Chemistry 1b

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  • Created on: 23-03-14 15:24

Cracking Crude Oil

Long-chain bydrocarbons form thick gloopy liquids like tar, which aren't very useful. The longer molecules produced from fractional distillation are turned into smaller ones by a process called cracking. Some of the products from cracking are useful fuels.

Cracking also produces substances like ethene which is needed to make plastic.

Cracking is a thermal decomposition reaction, the breaking down of molecules by heating them. First its heated to vapourise it, then the vapour is passed over a powdered catalyst at a temperature of 400-700degrees. Aluminium oxide is the catalyst used. The longchain molecules split apart or 'crack' on the surface of the specks of catalyst.

Most products of cracking are alkanes and unsaturated hydrocarbons called alkenes.

long-chain hydrocarbon = shorter Alkane + Alkene

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

They are known as unsaturated because they can make more bonds - the double bond can open up, allowing the two carbon atoms to bond with other atoms.

The first two alkenes are ethene and propene.

All alkenes have the general formula CnH2n - They have twice as many hydrogens as carbons.

You can test for an alkene by adding bromine water. An alkene will decolourise the bromine water from orange to colourless.

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Ethene C2H4 can be hydrated with steam in the presence of a catalyst to make ethnol. This is a cheap process, because ethene's fairly cheap and not much is wasted. Ethene's produced from crude oil, it's a non-renewable source so it could soon run out. This means that soon making ethanol will become expensive.

The alcohol in beer etc isn't made from ethene, it's made by fermentation. The raw material for fermentation is sugar. Sugar is converted into ethanol using yeast.

sugar = carbon dioxide + ethanol

It needs lower temperature than using ethene. Also it's a renewable source, as sugar is grown as a crop. It can also be a cheap fuel for countries that don't have oil reserves for making petrol.

It isn't very concentrated if you want to increase it you have to distil it, it also needs to be purified.

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Alkenes to make polymers

A useful thing to do with Alkenes is polymerisation. It is where you join together lots of small alkene molecules (monomers) to form very large molecules called polymers.

Ethene can be  joined up to make poly(ethene).

The properties of polymers depend on what it's made from, A polymer's physical properties are also affected by the temperature and pressure of polymerisation.

For example poly(ethene) made at 200degrees and 2000atmospheres pressure is flexible and has low density. But if it was made at 60degrees and a few atmospheres pressure with a catalyst is rigid and dense.

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Plant Oils

Fruits and seeds contain a lot of oil. They can be extracted and used for food or fuel. To get the oil out the plant is crushed. The next step is to press the crushed plant material between metal plates and squash the oil out.

Oil can be separated from crushed plant material by a centrifuge.  Or solvents can be used to get oil. Distillation refines oil, and removes water, solvents and impurities.

Vegetable oils produce a lot of energy, some contain different vitamins. Vegetable oils contain essential fatty acids, needed for many metabolic processes. They have higher boiling poijnts than water so they can cook at faster speeds. It also gives food flavour, although its also down to the fact that many flavours come from chemicals that are soluble in oil.

Vegetable oils can be processed and turned into fuels, as they provide a lot of energy. The fuel is called biodiesel.

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Unsaturated Oils

Oils and fats contain long-chain molecules with lots of carbon atoms. Oils and fats are either saturated or unsaturated. Unsaturated oils contai double bonds between some of the carbon atoms in the chains. An unsaturated oil will decolourise bromine water. Monosaturated fats contain one C=C double bond somewhere in the chains. Polyunsaturated fats contain more that one C=C double bond.

Unsaturated oils can be hydrogenated. The can be hardened by reacting them with hydrogen over a nickel catalyst at 60degrees, it's called hydrogenation. The hydrogen reacts with the double bonded carbon atoms adn opens out the double bonds.

Hydrogenated oils have higher melting points than unsaturated, so they're more solid. Magarine is made from partially hydrogenated vegetable oil. Partially hydrogenated oils are a lot cheaper but they keep longer, but you end up with a lot of trans-fatss and they're very bad for you.

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Oil and water mixtures usually separate out. Emulsifiers are molecules with one part thats attracted to water, hydrophilic, and another part thats attracted to oil, hydrophobic.

Emulsifiers stopemulsions from spreading out giving them a longer shelf-life. They allow companies to produce food thats low fat with a good texture. The downside is that some people are allergic to certain emulsifiers.

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Plate tectonics

Wegeners theory:

Alfred wegener came across the fossil listing of very similar plants and animals which have been found on opposite sides of the ocean. He foundother cases of this happening too. He had explained that there had been landbridges between the continents, linking them, so animals would be able to cross.

He also noticed that south america and africa seemed to match like pieces of a jigsaw. There were also matching layers of rock in the continents. He published his theory of continental drift.

He said that 300million years ago there was a supercontinent, Pangaea, broke into smaller chunks which moved apart. He claimed that these chunks were the modern day continents.

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Tectonics of plates

Wegener thought that continents movement was caused by tidal forces and the earths rotation.

Other geologists said that it was impossible, some said that forces needed to move the continents would have stopped the earth rotating.

Wegener had inaccurate data in his calculations, so he made wild predictions.

No one saw a reason to believe such a strange theory. He wasn't a proper geologist, he studied astronomy.

In the 1950's they were allowed to investigate the ocean floor, new evidence was fiun d tosupport the theory, by the 1960's geologists were convinced, the earths crust wwas made of several chunks called tectonic plates.

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The Earth's Structure

The earth is almost spherical. The bit we live on is called the crust, it's very thin and is surrounded by the atmosphere. Below is the mantle, it has all the properties of a solid except that it can flow slowly. Within the mantle, radioactive decay takes place, it produces a lot of heat which causes the mantle to flow in convection currents. At the centre of the earth is the core which we think is made from iron and nickel.

The crust and uppermantle are cracked into pieces called tectonic plates. They don't stay in one place because the convection currents cause it to drift. When they move very suddenly they can cause an earthquake. They occur at plate boundaries.

It's impossible to predict when they'll move. But scientists can see clues, like strain in rocks, or small buges in the ground but they only make it likely and not certain that these things will happen.

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The Atmosphere

The atmosphere is made up of 78% nitrogen, 21% oxygen and small amounts of other gases, carbon dioxide, noble gases and water vapour.

First the volcanoes gave out gases and the atmosphere would have been mainly carbon dioxide with virtually no oxygen. The oceans formed when the water vapour condensed.

Green plants evolved and produced oxygen. A lot of the carbon dioxide dissolved in to the oceans, also some of it was absorbed by plants and produced oxygen by photosynthesis. The carbon and hydrocarbons locked up in sedimentary rocks as insoluble carbonates (e.g limestone) and fossil fuels.

Ozone layer allows evolution of complex animals. The build of oxygen killed off some early organisms that couldn't tlerate it. It also created the ozone layr which blocks harmful rays from the sun and enables more complex organisms to evolve.

There is virtually no carbon dioxide left now.

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Primordial Soup

The primordial soup theory states that billions of years ago the earht's atmosphere was rich in nitrogen, hydrogen, ammonia and methane. Lightning struck causing chemical reactions between the gases resulting in amino acids.They collected together in a 'primordial soup' and gradually combined to produce organic matter, it eventually evolved into living organisms.

In the 1950's miller and urey carried an experiment to prove the theory. They sealed gases in their apparatus and applied an electrical charge for a week. They found amino acids were made but not as many as on earth.

We have to fractionally distil air to get a variety of products:

Air is filtered removing dust. It's then cooled around -200degrees and becomes a liquid. Water vapour condenses and is removed. Carbon dioxide freezes and is removed. The liquified air enters the fractional distillation column and is heated slowly. The remaining gases separate. Oxygen and argon come out together so another column is used.

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