B. alkanes Flashcards
(36 cards)
What are the conditions for thermal cracking
450 - 750*C
70 atm
no catalyst required
What is the general formula for an alkane
CnH2n+2
What intermolecular forces occur between alkanes
only london forces
How does increased chain length of an alkane affect its boiling point
london forces increase in strength due to there being more electrons in the molecule
more energy is required to overcome these forces
so boiling point increases
How does branching affect an alkanes boiling point
if there is more branching, london forces are weaker due to fewer points of contact
therefore boiling point is lower
Why are alkanes insoluble in water
alkanes cannot form hydrogen bonds with water molecules
not enough energy is released to break the hydrogen bonds between water molecules
- so insoluble
What is fractional distillation
separating crude oil into different fractions based on individual molecules boiling point
Describe the process of fractional distillation
- crude oil is heated so it is vaporised
- vapour is passed into a tower with a temperature gradient of hot to cold from bottom to top
- as the vapour rises it cools
- molecules will condense at different heights as they have different boiling points
- the larger the molecule, the lower down the column it condenses
What are the major products in thermal cracking
mixtures of products containing high proportions of alkenes
What are the conditions for catalytic cracking
500*C
1-2 atm
zeolite catalyst (large surface area)
What are the major products of catalytic cracking
produces high proportions of branched alkanes and aromatic hydrocarbons e.g. benzene
How does a catalyst increase the rate of reaction
reactants adsorb onto surface of catalyst
weakens bonds in the molecules which lowers the activation energy
increases rate of reaction
Describe the set up of apparatus for cracking in the lab
horizontal test tube containing mineral wool soaked in parafin and a zeolite catalyst
tube from test tube into upside down submerged test tube in a beaker
bunsen burner underneath horizontal test tube
What is reforming
the processing of straight chain hydrocarbons into branched chain alkanes and cyclic hydrocarbons for efficient combustion
- hydrogen is produced as a by-product of the reaction
Why is reforming useful
it increases the octane number of petrol which prevents knocking in petrol engines
What is knocking
when hydrocarbons ignite too early in car engines, resulting in reduced engine performance
What are the conditions for reforming
around 500*C
around 20 atm
mixture of platinum and aluminium oxide catalyst
How is carbon monoxide formed and what problem does it cause
incomplete combustion of fuels containing C
toxic - binds to haemoglobin in lungs
How is soot formed and what problem does it cause
incomplete combustion of fuels containing C
can cause respiratory issues, global dimming
How is sulfur dioxide formed and what problem does it cause
combustion of S containing compounds in fuel
causes acid rain - makes soil too acidic
How are oxides of nitrogen formed and what problems can they cause
Reaction of nitrogen with oxygen in the air at very high temperatures ( in engines and furnaces )
causes acid rain, can cause respiratory issues
How can we reduce the problem caused by carbon monoxide
ensure a good supply of oxygen when burning fuels
- use a catalytic converter
How can we reduce the problem caused by soot
ensure a good supply of oxygen when burning fuels
How can we reduce the problem caused by sulfur dioxide
remove S from fuel before burning
- flue gas desulfurisation
