Carbon and silicon dioxides | Dioxides of Ge, Sn, Pb  |  Oxides of Ge, Sn, Pb  |  Pb3O4

 

  The dioxides of carbon and silicon are acidic; the dioxides and oxides of the remainder of group 4 are amphoteric.

  1. The oxides of carbon and of silicon; acidic oxides.

CO is often regarded as a neutral oxide. It does not react with water to give the predicted HCO2H or HCOOH, methanoic acid, though it will react with hot concentrated sodium hydroxide solution under pressure to give a solution of methanoate ions. It is really very weakly acidic.

 

 

 

CO (g)  +  OH – (aq)

ŕ

HCOO – (aq)

 

 

 

Carbon dioxide is acidic, though the molecule H2CO3 is only partially formed in aqueous solution. The reaction between water and carbon dioxide is very slow, and a significant amount of CO2(aq) is present:

 

 

 

CO2(aq)  +  H2O (l)

ŕ

H+ (aq)  +  HCO3 – (aq)

 

 

 

Silicon dioxide is acidic, but is so water-insoluble (owing to its giant covalent structure) that it reacts only with hot concentrated sodium hydroxide solution:

 

 

 

SiO2 (s)  +  2OH – (aq)

ŕ

SiO3 2 – (aq) +  H2O (l)

 

 

 

The reaction of silica with calcium oxide in the blast furnace is an acid-base reaction of the Lewis type:

 

500oC

 

SiO2 (s)  +  CaO (s)

ŕ

CaSiO3 (l)

 

 

   slag

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2. The oxides of germanium, tin and lead; amphoteric oxides.

  The Edexcel specification seems to include the oxides of germanium, wholly unimportant, and exclude those of tin which are perhaps more important. Fortunately they behave more or less similarly for all three elements, though the balance between acidic and basic properties changes with each element.

An important note.

The solutions of substances described as ‘germanates’, ‘stannates’ or ‘plumbates’, that is the products from the reaction of the oxides with alkali, are often not very well-characterised. Formulae for them were suggested many decades ago, but little work has been done on them and in any case their composition may well be variable depending on how they are made. Thus plumbates(II), produced from the reaction of PbO with alkali, may be represented [Pb(OH)6]4 – , HPbO2  – , PbO2 2 – , or PbO3 4 – . What teachers say they ‘are’ is usually related to what they were taught or to what their favourite textbook says. Any recognised representation is acceptable in an examination.

(a) The dioxides MO2. The acidic character of the dioxides decreases Ge < Sn < Pb.

 

heat

 

GeO2 (s)  +  2OH – (aq)  +  2H2O (l)

ŕ

[Ge(OH)6] 2 – (aq)

 

 

 

The corresponding reaction for tin(IV) oxide requires concentrated alkali solution:

 

 

 

 

heat

 

SnO2 (s)  +  2OH – (aq)  +  2H2O (l)

ŕ

[Sn(OH)6] 2 – (aq)

 

 

 

The reaction with lead(IV) oxide requires molten alkali and gives a somewhat different product:

 

 

 

PbO2 (s)  +  2NaOH(l) 

ŕ

Na2PbO3 (s)  +  H2O (g)

 
The basic character of the dioxides is illustrated by their reaction with concentrated HCl. The use of concentrated acid suppresses the hydrolysis of the chloride produced.

GeO2 (s)  +  4HCl(aq)

ŕ

GeCl4 (aq)  +  2H2O(l)

 

 

 

SnO2 (s)  +  4HCl(aq)

ŕ

SnCl4 (aq)  +  2H2O(l)

 

 

 

 

<0oC

 

PbO2 (s)  +  4HCl(aq)

ŕ

PbCl4 (l)  +  2H2O(l)

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(b) The oxides MO. Their acidic properties are shown by the reactions with aqueous alkali:

GeO (s)  +  2OH – (aq)

ŕ

GeO2 2 – (aq)  +  H2O (l)

 

 

 

SnO (s)  +  2OH – (aq)

ŕ

SnO2 2 – (aq)  +  H2O (l)

 

 

 

PbO (s)  +  2OH – (aq)

ŕ

PbO2 2 – (aq)  +  H2O (l)

  Their basic properties are shown in the reaction with concentrated HCl: apart from the case of PbO, this is shown simply as H+ (aq).

GeO (s)  +  2H+(aq)

ŕ

GeCl2(aq)  +  H2O (l)

 

 

 

SnO (s)  +  2H+(aq)

ŕ

SnCl2(aq)  +  H2O (l)

 

 

 

Lead(II) chloride is insoluble in water; in the presence of concentrated HCl it forms soluble complexes:

 

 

 

PbO (s)  +  2HCl(aq)

ŕ

PbCl2(s)  +  H2O (l)

 

 

 

PbCl2(s)  +  Cl – (aq)

ŕ

PbCl3 – (aq)

PbCl2(s)  +  2Cl – (aq)

ŕ

PbCl4 2 – (aq)

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(c) Trilead tetroxide, Pb3O4. This beautiful scarlet powder is a mixed oxide, containing both lead(II) and lead(IV) ions. It behaves as PbO2.2PbO.

  With dilute nitric acid, the PbO part reacts as a base to give lead(II) nitrate; PbO2 remains, since it is not basic enough to react with nitric acid under these conditions:

PbO2.2PbO (s)  +  4HNO3 (aq)

ŕ

PbO2 (s)  + 2Pb(NO3)2 (aq)  +  2H2O(l)

Its reaction with alkali is unimportant.

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