Oxides Of Group 14 - Carbon & Silicone

Question 1

What are the enthalpies of formation of carbon monoxide and carbon dioxide?

Answer 1

∆fH˚(CO) = -110.5 KJ/mol
- when C is burned in limited oxygen

∆fH˚(CO2) = -393.5 KJ/mol
- when C is burned is excess oxygen

Question 2

How do you calculate the enthalpy change of reaction?

Answer 2

∆rxnH˚ = ∑∆fH˚(products) - ∑∆fH˚(reactants)

Question 3

What are the properties of carbon monoxide?

Answer 3

Colourless, odourless gas

Lewis base - lone pairs on both C and O
- ligand for transition metals

Question 4

What’s the danger with CO?

Answer 4

Highly toxic

Binds 300x more strongly than O2 to haemoglobin

Question 5

How can CO be used in the chemical industry?

Answer 5

Used to form methanal with H2

Used to form propanal with ethene - OXO process

Question 6

What’re the properties of carbon dioxide?

Answer 6

Colourless, odourless gas

No liquid state at atmospheric pressure - sublimation
Liquid at 67 atm / 298K (room temp)

CO2 is a Lewis acid —> hydrolysed to form carbonic acid - weak Bronsted acid

Question 7

What’re the uses of CO2?

Answer 7

Solid CO2 used as a refrigerant: 50%

Carbonated drinks: 25%

Supercritical CO2 used to extract caffeine from tea/coffee - non-toxic solvent

Question 8

Why does C form 2 x C=O but Si forms 4 x Si-O?

Answer 8

Due to poor orbital overlap due to disparity in size between Si and O p orbitals.

Si-O is also a very polar bond, greater bond enthalpy than C-O

Question 9

Why is SiO2 closer to ionic on the Ketelaar triangle than CO2?

Answer 9

Because of the difference in electronegativity between Si and O

Hence more polar bond

Question 10

What structure can Si and O form?

Answer 10

ß quartz - polymeric solid with Si-O single bonds

Cristobalite

Question 11

Describe the structure of Critobalite

Answer 11

Si O compound similar to diamond structure

Very high MP = 1705˚C

Question 12

What’re the special properties of quartz?

Answer 12

Quartz crystals are chiral

They are also piezoelectric: charge separation on application of pressure
- reversible process

Question 13

What is ‘water glass’?

Answer 13

A salt that is made from glass but can dissolve in water

Question 14

What’s the significance of the orthosilicate [SiO4}4- ion?

Answer 14

Highly charged anion - condenses to form a species with lower charge per Si atom

Question 15

What happens when orhtosilicate is expensed to acid?

Answer 15

Orthosilicate ions dimerise to form pyrosilicate and water

Now has a lower charge per Si atom

Question 16

What happens when orthosilicate is exposed to additional acid?

Answer 16

It forms cyclic metasilicates

2 -ve charges per Si atom

Can from infinite chains, sheets or ribbons by condensation reactions

Question 17

What structure do silicate minerals have?

Answer 17

Layered structure of a sheet of silicate - repeat unit = [Si2O5]2-

Question 18

How is talc different to white asbestos chemically?

Answer 18

White asbestos is a hard substance with ABABA structure sequence

Talc is a very soft substance with ABAABA structure sequence
- there is very little interaction between layers in talc

Question 19

What are zeolites?

Answer 19

They are derived from SiO2 structures by replacing Si(IV) with Al(lll)

Results in excess -ve charge which must be neutralised by binding an appropriate number of other cations.

Question 20

Give an example of a zeolite

Answer 20

Zeolite A - Faujasite

Based on sodalite Si24O48 - replace 12 Si atoms with Al
[Si12Al12O48]12-

Question 21

What shape is a sodalite cage?

Answer 21

Truncated cubotahedron

Square faces join to form an extended porous framework

Forms supercages and cubic cages between sodalite cages

Question 22

What are the applications of zeolites?

Answer 22

Involved in ion exchanges - water softener in detergents, can be selectively removed

Drying agent - H2O strongly bound inside super cages, used for drying gases / solvents

Question 23

What’s the importance of size selective catalysis by zeolites?

Answer 23

Diameter of octagonal face of cage = 4.2Å only molecules smaller can enter

Compounds can undergo isomerisation so that they are able to fit into cage

Catalyst is a powerful Bronsted acid

Question 24

What’s the importance of silicones (polysiloxanes)?

Answer 24

Si=O cannot exist because of poor overital overlap so long polymer chains form

Question 25

What are the applications of silicones?

Answer 25

Silicone oils: high thermal stability - hydraulic fluids

Silicone elastomers: silicone rubber boots used in moon missions

O2-porous silicone elastomer: soft contact lenses

Dimethicone: conditioners and other care products

‘Silly putty’