3.2- Alkanes Flashcards
Give all the different steps of fractional distillation
. Oil is pre-heated
• then passed into column.
• The fractions condense at different heights
• The temperature of column decreases upwards
• The separation depends on boiling point.
• Boiling point depends on size of molecules.
• The larger the molecule the larger the van der waals forces
• Similar molecules (size, bp, mass) condense together
• Small molecules condense at the top at lower temperatures
• and big molecules condense at the bottom at higher temperatures
What is a vacuum distillation unit
.Heavy residues from the fractionating column are distilled again under a vaccum
. Lowering the pressure over a liquid will lower its boiling point
. Vaccum distillation allows heavier fractions to be further seperated without high temperatures which could break them down
Give the method of fractional distillation in the laboratory
. Heat the flask with a bunsen burner or electric mantle
. This causes vapours of all the components in the mixture to be produced
.Vapours pass up the fractionating column
. The vapour of the substance with the lower boiling point reaches the top of the fractionating column first
. The themometer 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 through condenser
.The condenser cools the vapour and condenses to a liquid and is collected
What is cracking
Cracking is the conversion of large hydrocarbons to smaller hyrdrocarbon molecules by breakage of C-C bonds
What are the economic reasons for cracking
. The petroleum fracions with shorter C chains like petrol and naptha are in more demand than larger fraction
. To make use of larger hydrocarbons and to supply demand for shorter ones the longer ones are cracked
. The products of cracking are more valuable than the starting materials
What are the conditions to thermal cracking and the products
.7000 kPa
.400C - 900C
. Produces mostly alkenes, and sometimes produces hydrogen that is used in the haber process
What are the conditions for catalytic cracking and the products
.Slight or moderate pressure
.High temperature 450C
.Zeolite catalyst
. Produces branched and cyclic alkanes and aromatic hyrdrocarbons
. Branched and cyclic hydrocarbons burn more cleanly, cheaper than thermal cracking becuase it saves every because of the low temp and pressure
What are the products of combustion and incomplete combustion for an alkane
The products are carbon dioxide and water for combustion, and for incomplete combustion the products are carbon and water
What are the impurities from combustion
.Sulfur containing impurities are found in petroleum fractions which produce SO2 when they are burned
. SO2 will dissolve in atmospheric water and can produce acid rain
. Coal has high sulfur content, large amounts of sulfur oxides are emitted from power stations
How can you remove SO2 from waste gases
.Flue gas desulfurisation
.The gases pass through a scrubber containing basic calcium oxide wheich with the acidic sulfur dioxide in a neutralisation
. The calcium sulfite which is fomred can be used to make calcium sulfate for plasterboard
How is nitrogen oxide formed in a car engine
From the reaction of N2 and O2
Give the different pollutants formed by hydrocarbons and NO2 and the enviromental consequence
.Nitrogen oxides- toxic and can form acidic gas NO2, NO2 toxic and acidic and forms acid rain
. Carbon monoxide- Toxic
. Carbon dioxide- Contributes towards global warming
. Unburnt hydrocarbons- Contributes towards formation of smog
. Soot- Global dimming and respiratory problems
How do catalytic converters work
These remove CO, NO and unburnt hydrocarbons from the exhause gases turning them into harmless gases lilke H2O and CO2
2 CO + 2 NO > 2 CO2 + N2
C8H18 + 25 NO > 8 CO2 + 12½ N2 + 9 H2O
.Converters have a ceramic honeycomb coated with a thin layer a catalyst metals, platinum, palladium, rhodium to give a large surface area
What are the different greenhouse gases
. Carbon dioxide, methane, water vapour
What is the mechanism for the greenhouse effect
.UV wavelength radiation passes through the atomsphere to the earths surace and heats up the earth
. The earth radiates out infared long wavelength radiation
. The = bond in the CO2 absorb infared radiation so the IR radiation does not escape from the atmosphere
. This energy is transferred to other molecules by collisions within the atmosphere so it becomes warmer
