Chemistry 1b

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AQA Chemistry unit 1b CGP revision guide.

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What is cracking and what does it do? 1) Cracking means splitting up long-chain hydrocarbons. 2) Long-chain hydrocarbons form thick gloomy liquids like tar which aren't all that useful so…. 3)….. a lot of the longer molecules produced from fractional distillation are turned into smaller ones by a process called cracking. 4) Cracking also produces substances like ethene, which are needed for making plastics. 5) e.g. diesel cracked makes petrol, paraffin and ethene for plastics.
Describe the process of cracking crude oil. 1) Cracking is a thermal decomposition reaction - braking molecules down by heating them. 2) The first step is to heat the long-chain hydrocarbon to vaporise it (turn it into a gas). 3) Then the vapour is passed over a powdered catalyst at a temperature of about 400 degrees to 700 degrees. 4) Aluminium oxide is the catalyst used. 5) The long-chain molecules split apart or "crack" on the surface of the specks of catalyst. 6) Most of the products of cracking are alkanes and unsaturated hydrocarbons called alkenes.
What is produced when kerosene is produced? Kerosene (ten C atoms) too much of this in crude oil is cracked to form Octane (single bonds) (eight C atoms) which is used for petrol + ethene (two C atoms) (double bond between the two C atoms) for making plastics.
What is another way of cracking long-chain hydrocarbons? An alternative way of cracking long-chain hydrocarbons is to mix the vapour with steam at a very high temperature.
What are alkenes? 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 bond can open up, allowing the two carbon atoms to bond with other atoms.
What are the first two alkenes? The first two alkenes are Ethene (C2H4) and Propene (C3H6).
What is the general formula for alkenes? All alkenes have the general formula: Cn+H2n - they have twice as many hydrogens as carbons.
Draw Ethene and Propene. 1) Ethene C2H4 has 2 carbons which are connected with a double bond so each carbon is still making four bonds. The carbon atoms always have four bonds by hydrogen atoms only make 1. 2 hydrogen atoms are connected to each carbon. 2) Propene C3H6. Has 3 carbon joined with one single and one double bond. The first carbon has 3 hydrogens attached to it, the second has one and the last has two hydrogen atoms attached to it.
How can you test for alkenes? 1) 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 bond with the bromine.
How can ethene be reacted to produce ethanol? 1) Ethene (C2H4) can be hydrated with stem (H2O) in the presence of a catalyst to make ethanol. 2) At the moment this is a cheap process, because ethene's fairly cheap and not much of it is wasted. 3) The trouble is that ethene'd 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.
How can ethanol also be produced from renewable resources? 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.
What are the advantages and disadvantages of making ethanol from renewable resources? 1) This process needs a lower temperature and simpler equipment than when using ethene. 2) 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. 3) The ethanol produced this way can also be used as quite a cheap fuel in countries which don't have oil reserves for making petrol. 4) There are disadvantages though. The ethanol you get from this process isn't very concentrated , so if you want to increase its strength you have to distil it (as in whisky distilleries). It also needs to be purified.
How can alkenes be used to make polymers? 1) Probably the most 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.
Explain how you make the polymer polythene. Many ethene molecules can be joined up to produce poly(ethene) or "polythene". The polymers are often written without the brackets e.g. polyethene. Many monomers with pressure and a catalyst form the polymer. n(C=C) many single ethenes = -(C-C)-n poly(ethene). 2) In the same way, if you join propene molecules together you've got poly(propene).
Why do 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 polymerisation. Poly(ethene) made at 200 degrees and 2000 atmosphere pressure is flexible, and has low density. But poly(ethene) made at 60 degrees and a few atmospheres pressure with a catalyst is rigid and dense.
How can the different properties of polymers make them suitable for various different uses? 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 Lyrca fibre for tights. 2) New uses are developed all the time. Waterproof coatings for fabrics are made of polymers. Dental polymers are used in resin tooth fillings. Polymer hydrogel wound dressings keep woulds 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 when you lie on them.
Why are the price of polymers changing and why don't they rot? 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 thing made from metals. 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 crude oil for fuel and plastics and other uses. Choosing how to use the oil that's left means weighing up advantages and disadvantages on all sides.
How can we extract oils from plants? 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). 2) These oils can be extracted and used for food or for fuel. 3) To get the oil out, the plant material 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 separated 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.
How are vegetable oils used? 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. 4) Vegetable oil are used in food.
What are the benefits of using vegetable oils for cooking? 1) Vegetable oils have higher boiling points than water. This means they can be used to cook foods at high temperatures and at faster speeds. 2) Cooking with vegetable oil gives food a different labour. This is because of the oil's own flavour, but it's also down to the fact that many flavours come from chemicals that are soluble 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.
How can vegetable oils be used to produce fuels? 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 oils 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.
How does the amount of bonds change in different oils and fats? 1) Oils and fats contain long-chain molecules with lots of carbon atoms. 2) Oils and fats are either saturated or unsaturated. 3) Unsaturated oil contain double bonds between some of the carbon atoms in their carbon chains. 4) So, an unsaturated oil will decolourise bromine water (as the bromine opens up the double bond and joins on). 5) Monounsaturated fats contain one C=C double bond somewhere in their carbon chain. Polyunsaturated fats contain more than one C=C double bond.
How can unsaturated oils be hydrogenated? 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. This is called hydrogenation. The hydrogen reacts with the double-bonded carbons and opens out the double bonds.
What happens to the oil when it is hydrogenated? 1) Hydrogenated oils have high 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.
How is hydrogenating vegetable oil helpful to margarine? 1) Margarine is usually made from partially hydrogenated vegetable oil - turning all the double bonds in vegetable oil to single bonds would make margarine too hard and difficult to spread. Hydrogenating most of them gives margarine a nice, buttery, spreadable consistency. 2) Partially hydrogenated vegetable oils are often 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. 3) But partially hydrogenated vegetable oils means you end up with a lot of so-called trans fats. And evidence shows that trans fats are very bad for you.
How can vegetable oils in food affect health? 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 fats increase the amount of cholesterol in the blood, which can block up 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 saturate, unsaturated or partially unsaturated makes it more fattening.
How can emulsions be made from oil and water? 1) Oils dissolve in water. 2) However, you can mix oil with water to make an emulsion. Emulsions are made up of lots of droplets of one liquid suspended in another liquid. You can have an oil-in-water emulsion (oil droplets suspended in water) or a water-in-oil emulsion (water droplets suspended in oil). 3) 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.
How do the properties of emulsions change and how are the properties good for different uses? 1) 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 vinegar. 2) 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% of oil in full fat milk. Single cream has a bit more oil - about 18%. Double cream has lots of oil - nearly 50%. 3) Whipped cream and ice cream are oil-in-water emulsions with an extra ingredient - air. Air is whipped into 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 it easier to scoop out of the tub. 4) Emulsions also have non-food uses. Most moisturising lotions are oil-in-water emulsions. The smooth texture of an emulsion make it easy to rub into the skin.
Why do some foods contain emulsifiers? Oil and water mixtures naturally separate out. But here's where emulsifiers come in… 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 cosies up to oil molecules. 4) When you shake 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 oil droplets are repelled by the hydrophilic bit of the emulsifier, while water molecules latch on so the emulsion won't separate out.
What are the pros and cons of using emulsifiers? 1) Emulsifiers stop emulsions from separating out and this gives them a longer shelf-life. 2) Emulsifiers allow food companies to produce food that's 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 very carefully.
What was the first part of Wegener's theory of continental drift? 1) Alfred Wegener came across some work listing fossils of very similar plants and animals which had been found on 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 probably 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 has 'sunk' or been covered over since then.
What was the second part of Wegener's theory? 1) But Wegener has also noticed that the coastlines of Africa and South America seemed to 'match' like the 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. 2) He found that there were matching layers in the rocks of different continents. 3) 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 clearly have killed them off. 4) In 1915, Wegener felt he had enough evidence. He published his theory of "continental drift". 5) Wegener said that about 300 million years ago, there had been just one 'supercontinent'. This landmass, Pangaea, broke into smaller chunks - our modern-day continents - were still slowly 'drifting' apart.
Why wasn't Wegener's theory accepted? The reaction from other scientists was mostly very hostile. The main problem was that Wegener's explanation of how the 'drifting' happened wasn't very convincing. 1) Wegener though that the continents were 'ploughing through' the sea bed, and that their movement was caused by tidal force and the earth's rotation. 2) Other geologists said this was impossible. One scientist calculated the forces needed to move continents like this would have stopped the earth rotating. 3) Wegener has used inaccurate data in his calculations, so he'd made some rather wild predictions 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.
What was the outcome of Wegener's theory? 1) 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. 2) By the 1960s, geologists were convinced. We now think 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.
What is the Earth made up of? 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 id the core, which we think is made of iron and nickel.
What is the Earth's surface made up of? 1) The crust and the upper part of the mantle are cracked into a number of large pieces called tectonic plates. Theses 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 of the plates are moving at speeds of a few cm per year relative to each other. 4) Occasionally, the plates move very suddenly, causing an earthquake. 5) Volcanoes and earthquakes often occur at the boundaries between two tectonic plates.
Why can't scientists predict earthquakes and volcanic eruption? What are they doing to help predict them? 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's likely to happen, not exactly when it'll happen. 3) There are some clues that say a volcanic eruption 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 erupting, so mini-earthquakes can be a false alarm.
What is phase 1 of the evolution of the atmosphere? 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 erupting. 3) The volcanoes gave out lots of gas. We think this was how the oceans and atmosphere were formed. 4) The early atmosphere was probably mostly carbon dioxide, with virtually no oxygen. There may also have been water vapour, and small amounts of methane and ammonia. This is quite like the atmospheres of Mars and Venus today. 5) The oceans formed when the water vapour condensed.
What is phase 2 of the evolution of the atmosphere? 1) Green plants and algae evolved over most of the Earth. They were quite happy in the carbon dioxide atmosphere. 2) A lot of the early carbon dioxide dissolved into the oceans. The green plants and algae also absorbed some of the carbon dioxide and produced oxygen by photosynthesis. 3) Plants and algae died and were buried under layers of sediment, along with the skeletons and shells of marine organisms that had slowly evolved. The carbon and hydrocarbons inside them became 'locked up' in sedimentary rocks and insoluble carbonates (e.g. limestone) and fossil fuels. 4) When we burn fossil fuels today, this 'locked up' carbon is released and the concentration of carbon dioxide in the atmosphere rises.
What is phase 3 of the evolution of the atmosphere? 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. 3) There is virtually no carbon dioxide left now.
What is the theory of primordial soup? 1) The primordial soup theory stated 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, resulting 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 the 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 the theory could be along the right line, but isn't quite right.
How does the earth have all resources humans need? The Earth's crust, oceans and atmosphere are the ultimate source of 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 and becomes a liquid. 3) During cooling water vapour condenses and is removed. 4) Carbon dioxide freezes and is removed. 5) The liquified air then enters the fractionating column and is heated slowly. 6) The remaining gases are separated by fractional distillation. Oxygen and argon come out together so another column is used to separate them.
How does increasing carbon dioxide levels affect the climate and the oceans? Burning fossil fuels releases carbon dioxide - and as the world's become more industrialised, more fossil fuels have been burnt in power stations and in car engines. This carbon dioxide 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 carbon dioxide - they absorb it from the atmosphere. However the extra carbon dioxide 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 any more carbon dioxide.
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