Oxides Of Group 14 - Carbon & Silicone Flashcards
What are the enthalpies of formation of carbon monoxide and carbon dioxide?
∆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
How do you calculate the enthalpy change of reaction?
∆rxnH˚ = ∑∆fH˚(products) - ∑∆fH˚(reactants)
What are the properties of carbon monoxide?
Colourless, odourless gas
Lewis base - lone pairs on both C and O
- ligand for transition metals
What’s the danger with CO?
Highly toxic
Binds 300x more strongly than O2 to haemoglobin
How can CO be used in the chemical industry?
Used to form methanal with H2
Used to form propanal with ethene - OXO process
What’re the properties of carbon dioxide?
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
What’re the uses of CO2?
Solid CO2 used as a refrigerant: 50%
Carbonated drinks: 25%
Supercritical CO2 used to extract caffeine from tea/coffee - non-toxic solvent
Why does C form 2 x C=O but Si forms 4 x Si-O?
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
Why is SiO2 closer to ionic on the Ketelaar triangle than CO2?
Because of the difference in electronegativity between Si and O
Hence more polar bond
What structure can Si and O form?
ß quartz - polymeric solid with Si-O single bonds
Cristobalite
Describe the structure of Critobalite
Si O compound similar to diamond structure
Very high MP = 1705˚C
What’re the special properties of quartz?
Quartz crystals are chiral
They are also piezoelectric: charge separation on application of pressure
- reversible process
What is ‘water glass’?
A salt that is made from glass but can dissolve in water
What’s the significance of the orthosilicate [SiO4}4- ion?
Highly charged anion - condenses to form a species with lower charge per Si atom
What happens when orhtosilicate is expensed to acid?
Orthosilicate ions dimerise to form pyrosilicate and water
Now has a lower charge per Si atom
What happens when orthosilicate is exposed to additional acid?
It forms cyclic metasilicates
2 -ve charges per Si atom
Can from infinite chains, sheets or ribbons by condensation reactions
What structure do silicate minerals have?
Layered structure of a sheet of silicate - repeat unit = [Si2O5]2-
How is talc different to white asbestos chemically?
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
What are zeolites?
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.
Give an example of a zeolite
Zeolite A - Faujasite
Based on sodalite Si24O48 - replace 12 Si atoms with Al
[Si12Al12O48]12-
What shape is a sodalite cage?
Truncated cubotahedron
Square faces join to form an extended porous framework
Forms supercages and cubic cages between sodalite cages
What are the applications of zeolites?
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
What’s the importance of size selective catalysis by zeolites?
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
What’s the importance of silicones (polysiloxanes)?
Si=O cannot exist because of poor overital overlap so long polymer chains form
What are the applications of silicones?
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’
