Alkanes PDF - AS Chemistry 3.3.2

AS CHEMISTRY 3.3.2 ALKANES ALKANES OVERVIEW DESCRIPTION General CnH2n+2 Formula Description Saturated. Only contain c...

AS CHEMISTRY 3.3.2 ALKANES ALKANES OVERVIEW DESCRIPTION General CnH2n+2 Formula Description Saturated. Only contain carbon & hydrogen. Polarity Non-polar Highly unreactive due to being saturated & non-polar. Reactions - Free Radical Substitution Solubility in Insoluble. Non-polar alkanes are not miscible with polar Water H2O. Increase with increased chain length. Larger molecules have more electrons involved in the induced-dipole IMF’s making them stronger, so more energy is required to break them. Branched chain isomers of straight chain alkanes have lower melting and boiling points. e.g. methylpropane butane Melting & Boiling Points Boiling point: -11.7oC -1oC Branched chain molecules have a smaller surface area, so fewer interaction points. This weakens the induced-dipole forces between the molecules so they require less energy to break. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES FRACTIONAL DISTILLATION Crude oil (unrefined oil) is made up of a mixture of many different alkanes. I order to make them useful, Fractional Distillation is used to separate out the different groups of alkanes by their boiling point. These groups of alkanes are known as fractions, and all have similar boiling points. Boiling points of alkanes increase with increased chain length. Larger molecules have more electrons involved in the induced-dipole IMF’s making them stronger, so more energy is required to break them. NAME USE Fuel Gases Fuel e.g. barbecues Coolest Petrol / Fuel for transport Gasoline Naphtha Use to make chemicals Paraffin Heating, lighting & jet fuel Diesel Oil Fuel for diesel engines Fuel Oil Fuel for ships Crude and factories Oil Vapours Lubricating Oil For moving Hottest parts in engines Residue / For making Bitumen roads (tarmac) 1. Crude oil is evaporated (turned into a gas). 2. The very large alkanes (bitumen) remain as liquids and are removed at the bottom. 3. The evaporated alkanes rise up the column, cool down and condense (turn into a liquid). 4. The larger the molecules, the higher the temperature at which they condense. 5. So, the larger the molecule, the lower down the column they condense. 6. The smallest molecules condense at the top (the coolest part of the column). 7. Some remain as gases are are collected at the top. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES CRACKING Short chain alkanes tend to be more useful than long chain alkanes (e.g. as fuels). Cracking is the process of breaking long chain alkanes into more useful. Smaller ones. There are two main methods of doing so which you need to know: Thermal Cracking Temperature: 900oC Pressure: 7000kPa High temperatures and pressures are used to break the C-C bonds in the long chain alkanes. Produces: Mainly alkenes, small alkanes and hydrogen gas. Catalytic Cracking Temperature: 500oC Catalyst: Zeolite (Al2O3 + ZnO) A lower temperature and a catalyst is used are used to break the C-C bonds in the long chain alkanes. Less energy is required due to the lower temperature, soothes is considered to be a more environmentally friendly method. Produces: Mainly small alkanes The process of cracking requires a lot of energy, but it is worth going through the process for oil companies as short chain alkanes are more valuable than long chain alkanes. The short chain alkanes produced are sold as fuels for vehicles etc. The alkenes produced from thermal cracking can be used to make polymers (plastics) with. More on these in the alkenes section! How to Write Equations for Cracking AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES COMBUSTION OF ALKANES Many alkanes are used as fuels. In order to release the energy stored in the molecules, they must be burned in oxygen (AKA combustion). This reaction is highly exothermic (-ΔH). Complete Combustion (in excess O2) alkane + oxygen carbon dioxide + water CH4 + 2 O2 CO2 + 2 H2O Short chain alkanes are easier to burn, than long chain alkanes. Long chain alkanes release more energy per mole than short chain alkanes. Incomplete Combustion (in limited O2) alkane + oxygen carbon monoxide + water CH4 + 1½ O2 CO + 2 H2O Carbon monoxide is highly toxic. It permanently bonds to the haemoglobin in red blood cells and prevent s the body from transporting oxygen. How to Balance Combustion Equations AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES POLLUTANTS FROM COMBUSTION Alkane based fuels can also contain impurities. During combustion, these impurities can also react with oxygen to produce pollutants. You need to know what these are and what environmental issue they cause. Environmental Issue CO2 Greenhouse gas. Contributes to global warming. H2O(g) Greenhouse gas. Contributes to global warming. Sulfur impurities in fuels produce SO2 when burned. This SO2 dissolves in water vapour in the atmosphere to cause acid rain which impacts on forests, rivers and lakes. Nitrogen impurities in fuels produce various nitrogen oxides when burned. This also dissolves in water vapour in NOx the atmosphere to cause acid rain which impacts on forests, rivers and lakes. Produced during the incomplete combustion of alkanes. CO The is highly toxic. Some fuel may not be burned and evaporate into the Unburned atmosphere. These are greenhouse gases and cause Hydrocarbons global warming. Unburned carbon atoms (particulates) can be released into Carbon the atmosphere. These contribute to “smog” and can Particulates cause serious health issues in humans, including cancers. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES REDUCING POLLUTION It is important to minimise the output of pulling gases. Here’s a couple of methods / examples you should know… Catalytic Converters in Cars These use a catalyst (Platinum, palladium & rhodium) to react carbon monoxide and nitrogen oxides together to form less harmful waste gases. e.g. 2CO + 2NO CO2 + N2 Desulfurisation of Flue Gases The “flue” of an coal fired power station is essentially its “exhaust”. Sulfur Dioxide (SO2) is a powerful pollutant so needs removing from the waste gases. Calcium oxide or calcium carbonate is used to line the flue. These react with the SO2 to produced less harmful pollutants (as seen in the Group 2 section) CaO (s) + SO2(g) → CaSO3 (s) (calcium sulfide) Bad!! CO2 is a CaCO3 (s) + SO2(g) → CaSO3 (s) + CO2(g) greenhouse gas! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES CHLORINATION OF ALKANES Being non-polar, saturated molecules, alkanes are very unreactive, so it takes another very reactive species to react with it. Free radicals are highly reactive species. They are species that possess a single, unpaired electron. They are produced via a process known as homolytic fission. For example, chlorine free radicals are formed when U.V. light causes the Cl-Cl bond in a Cl2 molecule to break. Cl — Cl Cl + Cl Homolytic Fission: Homo = “the same” Fission = ”to break” i.e. where a covalent bond breaks to form two species that are the same. Bottom line here, 1 electron from the pair goes to one Cl atom and the other goes to the other Cl atom to form two free radicals. This is the only example of homiletic fission you need to know! The represents the single, unpaired electron in the free radical. FREE RADICAL SUBSTITUTION This reaction causes a hydrogen atom on an alkane molecule to be substituted by a chlorine atom to form a halogenoalkane. As chlorine free radicals are required for this reaction, it can only take place in the presence of U.V. light, as this produces the chlorine free radicals from Cl2. There is a 3 step process that you need to learn to show how this occurs: 1. Initiation: Starting the reaction by creating the Cl free radicals 2. Propagation: A two step process of substitution 3. Termination: Three possible ways in which the reaction could end AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.2 ALKANES The Free Radical Substitution of methane: Initiation Cl2 Cl + Cl Propagation CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH4 + Cl2 CH3Cl + HCl Overall Reaction Termination Cl + Cl Cl2 CH3 + Cl CH3Cl CH3 + CH3 C2H6 HINTS | TIPS | HACKS The propagation steps are a “chain” reaction as the Cl free radical is reproduced. How to Write Free Radical The overall reaction is a combination of the Substitution Equations propagation equations minus the free radicals. The reactions can only terminate when the free radicals run out by reacting with each other. We cannot control where on the alkane the substitution occurs nor how many are substituted. 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