Alkanes and alkenes Flashcards
What type of compounds are alkanes and alkenes?
- Covalent compounds
- bond formed btwn non-metals
Physical properties of alkanes and alkenes
- Low MP/BP
- Insoluble in water
- Soluble in organic solvents
- Do not conduct electricity in any state
How physical properties change going down the series
- MP/BP increase
- larger molecular size
- stronger intermolecular forces - More viscous
- Densities increase
- Flammability decreases
How physical properties change going down the series
- MP/BP increase
- larger molecular size
- stronger intermolecular forces - More viscous
- Densities increase
- Flammability decreases
Why alkenes burn with smokier flame
- Alkenes have higher percentage of carbon
- Need more O2 for complete combustion
- Higher chance of incomplete combustion, more carbon left as soot, smokier flame
Chemical Properties of Alkanes
- Generally unreactive
- Combustion
- Substitution rxn
Why are alkanes generally unreactive
- consists of only C-C bonds (Single), C-H bonds
- bonds are strong, difficult to overcome
Combustion (Alkanes)
- ignited by spark/flame
- burn readily in xs air
- produce CO2 and water vapour
- highly exothermic
- alkanes –> gd fuels
Substitution rxns
- alkanes react with halogens
- e.g. chlorine, bromine
- presence of UV light
- methane + chlorine –> chloromethane + hydrogen chloride
Cracking
- breaking down of long-chain hydrocarbons
- into smaller molecules
Products of cracking
- Small alkanes
- Small alkenes
- Hydrogen gas
Products of cracking
- Small alkanes
- Small alkenes
- Hydrogen gas
2 ways of cracking
- Big alkane –> small alkane + small alkene
2. Big alkane –> small alkene + H2
Uses of cracking
- Produce smaller chain of alkenes (starting materials for alcohols, plastics)
- Produce H2 needed for fertilisers
- Produce petrol frm lubricating oil and diesel, more useful, higher demand
Evidence of cracking
- bubbles of gas/effervescence
- shows liquid pertoleum (higher bp) converted into smaller alkanes and alkenes (lower bp)
Chemical properties of alkenes
- Addition of hydrogen/Hydrogenation
- Addition of Bromine/Bromination
- Addition of steam/Hydration
- Addition polymerisation
Hydrogenation (Alkenes)
- 200 degrees
- presence of catalyst ie. nickel
- alkenes react with hydrogen, form alkanes
Bromination (Alkenes)
- bromine soln = reddish brown
- alkene + bromine –> decolourizes immediately, colourless liquid formed
- alkane formed
Hydration (Alkenes)
- react with steam, produce alcohols
- 300 degrees
- 60 atm
- phosphoric acid H3PO4 catalyst
Addition polymerisation (Alkenes)
- high temp and pressure
- presence of catalyst
- ethane molecules react w each other
- form long molecule called polyethene
- draw out structural formula
Fats and Oils
- some are polyunsaturated
- hydrocarbon chains contain more than 1 C=C bond
