F322: Alkanes

Introduction

Alkanes are saturated hydrocarbons, meaning they have all single C-C bonds.
General Formula = CnH2n+2
General Formula of a cycloalkane = CnH2n
-These are unreactive because only have C-H and C-C bonds, which are very strong.
-Also are non-polar, meaning they don't attract attacking species.

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As carbon chain length (Also Mr) increases, so does boiling/melting point:
-Number of electrons increases.
-Strength of van der Waals' forces increases.
As branching increases the melting point/boiling point decreases:
-Molecules cannot pack as tight, so less surface contact.
-So weaker and fewer van der Waals' forces.
Therefore Octane has a hight mpt/bpt than butane.
And 2,3-diethylpentane has a lower mpt/bpt than hexane.

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Crude oil is seperated by fractional distillation, which is: the seperation of components in a liquid mixture into fractions which differ by boiling points by means of distillation.
Different fractions have different boiling points due to chain length and branching.

-Uses: Fuels = Methane for domestic use, Butane for camping gas, and Octane for petrol.
Petrochemicals = Converted into alkenes for plastics etc.

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Cracking = The breaking down of long-chained alkanes into shorter-chained ones and alkenes.
E.g. C10H22 -> C2H4 (solvents) + C8H18 (petrol) {ZEOLITE CATALYST, 450 DEGREES}
Reforming = Conversion of unbranched into cyclic or aromatic, by removing H.
E.g. Hexane (C6H14) -> Cyclohexane (C6H12) + H2
Cyclohexane -> Benzene (C6H6, an arene) +3H2
Isomerism = Conversion of unbranched into branched alkanes.
E.g. Octane (C8H18) -> 2,3,4-triethylpentane (C8H18)
Alkanes formed by reforming and isomerism are added to fuel to increase efficiency.

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ALKANE + O2 -> CO2 + H2O
E.g. C4H10 + O2 -> CO2 + H2O
-Incomplete sometimes occurs, which produces CO rather than CO2; this happens frequently in car engines.
-CO is a colourless and odourless gas, toxic because it inhibits the haemoglobin from binding with O2, often fatal.
-Plentiful O₂: C₃H₈ + 5O₂ -> 3CO₂ + 4H₂O
-Limited O₂: C₃H₈ + 7/2 O₂-> 3CO + 4H₂O
Hydrocarbons make good fuels because they:
-Release large amounts of energy.
-Readily available.
Problems:
-Non-renewable (Becoming more expensive).
-Produces pollutants (NO, CO, SO2 which affect acid rain and destruction of forests).
-They can add to climate change.

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A biofuel which is a fuel made from recently living material such as plants/animal waste.
E.g. -Sugar cane grown for energy production.
-Ethanol made by fermentation, used to burn or added to fuels.
-Biodisel derived from plants.

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A type of substitution where a radical replaces a different atom or group of atoms.
Overall reaction for methane: CH4 + Cl2 -> CH3Cl + HCl
Mechanism:
INITIATION - Formation of radicals by homolytic fission by UV light.
Cl₂  -> 2Cl^.PROPOGATION - Radicals used up and regenerated.

CH₄ +    Cl^.   -> CH₃^. +    HCl
CH₃^. +    Cl₂  -> CH₃Cl    +    Cl^.

TERMINATION - Radicals used up to form product.
CH₃^. +    CH₃^. -> C₂H₆CH₃^. +    Cl^.   -> CH₃Cl

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