AQA Chemistry 1b

Cracking means splitting up long-chain hydrocarbons.

1) Long-chain hydrocarbons form thick gloppy liquids like tar which aren't all that useful.

2) So a lot of the longer molecules produced from fractional distilation are turned into small ones by a process called cracking.

3) Some of the products of cracking are useful as fuels, e.g. petrol for cars and parrafin for jet fuel.

4) Cracking also produces substances like ethene, which are needed for making plastics.

1) Cracking is a thermal decompositon reaction - breaking molecules down by heating them.

2) The first stepis to heat the long-chain hydrocarbon to vapourise it (turn it into gas).

3) Then the vapour is passed over a powdered catalyst at a temperature of about 400 - 700 degrees centigrade.

4) Aluminium oxide is the catalyst used.

5) The long chain molecules split apart or "crack" on the surface of th specks of catalyst.

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

Kerosene (ten C atoms) --> Octane (eight C atoms) + ethene (two C atoms)

An alternative way of cracking long-chain hydrocarbons is to mix the vapour with steam at a very high temperature.

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

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

3) The first two alkenes are ethene and propene.

4) All alkenes have the general formula: CnH2n - they have twice as many hydrogens as carbons.

5) You can test for an alkene by adding the substance to bromine water. An alkene will decolourise the bromine water, turning it from orange to colourless. This is because the double bond has opened up and formed bonds with the bromine.

bromine water + alkene --> decolourised

Ethene can be reacted with steam to produce ethanol.

1) Ethene (C2H4) can be hydrated with steam (H2O) in the presence of a catalyst to make ethanol.

2) At the moment this is a cheap process, because ethene's fairy cheap and not much of it is wasted.

3) The trouble is that ethene's produced from crude oil, which is a non-renewable resource that could start running out fairly soon. This means using ethene to make ethanol will become very expensive.

The alcohol in beer and wine, etc. isn't made from ethene - it's made by fermentation.

1) The raw material for fermentation is sugar. This is converted into ethanol using yeast. The word equation for this is: sugar --> carbon dioxide + ethanol

2) This process needs a lower temperature and similar equipment than when using ethene.

3) Another advantage is that the raw material is a renewable resource. Sugar is grown as a major crop in several parts of the world, including many poorer countries.

4) The ethanol produced this way can also be used as a quite cheap fuel in countries which don't have oil reserves for making petrol.

5) There are disadvantages though. The ethanol you get from this process isn't very concentrated, so if you want to increase it's strength you have to distil it (as in whiskey distilleries). It also needs to be purified.

Alkenes can be used to make polymers.

1) Probably themost useful thing you can do with alkenes is polymerisation. This means joining together lots of small alkene molecules (monomers) to form very large molecules - these long-chain molecules are called polymers.

2) For instance, many ethene molecules can be joined up to produce poly(ethene) or "polythene".

many monomers -- (pressure and catalyst) --> polymer

3) In the same way, if you join lots of propene molecules together, you've got poly(propene).

Different polymers have different physical properties.

1) The physical properties of a polymer depend on what it's made from. Polyamides are usually stronger than poly(ethene) for example.

2) A polymer's physical properties are also affected by the temperature and pressure of the polymeristaion. Poly(ethene) made at 200 degrees C and 2000 atmospheres pressure is flexible, and has low density. But poly(ethene) made at 60 degrees C and a few atmospheres pressure with a catalyst is rigid and dense.

1) Light, stretchable polymers such as low density poly(ethene) are used to make plastic bags. Elastic polymer fibres are used to make super-stretchy LYCRA fibre for tights.

2) New uses are developed all the time. Waterproof coatings for fabrics are made from polymers. Dental polymers are used in resin tooth fillings. Polymer hydrogel wound dressings keep wounds moist.

3) New biodegradable packaging materials made from polymers and cornstarch are being produced.

4) Memory foam is an example of a smart material. It's a polymer that gets softer as it gets warmer. Mattresses can be made of memory foam - they mould to your body shape when you lie on them.

1) Most polymers aren't "biodegradable" - they're not broken down by microorganisms, so they don't rot.

2) It's difficult to get rid of them - if you bury them in a landfill site, they'll still be there years later. The best thing is to re-use them as many times as possible and then recycle them if you can.

3) Things made from polymers are usually cheaper than things made from metal. However, as crude oil resources get used up, the price of crude oil will rise. Crude oil products like polymers will get dearer.

4) It may be that one day there won't be enough oil for fuel AND plastics AND all the other uses. Choosing how to use the oil that's left means weighing up advantages and disadvantages on all sides.

Naturally occuring polymers: silk & rubber

Synthetic polymers: polyester & PVC

1) Some fruits and seeds contain a lot of oil. For example, avocados and olives are oily fruits. Brazil nuts, peanuts and sesame seeds are oily seeds (a nut is just a big seed really).

2) These oils can be extracted and used for food or for fuel.

3) To get the oil out, the plant meterial is crushed. The next step is to press the crushed plant material between metal plates and squash the oil out. This is the traditional method of producing olive oil.

4) Oil can be seperated from crushed plant material by a centrifuge - rather like using a spin dryer to get water out of wet clothes.

5) Or solvents can be used to get oil from plant material.

6) Distillation refines oil, and removes water, solvents and impurities.

1) Vegetable oils provide a lot of energy - they have a very high energy content.

2) There are other nutrients in vegetable oils. For example, oils from seeds contain vitamin E.

3) Vegetable oils contain essential fatty acids, which the body needs for many metabolic processes.

1) Vegetableoils have higher boiling points than water. This means they can be used to cook foods at higher temperatures and faster speeds.

2) Cooking with vegetable oil gives food a different flavour. This is because of the oil's own flavour,  but it's also down to the fact that many flavours come from the chemicals that are soluable in oil. This means the oil 'carries' the flavour, making it seem more intense.

3) Using oil to cook food increases the energy we get from eating it.

1) Vegetable oils such as rapeseed oil and soybean oil can be processed and turned into fuels.

2) Because vegetable oils provide a lot of energy they're really suitable for use as fuels.

3) A particularly useful fuel made from vegetable oil is called biodiesel. Biodiesel has similar properties to ordinary diesel fuel - it burns in the same way, so you can use it to fuel a diesel engine.

Oils are usually quite runny at room temperature - hydrogenate the oil to make margarine.

1) Oils and fats contain long chain molecules with lots of carbon atoms.

2) Oils and fats are either saturated or unsaturated.

3) Unsaturated oils contain double carbon bonds between some of the carbon atoms in their carbon chains.

4) So, an unsaturated oil will decolourise bromine water (as the bromine opens the double bond and joins on).

5) Monounsaturated fats contain on C=C double bond in their carbon chains. Polyunsaturated fats contain more than one C=C double bond.

1) Unsaturated vegetable oils are liquid at room temperature.

2) They can be hardened by reacting them with hydrogen in the presence of a nickel catalyst at about 60 degrees C. This is called hydrogenation. The hydrogen reacts with the double-bonded carbons and opens out the double bonds.

3) Hydrogenated oils have higher melting points than unsaturated oils, so they're more solid at room temperature. This makes them useful as spreads and for baking cakes and pastries.

4) Margarine is usually made from partially hydrogenated vegetable oil - turning all the double bonds in vegetable oil to single bonds would makemargarine too hard and difficult to spread. Hydrogenating most of them gives margarine a nice, buttery, spreadable consistency.

5) Partially hydrogenated vegetable oils areoften used instead of butter in processed foods, e.g. biscuits. These oils are a lot cheaper than butter and they keep longer. This makes biscuits cheaper and gives them a long shelf life.

6) But partially hydrogenating vegetable oils means you end up with a lot of so-called trans fats. And there is evidence to suggest that trans fats are very bad for you.

1) Vegetable oils tend to be unsaturated, while animal fats tend to be saturated.

2) In general, saturated fats are less healthy than unsaturated fats (as saturated fat increases the amount of chloesterol in the blood, which can block the arteries and increase the risk of heart disease).

3) Natural unsaturated fats such as olive oil and sunflower oil reduce the amount of blood cholesterol. But because of trans fats, partially hydrogenated vegetable oil increases the amount of cholesterol in the blood. So eating a lot of foods made with partially hydrogenated vegetable oils can actually increase the risk of heart disease.

4) Cooking food in oil, whether saturated, unsaturated or partially hydrogenated, makes it more fattening.

1) Oils don't dissolve in water. However you can mix an oil with water to make an emulsion. Emulsions are made up of lots of tiny droplets of one liquid suspended in another liquid. You can have an oil-in-water emulsion or a water-in-oil emulsion.

2) Emulsions are thicker than either oil or water. E.g. mayonnaise is an emulsion of sunflower oil (or olive oil) and vinegar - it's thicker than either.

3) The physical properties of emulsions make them suited to lots of uses in food - e.g. as salad dressings and in sauces. For instance, a salad dressing made by shaking olive oil and vinegar together forms an emulsion that coats salad better than plain oil or plain vingar.

4) Generally the more oil you've got in an oil-in-water emulsion, the thicker it is. Milk is an oil-in-water emulsion with not much oil and a lot of water - there's about 3% oil in full-fat milk. Single cream has a bit more oil - about 18%. Double cream has lots of oil - nearly 50%.

5) Whipped cream and ice cream are oil-in-water emulsions with an extra ingredient - air. Air is whipped into the cream to give it a fluffy, frothy consistency for use as a topping. Whipping air into ice cream gives it a softer texture, which makes in easier to scoop out of the tub.

6) Emulsions also have non-food uses. Most moisturising lotions are oil-in-water emulsions. The smooth texture of an emulsion makes it easy to rub into the skin.

Oil and water mixtures naturally seperate out.

1) Emulsifiers are molecules with one part that's attracted to water and another part that's attracted to oil or fat. The bit that's attracted to water is called hydrophilic, and the bit that's attracted to oil is called hydrophobic.

2) The hydrophilic end of each emulsifier molecule latches onto water molecules.

3) The hydrophobic end of each emulsifier molecule latches onto the oil molecules.

4) When you shake the oil and water together with a bit of emulsifier, the oil forms droplets, surrounded by a coating of emulsifier... with the hydrophilic bit facing outwards. Other droplets are repelled by the hydrophilic bit of the emulsifier, while water molecules latch on. So the emulsion won't seperate out.

1) Emulsifiers stop emulsions from separating out and this gives them a longer shelf life.

2) Emulsifiers allow food companies to produce food that is lower in fat but that still has a good texture.

3) The down side is that some people are allergic to certain emulsifiers. For example, egg yolk is often used as an emulsifier - so people who are allergic to eggs need to check the ingredients list very carefully.

1) Alfred Wegener came across some work listing the fossils of very similar plants and animals which had been found on the opposite sides of the Atlantic Ocean.

2) He investigated further and found other cases of very similar fossils on opposite sides of oceans.

3) Other people had previously noticed this too. The accepted explanation was that there had once been land bridges linking the continents - so animals had been able to cross. The bridges had 'sunk' or been covered over since then.

4) But Wegenger has also noticed that the coastlines of Africa and South America seemed to 'match' like pieces of a jigsaw. He wondered if these two continents had previously been one continent which then split.He started to look for more evidence, and found it...

5) There were matching layers in the rocks in different continents. 

6) Fossils had been found in the 'wrong' places - e.g. fossils of tropical plants had been discovered on Arctic islands, where the present climate would have killed them off.

In 1915, Wegener felt he had enough evidence. He published his theory of "continental drift".

Wegener said that about 300 million years ago, there had been just one 'supercontinent'. This land mass, Pangaea, broke into smaller chunks which moved apart.

He claimed that these chunks - our modern-day continents - were slowly 'drifting' apart.

The reaction from other scientists was mostly very hostile. Wegener's explanation couldn't explain how the 'drifting' happened.

1) Wegener thought that the continents were 'ploughing through' the sea bed, and that their movement was caused by tidal forces and the earth's rotation.

2) Other geologists said this was impossible. One scientist calculated that the forces needed to move the continents like this would also have stopped the Earth rotating. (Which it hadn't.)

3) Wegener had used inaccurate data in his calculations, so he'd made some rather wild prdicitions about how fast the continents ought to be moving apart.

4) A few scientists supported Wegener, but most of them didn't see any reason to believe such a strange theory. It probably didn't help that he wasn't a 'proper' geologist - he'd studied astronomy.

5) Then in the 1950s, scientists were able to investigate the ocean floor and found new evidence to support Wegener's theory. He wasn't right about everything, but his main idea was correct.

6) By the 1960s, geologists were convinced. We now think that the Earth's crust is made of several chunks called tectonic plates which move about, and that colliding chunks push the land up to create mountains.

The Earth is almost spherical and it has a layered structure, a bit like a scotch egg. Or a peach.

1) The bit we live on, the crust, is very thin (it varies between 5km and 50km) and is surrounded by the atmosphere.

2) Below that is the mantle. The mantle has all the properties of a solid, except that it can flow very slowly.

3) Within the mantle, radioactive decay takes place. This produces a lot of heat, which causes the mantle to flow in convection currents.

4) At the centre of the Earth is the core, which we think is made of iron and nickel.

1) The crust and the upper part of the mantle are cracked into a number of large pieces called tectonic plates. These plates are a bit like big rafts that 'float' on the mantle.

2) The plates don't stay in one place though. That's because the convection currents in the mantle cause the plates to drift.

3) Most plates are moving at a speed of a few cm per year relative to each other.

4) Occassionally, the plates move very suddenly, causing an earthquake.

5) Volcanoes and earthquakes often occur at the boundaries between two tectonic plates.

1) Tectonic plates can stay more or less put for a while and then suddenly lurch forwards. It's impossible to predict exactly when they'll move.

2) Scientists are trying to find out if there are any clues that an earthquake might happen soon - things like strain in underground rocks. Even with these clues they'll only be able to say an earthquake is likely to happen, not exactly when it's likely to happen.

3) There are some clues that say a volcanic erruption might happen soon. Before an eruption, molten rock rises up into chambers near the surface, causing the ground surface to bulge slightly. This causes mini-earthquakes near the volcano.

4) But sometimes molten rock cools down instead of errupting, so mini-earthquakes can be a false alarm.

For 200 million years or so, the atmosphere has been about how it is now:

78% nitrogen, 21% oxygen and small amounts of other gases, mainly carbon dioxide, noble gases and water vapour.

It hasn't always been like this.

Here's how the past 4.5 billion years may have gone:

1) The Earth's surface was originally molten for many millions of years. It was so hot that any atmosphere just 'boiled away' into space.

2) Eventually things cooled down a bit and a thin crust formed, but volcanoes kept errupting.

3) The volcanoes gave out lots of gas. We think this is how the oceans and atmosphere were formed.

4) The early atmosphere was probably mostly CO2, with virtually no oxygen. There may also have been water vapour, and small amounts of methane and ammonia.

5) The oceans formed when the water vapour condensed.

1) Green plants and algae evolved over most of the Earth. They were quite happy in the CO2 atmosphere.

2) A lot of the early CO2 dissolved into the oceans. The green plants and algae also absorbed some of the CO2 and produced O2 by photosynthesis.

3) Plants and algae died and were burried under layers of sediment, along with skeletons and shells of marine organisms that had slowly evolved. The carbon and hydrocarbons inside them became 'locked up' in sedimentary rocks as insoluable carbonates (e.g. limestone) and fossil fuels.

4) When we burn fossil fuels today, this 'locked up' cabron is released and the concentration of CO2 in the atmosphere rises.

1) The build up of oxygen in the atmosphere killed off some early organisms that couldn't tolerate it, but allowed other, more complex organisms to evolve and flourish.

2) The oxygen also created the ozone layer (O3) which blocked harmful rays from the Sun and enabled even more complex organisms to evolve - us, eventually.

3) There is virtually no CO2 left now.

1) The primordial soup theory states that billions of years ago, the Earth's atmosphere was rich in nitrogen, hydrogen, ammonia and methane.

2) Lightning struck, causing a chemical reaction between the gases, resultuing in the formation of amino acids.

3) The amino acids collected in a 'primordial soup' - a body of water out of which life gradually crawled.

4) The amino acids gradually combined to produce organic matter which eventually evolved into simple living organisms.

5) In the 1950s, Miller and Urey carried out an experiment to prove this theory. They sealed the gases in their apparatus, heated them and applied an electrical charge for a week.

6) They found that amino acids were made, but not as many as there are on Earth. This suggests that the theory could be along the right lines, but isn't quite right.

The Earth's crust, oceans and atmosphere are the ultimate sourceof minerals and resources - we can get everything we need from them. For example, we can fractionally distil air to get a variety of products (e.g. nitrogen and oxygen) for use in industry:

1) Air is filtered to remove dust.

2) It's then cooled to around -200 degrees C and becomes a liquid.

3) During cooling water vapour condenses and is removed.

4) The liquified air then enters the fractionating column and is heated slowly.

5) The remaining gases are seperated by fractional distilation. Oxygen and argon come out together so another column is used to separate them.

Burning fossil fuels releases CO2 - and as the world's become more industrialised, more fossil fuels have been burnt in power stations and in car engines. This CO2 is thought to be altering our planet.

1) An increase in carbon dioxide is causing global warming - a type of climate change.

2) The oceans are a natural store of CO2 - they absorb it from the atmosphere. However the extra CO2 we're releasing is making them too acidic. This is bad news for coral and shellfish, and also means that in the future they won't be able to absorb anymore carbon dioxide.