Alkenes Flashcards
What happens during cracking?
Long-chain hydrocarbons are broken down into short-chain hydrocarbons (smaller Alkane molecules, alkene molecules and sometimes hydrogen molecules)
Cracking involves the cleavage of Alkanes by heat (thermal decomposition)
What are the conditions for cracking?
600 degrees, silicon (IV) oxide aka silica and aluminium oxide catalyst
What is the purpose of cracking?
To obtain hydrogen: fuel and raw material to produce ammonia- Haber process
To obtain short chain Alkenes: as starting materials for making ethanol and plastics
Melting and boiling points of Alkenes
Increase as molecular size increases
Why does melting and boiling point increase with increase in molecular size?
Increased length of chain increases the surface area. Attractive forces (Van Der Waal’s forces) between alkene molecules become stronger as molecule becomes larger
Why do Alkenes have lower melting and boiling point compared to Alkanes with the same number of carbon atoms?
Each alkene has 2 fewer electrons than the Alkane with the same number of carbon atoms. Hence Van der Waal’s forces, which is dependent on the number of electrons between alkene molecules, are weaker.
What about other physical properties of Alkenes? (Eg density, viscosity, flammability, solubility etc)
They are comparable to those of Alkanes
What are the 2 main chemical properties of Alkenes?
Alkenes undergo combustion and addition reactions
Complete VS incomplete combustion (Alkenes can undergo both)
Complete combustion: forms water and CO2
Incomplete combustion: forms water and CO or water and C
What kind of combustion are Alkenes more likely to undergo?
Alkenes are more likely to undergo incomplete combustion and burns with a smokier flame than Alkanes with similar number of carbon atoms because there is a higher percentage of carbon in Alkenes (this is not always applicable)
Why are Alkenes more reactive than Alkanes?
Alkenes are more reactive than Alkanes because of the greater electron density at the carbon-carbon double bond
What happens generally during an addition reaction?
In an addition reaction, carbon-carbon double bonds become single bonds (an unsaturated organic compound becomes saturated)
Name the 5 types of addition reactions
1) Hydrogenation (addition of hydrogen)
2) hydration (addition of steam)
3) halogenation (addition of halogens- eg bromination)
4) hydro halogenation (addition of hydrogen halides)
5) addition polymerisation (addition of many small molecules- monomers- to form very large molecules-polymers)
What are the conditions required for hydrogenation?
200 degrees, nickel catalyst to speed up rate of reaction
What is hydrogenation used for?
Hydrogenation in used in the production of margarine from vegetable oil
