DF 5 - getting the right sized molecules Flashcards
specification reference - (i) (h) (j) (l)
what is problem with the production of gasoline from crude oil
- the straight run gasoline from the primary distillation makes poor petrol. some is used directly in petrol but most is treated further.
- problem of supply and demand. crude oil has a surplus of the high boiling fractions such as the gas oil and residue and not enough of the lower boiling points such as gasoline
how has the demand for gas oil been increased
the gas oil has been cracked and used in car petrol.
which is greater for petrol and diesel - the demand or supply
the demand
what is the job of the refinery
to convert crude oil into useful components.
how has crude oil been converted into useful products
in order to do this, the structure of the alkane molecules present must be altered to produce different alkanes. the alkanes are also converted into other types of hydrocarbon used in petrol.
which hydrocarbons are present in petrol
arenes, cycloalkanes and alkenes.
what is cracking
Cracking is the breakdown of large hydrocarbons into smaller, more useful alkanes and alkenes.
why is cracking needed
to break down alkanes with large molecules that cannot be used in petrol into shorter chain alkanes that can be used in petrol
what is special about the shorter chain alkanes
they tend to be branched
what does petrol made by cracking have
a high octane number
what is octane number
a standard measure of a fuels ability to withstand compression in an internal combustion engine without causing engine knocking
give an equation to show cracking
C11H24 ——> C8H18 + C3H6
long chain alkene from kerosene is made into octane (for car petrol) and propene
why is propene unsaturated
it is an alkene so has a carbon=carbon double bond. this makes it unsaturated as it doesnt have as many hydrogen atoms as it could for the carbon atoms
give some examples of reactions involving cracking
alkanes —-> branched alkanes + branched alkenes
alkanes —-> smaller alkanes + cycloalkanes
cycloalkanes —-> alkenes + branched alkenes
alkenes —-> smaller alkenes
what is catalytic cracking
cracking is done by heating heavy oils such as gas oil in the presence in the catalyst
what happens with the alkenes produced from cracking
they are important starting materials for other parts of the petrochemical industry.
what needs to happen with the products of cracking and why
cracking always produces many different products which need to be separated in a fractionating column
how does cracking take place in a modern catalytic cracker
the cracking takes place in a 60m high vertical tube about 2m in diameter.
why is a catalytic cracker called a riser reactor
its called a riser reactor because the hot vaporised hydrocarbons and zeolite catalyst are fed into the bottom of the tube and forced upwards by steam. the mixture contains solid particles which flow like liquids
how long does the mixture in a cc take to flow from the bottom to the top of the tube
about two seconds
—-> the hydrocarbons are in contact with the catalyst for a short period of time
what is the problem with catalytic cracking
in addition to all the other reactions, coke (carbon from the decomposition of hydrocarbon molecules) forms on the catalyst surface
what does the carbon do the catalyst
causes the catalyst to become inactive.
how do we overcome the catalyst becoming inactive
the powdery catalyst needs to be regenerated
what happens after the mixture passes through the riser reactor
the mixture passes into a separator where steam carries away the cracked products leaving behind the solid catalyst.
how is the catalyst regenerated
the catalyst goes into the regenerator where it takes about 10 minutes for the coke to burn off in the hot air of the regenerator. the catalyst can then be reused
what does the energy from burning coke do to the catalyst and happens to the energy
it heats up the catalyst. the catalyst then transfers the energy to the feedstock so that crackking can occur without additional heating
what is a catalyst
a substance which speeds up a reaction but can be removed chemically unchanged at the end
what is catalysis
the process of speeding up a chemical reaction using a catalyst
does anything happen to the catalyst during the reaction
it doesnt undergo any permanent change but can be changed physically. the surface of a solid catalyst may crumble or become roughened.
what does the physical change in the catalyst suggest
the catalyst is taking part in the reaction but is being regenerated
how much of the catalyst is usually needed in a reaction
only small amounts of it is needed usually
does a cataluyst affect the amount of product formed
no but it does affect the rate at which the product is formed
are catalysts reactants
no
what are the two types of catalysts
homogenous and heterogenous
what is homogenous catalysis
if the reactants and catalysts are in the same physical state
give an example of where homogenous catalysis is used
enzyme catalysed reactions in cells
what is hetergenous catalysis
if the reactants and catalysts are in different physical states
give an example of where heterogenous catalysis is used
many immportant industrial processes
where does the reaction occur on a solid catalyst
on the surface
state the model for heterogenous catalysis
1) reactants are adsorbed onto the catalyst surface
2) bonds are weakened and then they break
3) new bonds are made
4) the product diffuses away from the catalyst
explain the first step of heterogenous catalysis
the reactants form bonds with atoms on the surface of the catalyst - they are adsorbed onto the surface
explain the second step of heterogenous catalysis
as a result of the 1st step, bonds in the reactant molecules are weakened and they break
explain the 3rd step of heterogenous catalysis
new bonds form between the reactants held close together on the surface to form the products
explain the 4th step of heterogenous catalysis
this in turn weakens the bonds to the catalysts surface and the product molecules are released and diffuse away from the catalyst,.
what must a catalyst in order to perform its function well and how does it achieve this
it must have a large surface area for contact with reactants. for this reason, solid catalysts are used in a finely divided form or as a fine wire mesh. sometimes the catalyst is supported on a porous material to increase its surface area and prevent it from crumbling. this happens in catalytic converters fitted to car exhaust sytems.
give an example of a hetergenous catalyst
zeolite
what are many of the hetergenous catalysts
transition metal compounds
what is catalyst poisoning
the partial or total deactivation of a catalyst by a chemical compound.
how can catalysts be poisoned in hetergenous catalysis
the ‘poison molecules’ are adsorbed more strongly to the catalyst surface than the reactant molecules. the catalyst cannot catalyse a reaction of the poison and becomes inactive with poison molecules blocking the active sites on its surface
why can’t leaded petrol be used in cars fitted with catalytic converters
lead is strongly adsorbed onto the surface of the catalyst
why can’t we replace the very costly metals such as platinium with cheaper metals such as copper as catalysts
because these cheaper metals are vulnerable to poisoning by the trace amounts of SO2 always present in car exhaust gases.
what happens once the catalyst in a converter becomes poisoned
it becomes inactive so it cannot be regenerated so new converter has to be fitted (expensive)
how is the hydrogen for the Haber Process prepared
by steam reforming of methane
how does steam reforming of methane work
methane reacts with steam in the presence of a nickel catalyst
what happens if the feedstock of methane contains sulfur compounds
they must be removed first to prevent severe catalyst poisoning
how can the surface of a catalyst be cleaned or regenerated
in the catalytic cracking of long chain hydrocarbons for e.g. carbon is produced and surface of the zeolite catalyst becomes coated in a layer of soot. this blocks the adsorption of reactant molecules and the activity of the catalyst is reduced. the catalyst is constantly recycled through a separate container where hot air is blown through the zeolite powder. the oxygen in the air converts the C to CO2 and cleans the catalyst surface
