09b p-block (group iv) - hkedcity.net€¢ outermost electronic configuration: ns2 np2 group iv...

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p-BLOCK ELEMENTS

(Part II)

Content

• Group IV elements– variation in m.p. / b.p.– dissimilarity in properties

• oxides• chlorides

• Silicon & silicates

[essay: 96-1-6]

Group IV Elements• The entire family consists of

– the elements• Carbon, C• Silicon, Si• Germanium, Ge• Tin, Sn• Lead, Pb

– their compounds (oxidation state = +4/+2)• oxide• chloride

• Outermost electronic configuration: ns2 np2

Group IV Elements• Group IV exhibit the most contrasting properties

among the same group, best illustrated by the– increasing metallic character down the group

• C (non-metal � graphite as conductor; diamond as insulator)• Si, Ge (metalloid � semi-conducting)• Sn, Pb (metals � conductor)

– variation in m.p. / b.p.

b.p.

m.p.

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Variation in m.p. / b.p.

• Interpretation of variation in m.p. / b.p. of elements in terms of structure & bonding– C / Si / Ge : giant covalent structure

• melting involves breaking a large number of strong X-X covalent bonds between X atoms

• X-X bond strength decreases down the group– atomic size of X increases

– Sn / Pb : metallic structure• melting involves breaking a smaller proportion of metallic

bonds between X2+ ions and delocalized electrons

– 02-2-2c(i), 93-2-4b(i), 03-AS-7b/c, 06-AS-5a

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• structure of Si– sp3-hybridized / co-ordination no. = 4 /

tetrahedral arrangement– cubic unit cell similar to ZnS

Carbon Allotropes

• allotrope: same element with different structures / forms / properties

• 06-1-2a, 98-2-2a, 00-1-1a, 03-AS-7

diamond graphite

Carbon Allotropes

• both have high m.p. / b.p.– giant covalent network with millions of strong

C-C bonds between C atoms– graphite (3700°C) has higher m.p. than

diamond (3550°C)• C atoms in graphite & diamond are sp2- & sp3-

hybridized respectively– C-C bond in graphite has multiple bond character owing

to the sideway overlap of unhybridized p-orbitals– more s-character � e- being held more tightly

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Carbon Allotropes

• contrasting properties– hardness

• strong & directional C-C bonds in diamond restrictsthe relative motion between C atoms

• weak van der Waals’ forces between layers of graphite allow the layers to slip over each other

– electrical conductivity• all electrons in diamond are localized• each C atom has a delocalized p-electron to

conduct e- along the same layer

Group IV Oxides

• Formulae of Group IV oxides– C : CO, CO2

– Si : SiO2 only– Ge / Sn / Pb : both XO2 (O.S. +4) & XO (O.S. +2)

Group IV Oxides

• Dissimilarity in properties as illustrated by the following trends down the group :– increasing ionic character

• e.g. variation in m.p. / b.p.

– increasing basic nature• e.g. rxn with H2O (hydrolysis)

– increasing stability of O.S. = +2• e.g. thermal stability of divalent / tetravalent oxides

Variation in m.p. / b.p.

• increasing ionic character (or decreasing covalent character) as illustrated by the variation in m.p./ b.p.– CO / CO2 : simple molecule– SiO2 / GeO2 : giant covalent network– SnO2 / PbO2 : ionic structure

• [02-1-3c, 04-1-7a(ii), 04-AS-6, 95-AS-12b]

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Reaction with Water

• increasing basic nature as illustrated by rxn with H2O (hydrolysis)– CO : neutral � no rxn– CO2 : acidic � react with water & alkali

CO2 + H2O

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2 H+ + CO32-

CO2 + Ca(OH)2

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CaCO3 + H2O (turns limewater milky)

CaCO3 + CO2 + H2O

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Ca(HCO3)2 (then clear again)

– SiO2 : acidic � react with alkaliSiO2 + 2 NaOH → Na2SiO3 + H2O

• 96-1-4a(ii), 91-2-5a, 95-AS-12b(i), 04-AS-2d

Reaction with Water

• increasing basic nature as illustrated by rxn with H2O (hydrolysis)– SnO / PbO : amphoteric � react with alkali & acid

SnO + 2 OH- + H2O → [Sn(OH)4]2-

SnO + 2 HCl → SnCl2 + H2O

PbO + 2 OH- + H2O → [Pb(OH)4]2-

PbO + 2 HNO3 → Pb(NO3)2 + H2O

• 91-2-5a, 92-2-5b(iii), 94-2-6d(i), 97-AS-3c

Reaction with Water• hydrolysis for basic oxides

Na2O + H2O → 2 Na+ + 2 OH-

Na2O2 + 2 H2O → 2 NaOH + H2O2MgO + H2O

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Mg2+ + 2 OH-

• hydrolysis for acidic oxides SO2 + H2O → 2 H+ + SO3

2-

2 NO2 + H2O → HNO2 + HNO3F2O + H2O → 2 HF + O2

Cl2O + H2O → 2 HOClCl2O7 + H2O → 2 HClO4

• hydrolysis for amphoteric oxidesBeO + 2 H+ → Be2+ + H2O / BeO + 2 OH- + H2O → [Be(OH)4]2-

Al2O3 + 6 H+ → 2 Al3+ + 3 H2O / Al2O3 + 2 OH- + 3 H2O → 2 [Al(OH)4]-

• 01-2-3c, 07-2-4a(i), 08-2-1b

Thermal Stability

• increasing thermal stability of divalent (O.S.+2) or decreasing stability of tetravalent (O.S.+4) down the group

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Thermal Stability

• example 1– PbO2 decomposes on heating to give PbO,

whereas PbO does not :2 PbO2(s) → 2 PbO(s) + O2(g)

• observation :– colorless gas relighting glowing splint– solid turns brown when hot & yellow when cold

• 05-AS-9c, 98-AS-9b, 97-AS-3b, 95-AS-12b(iii)

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Thermal Stability

• example 2– redox strength :

• Sn2+ is a reducing agentSn2+ → Sn4+ + 2e-

• PbO2 is an oxidizing agentPbO2 + 2e- → Pb2+

• 05-AS-9c, 93-1-1d, 91-1-3g, 98-AS-9b, 94-2-6e, 92-2-5b

Thermal Stability

• explanation– bond enthalpy

• M-Cl bond becomes weaker down the group

� MCl4 less stable than MCl2– inert pair effect

• increasing stability of the n s2 electron pair to remain un-removed down the group

� O.S.+2 (n s2) more stable than O.S.+4 (n s2 n p2 )

• 02-2-5b(v), 06-2-4d, 98-AS-9a, 95-AS-12b(iii)

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Group IV Chlorides

• all XCl4 are covalent molecules with tetrahedral shape, whereas XCl2 are ionic compounds with covalent character [96-2-4d]

• increasing stability of X2+ down the group– only Sn & Pb forms XCl2– explanation: inert-pair effect– 92-2-5b(vi)

Group IV Chlorides

• All Group IV tetrachlorides (except CCl4) undergo hydrolysis

XCl4 + 4 H2O → X(OH)4 + 4 HCl– hydrolysis involves nucleophilic attack of H2O

molecules on the central atom of XCl4– X has vacant and low-lying d-orbitals to accept lone-

pair electrons from O atom in H2O– C has no vacant & low-lying 2d-orbitals for H2O

molecules to attack– 06-2-1b(ii), 01-AS-5c, 97-AS-3a, 96-2-4d, 94-2-6d

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Silicon & Silicates

• Si : 2nd most abundant element in Earth’s crust (e.g. sand, rocks)

• Use of silicon– semi-conductor

• e.g. computer chips

– construction materials• e.g. concrete, bricks• manufacture of glass & ceramic pottery

– abrasive [04-AS-6c]

Bonding & Structures of Silicates

• all silicates are built from the basic tetrahedral structural unit (SiO4

4-) with a Si atom at the centre and O atoms at each of the 4 corners

SiO44- Si2O7

6-

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• the simplest form of silicates contain SiO44-

anions balanced by 2 M2+ cations• properties similar to most ionic cpds

Ca2SiO4 (orthosilicate)Sc2Si2O7 (pyrosilicate)


• complicated silicates are built up from simple tetrahedral units by 3 ways:

1) chain silicates• each Si atom shares 2 O atoms with Si atoms in adjacent

tetrahedra to form a long chain of silicates• 2 chains can link laterally into a ribbon (e.g. asbestos)• exist in fibres (

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weak van der Waals’ forces % strands)

tetrahedra join by corners only, never by edges / faces


2) sheet silicates• each Si atom shares 3 of its 4 O atoms with other

tetrahedra to form 2-dimensional layer (e.g. mica)


2) sheet silicates• mica layers easily flake off

– ionic attraction within sheets is stronger than between sheets

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3) network silicates• each Si atom shares all 4 O

atoms with other tetrahedrato form a 3-dimensional network of silicates (e.g. quartz)

• properties– crystalline solid– v. high m.p.– quite hard– non-conductor


3) network silicates• feldspar is a group of minerals

important in rock formation– account for 60% of Earth’s crust– result when some Si in the

network are replaced by Al (aluminosilicate), plus some alkali & alkaline earth ions for charge balance


Why silicates occur in a wide variety of structures ?• Si-O bond (466 kJ/mol) is very strong• SiO4

4- tetrahedral units can join together by sharing O atoms in a variety of ways to form chains, rings, sheets & 3-D silicates

• some of the Si(IV) inside silicates can be replaced by other metal ions (e.g. Al3+) to form a wide variety of silicate analogues

09b p-block (group iv) - hkedcity.net€¢ outermost electronic configuration: ns2 np2 group iv...

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