Alkanes Flashcards
(32 cards)
Fractional distillation
• oil is heated
• then passed into column
• the fractions condense at different heights
• the temperature of the column decreases upwards
• the separation depends on boiling point
• boiling point depends on size of molecules
• the larger the molecule the larger the VDWs forces
• similar molecules condense together
• small molecules condense at the top at lower temps
• larger molecules condense at the bottom at higher temps
What kind of process is fractional distillation
A physical process involving the splitting of VDWs forces between molecules
What does vacuum distillation allow
Heavier fractions to be further separated without high temps which could break them down
Vacuum distillation
• heavy residues from the fractioning column are distilled again under a vacuum
• lowering the pressure over a liquid will lower its boiling point
Fractional distillation in a lab
- Heat the flask with a Bunsen burner
- This causes vapours of all the components in the mixture to be produced
- Vapours pass up the fractioning column
- The vapour of the substance with the lower bp reaches the top of the fractioning column first
- The thermometer should be at or below the boiling point of the most volatile substance
- The vapours with higher boiling points condense back into the flask
- Only the most volatile vapour passes into the condenser
- The condenser cools the vapours and condenses to a liquid and is collected
What is cracking
Conversion of large hydrocarbons to smaller hydrocarbon molecules by breaking C-C bonds
Large alkanes to smaller alkanes and alkenes and hydrogen
What kind of process is cracking
A chemical process involving the splitting of strong covalent bonds so requires high temps
Economic reasons for cracking
• the petroleum fractions with shorter C chains are in more demand than larger fractions
• the make use of excess larger hydrocarbons and to supply demand for shorter ones, larger hydrocarbons are cracked
• the products of cracking are more valuable than the starting materials
What are the 2 main types of cracking
Thermal and catalytic
Thermal cracking
Conditions:
• high pressure (7000kPa)
• high temperature (400-900*C)
Produces mostly alkanes and sometimes hydrogen (used in the haber process)
Catalytic cracking
Conditions:
• moderate pressure
• high temperature (450*C)
• zeolite catalyst
Produces branched and cyclic alkanes and aromatic hydrocarbons
Used for making motor fuels
Branched and cyclic hydrocarbons burn more cleanly
Why is catalytic cracking cheaper than thermal
Be chase it saves energy as lower temps and pressures are used
Fuel
Release energy when burnt
Complete combustion
In excess oxygen alkanes will burn with complete combustion
Products: CO2 and H2O
Incomplete combustion
If there’s a limited amount of oxygen then incomplete combustion occurs, producing CO and/or C
Why are alkanes used as fuels
They readily burn in the presence of oxygen making their combustion highly exothermic
What is a free radical
A reactive species which possesses an unpaired electron
How is SO2 produced
Sulfur containing impurities are found in petroleum fractions which produce SO2 when burnt
S + O2 -> SO2
How is acid rain produced
• SO2 will dissolve in atomospheric water and can produce acid eain
How can SO2 be removed from the waste gases from furnaces
By flue gas desulfurisation
-> the gases pass through a scrubber containing basic calcium oxide which reaches with the acidic SO2 in a neutralisation reaction
SO2 + CaO -> CaSO3
This calcium sulfite produced can be used to make calcium sulfate for plasterboard
How do nitrogen oxides form
From the reaction between nitrogen and oxygen inside the car engine
-> the high temp and spark in the energy provides sufficient energy to break strong nitrogen bond
N2 + O2 -> 2NO
N2 + 2O2 -> 2NO2
Environmental consequence of nitrogen oxides
NO is toxic and can form acidic gases NO2
NO2 is toxic and acidic and forms acid rain
Environmental consequence of carbon monoxide
Toxic
