.262 \times 55.85 = 699\ \mathrm{g}$ . Aluminium needed: $2 \times 6.262 \times 26.98 = 338\ \mathrm{g}$ .
The reaction is so exothermic because aluminium bonds to oxygen far more strongly than iron does.

Warning

Common mistakes. (1) Saying aluminium is unreactive because it does not corrode: it is very reactive, and the oxide film hides this. (2) Writing $\mathrm{Al + NaOH \rightarrow}$ " $\mathrm{NaAl + H_2O}$ ": the products are hydrogen and the aluminate ion, $\mathrm{[Al(OH)_4]^-}$ . (3) Writing $\mathrm{TlCl_3}$ as the product of thallium and chlorine: thallium gives TlCl. (4) Calling $\mathrm{AlCl_3}$ ionic because aluminium is a metal: the anhydrous chloride is covalent and sublimes at 180 °C.

Safety. Anhydrous aluminium chloride reacts violently with water and releases hydrogen chloride gas: handle it dry, in a fume cupboard, with eye protection and gloves. Hydrogen from aluminium and alkali forms explosive mixtures with air, so keep flames away. Hot sodium hydroxide solution is corrosive to skin and eyes; wear goggles, and if any splashes on the skin or in the eyes, rinse at once with plenty of running water and get medical help. The thermite reaction throws out molten iron and is only demonstrated by a trained teacher, outdoors, behind a safety screen.

Tip

To predict a Group 13 reaction, ask three questions: Is the oxide film in the way? (aluminium) Is the element a non-metal at heart? (boron: no reaction with water or dilute acids, covalent halide) Is the +1 state taking over? (thallium: TlCl, $\mathrm{Tl_2O}$ ).

Royal Society of Chemistry: Thermite in a flowerpot (5 min). Watch how a little magnesium ribbon is needed to start the reaction, and how much light, heat and molten iron one small charge of aluminium and iron oxide produces. Note the safety precautions the demonstrator takes.

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Practice

Write balanced equations, with state symbols, for (a) aluminium burning in oxygen; (b) aluminium reacting with dilute sulphuric acid; (c) gallium reacting with dilute hydrochloric acid.
Explain why a piece of aluminium foil dropped into dilute hydrochloric acid reacts slowly at first and then much faster.
Write the equation for aluminium dissolving in sodium hydroxide solution, and name the aluminium-containing ion formed.
Calculate the volume of hydrogen, at room temperature and pressure, formed when 0.405 g of aluminium reacts with excess hydrochloric acid. $A_r(\mathrm{Al}) = 26.98$ ; molar volume 24.0 dm³ mol⁻¹.
Draw the structure of $\mathrm{Al_2Cl_6}$ and explain why the bonds drawn as arrows form.
Aluminium fluoride sublimes at about 1290 °C, but aluminium chloride sublimes at 180 °C. Explain this difference in terms of polarisation.
Chromium(III) oxide, $\mathrm{Cr_2O_3}$ , is reduced by aluminium in the same way as iron(III) oxide. Write the equation, and calculate the mass of aluminium needed to make 1.00 kg of chromium. $A_{r}$ : Cr 52.00, Al 26.98.

Show answersHide answers

(a) $\mathrm{4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)}$ ; (b) $\mathrm{2Al(s) + 3H_2SO_4(aq) \rightarrow Al_2(SO_4)_3(aq) + 3H_2(g)}$ ; (c) $\mathrm{2Ga(s) + 6HCl(aq) \rightarrow 2GaCl_3(aq) + 3H_2(g)}$ .
The acid must first dissolve the protective oxide film. Once bare metal is exposed the reaction is fast, and as it is exothermic the solution warms up, which speeds it further.
$\mathrm{2Al + 2NaOH + 6H_2O \rightarrow 2Na[Al(OH)_4] + 3H_2}$ ; the tetrahydroxoaluminate (aluminate) ion, $\mathrm{[Al(OH)_4]^-}$ .
$n(\mathrm{Al}) = \frac{0.405}{26.98} = 0.01501$ mol; $n(\mathrm{H_2}) = 0.02252$ mol; $V = 0.02252 \times 24.0 = 0.540\ \mathrm{dm^3} = 540\ \mathrm{cm^3}$ .
As Figure 8.3: two Al atoms bridged by two Cl atoms, each Al also holding two terminal Cl. In $\mathrm{AlCl_3}$ the aluminium has only six outer electrons and an empty orbital; a lone pair on a chlorine of the other molecule is donated into it, giving each Al an octet.
$\mathrm{Al^{3+}}$ is small and highly charged, so it strongly polarises (distorts) an anion's electron cloud. The large chloride ion is easily polarised, so electron density is pulled between the nuclei and the bonding is mainly covalent: simple molecules ( $\mathrm{Al_2Cl_6}$ ) with weak forces between them. The small fluoride ion is hardly polarised, so $\mathrm{AlF_3}$ is a giant ionic lattice that needs much more energy to break.
$\mathrm{2Al + Cr_2O_3 \rightarrow Al_2O_3 + 2Cr}$ : 1 mol Al per mol Cr. $n(\mathrm{Cr}) = \frac{1000}{52.00} = 19.23$ mol, so mass of Al $= 19.23 \times 26.98 = 519\ \mathrm{g}$ .

Quick check

Aluminium is above iron in the reactivity series. Why does an aluminium window frame not corrode away in moist air?

Key points

Aluminium is reactive but passivated by a thin oxide film; with the film removed it reacts with water. It dissolves in dilute acids and in alkalis, giving hydrogen and $\mathrm{Al^{3+}}$ or $\mathrm{[Al(OH)_4]^-}$ .
Boron behaves as a non-metal: no reaction with water or dilute acids; covalent, acidic $\mathrm{B_2O_3}$ and gaseous $\mathrm{BCl_3}$ .
Anhydrous aluminium chloride is covalent and exists as $\mathrm{Al_2Cl_6}$ dimers held by dative bonds, because $\mathrm{Al^{3+}}$ strongly polarises chloride ions; $\mathrm{BCl_3}$ and $\mathrm{AlCl_3}$ monomers are electron-deficient Lewis acids.
Thallium favours +1 (TlCl, $\mathrm{Tl_2O}$ ). Aluminium reduces metal oxides such as $\mathrm{Fe_2O_3}$ in the strongly exothermic thermite reaction ( $\Delta H = -851.5\ \mathrm{kJ\,mol^{-1}}$ ).

Sources and credits

Image: Anhydrous aluminium(III) chloride powder.jpg by Leiem, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Image: Thermite welding 07.jpg by PetrS., via Wikimedia Commons, licensed under CC BY-SA 3.0.
Data: sublimation temperatures from Wikipedia's Aluminium chloride and Aluminium fluoride articles; boiling point of $\mathrm{BCl_3}$ from Boron trichloride; enthalpies of formation from Aluminium oxide and Iron(III) oxide (CRC Handbook values); all Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.
First aid for chemical splashes: NIOSH Pocket Guide, first aid, CDC.
Video: Thermite in a flowerpot, Royal Society of Chemistry, embedded from YouTube.
The text, structure diagram, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A reactive metal that does not corrode

2Reaction with oxygen

3Reaction with water

4Reaction with acids

5Reaction with alkalis

6Reaction with the halogens

7Reaction with nitrogen and with metal oxides