Organic Chemistry



A hydrocarbon is molecule made up of only hydrogen and carbon atoms.

Simplest types of hydrocarbons=Alkanes. Alkanes made up of chains of carbon atoms surrounded by hydrogen atoms. Carbon atoms forms 4 bonds and hydrogen atoms form only one bond. In alkanes=no carbon-carbon double bonds so all atoms have formed bonds with as many other atoms as they can = saturated. Different alkanes=different chain lengths

Alkenes=another type of hydrocarbon. Are more reactive than alkanes=useful starting materials for making other organic compounds + polymers.

As alkenes more reactive than alkanes=can distinguish between them using bromine water. When orange bromine water=added to alkane, no reaction will happen and it'll stay bright orange. If added to alkene, reaction occurs and bromine water turns colourless. Bromine water=test for alkenes

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Properties of Hydrocarbons

The properties of hydrocarbons change depending on how long the carbon chain is. Are tends in properties of hydrocarbons that you need to know:

  • the shorter the molecules, the less viscous the hydrocarbon is
  • the shorter the molecules, the lower the boiling point - shorter molecules=the lower the temp at which they vaporise/condense
  • the shorter the molecules, the more flammable the hydrocarbon is
  • Hydrocarbons with long carbon chains=viscous, have high boiling points + not very flammable

Complete combustion of hydrocarbons - if you burn hydrocarbons, carbon + hydrogen react with oxygen from air to form carbon dioxide + water vapour. Equation: hydrocarbon + oxygen --> carbon dioxide + water vapour

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Fractional Distillation of Crude Oil

Crude oil=mixture of many different compounds - formed of remains of plants + animals (mainly plankton) that dies millions of yrs ago + were buried in mud. Over millions of yrs with high temp + high pressure = turned into crude oil=can be drilled up. Because takes so long to form=said to be finite resource.

Most of compounds in crude oil=hydrocarbon molecules + majority of them=alkanes

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Fractional Distillation

Crude oil can be split up ito separate groups of hydrocarbons using technique=fractional distillation. Crude oil=pumped into fractionating column=works continuously. Fractionating column has temp gradient running through it - hottest at bottom + coldest at top. 

Crude oil is first heated so evaporates + piped into bottom of column, gas rises up column =gradually cools. Different compounds in mixture=different boiling points (b.p)=condense at different levels.

Hydrocarbons=similar no. of carbon atoms=similar boiling points=condense at similar levels in column. (e.g hydrocarbons with lots of carbon atoms=high b.p=condense near bottom but hydrocarbons with small no. of carbon atoms=low b.p=condense near top)

The groups of hydrocarbons that condense together=fractions. Various fractions constantly tapped off from column at different levels where they condense.

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Process of fractional distillation


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Uses of Crude Oil

Fractions of crude oil can be processed to provide fuel for most modern transport. Diesel, petrol, kerosene, LPG.. are used to fuel cars, trains, planes + other transport. Uses of hydrocarbons depend on their properties

Volatility (tendency of substance to vaporise) helps decide what fraction used for:

  • LPG fraction=lowest b.p=gas at room temp=ideal for bottled gas. Stored under pressure as liquid in bottles. When tap on bottle opened fuel vaporises + flows to burner where is ignited
  • Petrol fraction=higher b.p=liquid at room temp=ideal for storing in fuel tank of car. Can flow to engine where easily vaporised to mix with air before ignited

Viscosity also helps decide what is used for (hydrocarbons): really gloopy, viscous hydrocarbons=for lubricating engine parts/covering roads.

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Uses of Crude Oil..(cont)

Petrochemical industry uses some of hydrocarbons from crude oil as feedstock (raw material used to produce other substance through industrial processes) to make new compounds for use in things like polymers, solvents, lubricants + detergents.

Products from crude oil=examples of organic compounds. The reason you get such large variety of products=because carbon atoms can bond together to form different groups called homologous series. These groups contain similar compounds that all have same general formula + many properties in common. Alkanes, alkenes as well as other families e,g alcohols, carboxylic acids =all of different homlogous series.

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Cracking Crude Oil

Cracking=process of turning long-chained hydrocarbons into short-chained more useful hydrocarbons. Some products of cracking=useful fuels (petrol for cars/paraffin for jet fuel). Cracking also produces substances like ethene=needed for making plastics.

How cracking works - Cracking=thermal decomposition (exothermic) - breaking down molecules by heating them. 2 methods used to crack alkanes=steam cracking or catalytic cracking. In both methods, first step=vaporise long-chained hydrocarbons. In catalytic cracking, vapour then passed over hot, powdered catalyst (powdered because larger SA=faster reaction). Aluminium oxide e.g of one catalyst used. Long-chain molecules 'crack' on surface of speeds of catalyst. Alternatively, in steam cracking vapour=mixed with steam + heated to very high temp. Will also lead to thermal decomposition of long-chain hydrocarbon molecules to form smaller ones. Products of cracking - most=alkanes + alkenes

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