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
Conditions required for hydration
300 degrees, 65 tam, Phosphoric (V) acid H3PO4 as catalyst
What are the conditions needed for halogenation/bromination?
Absence of UV light, inert solvent (tetrachloromethane) and room temp and pressure
What happens to bromine in the presence of Alkenes?
Reddish-brown (concentrated) or yellow (diluted) bromine rapidly forms a Colourless oil
What happens when bromine solution is added to oil containing unsaturated hydrocarbons?
Bromine solution is decolourised when added to oil containing unsaturated hydrocarbons
What can bromination be used for?
Bromination can be sued to test for presence of unsaturated hydrocarbons and distinguish Alkanes form Alkenes
How will Alkanes react with bromine?
Alkanes decolourise bromine slowly only in presence of UV light (free radical substitution)
What are the conditions required for hydro halogenation?
Room temp and pressure
What are the conditions required for addition polymerisation?
High temperature and pressure, catalyst
Define addition polymerisation
It is the process by which one molecule (monomer) is directly added to another without any loss of any portion of the monomer to form a very large molecule (polymer)
What is a monomer?
Smallest unit from which the polymer is made
What is a polymer?
Long chain molecule with high molecular mass
Diff between saturated and unsaturated fats
Saturated fats contain no carbon-carbon double bonds in their fat molecules. unsaturated fats contain carbon-carbon double bond in their fat molecules.
Diff between monounsaturated and polyunsaturated fats?
Monounsaturated fats: only one carbon-carbon double bond
Polyunsaturated fats: more than one carbon-carbon double bond
Diff between fats and oils
Fats contain mainly saturated fat molecules and are solid at room temp and pressure. Oils contain a larger proportion of unsaturated fat molecules and are liquids at room temp and pressure.
How are Alkenes manufactured?
Manufactured by the cracking of hydrocarbons from petroleum
