alkanes Flashcards

Question

what is thermal cracking

Answer 1

thermal cracking involves heating alkanes to a high temperature, 700K -1200K (1000C) under high pressure, up to 7000kPa (70atm) the C-C bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom These fragments are called free radicals Free radicals are highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules

Answer 2

any number of C - C may break and the chain does not necessarily break in the middle -Hydrogen may also be produced

Answer 3

as there is not enough hydrogen atoms to produce two alkanes, one of the new chains must have a C = C and therefore be an alkene

Answer 4

thermal cracking tends to produce a high proportion of alkenes to avoid too much decomposition (ultimately to carbon and hydrogen) the alkanes are kept in these conditions for a very short time (typically one second)

Answer 5

Catalytic cracking involves catalysts to crack hydrocarbons This form of cracking is used to produce motor fuels

Answer 6

catalytic cracking takes place at a lower temperature (approximately 500C ) and lower pressure ( but more than atmosphere)

Answer 7

it uses zeolite catalyst, consisting of silicon dioxide and aluminium oxide (aluminium silicates)

Answer 8

zeolites have a honeycomb structure with an enormous surface area. They are also acidic.

Answer 9

the products are mostly branched alkanes, cycloalkanes (rings) and aromatic compounds

Answer 10

it is cheaper because it does not require high pressure or temperatures Catalysts speed up reactions so less time and money is spent

Answer 11

in the laboratory, catalytic cracking can be carried out using an aluminium oxide catalyst the products are mostly gases, showing that they have chain lengths of less than C5 and the mixture decolourises bromine solution. This test is a test for carbon-carbon double bond showing that the product contains alkenes

Answer 12

in a limited supply of oxygen, the poisonous gas carbon monoxide, CO is formed This is called incomplete combustion with even less oxygen, carbon (soot) is produced e.g. when a bunsen burner is used with a closed air hole, the flame is yellow and a black sooty deposit appears on the apparatus water is also produced in each of these reactions

Answer 13

incomplete combustion often happens with longer chain hydrocarbons, which need more oxygen to burn compared with shorter chains

Answer 14

all hydrocarbon-based fuels derived form crude oil may produce polluting when they burn: They include the following: 1. nitrogen oxides 2. sulfur dioxide 3. carbon particulates 4. unburnt hydrocarbons 5. CO2

Answer 15

nitrogen oxides (NOx) is produced when there is enough energy for nitrogen and oxygen in the air to combine e.g. N2(g) + O2(g) - 2NO(g) The nitrogen monoxide can reach further to produce nitrogen dioxide: 2NO(g) + O2 - 2NO2 This happens in a petrol engine at the high temps present, when the sparks ignite the fuel. These oxides react with water vapour and oxygen in the air to form acid. They are therefore contributors to acid rain and photochemical smog Nitrogen monoxide is produced when the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms in the air to react

Answer 16

Some fossil fuels contain sulfur When they are burnt, the sulfur reactor formsukfur dioxide gas (SO2) When sulfur dioxide gets in the atmosphere, it dissolves the moisture and is converted to sulfuric acid. This is what causes acid rain- the same process occurs with nitrogen oxides These oxides combine with water vapour and oxygen in the air to form sulfuric acid

Answer 17

acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes

Answer 18

carbon particles, are called particulates which can exacerbate asthma and cause cancer

Answer 19

Engined don't burn all the fuel molecules | Some these come out as unburned

Answer 20

CO2, is a greenhouse gas. It is always produced hydrocarbons burn Greenhouse gases in the atmosphere are very good at absorbing infrared energy. Try emit some of the energy back towards the earth keeping it warm- this js called the greenhouse effect Although carbon is necessary in the atmosphere, its level is rising and this is a cause of the earth's temperature and consequent climate change

Answer 21

large numbers of power stations generate electricity by burning fossil fuels such as coal or natural gas These compounds contain sulfur compounds and one of the products of their combustion is sulfur dioxide, SO2, a gas that causes acid rain by combining with oxygen and water in the atmosphere to form sulfuric acids SO2(g)+1/2O2(g) +H2(l) - H2SO4 these gases that are called flue gases so the process of removing the sulfur dioxide is called flue gas desulfuration

Answer 22

one method: a slurry of calcium oxide (lime =) and water is sprayed onto the flue gas which reacts with the calcium oxide ad water to form calcium sulfite, which can be further oxidised to calcium sulfate CaO(s) +2H2O(l) +SO2 +1/2O - CaSO4 .2H2O Alternatively: you can use calcium carbonate rater than calcium oxide: CaCO3(s+ 1/2O3(g) +SO(g) - CaSO4+CO2

Answer 23

internal combustion engines produce most of the pollutants listed previously (except sulfur dioxide because sulfur is removed from petrol) therefore all new cars with petrol engines are now equipped with catalytic converters in their exhaust systems to reduce the output of carbon monoxide, nitrogen oxides, and unburnt hydrocarbons in the exhaust gas mixture

Answer 24

honeycomb made of ceramic material coated with platinum and rhodium metals are the catalysts

Answer 25

honeycombs provide an enormous surface are, so a little of these expensive metals goes a long way

Answer 26

as the polluting gases pass over the catalyst, they react with each other to form less harmful products by the following reactions: 2CO + 2NO(g) - N2(g) +2CO2 carbon monoxide+ nitrogen oxide - nitrogen + carbon dioxide hydrocarbon + nitrogen oxide - nitrogen + carbon dioxide + water 2NO(g) - N2 + O2

Answer 27

greenhouses (CO2) traps infrared radiation so the earth's atmosphere heats up This is the greenhouse effect

Answer 28

the greenhouse effect it is impostant for life because, without other greenhouse gases, the earth would be too cold to sustain human life

Answer 29

due to the industrial revolution, fossil fuels have been used to fuel industrial plants and the levels of CO2 has been rising. This causes the eath's temp has been rising too and scientists believe this is due to the increased level of CO2

Answer 30

When you put a mixture of an alkane and a halogen into bright sunlight (UV light) to form a halogenalkane This reaction is called a photochemical reaction as it is started by uv light

Answer 31

if you put a mixture of hexane and a little liquid of bromine into a test tube and leave it in the dark, it will stay a red-brown colour ( the colour of bromine) However, if we shine ultraviolet light onto it, the mixture becomes colourless and mist fumes of hydrogen bromide appear This is a substitution reaction : one or more of the hydrogen atoms in the alkane has been replaced by a bromine atom and hydrogen bromide is given off as a gas the haloalkane is also produced

Answer 32

the reaction from above: an alkane and bromine is a chain reaction the three stages are: initiation, propagation and termination the reaction between any alkane and halogen goes by the same mechanism

Answer 33

imagine a reaction between methane and chlorine first, (initiation) step of the reaction is breaking the CL -CL bond to form two chlorine atoms the chlorine molecule absorbs the energy of a single quantum of ultraviolet (UV) light The energy of one quantum of UV light is more than the CL -CL bond energy, so the bond will break Since both atoms are the same, the CL- CL bond breaks homolytically, that is one electron going to each chlorine atom This results in two separate chlorine atoms. They are called free radicals. The dot next to the CL is used to show the unpaired electron free radicals are highly reactive The C-H bond in the alkane needs more energy to break than is available in a quantum of UV radiation and therefore does not break

Answer 34

we are still considering the chlorine and alkane reaction propagation takes place in two stages: 1. the chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, a stable compound. This leaves a methyl free radical CH3 2. The methyl free radical is also very reactive and reacts with a chlorine molecule. This produces another chlorine free radical and a molecule of chloromethane - a stable compound the effect of these two steps is to produce hydrogen chloride and chloromethane, and a new CL free radical. This is ready to react with more methane and the two steps are repeated. This is the chain part of the chain reaction These steps may take place thousand of times before the radicals are destroyed in the termination

Answer 35

Termination is when free radicals are removed. This can happen in any of the following three ways ( in regards to the reaction with alkanes and chlorine) : 1. Two chlorine free radicals react together to give chlorine CL2 2. A chlorine free radical and a methyl free radical react to give chloromethane 3. Two methyl free radical react together to give ethane

Answer 36

the main ones are chloromethane and hydrogen chloride some ethane dichloromethane - in the propagation stage if a chlorine radical reacts with some chloromethane that has already formed with longer chain alkanes there w9ill be many isomers formed because the CL can replace any of the hydrogen atoms chain reactions are not very useful because they produce such a mixture of products. They will also occur without light at high temperatures

Answer 37

chain reactions are not very useful because they produce such a mixture of products. They will also occur without light at high temperatures

Answer 38

ozone is a molecule made from three oxygen atoms O3 The ozone layer is important because it protects the earth from the harmful exposure to too many ultraviolet (UV) rays. Ultraviolet can cause skin cancer in people by damaging DNA

Answer 39

chlorine free radicals are formed from CFCs because the C - CL bonds breaks homolytically in the presence of UV radiation to produce free radicals, CL Cl•+ O3• - O2(g) + ClO• ClO• + O3 - 2O2 + Cl• Because the chlorine free radical is regenerated, it only takes a little of it to destroy loads of ozone molecules

Answer 40

These hydrocarbons react with nitrogen oxides in the presence of sunlight to form ground level ozone (O3) This is a major component of smog Ground level ozone irritates people's eyes, aggravates respiratory problems and even causes lung damage

Answer 41

Nitrogen oxides Unburnt hydrocarbons Carbon monoxide Catalytic convertes of Cars remove these pollutants from the exhaust

Answer 42

It is just the equation for the complete combustion of a hydrocarbons