Alkanes Flashcards
what is a catalytic converter
-device fitted in a car
-reduces amount of emissions from an internal combustion engine
-uses expensive metals like platinum and rhodium as the heterogeneous catalyst
-catalyst is mounted on a ceramic honeycomb to maximise the surface area
Catalytic cracking
-type of cracking that takes place at:
-a slight pressure
-high temperature
-in the presence of a zeolite catalyst
-used mainly to produce motor fuels and aromatic hydrocarbons
cracking
A process which involves breaking C-C bonds in alkanes to produce shorter chained alkanes and alkenes.
thermal cracking
-type of cracking that takes place at a high pressure and high temperature
-produces a high percentage of alkenes
difference between saturated and unsaturated
saturated: all the carbon-carbon bonds are single C-C bonds
unsaturated: have at least one double carbon bond (C=C)
Compare catalytic and thermal cracking
Catalytic:
-uses zeolite catalyst, so faster rate contributes to it being cheaper
-high temperature (around 450 degrees), but lower than thermal so cheaper
-low pressure, so cheaper (not as high as thermal)
-produces alkanes and aromatic hydrocarbons
Thermal:
-high pressure (up to 7000kPa)
-produces mainly alkenes, which can be used to make polymers
-also produces some alkanes
-very high temperature (500-950 degrees)
Steps of fractional distillation
-heat crude oil to a gas
-put gas in fractionating column, which has a temperature gradient – warmer at the bottom; cooler at the top
-the gases rise up the column, cool and condense at their boiling points
-the smaller the molecules are, the lower their boiling point, due to weaker Van der Waals forces between the molecules, so less energy is needed to separate them
-smaller molecules condense near the top of the column, where it is cooler
-the liquids are removed from the column as different fractions
-the smallest fractions are taken off at the top of the column as gases
-most of the shorter chains are used as fuels
-the longer chains are used as oil
Order of fractions (top to bottom of fractionating column)
Remember: Real Guns Never Kill Deer by Firing Bananas
Viscosity ______ down the column
Flammability ______ down the column
Size of molecules ______ down the column
Boiling points ______ down the column
Shorter chain alkanes (refinery gases)
Petrol (used for cars)
Naphtha (used in cleaning products)
Kerosene (jet fuel)
Diesel (diesel engines)
Fuel oil (fuel for ships/factories)
Bitumen (contains paraffin wax and some other substances)
Viscosity increases down the column
Flammability decreases down the column
Size of molecules increases down the column
Boiling points increases down the column
Explain why the sulfur compounds found in crude oil should be removed from the fractions before they are used for central heating fuel. (2 marks)
(On combustion) SO2 produced
Allow equation to produce SO2. Ignore sulfur oxides.
Which causes acid rain
If formula shown it must be correct
M2 is dependent on M1.
But if M1 is sulfur oxides, allow M2.
For M2 allow consequence of acid rain or SO2.
Ignore greenhouse effect and toxic
Suggest one important substance manufactured on a large scale from propene (1 mark)
polypropene / propan(-1 or 2-)ol / propane(-1,2-)diol / isopropanol / propanone / propanal
Accept alternative names Ignore plastic and polymer
Oxides of nitrogen are also produced during the combustion of paraffin in air.
Explain how these oxides of nitrogen are formed. (2 marks)
M1 Nitrogen and oxygen (from air) react / combine / allow a correct equation
If nitrogen from petrol / paraffin / impurities CE = 0 / 2.
M2 at high temperatures
Allow temperatures above 1000 °C or spark.
Not just heat or hot.
M2 dependent on M1.
But allow 1 mark for nitrogen and oxygen together at high temperatures.
Write an equation to show how nitrogen monoxide in the air is converted into nitrogen dioxide. (1 mark)
2NO + O2 2NO2
Allow multiples.
Nitric acid (HNO3) contributes to acidity in rainwater.
Deduce an equation to show how nitrogen dioxide reacts with oxygen and water to form nitric acid. (1 mark)
4NO2 + 2H2O + O2 —> 4HNO3
Explain why the melting point of dodecane is higher than the melting point of the straight-chain alkane produced by cracking dodecane. (2 marks)
Larger molecule / longer carbon chain / more electrons / larger surface area
More / stronger van der Waals’ forces between molecules
Allow dispersion forces / London forces / temporary induced dipole-dipole forces between molecules.
If breaking bonds, CE = 0 / 2.
Dodecane can be converted into halododecanes.
(2)
Deduce the formula of a substance that could be reacted with dodecane to produce 1-chlorododecane and hydrogen chloride only.
Cl2
Pent-1-ene is a member of the alkene homologous series. Pent-1-ene can be separated from other alkenes.
State the physical property of alkenes that allows them to be separated from a mixture by fractional distillation. (1 mark)
(Different) boiling points
Ignore mp’s, references to imf, different volatilities
The cracking of one molecule of compound X produces pent-1-ene, ethene and butane in a 1:2:1 mol ratio.
Deduce the molecular formula of X and state a use for the ethene formed. (2 marks)
C13H28
Making plastics/used to make polymers or polythene/used to make antifreeze/make ethanol/ripening fruit/any named additional polymer
not used as a plastic/polymer/antifreeze
not just ‘polymers’ – we need to see that they are being made
State the type of cracking that produces a high proportion of ethene and propene.
Give the two conditions for this cracking process (2 marks)
Thermal (cracking)
If not thermal cracking CE = 0 / 2
High pressure AND high temperature
If blank mark on Allow high P and T
OR
Pressure of ≥ 10 atm, ≥ 1 MPa ≥ 1000 kPa
AND temp of 400 °C ≤ T ≤ 1000 °C or 650 K ≤ T≤ 1300 K
Do not allow high heat
If no units for T, then range must be 650 − 1000
Outline the essential features of the fractional distillation of crude oil that enable the crude oil to be separated into fractions. (4 marks)
(1) Fractions or hydrocarbons or compounds have different boiling points/ separation depends on bp;
Ignore mp and vdw
(2) bp depends on size/ Mr/ chain length;
If refer to bond breaking/cracking/ blast furnace/oxygen/air 2
max
(3) Temp gradient in tower or column / cooler at top of column or vice versa;
QWC
(4) Higher bp / larger or heavier molecules at bottom (of column) or vice versa;
Not increasing size of fraction Not gases at top
C8H18 is obtained by the catalytic cracking of suitable heavy fractions.
State what is meant by the term cracking and name the catalyst used in catalytic cracking.
Write an equation to show how one molecule of C14H30 is cracked to form one molecule of C8H18 and one molecule of another hydrocarbon.
Explain why oil companies need to crack ‘suitable heavy fractions’.
(4 marks)
Large molecules or compounds or long chain hydrocarbons (broken) into smaller molecules or compounds or smaller chain hydrocarbons;
QWC
Zeolite or aluminosilicate (catalyst);
C14H30 →C8H18 +C6H12;
Smaller chain molecules are in more demand or have higher value or vice versa;
Write an equation for the incomplete combustion of C8H18 to form carbon monoxide and water only.
A catalytic converter is used to remove carbon monoxide from the exhaust gases in a car. Identify a catalyst used in the catalytic converter.
Write an equation to show how carbon monoxide is removed in a catalytic converter.
State why the water produced in the exhaust gases may contribute to global warming.
(4 marks)
C8H18 +81⁄2O2 →8CO+9H2O;
Allow multiples
Rh/ Pd/Pt/lr or in words;
Penalise contradiction of name and symbol
2CO+2NO→2CO2 +N2 / 2CO+O2 →2CO2;
Allow multiples
Greenhouse gas/ absorbs infrared radiation;
When some petrol was accidentally contaminated in 2007, the sensors in the affected cars caused a decrease in the supply of petrol to the engine.
Suggest the effect that the contaminated fuel would have on the performance of the cars.
State how the oil company might have recognised the problem before the petrol was sold. (2 marks)
car less powerful/ car stops/ reduced performance/ won’t run smoothly/ can’t accelerate;
Not incomplete combustion or bad effect on engine. Not doesn’t go as far.
Test it (before sale) /Quality control etc;
Crude oil is separated into fractions by fractional distillation. Outline how different fractions are obtained in this process (3 marks)
Crude oil is heated to vaporise it / oil vaporised (1)
(Vapour passed into fractionating) tower / column (1)
Top of tower cooler than bottom or negative temperature gradient (1)
fractions separated by b.p
OR condensed at different temperatures
OR levels
OR low boiling fractions at the top
OR at the top small molecules or light components (1)
Give the two main types of product obtained by catalytic cracking. (2 marks)
Motor fuels
Aromatic hydrocarbons
Branched alkanes / hydrocarbons
Cycloalkanes
Any two (2)
Ignore specific fractions, alkanes, shorter alkanes, penalise alkenes, and hydrogen
