CIE AS Chemistry Alkanes PDF

Head to www.savemyexams.com for more awesome resources CIE AS Chemistry Your notes 14.1 Alkanes Contents Producing Alkanes Combustion & Free Radical Substitution of Alkanes Free Radical Substitution Crac...

Head to www.savemyexams.com for more awesome resources CIE AS Chemistry Your notes 14.1 Alkanes Contents Producing Alkanes Combustion & Free Radical Substitution of Alkanes Free Radical Substitution Cracking of Alkanes Chemical Reactivity of Alkanes Combustion of Alkanes Page 1 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Producing Alkanes Your notes Production of Alkanes: Hydrogenation & Cracking Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains Production of alkanes from addition reactions Alkenes are unsaturated organic molecules and contain C-C double bonds When hydrogen gas and an alkene are heated and passed over a finely divided Pt / Ni catalyst, the addition reaction produces an alkane: The Pt / Ni catalyst is finely divided to increase its surface area and therefore increase the rate of reaction E.g. butane from 1-butene The hydrogenation reaction Hydrogen gas is added to 1-butene which is then heated and passed over a Pt / Ni catalyst to produce butane The addition reaction of alkenes with hydrogen is called hydrogenation Hydrogenation is often used in the manufacture of margarine from vegetable oil Vegetable oil is an unsaturated organic molecule with many C-C double bonds When these are partially hydrogenated, their hydrocarbon chains become straighter This raises the melting point of the oils which is why margarine is a soft solid and vegetable oil a liquid at room temperature Production of alkanes from cracking In cracking large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules The large hydrocarbon molecules are fed into a steel chamber, heated to a high temperature and then passed over an aluminium oxide (Al2O3) catalyst The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide Page 2 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed E.g. octane and ethene from decane Your notes An example of cracking Long hydrocarbons are cracked by heating them and using an aluminium oxide catalyst into smaller hydrocarbons and an alkene Exam Tip Remember that hydrogenation is an exothermic reaction and cracking is an endothermic reaction Page 3 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Combustion & Free Radical Substitution of Alkanes Your notes Combustion & Free Radical Substitution of Alkanes Alkanes are combusted (burnt) on a large scale for their use as fuels They also react in free-radical substitution reactions to form more reactive halogenoalkanes Complete combustion When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively For example, the complete combustion of octane to carbon dioxide and water The complete combustion of alkanes Complete combustion involves a plentiful supply of oxygen Incomplete combustion When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised Some carbon is only partially oxidised to form carbon monoxide For example, the incomplete combustion of octane to form carbon monoxide The incomplete combustion of alkanes Incomplete combustion involves a limited supply of oxygen Page 4 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Carbon monoxide is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind to oxygen As no oxygen can be transported around the body, victims will feel dizzy, lose consciousness and if not Your notes removed from the carbon monoxide, they can die Carbon monoxide is extra dangerous as it is odourless (it doesn’t smell) and will not be noticed Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present Free-radical substitution of alkanes Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine) Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur The free-radical substitution reaction consists of three steps: In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals These radicals create further radicals in a chain type reaction called the propagation step The reaction is terminated when two radicals collide with each other in a termination step Page 5 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Free Radical Substitution Your notes Free Radical Substitution Mechanism Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine) Ultraviolet light (sunlight) is needed for this substitution reaction to occur Reacting an alkane with bromine The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place Page 6 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The free-radical substitution reaction consists of three steps: Initiation step Your notes In the initiation step, the Cl-Cl or Br-Br is broken by energy from the UV light This produces two radicals in a homolytic fission reaction Free radical initiation The first step of the free-radical substitution reaction is the initiation step in which two free radicals are formed by sunlight Propagation step The propagation step refers to the progression (growing) of the substitution reaction in a chain type reaction Free radicals are very reactive and will attack the unreactive alkanes A C-H bond breaks homolytically (each atom gets an electron from the covalent bond) An alkyl free radical is produced This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical This free radical can then repeat the cycle Free radical propagation Page 7 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The second step of the free-radical substitution reaction is the propagation step in which the reaction grows in a chain type reaction This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products are formed If there is enough chlorine/bromine present, all the hydrogens in the alkane will eventually get substituted (eg. ethane will become C2Cl6/C2Br6) Free radical further substitution Page 8 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The free-radical substitution reaction gives a variety of products and not a pure halogenoalkane Termination step The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule Multiple products are possible Free radical termination The final step in the substitution reaction is to form a single unreactive molecule Page 9 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Exam Tip Your notes You could be asked to draw the mechanism for initiation and termination steps for free radical substitution This mechanism will use half-headed arrows to show the movement of one electron (double-headed arrows show the movement of a pair of electrons) A half-headed arrow is known as a ‘fish hook’ arrow. Initiation: Termination: The key is the use of the ‘fish hook’ arrow to show the homolytic fission of the bond in initiation and the formation of the bond in termination. Page 10 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cracking of Alkanes Your notes Obtaining Useful Compounds by Cracking Crude oil Crude oil is a mixture of hydrocarbons containing alkanes, cycloalkanes and arenes (compounds with a benzene ring) The crude oil is extracted from the earth in a drilling process and transported to an oil refinery At the oil refinery, the crude oil is separated into useful fuels by fractional distillation This is a separating technique in which a wide range of different hydrocarbons are separated into fractions based on their boiling points A fractional distillation column Page 11 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Crude oil is initially separated into fractions with similar boiling points in a process called fractional distillation Your notes However, the smaller hydrocarbon fractions (such as gasoline fractions) are in high demand compared to the larger ones Therefore, some of the excess heavier fractions are broken down into smaller, more useful compounds These more useful compounds include alkanes and alkenes of lower relative formula mass (Mr) This process is called cracking The process of cracking The heavier fractions that are obtained in fractional distillation are further cracked into useful alkanes and alkenes with lower Mr values Cracking The large hydrocarbon molecules are fed into a steel chamber, heated to a high temperature and then passed over an aluminium oxide (Al2O3) catalyst The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed E.g. octane and ethene from decane Cracking of long-chain hydrocarbons Page 12 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The long hydrocarbon fraction is cracked into two smaller ones The low-molecular mass alkanes formed make good fuels and are in high demand The low-molecular mass alkenes are more reactive than alkanes due to their double bond This makes them useful for the chemical industry as the starting compounds (feedstock) for making new products E.g. they are used as monomers in polymerisation reactions Using alkenes to form other useful compounds Page 13 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Alkenes are reactive molecules and can undergo many different types of reactions making them useful as starting compounds Your notes Page 14 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Chemical Reactivity of Alkanes Your notes Unreactivity of Alkanes Strength of C-H bonds Alkanes consist of carbon and hydrogen atoms which are bonded together by single bonds Unless a lot of heat is supplied, it is difficult to break these strong C-C and C-H covalent bonds This decreases the alkanes’ reactivities in chemical reactions Lack of polarity The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same This means that both atoms share the electrons in the covalent bond almost equally The Pauling Scale The Pauling Scale shows that the difference in electronegativity between carbon and hydrogen is only 0.4 As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ+ and δ- respectively) Alkanes therefore do not react with polar reagents They have no electron-deficient areas to attract nucleophiles They also lack electron-rich areas to attract electrophiles Examining bond polarity in ethane Page 15 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Ethane is an example of an alkane that lacks polarity due to almost similar electronegativities of the carbon and hydrogen atoms Due to the unreactivity of alkanes, they only react in combustion reactions and undergo substitution by halogens Exam Tip Remember: nucleophiles are negatively charged and are attracted to electron-deficient regions Electrophiles are positively charged and attracted to electron-rich regions Page 16 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Combustion of Alkanes Your notes Combustion of Alkanes & the Environment Cars’ exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO2) and volatile organic compounds (VOCs) When released into the atmosphere, these pollutants have drastic environmental consequences damaging nature and health Carbon monoxide Carbon monoxide is formed in the incomplete combustion of alkanes inside a car engine Due to a lack of enough oxygen in the engine, some of the carbon is only partially oxidised to CO instead of carbon dioxide (CO2) Word equation for incomplete combustion Incomplete combustion of alkanes is caused by a limited supply of oxygen CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death The CO binds to haemoglobin which therefore cannot bind oxygen and carbon dioxide Oxygen is transported to organs Carbon dioxide is removed as waste material from organs The effect of carbon monoxide on haemoglobin Page 17 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The high affinity of CO to haemoglobin prevents it from binding to O2 and CO2 Oxides of nitrogen Normally, nitrogen is too unreactive to react with oxygen in air However, in a car’s engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place: N2 (g) + O2 (g) → 2NO (g) N2 (g) + 2O2 (g) → 2NO2 (g) The oxides of nitrogen are then released in the car’s exhaust fumes into the atmosphere Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs) In the air, the nitrogen oxides can react with these VOCs to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog PAN is also harmful to the lungs, eyes and plant life Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain Acid rain can cause corrosion of buildings, endanger plant and aquatic life (as lakes and rivers become too acidic) and directly damage human health Catalytic removal To reduce the amount of pollutants released in cars’ exhaust fumes, many cars are now fitted with catalytic converters Page 18 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Precious metals (such as platinum) are coated on a honeycomb to provide a large surface area The reactions that take place in the catalytic converter include: Oxidation of CO to CO2: Your notes 2CO + O2 → 2CO2 or 2CO + 2NO → 2CO2 + N2 Reduction of NO/NO2 to N2: 2CO + 2NO → 2CO2 + N2 Oxidation of unburnt hydrocarbons: CnH2n+2 + (3n+1)[O] → nCO2 + (n+1)H2O Pollutants, their effect & removal table Environmental Formation Catalytic Removal Consequence Oxidation to CO2 Incomplete 2CO + O2 → 2CO2 Carbon combustion of Toxic gas Monoxide OR alkanes in car engines 2CO + 2NO → 2CO2 + N2 Dissolve in and react in Reduction to nitrogen gas: Oxides of Oxidation of nitrogen water with oxygen to Nitrogen in car engines 2CO + 2NO → 2CO2 + N2 form acid rain Unburnt React with oxides of Oxidise unburnt hydrocarbons to carbon hydrocarbons from nitrogen in the dioxide and water VOCs fuels and their oxides atmosphere to produce CnH2n+2 + (3n + 1)[O] → nCO2 + (n + 1)H2O formed in car engines PAN PAN From the Photochemical smog Oxidise unburnt hydrocarbons and reduce photochemical nitrogen oxides to prevent the formation of Page 19 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources reaction of VOCs and PAN in the atmosphere nitrogen oxides in the atmosphere Your notes Exam Tip Although CO2 is not a toxic gas, it is still a pollutant causing global warming and climate change Page 20 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers