Metals are very useful. Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them: most everyday metals are mixtures called alloys.
Methods of extracting metals
The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To become useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in high enough concentration to make it economic to extract the metal. Ores are mined. They may need to be concentrated before the metal is extracted and purified. The economics of using a particular ore may change over time. For example, as a metal becomes rarer, an ore may be used when it was previously considered too expensive to mine. Reactivity and extraction method: Metals are produced when metal oxides are reduced (have their oxygen removed). The reduction method depends on the reactivity of the metal. For example, aluminium and other reactive metals are extracted by electrolysis, while iron and other less reactive metals may be extracted by reaction with carbon or carbon monoxide.
Phytomining, bioleaching and scrap iron
Some plants absorb copper compounds through their roots. They concentrate these compounds as a result of this. The plants can be burned to produce an ash that contains the copper compounds. This method of extraction is called phytomining. Some bacteria absorb copper compounds. They then produce solutions called leachates, which contain copper compounds. This method of extraction is calledbioleaching. Copper can also be extracted from solutions of copper salts using scrap iron. Iron is more reactive than copper, so it can displace copper from copper salts. For example: iron + copper sulfate → iron sulfate + copper
Aluminium and titanium Aluminium and titanium are two metals with a low density. This means that they are lightweight for their size. They also have a very thin layer of their oxides on the surface, which stops air and water getting to the metal, so aluminium and titanium resist corrosion. These properties make the two metals very useful. Aluminium is used for aircraft, trains, overhead power cables, saucepans and cooking foil. Titanium is used for fighter aircraft, artificial hip joints and pipes in nuclear power stations.
Extraction Unlike iron, aluminium and titanium cannot be extracted from their oxides by reduction with carbon. You do not need to know any details of how these metals are extracted, but existing methods are expensive because: the processes have many stages large amounts of energy are needed Recycling Aluminium is extensively recycled because less energy is needed to produce recycled aluminium than to extract aluminium from its ore. Recycling preserves limited resources and requires less energy, so it causes less damage to the environment. Alloys The properties of a metal are changed by adding other elements to it. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.
Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels.
Crude oil forms naturally over millions of years from the remains of living things. Most of the compounds in crude oil are hydrocarbons. These are compounds that contain hydrogen and carbon atoms only, joined together by chemical bonds called covalent bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.
The alkanes are a family of hydrocarbons that share the same general formula. This is:
The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6.
Alkane molecules can be represented by displayed formulas. In a displayed formula, each atom is shown as its symbol (C or H) and each covalent bond by a straight line. This table shows four different alkanes.
Distillation Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water. The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates. This is the sequence of events in distillation: heating → evaporating → cooling → condensing Fractional distillation: Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
Several waste products are released when fuels burn. These do not just disappear and they can harm the environment by contributing to global warming, global dimming and acid rain.
Fuels burn when they react with oxygen in the air. If there is plenty of air, complete combustion happens. Coal is mostly carbon. During complete combustion, carbon is oxidised to carbon dioxide: carbon + oxygen → carbon dioxide Carbon dioxide is a greenhouse gas. Increasing concentrations of it in the atmosphere contribute to global warming. Hydrocarbon fuels contain carbon and hydrogen. During combustion, hydrogen is oxidised to water (remember that water, H2O, is an oxide of hydrogen). In general: hydrocarbon + oxygen → carbon dioxide + water The combustion of a fuel may release several gases into the atmosphere, including:
If there is insufficient air for complete combustion, incomplete combustion (also called partial combustion) happens. Hydrogen is still oxidised to water, but carbon monoxide forms instead of carbon dioxide. Carbon monoxide is a toxic gas, so adequate ventilation is important when burning fuels.
Solid particles (particulates) are also released. These contain carbon and are seen as soot or smoke. Particulates cause global dimming. They reduce the amount of sunlight reaching the Earth’s surface.
Carbon dioxide dissolves in water in the atmosphere to form a weakly acidic solution. This means that rainwater is naturally slightly acidic. However, some of the products from burning fuels make rainwater more acidic than normal. This is acid rain.
Acid rain reacts with metals and rocks such as limestone, causing damage to buildings and statues. Acid rain damages the waxy layer on the leaves of trees. This makes it more difficult for trees to absorb the minerals they need for healthy growth and they may die. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.
Coal and most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide:sulfur + oxygen → sulfur dioxide This gas dissolves in water to form an acidic solution. It is a cause of acid rain. Sulfur can be removed from fuels before they are used. ‘Low sulfur’ petrol and diesel are widely available at filling stations to use in vehicles. In power stations, sulfur dioxide can be removed from the waste gases before they are released from chimneys. The waste gases are treated with powdered limestone. The sulfur dioxide reacts with it to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.
Coal and crude oil are non-renewable resources. They take so long to form that they cannot be replaced once they have all been used up. This means that these fossil fuels are likely to become more expensive as they begin to run out. Petrol, diesel and other fuels produced from crude oil make a range of harmful substances when they are burned, including:
· carbon dioxide
· carbon monoxide
· water vapour
· particulates (solid particles)
· sulfur dioxide
· oxides of nitrogen or NOx.
Biofuels are fuels produced from plant material. They have some advantages and disadvantages compared to fossil fuels.
Biodiesel is made from rapeseed oil and other plant oils. It can be used in diesel-powered vehicles without needing any modifications to the engine.
Ethanol, C2H5OH, is not a hydrocarbon because it contains oxygen as well as hydrogen and carbon. However, it is a liquid fuel that burns well. Bioethanol is made by fermenting sugars from sugar cane, wheat and other plants. It cannot be used on its own unless the engine is modified. However, modern petrol engines can use petrol containing up to 10 percent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol.
There are ethical issues surrounding the use of biofuels. For example, crops that could be used to feed people are used to provide the raw materials for biofuels instead. This could cause food shortages or increases in the price of food. There are other economic issues surrounding the use of biofuels, including:
· human resources -more people are needed to produce biofuels than are needed to produce petrol and diesel
· increased income - for farmers
· lower fuel prices - biofuels limit the demand for fossil fuels, helping to reduce increases in fuel prices.
There are environmental issues surrounding the use of biofuels. Biodiesel naturally contains little sulfur. For example, it may be said that they are carbon neutral – the amount of carbon dioxide released when they are used is the same as the amount absorbed by the plants as they grew. If so, this would reduce the production of this greenhouse gas. However, while biofuels produce less carbon dioxide overall, they are not carbon neutral. This is because fossil fuels are used in their production, for example in making fertilisers for the growing plants