DF 5 - getting the right sized molecules

Question 1

what is problem with the production of gasoline from crude oil

Answer 1

• 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

Question 2

how has the demand for gas oil been increased

Answer 2

the gas oil has been cracked and used in car petrol.

Question 3

which is greater for petrol and diesel - the demand or supply

Answer 3

the demand

Question 4

what is the job of the refinery

Answer 4

to convert crude oil into useful components.

Question 5

how has crude oil been converted into useful products

Answer 5

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.

Question 6

which hydrocarbons are present in petrol

Answer 6

arenes, cycloalkanes and alkenes.

Question 7

what is cracking

Answer 7

Cracking is the breakdown of large hydrocarbons into smaller, more useful alkanes and alkenes.

Question 8

why is cracking needed

Answer 8

to break down alkanes with large molecules that cannot be used in petrol into shorter chain alkanes that can be used in petrol

Question 9

what is special about the shorter chain alkanes

Answer 9

they tend to be branched

Question 10

what does petrol made by cracking have

Answer 10

a high octane number

Question 11

what is octane number

Answer 11

a standard measure of a fuels ability to withstand compression in an internal combustion engine without causing engine knocking

Question 12

give an equation to show cracking

Answer 12

C11H24 ——> C8H18 + C3H6
long chain alkene from kerosene is made into octane (for car petrol) and propene

Question 13

why is propene unsaturated

Answer 13

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

Question 14

give some examples of reactions involving cracking

Answer 14

alkanes —-> branched alkanes + branched alkenes
alkanes —-> smaller alkanes + cycloalkanes
cycloalkanes —-> alkenes + branched alkenes
alkenes —-> smaller alkenes

Question 15

what is catalytic cracking

Answer 15

cracking is done by heating heavy oils such as gas oil in the presence in the catalyst

Question 16

what happens with the alkenes produced from cracking

Answer 16

they are important starting materials for other parts of the petrochemical industry.

Question 17

what needs to happen with the products of cracking and why

Answer 17

cracking always produces many different products which need to be separated in a fractionating column

Question 18

how does cracking take place in a modern catalytic cracker

Answer 18

the cracking takes place in a 60m high vertical tube about 2m in diameter.

Question 19

why is a catalytic cracker called a riser reactor

Answer 19

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

Question 20

how long does the mixture in a cc take to flow from the bottom to the top of the tube

Answer 20

about two seconds
—-> the hydrocarbons are in contact with the catalyst for a short period of time

Question 21

what is the problem with catalytic cracking

Answer 21

in addition to all the other reactions, coke (carbon from the decomposition of hydrocarbon molecules) forms on the catalyst surface

Question 22

what does the carbon do the catalyst

Answer 22

causes the catalyst to become inactive.

Question 23

how do we overcome the catalyst becoming inactive

Answer 23

the powdery catalyst needs to be regenerated

Question 24

what happens after the mixture passes through the riser reactor

Answer 24

the mixture passes into a separator where steam carries away the cracked products leaving behind the solid catalyst.

Question 25

how is the catalyst regenerated

Answer 25

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

Question 26

what does the energy from burning coke do to the catalyst and happens to the energy

Answer 26

it heats up the catalyst. the catalyst then transfers the energy to the feedstock so that crackking can occur without additional heating

Question 27

what is a catalyst

Answer 27

a substance which speeds up a reaction but can be removed chemically unchanged at the end

Question 28

what is catalysis

Answer 28

the process of speeding up a chemical reaction using a catalyst

Question 29

does anything happen to the catalyst during the reaction

Answer 29

it doesnt undergo any permanent change but can be changed physically. the surface of a solid catalyst may crumble or become roughened.

Question 30

what does the physical change in the catalyst suggest

Answer 30

the catalyst is taking part in the reaction but is being regenerated

Question 31

how much of the catalyst is usually needed in a reaction

Answer 31

only small amounts of it is needed usually

Question 32

does a cataluyst affect the amount of product formed

Answer 32

no but it does affect the rate at which the product is formed

Question 33

are catalysts reactants

Answer 33

no

Question 34

what are the two types of catalysts

Answer 34

homogenous and heterogenous

Question 35

what is homogenous catalysis

Answer 35

if the reactants and catalysts are in the same physical state

Question 36

give an example of where homogenous catalysis is used

Answer 36

enzyme catalysed reactions in cells

Question 37

what is hetergenous catalysis

Answer 37

if the reactants and catalysts are in different physical states

Question 38

give an example of where heterogenous catalysis is used

Answer 38

many immportant industrial processes

Question 39

where does the reaction occur on a solid catalyst

Answer 39

on the surface

Question 40

state the model for heterogenous catalysis

Answer 40

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

Question 41

explain the first step of heterogenous catalysis

Answer 41

the reactants form bonds with atoms on the surface of the catalyst - they are adsorbed onto the surface

Question 42

explain the second step of heterogenous catalysis

Answer 42

as a result of the 1st step, bonds in the reactant molecules are weakened and they break

Question 43

explain the 3rd step of heterogenous catalysis

Answer 43

new bonds form between the reactants held close together on the surface to form the products

Question 44

explain the 4th step of heterogenous catalysis

Answer 44

this in turn weakens the bonds to the catalysts surface and the product molecules are released and diffuse away from the catalyst,.

Question 45

what must a catalyst in order to perform its function well and how does it achieve this

Answer 45

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.

Question 46

give an example of a hetergenous catalyst

Answer 46

zeolite

Question 47

what are many of the hetergenous catalysts

Answer 47

transition metal compounds

Question 48

what is catalyst poisoning

Answer 48

the partial or total deactivation of a catalyst by a chemical compound.

Question 49

how can catalysts be poisoned in hetergenous catalysis

Answer 49

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

Question 50

why can’t leaded petrol be used in cars fitted with catalytic converters

Answer 50

lead is strongly adsorbed onto the surface of the catalyst

Question 51

why can’t we replace the very costly metals such as platinium with cheaper metals such as copper as catalysts

Answer 51

because these cheaper metals are vulnerable to poisoning by the trace amounts of SO2 always present in car exhaust gases.

Question 52

what happens once the catalyst in a converter becomes poisoned

Answer 52

it becomes inactive so it cannot be regenerated so new converter has to be fitted (expensive)

Question 53

how is the hydrogen for the Haber Process prepared

Answer 53

by steam reforming of methane

Question 54

how does steam reforming of methane work

Answer 54

methane reacts with steam in the presence of a nickel catalyst

Question 55

what happens if the feedstock of methane contains sulfur compounds

Answer 55

they must be removed first to prevent severe catalyst poisoning

Question 56

how can the surface of a catalyst be cleaned or regenerated

Answer 56

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