Chem6

Chem6 topics - crude oil, cracking, fuels, hydrocarbons, polymers

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Crude Oil

  • Crude oil is an ancient biomass found in rocks from which many useful materials can be produced
  • Crude oil is a mixture of hydrocarbons
  • Hydrocarbons are compounds (or molecules) which contain hydrogen and carbon atoms ONLY
  • Most of the hydrocarbons in crude oil are alkanes or saturated hyrdocarbons
  • The general formula for an alkane is CnH2n+2
  • Crude oil can be fractionally distilled
  • Fractional distillation is the separation of a liquid into fractions , each of which contains molecules with a similar number of carbon atoms, by evaporating the oil and allowing it to condense at a number of different temperatures, according to the different boiling points of the compounds
  • Some of the fractions can be used as fuel
  • Some properties of hydrocarbons depend on the size of their molecules. These properties influence how hydrocarbons are used as fuels
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Properties of Hydrocarbons

Short chain molecules:

  • Low boiling point
  • Low viscosity
  • Very flammable
  • Non-smokey flame

Long chain molecules:

  • High boiling point
  • Very viscose
  • Not very flammable
  • Smokey flame
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Burning fuels

  • Hydrocarbons burn in air to form carbon dioxide and water
  • Most fuels contain carbon and/or hydrogen and some contain sulphur
  • Gases released into the atmosphere when a fuel burns include CO2, water, CO and sulphur dioxide
  • The carbon monoxide occurs due to the incomplete combustion of the hydrocarbon. Sulphur dioxide occurs due to the combustion of sulphur impurities
  • Particles may also be released, which includes soot and unburnt fuels
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Cracking

  • Hydrocarbons can be broken down or cracked to produce smaller, more useful molecules. Fractions from the distillation of crude oil can be cracked to make molecules including unsaturated hydrocarbons such as ethene
  • This process involves heating hydrocarbons to vaporise them and then passing these vapours over a hot catalyst. A thermal decomposition occurs
  • The products of cracking include alkanes and alkenes (unsaturated hydrocarbons)
  • Alkenes have the general formula CnH2n
  • Alkenes have a carbon-carbon double bond
  • Some of the products of cracking are useful as fuels

           eg octane -> hexane + ethene

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Environmental issues

Cons:

  • Sulphur dioxide causes acid rain. Acid rain will react with metals and rocks, such as limestone, and damage buildings. It also makes lakes and rivers too acidic for aquatic life to survive
  • Carbon dioxide causes global warming and could change the Earth's climate, which will change global weather patterns and cause flooding, drought and erosion.
  • Particles cause global dimming which can change rainfall patterns
  • Carbon monoxide is poisonous and can affect the amount of oxygen our blood can carry
  • Cheap fuel encourages car travel and can lead to crowded roads and dirty/polluted town centres

Pros:

  • Sulphur can be removed from fuels before they are burnt e.g. in vehicles
  • Sulphur dioxide can be removed from waste gases after combustion e.g. in power stations
  • Oil-rich countries sell their crude oil to oil-poor countries, which boosts their economy and can lead to money spent on amenities such as hospitals and schools
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Vegetable Oils as fuels

Vegetable oils are produced from plants and can be used to make fuels. Biodiesel can be produced from vegetable oils.

Advantages of biodiesel:

  • The crops used to make diodisel absorb CO2 as they grow - the amount of CO2 released is nearly the same as that absorbed
  • Renewable
  • A very clean fuel and produces very little sulphur dioxide or particulates
  • Less harmfule to plants and animals than diesel
  • If spilled, it breaks down more quickly than diesel
  • As diesel prices increase, biodiesel is cheaper

Disadvantages of biodiesel:

  • Freezes at a higher temperature and turns to sludge
  • At high temperatures, the oil molecules oxidise and can form polymers which block engines
  • Large areas of farmland needed to grow crops cause the destruction of habitats and endanger species
  • There is not sufficient land available to use biodisel only in this country
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Vegetable Oils in foods

  • Vegetable oils can be used at a much higher temperature than boiling water, so the chemical reactions which occur are different.
  • The food cooks more quickly and the outside of the food becomes crispier and will change colour. The oil adds flavour.
  • Vegetable oils have a high energy content. The food absorbs come of the oil, which makes the energy content of the food higher. This is why fried food is bad for your health and can make you overweight.
  • You need some oil in your diet and you need energy to live
  • The saturated (no carbon-carbon double bonds) fats in things like animal fats, butter and cheese make the blood vessels in the heart clogged up with fat.
  • The un-saturated (carbon-carbon double bonds) fats in vegetable oils help keep arteries clear and reduce the risk of heart disease.
  • People who eat a lot of vegetable oil have a healthier level of cholesterol in their blood. 
  • Butter and hard margarines contain lots of saturated fats, whereas soft margarines contain more polyunsaturated fats
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Plant Oil

  • Some fruit, seeds and nuts are rich in oil that can be extracted. The plant material is crushed and the oil removed by pressure or in some cases by distillation
  • Vegetable oils are important foods and fuels as they provide a lot of energy and nutrients
  • Vegetable oils that are unsaturated can be detected using bromine or iodine. They turn from their initial colour - orange and reddy brown respecitvely - to colourless if a carbon-carbon double bond is present
  • Unsaturated vegetable oils can be hardened to make margarine by reacting them with hydrogen in the presence of a nickel catalyst at 60 degrees celcius. The hydrogenated oils have higher melting points so are soilds at room temperature, which makes them useful as spreads or in baking
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Additives

Additives are sometimes added to improve processed food. Additives must be listed in the ingredients and permitted additives are given E-numbers. Aditives can be identified through chromatography

Advantages:

  • Improve the appearance of food e.g. colourings
  • Improve the taste e.g. sugars, acids
  • Improve the shelf life e.g. antioxidants
  • Stop ingredients separating e.g. emulsifiers - oil and water will not dissolve together, but can form an emulsion which has better texture, coating ability and appearance
  • Sweetener has the came effect as sugar but doesn't damage teeth or contain the calories

Disadvantages:

  • Some additives are thought to affect children's behaviour making them lose concentration and become hyperactive
  • Some people may be allergic to some additives
  • They can be used to disguise substandard ingredients
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Ethanol - ethene and steam

Ethene  can be reacted with steam in the presence of a catalyst to produce ethanol. Ethanol can also be produced from sugars by fermentation.

Advantages of using ethene and steam:

  • It is a continuous process
  • Few workers are needed
  • Is a fast reaction
  • Produces a pure product

 Disadvantages of using ethene and steam:

  • It uses a non-renewable resource
  • It provides very few jobs
  • High temperature and pressure are needed for the reaction to occur
  • A lot of energy is needed

 

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Ethanol - sugars from plants

Advantages of using sugars from plant material:

  • It's renewable
  • Provides lots of jobs
  • Uses normal pressure and warm temperature
  • Better for alcoholic drinks
  • Only a little energy needed

Disadvantages of using sugars from plant material:

  • Batch production
  • Need to find a lot of workers
  • Slow rate of reaction
  • Produces an inpure product which needs treatment
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Polymers

  • Alkenes (unsaturated hydrocarbons) can be used to make polymers. In these reactions, many small molecules - monomers - join together to form very large molecules - polymers
  • The properties of polymers depend on what they're made from and the conditions in which they're made
  • Plasticisers are substances that let the polymer molecules slide over each other more easily. This makes the polymer softer and more flexible. For example, poly(chloroethene) or PVC is a hard poylmer which is used to make pipes  and window frames. However PVC with plasticisers is used make floor coverings, raincoats and car dashboards
  • Thermosetting polymers do not soften when heated, where as thermosoftening polymers do.
  • Polymers can be used for - waterproof coatings on fabrics, dental uses, wound dressings, hydrogels, food packaging and smart materials (e.g. shape memory polymers)
  • Polymers are unreactive, which is good for storing food and chemicals, but they are also non-biodegradable. This can lead to problems with waste disposal.
  • It is possible to include chemicals in polymers so that they break down more quickly
  • They can be incinerated which will produce a lot of energy which can be used to generate electricity for homes. However, CO2 is released.
  • Many polymers can be recycled which reduces crude oil used to make them, but it can be difficult and expensive to do so.
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