Hydrocarbons - Chapter 16

Hydrocarbons - Chapter 16 Definition of Homologous Series • A homologous series is a family of compounds that share the same functional group and thus, have the same general formula and similar chemical properties. Alkanes Alkanes are hydrocarbons that contain only carbon-carbon single bonds and carbon-hydrogen bond. 1. General Formula ------> 𝑪𝒏𝑯𝟐𝒏+𝟐 Name - Molecular Formula - Structural Formula • Methane - 𝑪𝑯𝟒 -- 𝑪𝑯𝟒 • Ethane - 𝑪𝟐𝑯𝟔 - CH3CH3 • Propane - C3H8 - CH3CH2CH3 Following General Formula, n=3(propane) 𝑪𝒏𝑯𝟐𝒏+𝟐 ---------> C3H2(3) +2 ------------>C3H8 (Propane) Combustion Alkanes burn excess oxygen to produce carbon dioxide and water Propane + Oxygen ----> Carbon Dioxide + Water C3H8 (g) + 5O2 (g)_-------> 3CO2 (g) + 4H2O (g) When there is insufficient oxygen carbon monoxide and soot are formed instead Propane + Oxygen ------> Carbon Monoxide + Water 2C3H8 (g) + 7O2 (g) --------> 6CO (g) + 8H2O (g) Propane + Oxygen --------> Carbon + Water C3H8 (g) + 2O2 (g) ----------> 3C (s) + 4H2O (g) * Alkenes are more likely to go through incomplete combustion • Incomplete Combustion --> More likely to give out carbon, carbon monoxide. • Incomplete Combustion --> Burn sootier flames than alkanes.

Substitution A chemical process where a halogen atom replaces a hydrogen atom to form a new organic product and a by-product. Reaction with halogens(chlorine or bromine) in UV Light/Sunlight Methane + Chlorine ---UV Light-----> Chloromethane + Hydrogen Chloride CH4 (g) + Cl2 (g) ---UV Light-----> CH3Cl (g) + HCl CH3Cl --> Chloromethane CH2Cl2 --> Dichloromethane CHCl3 --> Trichloromethane CCl4 --> Tetrachloromethane Alkenes Physical Properties of Alkenes Viscosity • Alkenes become more viscous as their molecular sizes increase. Solubility • Alkenes are insoluble in water but soluble in most organic solvents like tetrachloromethane (CCl4) Combustion of alkenes Combustion -Alkenes burn in excess oxygen to produce carbon dioxide and water. Ethene + oxygen -----> Carbon Dioxide + water C2H4 (g) + 3O2 (g) -----> 2CO2 (g) + 2H2O (g) * Compared to alkanes, alkenes have a higher percentage by mass of carbon. * They are more likely to undergo incomplete combustion * They burn with a sootier flame than alkanes

Addition Reactions --> The C=C bond are reactive. C=C bond breaks to form new single bonds. Reactants that can be added to alkenes including hydrogen, bromine and other alkenes. Addition Reaction (Hydrogenation) Ethene + hydrogen ----𝟏𝟓𝟎℃ + Nickel ---> Ethane C2H4 (g) + H2 (g) ----𝟏𝟓𝟎℃ + Nickel --->C2H6 Addition Reaction (Bromination) Ethene + Bromine -----> 1,2 – dibromoethane C2H4 (g) + Br2 (𝚤) ------> CH2BrCH2Br (𝚤) * In the reaction, the brown colour of bromine disappears. Addition Reaction (Polymerisation) Ethene ---high temperature and pressure + catalyst --> poly(ethene) n(CH2 = CH2) ---high temperature and pressure + catalyst --> (- CH2 – CH2 -)2 • A long molecule is formed • The process is used to manufacture materials like plastic Polymerisation is a process where a large number of unsaturated monomers join together to form a saturated large organic molecule. Cracking of Alkenes A process where larger hydrocarbon molecules, usually alkenes, are broken down into smaller hydrocarbon molecules. Hexane ---crack---> butane + ethene C6H14 –crack---> C4H10 + C2H4