Igcse Chemistry: Organic Chemistry Answers

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Homologous series
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A series of organic compounds that have the same general formula, similar chemical reactions and where each member differs from the next by a -CH₂- group
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Hydrocarbon
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A compound containing only the elements hydrogen and carbon
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Saturated
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An organic compound in which all the bonds are single bonds
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Unsaturated
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An organic compound that contains a carbon-carbon double bond
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General formula
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A formula that states the ratio of atoms of each element in the formula of every compound in a particular homologous series
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Isomerism
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Compounds that have the same molecular formula but different displayed formulae are said to exhibit this
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Alkanes
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A homologous series of compounds that have the general formula CnH2n+2
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CH₄
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Molecular formula for methane
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C₂H₆
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Molecular formula for ethane
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C₃H₈
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Molecular formula for propane
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C₄H₁₀
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Molecular formula for butane
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C₅H₁₂
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Molecular formula for pentane
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Combustion
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When oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat.
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Incomplete combustion
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When a lack of oxygen combines with another compound to form water and carbon monoxide. Carbon monoxide is poisonous – it reduces the capacity of the blood to carry oxygen.
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Substitution reaction
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When one atom is replaced by another atom in a reaction.
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CH₄ + Br₂ → CH₃Br + HBr
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Methane and bromine react together in the presence of ultraviolet light to form bromomethane.
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Alkenes
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A homologous series of compounds that have the general formula CnH2n. They are all unsaturated.
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C₂H₄
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Molecular formula for ethene
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C₃H₆
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Molecular formula for propene
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C₄H₈
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Molecular formula for butene
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C₅H₁₀
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Molecular formula for pentene
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Addition reactions
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When an unsaturated compound becomes saturated. The double bond breaks and two new atoms/groups are added to each of the previously double bonded carbon.
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Decolorization of bromine
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1,2-dibromomethane is colorless, so when bromine or bromine water is shaken with ethene the bromine will be decolorized. This can be used to test for the presence of unsaturation in a molecule.
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Fermentation
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When sugar or starch is dissolved in water and yeast is added, before being fermented for several days in the absence of air at room temperature. The excess yeast is filtered off to obtain a dilute solution of ethanol.
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Direct hydration of ethene
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When a mixture of ethene and steam is passed over a phosphoric acid catalyst at a temperature of 300°C and 60-70 atmospheres of pressure. Ethanol is produced and condensed as a liquid.
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Fermentation
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This ethanol production method uses renewable resources but is very slow and produces a solution that requires further processing if pure ethanol is required.
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Direct hydration of ethene
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This ethanol production method uses non-renewable resources but is continuous, fast and produces pure ethanol. However, high temperatures and pressures are required, increasing the cost.
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Dehydration of ethanol to ethene
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Ethanol can be dehydrated by passing ethanol vapor over a hot aluminum oxide catalyst. This can be used to manufacture ethene for the production of poly(ethene).

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