AQA Core Science: Oils, Earth and the atmosphere

The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet. The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today.

The Earth is almost a sphere. These are its main layers, starting with the outermost:

1. crust - relatively thin and rocky
2. mantle - has the properties of a solid, but can flow very slowly
3. outer core - made from liquid nickel and iron
4. inner core - made from solid nickel and iron

Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.

The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.

It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in places known for having earthquakes.

The theory of plate tectonics and continental drift were proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before his time it was believed that the planet's features, such as mountains, were caused by the crust shrinking as the Earth cooled after it was formed.

It took more than 50 years for Wegener’s theory to be accepted. This was because it was difficult to work out what the mechanism was that could make whole continents move, and it was not until the 1960s that enough evidence was discovered to support the theory fully.

Wegener's evidence states that plate tectonics explained why earthquakes and volcanoes were concentrated in specific places - around the boundaries of moving plates and that the match in shape between the east coast of South America and the west coast of Africa suggests both were once part of a single continent. There are similar patterns of rocks and similar fossils on both sides of the Atlantic - including the fossil remains of land animals that would have been unable to swim across an ocean.

You need to know the proportions of the main gases in the atmosphere. The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere. It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire, and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere. The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.
 

Gas syringes are used to measure the volume of gas in the experiment. The starting volume of air is often 100cm³ to make the analysis of the results easy, but it could be any convenient volume. In the simulation, there is 100cm³ of air at the start. The percentage of oxygen in the air can be measured by passing a known volume of air over hot copper, and measuring the decrease in volume as the oxygen reacts with it. Here are the equations for this reaction:

copper + oxygen    →    copper oxide

2Cu + O₂    →    2CuO

Argon makes up about 0.9 per cent of the air. It is one of a group of elements called the noble gases. The noble gases are in Group 0 of the periodic table.

The noble gases are all chemically unreactive gases.

• Helium is used in balloons and airships. It is much less dense than air, so balloons filled with it float upwards.
• Neon is used in advertising signs, it glows when electricity is passed through it. Different coloured neon lights can be made by coating the inside of the glass tubing of the lights with other chemicals.
• Argon is used in light bulbs. The very thin metal filament inside the bulb would react with oxygen and burn away if the bulb were filled with air instead of argon. Argon stops the filament burning away because it is unreactive.
• Krypton is used in lasers. Krypton lasers are used by surgeons to treat certain eye problems and to remove birthmarks.

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.

The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans. It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

The proportion of oxygen went up because of photosynthesis by plants.
The proportion of carbon dioxide went down because: it was locked up in sedimentary rocks, such as limestone, and in fossil fuels, absorbed by plants for photosynthesis and dissolved in the oceans. The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.

Vegetable oils are obtained from plants. They are important ingredients in many foods, and can be hardened through a chemical process to make, for example, margarine. They can also be used as fuels, for example as biodiesel.

Food additives are chemicals added by food manufacturers to certain foods, including vegetable oils, to improve their shelf-life, taste and appearance. Vegetable oils are natural oils found in seeds, nuts and some fruit.

The plant materials are crushed and pressed to squeeze the oil out. Olive oil is obtained this way. Sometimes the oil is more difficult to extract and has to be dissolved in a solvent. Once the oil is dissolved, the solvent is removed by distillation, and impurities such as water are also removed, to leave pure vegetable oil. Sunflower oil is obtained in this way.

Molecules of vegetable oils consist of glycerol and fatty acids. Glycerol has three carbon atoms and fatty acids have long chains of carbon atoms. Three long chains of carbon atoms are attached to a glycerol molecule, with its three carbon atoms and together they combine to make one molecule of vegetable oil.

The long fatty acid chains stop vegetable oils dissolving in water.

The fatty acids in some vegetable oils are saturated, and only have single bonds between their carbon atoms. Saturated oils tend to be solid at room temperature, and are sometimes called vegetable fats instead of oils. Lard is an example of a saturated oil.

The fatty acids in some vegetable oils are unsaturated, and have double bonds between some of their carbon atoms. Unsaturated oils tend to be liquid at room temperature, and are useful for frying food. They can be divided into two categories:

• Monounsaturated fats have one double bond in each fatty acid
• Polyunsaturated fats have many double bonds.

Unsaturated vegetable oils tend to be liquid at room temperature, but they can also be 'hardened', through a chemical process called hydrogenation, to make them solid at room temperature.

The carbon-carbon double bonds in unsaturated oils can be detected using the elements bromine or iodine. These elements react with the double bonds in the oils, and the more double bonds there are, the more bromine or iodine is used up.

You can check for unsaturated fats using a simple test with bromine water. The test is similar to one used to differentiate alkenes from alkanes.

Bromine water is a dilute solution of bromine, which is normally orange-brown in colour. It becomes colourless when shaken with an alkene, or with unsaturated fats. When shaken with alkanes or saturated fats, its colour remains the same.

During hydrogenation, vegetable oils are hardened by reacting them with hydrogen gas at about 60ºC. A nickel catalyst is used to speed up the reaction. The double bonds are converted to single bonds by the hydrogenation. In this way unsaturated fats can be made into saturated fats.

Saturated vegetable oils are solid at room temperature, and have a higher melting point than unsaturated oils. This makes them suitable for making margarine, or for commercial use in the making of cakes and pastry.

Vegetable oils are important nutrients and provide a lot of energy. You must be careful not to eat excessive amounts to avoid becoming overweight. Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste cooking oil and rapeseed oil. Such fuels are carbon neutral, which means that they release only as much carbon dioxide when they burn as was used to make the original oil by photosynthesis. This helps to reduce global warming. However, some people are concerned about whether it is ethical to use food crops in this way, instead of using them to feed hungry people.

Vegetable oils do not dissolve in water. If a mixture of oil and water is shaken, then left to stand, eventually a layer of oil will form on the surface of the water. If an emulsifier is added to the oil and water, a mixture called an emulsion forms. Emulsions are more viscous than oil or water on their own, and contain tiny droplets of one of the liquids spread through the other liquid. Examples of oil droplets in water: egg yolk, milk, salad cream and mayonnaise. Examples of water droplets in oil: margarine, butter, skin cream and moisturising lotion.

Partially hydrogenated vegetable oils may contain trans fats. These are thought to cause health problems such as heart disease in humans, and food manufacturers are being encouraged to reduce the amount of them in our food.

As we have seen, processed foods, including vegetable oils, may have chemicals added to them. These chemical additives have different jobs, including extending a food product’s shelf life and improving its taste and appearance. You can find additives listed on the ingredients label of such foods, and many of these additives have E numbers to identify them.

Colouring: tartrazine (E102) is used for orange colouring for soft drinks, sweets and sauces. Emulsifiers: lecithin (E322) allow oil and water mix to make margarine, ice cream and salad cream. Preservatives: benzoic acid (E210) are used in many foods to stop harmful micro-organisms growing. Sweeteners: Aspartame (E951) are used in low-calorie drinks and food.

Additives with an E number have been licensed by the European Union. Some are natural and some are artificial, but they have all been tested for safety and passed for use. The UK Food Standards Agency (FSA) has strict limits on the amount of colourings allowed in food. Some additives can lead people to have allergic reactions to them, and colourings are banned from use in baby foods.

In order for it to be useful to us, crude oil is broken down in oil refineries into its component parts, known as fractions, and these can then be used for many different purposes. Fractions that are produced by the distillation of crude oil can go through a process called cracking. This chemical reaction produces smaller hydrocarbons, including alkanes and alkenes. Ethene and other alkenes are unsaturated hydrocarbons and can be used to make polymers. Ethene can be used to make ethanol.

Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand - this is where cracking comes in. Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller hydrocarbon molecules. Cracking is an example of a thermal decomposition reaction.

Some of the smaller molecules formed by cracking are used as fuels, and some of them are used to make polymers in plastics manufacture.

The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula. This is C_nH_2n. The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C₂H₄ and propene is C₃H₆. Alkene molecules can be represented by displayed formulae, in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line. 

Alkenes are unsaturated hydrocarbons. They contain a double bond, which is shown as two lines between two of the carbon atoms. The presence of this double bond allows alkenes to react in ways that alkanes cannot. They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that. They can be used to make ethanol - alcohol - and polymers - plastics - two crucial products in today's world.

Propene: C₃H₆   Ethene: C₂H₄  

Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful as a fuel (for use in cars and other vehicles, usually mixed with petrol). Ethanol can be manufactured by reacting ethene (from cracking crude oil fractions) with steam. A catalyst of phosphoric acid is used to ensure a fast reaction.
ethene + steam ethanol

C₂H₄+ H₂O C₂H₅OH

Notice that ethanol is the only product. The process is continuous – as long as ethene and steam are fed into one end of the reaction vessel, ethanol will be produced. These features make it an efficient process, but there is a problem. Ethene is made from crude oil, which is a non-renewable resource. It cannot be replaced once it is used up and it will run out one day.

Ethanol can be made by reacting ethene with steam, but it can also be made by a process called fermentation. More than 90% of the world’s ethanol is made by fermentation. Sugar from plant material is converted into ethanol and carbon dioxide by fermentation. The enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this process happen:

C₆H₁₂O₆ 2C₂H₅OH + 2CO₂

Unlike ethene, sugar from plant material is a renewable resource.

Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers. In general:

lots of monomer molecules    →    a polymer molecule

Alkenes can act as monomers because they have a double bond:

• Ethene can polymerise to form poly(ethene), which is also called polythene.
• Propene can polymerise to form poly(propene), which is also called polypropylene.

Different polymers have different properties, so they have different uses.

Polymers have properties that depend on the chemicals they are made from, and the conditions in which they are made. Modern polymers have many uses, including: waterproof coatings, fillings for teeth, dressings for cuts, hydrogels for making soft contact lenses and disposable nappy liners and shape memory polymers for shrink-wrap packaging.

Polymer molecules can have branches coming off them, which change the properties of the polymer. 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 polymer. Unplasticised PVC, usually called uPVC, is used to make pipes and window frames. PVC with plasticisers is soft and flexible. It is used for floor coverings, raincoats and car dashboards.

Poly(ethenol) is a polymer that dissolves in water to make slime. The viscosity of the slime can be changed to make it thick or runny by varying the amount of water.

One of the useful properties of polymers is that they are unreactive, so they are suitable for storing food and chemicals safely. Unfortunately, this property makes it difficult to dispose of polymers.

Most polymers, including poly(ethene) and poly(propene) are not biodegradable. This means that micro-organisms cannot break them down, so they may last for many years in rubbish dumps. However, it is possible to include chemicals that cause the polymer to break down more quickly. Carrier bags and refuse bags made from such degradable polymers are already available. Polymers can be burnt or incinerated. They release a lot of heat energy when they burn and this can be used to heat homes or to generate electricity. There are problems with incineration. Carbon dioxide is produced, which adds to global warming. Toxic gases are produced unless the polymers are incinerated at high temperatures.

Many polymers can be recycled. This reduces the disposal problems and the amount of crude oil used. But the different polymers must be separated from each other first, and this can be difficult and expensive to do.