Alkanes Flashcards
What is the general formula for alkanes?
CnH2n+2
What is the functional group?
C-C single bonds
Are they saturated or unsaturated hydrocarbons?
Saturated Hydrocarbons (contain hydrogen and carbon ONLY)
How do they differ?
by CH2
Are they soluble or insoluble in water?
Insoluble in water
Are they reactive or unreactive?
Unreactive
What is the trend as you go down the homologous series?
As you go down the homologous series, the boiling point INCREASES. This is because as the chain length increases there is more surface area for intermolecular forces of attraction which needs to be overcome (van der vaals molecular forces of attraction)
What is cracking?
Cracking is the name given to breaking up large hydrocarbons into smaller and more useful bits.
How is cracking achieved?
This is achieved by using high pressures and temperatures without a catalyst or lower temperatures and pressures in the presence of a catalyst
Give some facts about cracking (reference temperature and pressures)
♡ Cracking breaks up large hydrocarbon molecules into smaller more useful bits. ♡This is achieved using high temperatures and high pressures without a catalyst or ………………. ♡ low temperatures and low pressures with a catalyst
Give the source of large hydrocarbon molecules
♡The source of large hydrocarbon molecules is often in the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). ♡These fractions are obtained from the distillation process as liquids but are re-vapourised before cracking
How are hydrocarbon molecules broken up?
♡ In a fairly random way to produce mixtures of smaller hydrocarbons some of which have carbon-carbon double bonds.
Write the reaction of the hydrocarbon C15H32
C15H32’——> 2C2H4 + C3H6 + C8H18 ethene propene octane . The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline).
Name the catalyst for Catalytic cracking:
Zeolites
What are zeolites?
These are complex aluminosillicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge. They are associated with positive ions such as sodium ions.
In catalytic cracking what are the temperatures and pressures used?
The alkane is brought into contact with the catalyst at a temperature of 500°C and moderately low pressures
What are the zeolites used for and what do they produce?
♡ to give high percentages of hydrocarbons with between 5 and 10 carbon atoms which is useful for peteol ♡ Produces high proportions of branched alkanes and aromatic hydrocarbons luke benzene
What is involved in catalytic cracking?
An ionic intermediate
What are the structures of the zeolite catalyst?
The zeolite catalyst has sites which can remove a hydrogen from an alkane togther with the two electrons which bound it to the carbon . ♡ Taht leaves the carbon atom with a positive charge. ♡ Ions like this are called carbonium ions
What are the temperatures and pressures used in thermal cracking?
High temperatures (450°C-750°C) and pressures (upto about 70 atmospheres) are uses to break the large hydrocarbons into smaller ones.
Describe thermal crackings mixtures of products
Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds - alkenes
Give one way on which thermal cracking is different from catalytic cracking
Thermal cracking doesn’t go via ionic intermediates like catalytic cracking. ♡ instead carbon-carbon double bonds are broken so tat each carbon atom ends up with a single electron. ♡ Free radicals are formed
