chemistry

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  • Created by: lewis0202
  • Created on: 16-01-18 16:29

alkenes and alkanes-Complete combustion

Complete combustion- When the elements in the fuel react fully with oxygen.

Fuels such as natural gas and petrol contain hydrocarbons which are compounds of hydrogen and carbon only. When hydrocarbons burn completely- The carbon oxidises to carbon dioxide - The hydrogen oxidises to water ( H2O is an oxide of hydrogen) 

Complete combustion

Hydrocarbon + oxygen ------ Carbon dioxide + water

E.g propane + oxygen ------ Carbon dioxide + water

C3H8 + 5O2 → 3CO2 + 4H2O

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alkenes and alkanes-Incomplete combustion

Incomplete combustion

Incomplete combustion occurs when the supply of air or oxygen is poor.Water is still produced but carbon monoxide and carbon are produced instead of carbon dioxide.

Equation

hydrocarbon + oxygen → carbon monoxide + carbon + water

The carbon is released as carbon soot. Carbon monoxide is a poisonous gas which binds to red blood cells alot easier than oxygen does causing repiratory problems.Gas fires and boilers have to be serviced regularly to ensure they do not produce carbon monoxide.

Example of Incomplete combustion of propane

propane + oxygen → carbon + water

C3H8 + 2O2 → 3C + 4H2O

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Alkanes

Alkanes are a homologus series of hydrocarbons. This means that they have similar chemical properties to each other and they trends in physical properties. As chain length increases, their boiling point also increases.

A straight chain of alkanes share a general formula:

The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms plus two. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the covalent bonds between them by a straight line.

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Alkanes

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Fractional distillation -Crude oil

Crude oil is made over a long period of time. 

Crude oil is a fossil fuel it is formed from the remains of plants and animals mainly plankton that died millions of years ago and were buried in mud. Over millions of years with high temperature and pressure, the remains turn to crude oil, which can be drilled up from rocks where it is found.

Fossil fuels like coal oil and gas are called non-renewable fuels as they take so long to make that they are being used up much faster than they're being formed.They are a finite resource.

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Fractional distillation- Separating hydrocarbons

Fractional distillation can be used to separate Hydrocarbon Fractions.

Crude oil is a mixture of lots of different hydrocarbons, most of which are alkanes.

The different compounds in crude oil are separated by fractional distillation.

How fractional distillation works

  • The oil is heated until most of it has turned to gas. The gas enters a fractionating column.
  • In the column thes a temperature gradient (it's hot at the bottom and gets coller as you go up).
  • The longer hydrocarbons have high boiling points. They condense back into liquids and drain out of the column early on, when they're near the bottom. The shorter hydrocarbons have lower boiling points. They condense and drain out much later on, near to the top of the column where it's cooler.
  • you will end up with the crude oil mixture separated out into different fractions. Each fraction contans a mixture of hydrocarbons that all contain a similar number of carbon atoms,so they have similar boiling points.
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Uses and Cracking of Crude Oil

Oil provides the fuel for most modern transport - cars, trains, planes the lot. Diesel oil, kerosene, Heavy fuel oil and LPG all come from crude oil.

The petrochemical industry uses some of the hydrocarbons from crude oil as feedstock to make new compounds for use in things like polymers, solvents , lubricants and detergents.

All the products you get from crude oil are examples of organic compounds. The reason that you get such a large variety of products is because carbon atoms can bond together to from different  groups called homologus series. These groups contain similar compunds with many properties in common. Alkanes and Alkenes.

Cracking of Crude oil

Cracking means splitting up Long-Chain Hydrocarbons

Short-chain hydrocarbons are flammable so they make good fuels and are in high demand. However long-chain hydrocarbons from thick liquids like tar which arent very useful this is why the longer chain hydrocarbons are turned into smaller chain hydrocarbons through cracking

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Uses and Cracking of Crude Oil

Cracking of Crude oil continued

Craking is a thermal decomposition reaction- breaking down molecules by heating them

The first step is to heat long chain hydrocarbons to vaporise them (turn them into a gas).

Then the vapour is passed over a hot powdered aluminium oxide catalyst.

The long chain molecules split apart on the surface of the pecks of catalyst- this is catalystic cracking.

you can also crack hydrocarbons if you vaporise them and mix them with steam and then heat them to a very high temperature. this is known as steam cracking.

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Alkenes

Alkenes have a C=C Double bond

Alkenes are hydrocarbons which have a double bond between two of the carbon atoms in their chain.

The C=C double bond means that alkenes have two fewer hydrogens compared to alkanes but they do contain the same number of carbon atoms this makes them unsaturated.

The C=C double bond can open up to make a single bond, allowing the two carbon atoms to bond with other atoms. This makes alkenes reactive far more reactive than alkanes.

The general formula for alkenes is CnH2n 

Alkenes burn with a smokey flame.

in large ammounts of oxygen alkenes combust completly to produce only water and carbon dioxide however there isnt enough oxygen in the air fot this so when you burn alkenes they tend to undergo incomplete combustion. Carbon dioxide and water are still produced but you can also get Carbon or Carbon monoxide

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Alkenes-Reactions of alkenes

Alkenes react by different addition reactions.

A functional group is a group of atoms in a molecule that determines how the molecule reacts. 

All alkenes have the fuctional group C=C so they all react in similar ways

Addition of hydrogen is known as Hydrogenation

Hydrogen can react with the double bonded carbons to open up the double bond and from the equivilent, saturated alkane. The alkene is reacted with hydrogen in the presence of a catalyst.

Steam can react with Alkenes to form Alcohols 

When alkenes react with steam water is added across the double bond and an alcohol is formed.

For example ethanol can be made by mixing ethene with steam then passing it over a catalyst.

The conversion of ethene to ethanol is one way of making ethanol industrally, After the reaction has taken place, the reaction mixture is passed from the reactor into a condenser. Ethanol and water have a higher boiling point than ethene so both condense whilst any unreacted ethene gas is recycled back into the reactor . The alcohol can then be purified from the mixture by Fractional distillation.

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Alkenes-Halogens reacting with Alkenes

Alkenes will also react in addition reactions with halogens such as bromine, chlorine and iodine.

the molecules formed from this addition reaction are saturated with the C=C double bonds each becoming bonded to a halogen atom.

For example bromine and ethene react together to form dibromoethane.

The addition of bromine to a double bond can be used to test for alkenes.

when orange bromine water  is added to a saturated compound like an alkane, no  reaction will happen and the water will stay bright orange.

Howver if bromine water is added to and alkene the bromine will add across the double bond, leaving a colourless dibromo-compound - so the bromine water is decolourised 

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