Topic 11
5 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Alkanes are saturated hydrocarbons with general formula CnH(2n+2). They undergo combustion and substitution reactions.
Explanation
Alkane series: methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10). General formula: CnH(2n+2). All single C-C bonds (saturated). Properties: - Non-polar, insoluble in water. - Flammable: used as fuels. - Relatively unreactive. Reactions: 1. Complete combustion: alkane + O2 to CO2 + H2O (blue flame). CH4 + 2O2 to CO2 + 2H2O 2. Incomplete combustion: insufficient O2 gives CO (toxic) and/or soot (C). 3. Substitution with halogens (in UV light): CH4 + Cl2 to CH3Cl + HCl (chloromethane) Fractional distillation of crude oil: - Crude oil is a mixture of hydrocarbons separated by fractional distillation. - Fractions: refinery gas, petrol, naphtha, kerosene, diesel, fuel oil, bitumen. - Shorter chains: lower boiling point, more volatile, more flammable, less viscous.
Practice Questions
Write a balanced equation for the complete combustion of propane (C3H8). [2 marks]
Key Idea
Alkenes are unsaturated hydrocarbons with a C=C double bond. General formula CnH(2n). They undergo addition reactions.
Explanation
Alkene series: ethene (C2H4), propene (C3H6), butene (C4H8). General formula: CnH(2n). Contain one C=C double bond (unsaturated). Test for unsaturation: - Bromine water (orange/brown) decolourises (turns colourless) in the presence of an alkene. - Alkanes do NOT decolourise bromine water. Addition reactions: 1. Addition of H2 (hydrogenation): C2H4 + H2 to C2H6 (with Ni catalyst, 150 degrees C). 2. Addition of Br2 (bromination): C2H4 + Br2 to C2H4Br2. Decolourises bromine water. 3. Addition of H2O (hydration): C2H4 + H2O to C2H5OH (ethanol). With H3PO4 catalyst, 300 degrees C, 60 atm. 4. Addition of HCl: C2H4 + HCl to C2H5Cl. Polymerisation: - Many alkene monomers join together to form a polymer. - n(CH2=CH2) to (-CH2-CH2-)n (polyethene/polythene). - Addition polymerisation: no other product formed.
Practice Questions
Which reagent is used to distinguish between an alkane and an alkene?
Key Idea
Alcohols contain the -OH functional group. General formula CnH(2n+1)OH. Ethanol is the most important alcohol.
Explanation
Alcohol series: methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH). Functional group: -OH (hydroxyl group). Properties: - Soluble in water. - Flammable: used as fuels. - Ethanol: solvent in perfumes, medicines; alcoholic drinks. Reactions of ethanol: 1. Combustion: C2H5OH + 3O2 to 2CO2 + 3H2O. 2. Oxidation to ethanoic acid: C2H5OH + [O] to CH3COOH + H2O (using acidified potassium dichromate, turns orange to green). 3. Dehydration to ethene: C2H5OH to C2H4 + H2O (with Al2O3 catalyst, 300 degrees C). 4. Esterification: alcohol + carboxylic acid to ester + water. Production of ethanol: 1. Fermentation: glucose to ethanol + CO2 (using yeast, 25-37 degrees C, anaerobic). C6H12O6 to 2C2H5OH + 2CO2 2. Hydration of ethene: C2H4 + H2O to C2H5OH (H3PO4 catalyst, 300 degrees C, 60 atm).
Practice Questions
Write a balanced equation for the fermentation of glucose to produce ethanol. State the conditions required. [3 marks]
Key Idea
Carboxylic acids contain the -COOH group. Esters are formed by reacting a carboxylic acid with an alcohol (esterification). Esters have fruity smells.
Explanation
Carboxylic acids: - Functional group: -COOH (carboxyl group). - Examples: methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (C2H5COOH). - Weak acids: partially ionise in water. - React with metals, metal carbonates, and bases. - Ethanoic acid is the acid in vinegar. Esterification: Carboxylic acid + alcohol to ester + water (reversible, with conc. H2SO4 catalyst) CH3COOH + C2H5OH to CH3COOC2H5 + H2O (ethanoic acid + ethanol gives ethyl ethanoate + water) Naming esters: - First part from alcohol (ethanol gives ethyl). - Second part from acid (ethanoic acid gives ethanoate). - Ethanol + ethanoic acid gives ethyl ethanoate. Uses of esters: - Flavourings and fragrances (fruity smells). - Solvents (nail varnish remover).
Practice Questions
Name the ester formed when methanol reacts with propanoic acid. Write the equation for this reaction. [3 marks]
Key Idea
Addition polymers form from alkene monomers (no other product). Condensation polymers form with loss of small molecules (H2O or HCl). Nylon and polyester are condensation polymers.
Explanation
Addition polymerisation: - Monomers: alkenes (contain C=C). - No other product formed. - Examples: poly(ethene) from ethene, poly(propene) from propene, PVC from chloroethene. Condensation polymerisation (Extended): - Two different monomers react, losing small molecules (H2O or HCl) each time. - Nylon (polyamide): diamine + dicarboxylic acid gives nylon + water. - Polyester: diol + dicarboxylic acid gives polyester + water. Disposal of plastics: - Non-biodegradable: persist in environment. - Landfill: waste of space. - Burning: produces toxic gases (HCl from PVC). - Recycling: preferred option. - Biodegradable plastics: made from plant materials, break down naturally.
Practice Questions
Which type of polymerisation produces nylon?