12 - Alkanes Flashcards
What is the general formula for an alkane?
C(n)H(2n+2)
Describe the polarity and intermolecular forces in alkanes.
Non-polar
Form only van der Waals
What are alkanes?
Saturated hydrocarbons.
What is petroleum?
A mixture containing mostly alkane hydrocarbons that can be separated by fractional distillation.
Describe the conditions for thermal cracking.
High (700-1200K) temperature, high pressure (up to 7000kPa/70 atm)
Give the conditions for catalytic cracking.
High but lower temperature (720K), slight pressure, zeolite catalyst.
What does catalytic cracking produce?
Motor fuels and aromatic compounds.
What occurs in thermal cracking?
C-C bonds are broken.
What does thermal cracking produce?
Mostly alkenes.
Where is the fractionating column hottest?
The bottom
Describe the process of fractional distillation. (3)
Crude oil vapourised
Gases enter fractionating column
Gases condense at different points due to their different boiling point
Each fraction is a mixture
Why is cracking done?
Shorter chains are more useful
Alkenes are more reactive and are used a chemical feedstock
What do alkanes completely combust to?
Carbon dioxide + water
What do alkanes incompletely combust to?
Carbon monoxide + C (soot) + water
Why are alkanes used as fuels?
They release high amounts of energy.
What harmful byproducts are produced by alkane combustion engines?
Carbon monoxide, nitrogen oxides
What do catalytic convertors do?
Remove nitrogen oxides, carbon monoxide and unburnt hydrocarbons (particulates) from exhausts into safer compounds ie CO2 or H2O or N2
By reacting them with each other
What metal catalyst do catalytic convertors use?
Rhodium catalyst
Why are unburnt hydrocarbons/carbon particulates bad?
They can cause respiratory problems
What can sulphur dioxide cause?
Acid rain
How is sulfur dioxide removed from flue gases?
Flue gas desulfurisation , using CaO and water to produce CaSO4. CaCO3 can also be used, producing CO2 and CaSO4
Name the three stages of a free radical substitution.
Initiation, Propagation, Termination
Under what condition do alkanes react with halogens to form halogenoalkanes?
UV light
What is propagation?
The replacement of a hydrogen with a halogen atom, and the radical halogen reforming.
What is a radical?
A highly reactive molecule with an unpaired electron.
What occurs in termination?
Two radicals react to form a stable compound with no unpaired electrons.
Why might values of enthalpy for combustion of a liquid alkane be different to that using an expression using bond enthalpies?
Enthalpy expression uses gaseous; alkane is not gaseous
