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Variation of properties across Period 3

A table is given below showing the formulae of the products of the reactions of the elements with oxygen, chlorine and water:

Oxygen

reaction reaction HOCl reaction

Notes: ( I ) with excess oxygen sodium peroxide is formed, cz> aluminium chloride is covalent, cJ> with excess chlorine, c4> with steam.

The hyd roxides become less basic and more acidic as the su rface charge density of the cation increases.

Sil icon dioxide is such a weak acid that molten sodium hyd roxide is req uired.

The oxides of the non-metals form increasingly strong acids as the n u m ber of oxygen atoms in the acid

Si02 weakly acidic: Si02(s) + 2NaOH (l) ---+ Na2Si03(l) + H20(g) P406 weakly acidic: P406 + 6H20 ---+ 4 H3P03 � 4H+(aq) + 4H2P03-P4010 strongly acidic: P4010 + 6H20 ---+ 4H3P04 -+ 4H.(aq) +

4H2P04-Ionic chlorides dissolve in water to form hyd rated ions.

These hyd rated ions may deprotonate, if the su rface charge density of the metal ion is high.

Covalent chlorides (of Period 3) react with water to form HCI and either the oxide or the oxoacid.

The trend in metallic character is caused by the decrease in ionisation energy as the rad ius of the atom increases, making it energetically favourable for the formation of ionic bonds.

Chlorides

weakly acidic: S02 + H20 - H2S03 � H•(aq) + HS03-strongly acidic: S03 + H20 - H2S04 - H•(aq) +

HS04-NaCl ionic solid, dissolves in water:

NaCl(s) + aq - Na·(aq) + Cr(aq) MgC12 ionic solid, dissolves in water:

MgC12(s) + aq - Mg2•(aq)

+

• Hydrated A1Cl3 ionic solid, deprotonated by water:

[Al(H20)b]3.(aq) + H20 - [Al(H20)5(0H)]2•(aq) + H30.(aq) Anhydrous A1Cl3 co alent solid, reacts with water:

A1Cl3(s) + 3 H20(l) - Al(OH)ls) + 3HCl(aq) SiCl� covalent liquid, reacts with water:

SiClJl) + 2H20(l) - Si02(s) + 4HCl(aq) PCll covalent liquid, react '\.\'ith water:

PCl l) + 3H: (l) - H P03(aq) + 3HCl(aq) PCl5 co alent olid, reacts with water:

PCls( ) + -1H20(1) - H PO-tCaq) + SHCl(aq)

Variation of properties down Group 4 The metallic character increases down the Group:

carbon and silicon are non-metallic (not malleable; only graphite conducts electricity)

germanium is semi-metallic (semi-conductor).

tin and lead are metallic (malleable and electrical conductors).

The +2 oxidation state becomes more stable relative to the +4 state. Lead(IV) is oxidising and is itself reduced to lead(II), whereas tin(II) is reducing and is itself oxidised to tin(IV). This is illustrated by:

lead(IV) oxide oxidises concentrated hydrochloric acid to chlorine:

Pb02 + 4HCl - PbC12 + Cl2 + H20 tin(II) ions are oxidised to tin(IV) ions by chlorine:

Sn2• + Cl2 - Sn-l• + zcr

Tetrachlorides. Both CC14 and SiCl4 are covalent and are tetrahedral molecules owing to the repulsion of the four bond pairs of electrons.

SiCl4 is rapidly hydrolysed by water. A lone pair of electrons from the oxygen in the water forms a dative bond into an empty 3d orbital in the silicon atom. The energy released is enough to overcome the activation energy barrier involved in the breaking of the Si-Cl bond.

Carbon's bonding electrons are in the 2nd shell and there are no 2d orbitals. The empty orbitals in the 3rd shell are of too high an energy to be used in bonding, and equally important is that carbon is such a small atom that water molecules are prevented from reaching it by the four much larger chlorine atoms arranged tetrahedrally around it.

Acidity of the oxides. This decreases down the Group:

Carbon dioxide is weakly acidic, and reacts with metal oxides and alkalis:

C02(g) + 20H-(aq) - C032-(aq) + H20(1)

Silicon dioxide is very weakly acidic, and reacts with molten sodium hydroxide:

Si02(s) + 2Na0H(l) - Na2Si03(1) + H20(g)

Lead oxides (and hydroxide) are amphoteric, and react with aqueous acids and alkalis, but lead(II) oxide is the strongest base of the Group 4 oxides:

PbO(s) + 2H.(aq) - Pb2\aq) + H20(1) PbO(s) + 20H-(aq) + H20 - Pb(OH)t(aq)

66 P E R I O D I C I TY, Q U A N T I TAT I V E E Q U I L I B R I A A N D F U N C T I O N A L G R O U P C H E M I S T R Y

P b O a n d Pb02 PbO

The answers to the numbered q uestions are on pages 1 32-1 33.

® ^Checklist

Before attempting the questions on this topic, check that, for the Period 3 elements, you:

Can write equations for the reactions of the elements with oxygen and with chlorine.

Can write equations for the reaction of the elements with water.

Can state and explain the acid/base nature of the metal hydroxides.

Can state and explain the acid/base nature of the non-metal oxides.

Know the formula of their chlorides and their reactions with water, and can relate the reactions to their bonding.

Before attempting the questions on this topic, check that, for the Group 4 elements, you:

u Know why the elements become more metallic down the Group.

Know that the +2 oxidation state becomes more stable down the Group.

Know why SiC14 is rapidly hydrolysed by water, whereas CCl4 is unreactive with water.

Can write equations to show the acid/base character of the oxides of carbon, silicon and lead.

:;g:l Test ing your knowledge and underst anding

For the following questions, cover the margin, write your answer, then check to see if you are correct.

State the formulae of lead(II) oxide and lead(IV) oxide.

State the formula of the most basic Group 4 oxide.

1 a Write equations for the changes caused by the addition of water to:

i sodium chloride ii silicon tetrachloride

111 phosphorus pentachloride.

b Relate these reactions to the bonding in the chlorides.

c Explain why silicon tetrachloride, SiCl4, reacts rapidly with water, but tetrachloromethane, CCl4, does not react even when heated with water to 1 00 °C.

2 Write two ionic equations to show that aluminium hydroxide is amphoteric.

3 Write ionic equations for the reactions, if any, of:

a aqueous acid with:

i magnesium hydroxide ii lead monoxide

111 sulphur dioxide b aqueous alkali with:

i magnesium hydroxide ii lead monoxide

111 sulphur dioxide.

4 a Write an equation to show lead(IV) oxide acting as oxidising agent.

b Write an equation to show aqueous tin(II) ions acting as a reducing agent.

The val ue of I( is specific to a reaction - and to the eq uation used to represent that reaction - and can only be altered by changing the temperature.