C2: The Revision Tackk...

C2a: The Structure of the Earth

The Earth is made of a layered structure. It has a thin, rocky crust, mantle, and a core, (containing iron). It's difficult to collect information about the structure of the Earth. The deepest mines and holed drilled into the crust are only a few kilometres into the thick crust. Scientists have to rely on studying the seismic waves, (vibrations), caused by earthquakes, to understand the structure of the Earth. Below is a diagram showing each layer of the Earth, and its approximate thickness.

The Earth's lithosphere is the relatively cold, rigid outer part of the Earth, made of the crust and the top part of the mantle. The top of the lithosphere is "cracked" into several large interlocking pieces called tectonic plates. Oceanic plates sit under the ocean, whereas continental plates form the continents. The plates sit on top of the mantle because they are less dense than the mantle. Plates move very slowly, (about 2.5cm per year). These movements cause earthquakes and volcanoes at the boundaries between plates. An earthquake will occur along the line where the two plates meet.

Just below the crust, the mantle is relatively cold and rigid. At greater depths, the mantle becomes hot and fluid, which means that it can flow. There are convection currents, formed by heat released from radioactive decay in the core. Convection currents cause magma, (molten rock), to rise to the surface at the boundaries of plates. When the molten rock solidifies, new igneous rock is formed. This slow movement of the magma causes the plates to move. Oceanic crust has a higher density than continental crust. When an oceanic plate collides with a continental plate, it dips down and slides underneath it. This is called subduction. The oceanic plate is partially re-melted as it goes under the continental plate. Below is a rather small diagram which describes the process of subduction.

Many theories, (ideas) have been put forward to explain changes in the Earth's surface. Earth scientists now accept the theory of plate tectonics. This theory is widely accepted, as it explains a range of evidence, and has also been discussed and tested by many scientists.

In 1914, a scientist called Alfred Wegener suggested that the surface of the Earth was changing. He developed a theory that, millions of years ago, all the continents were joined together. Wegener noticed several features on the surface of the Earth. Firstly, the continents looked like they would fit together, not unlike a jigsaw. The geology of Scotland and Canada was similar, as was the geology of Africa and South America. Similar animal species were found on either side of the Atlantic, for example, there are caribou in Canada and reindeer in Scandinavia. Initially, Wegener's ideas were not accepted by other scientists. But Wegener's theory was supported by studies in the 1960s, which looked at new rock formed at oceanic plate boundaries. The studies showed that the plates are moving apart; the age of rock increases as you move away from the boundary. So, Wegener's theory of plate tectonics has gradually become accepted.

Volcanoes form at places where magma, (molten rock underneath the Earth's surface), can find its way through weaknesses in the Earth's crust. This is often at plate boundaries, or where the crust is very thin. The magma rises through the crust because it has a lower density than the crust. Geologists study volcanoes to help understand the structure of the Earth. They also aim to predict when eruptions will occur, to give an early warning to people who live nearby. Living near a volcano can be very dangerous, because eruptions can't be predicted with accuracy. However, some people choose to live there because volcanic soil is very fertile.

Geologists are now better able to predict eruptions, but they still can't be 100% accurate. Below is a diagram depicting an erupting volcano.

The molten rock that erupts from a volcano is known as lava. Some volcanoes may have runny lava, whereas others will have thicker lava. Thick lava erupts more violently and catastrophically. When liquid rock cools, igneous rock is formed. Igneous rocks are very hard and have interlocking crystals of different sizes. Large crystals are made when the rock cools slowly, as in silica-rich granite and iron-rich gabbro. Small crystals are made when the rock cools quickly, as in silica-rich rhyolite and iron-rich basalt.

Volcanoes can produce two types of lava, which affects the type of eruption. Iron-rich basalt lava is quite runny and has fairly "safe" eruptions, whereas silica-rich rhyolite is thicker. Thicker lava results in more violent and catastrophic eruptions. Rhyolite lava makes pumice, volcanic ash and bombs.

C2b: Construction Materials

Many construction materials come from rocks found in the Earth's crust. Iron and aluminium are extracted from rocks called ores. Brick is made by baking clay that has been extracted from the Earth. Glass, concrete and cement are all made from sand, (small grains of rock). Limestone, marble, granite and aggregates, (gravel) are types of rock extracted from the Earth. These rocks just need to be shaped to be used as building materials. Limestone is the easiest to shape because it's the softest. Granite is hardest to shape.

Rocks differ in hardness because of the ways in which they were made. Limestone is a sedimentary rock. Marble is a metamorphic rock made from limestone that has been put under pressure and heated, which makes it harder. Granite is an igneous rock.

Limestone and marble are mainly made of calcium carbonate, (CaCO3). When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide.

Calcium Carbonate -> Calcium Oxide + Carbon Dioxide; CaCO3(S) -> CaO(S) + CO2(G)

This type of reaction is called a thermal decomposition reaction; one material breaks down into two or more new substances when heated. Clay and limestone can be heated together to make cement. Cement can be mixed with sand, gravel, (aggregates), and water and allowed to set to make concrete, which is very hard but not very strong. It can be strengthened by allowing it to set around steel rods to reinforce it. A reinforced material is a composite material.

A composite material combines the best properties of each component material. Reinforced concrete combines the strength and flexibility of the steel bars with the hardness of the concrete. Reinforced concrete has many more uses than ordinary concrete. The following is a diagram of reinforced concrete.

Rock is dug out of the ground in mines and quarries. Mining and quarrying companies have to try to reduce their impact on the local are and the Environment because mines and quarries can be noisy and dusty, take up land, change the shape of the landscape, and increase the local road traffic. A responsible company with also reconstruct, cover up and restore and area that has been worked on.

C2c: Metals and Alloys

Copper is extracted from naturally occurring copper ore by heating it with carbon.

Copper Oxide + Carbon -> Copper + Carbon Dioxide; 2CuO(S) + C(S) -> 2Cu(S) + CO2(G)

The process uses lots of energy, which makes it expensive. Oxygen is removed from the copper oxide. This process is called reduction. It is cheaper to recycle copper than to extract it from its ore. Recycling also conserves the world's limited supply of copper ore and uses less energy. But recycling copper can be more difficult if it has other metals stuck to it or mixed with it. If the copper is very impure, it can be purified using electrolysis, (which is an expensive process), before it can be used again.

Electrolysis uses an electric current to break down compounds into simpler substances. In electrolysis, electricity is passed through a liquid or a solution called an electrolyte, for example, copper(II) sulfate solution, to make simpler substances.

Electrodes are used to allow the electricity to flow through the electrolyte. The anode, (positive electrode), is made of impure copper).

Cu - 2e- -> Cu2+; This is an oxidisation process as electrons are lost.

The cathode, (negative electrode) is made of pure copper.

Cu2+ +2e- -> Cu; This is a reduction process as electrons are gained.

Below is a diagram, depicting the process of the electrolysis of copper.

An alloy is a mixture of a metal with another element, (usually another metal). Bronze and steel are alloys. Alloys improve the properties of a metal and make them more useful - they are often harder and stronger than the pure metal. For example, amalgam, (made using mercury) is used for fillings in teeth. Solder, (made of lead and tin) is used to join wires. Brass, (made of copper and zinc), is used in door handles, coins, and musical instruments.

A smart alloy, such as nitinol, (an alloy of nickel and titanium), can be bent and twisted. Nitinol will return to its original shape when it is heated; it has shape memory. This smart alloy is used for the frames of reading glasses.

C2d: Making Cars

Many different materials are used to make cars. For example, nylon fibre is used to make the seatbelts, because it is strong and flexible. Glass is used to make the windscreen because it's transparent. Copper is used for the wiring in the engine, as it is a good electrical conductor. Plastic is used for the trim, because it is rigid and does not corrode. Steel is used to make the body, because it is strong and malleable. Aluminium is used to make the alloy wheels, because it is lightweight and doesn't corrode in moist conditions.

Iron and aluminium are two different metals. Aluminium is not dense, whereas iron is; aluminium is not magnetic, whereas iron is; aluminium resists corrosion, whereas iron doesn't; both iron and aluminium are malleable, and both can conduct electricity. Aluminium can be mixed with other metals, such as copper and magnesium, to make an alloy. Alloys have different properties from the metals that they are made from. These properties make the alloy more useful.

For example, steel is an alloy of iron and carbon. It is used to make cars because it is harder and stronger than iron, and doesn't corrode as quickly as iron. Aluminium is used to make car bodies. In comparison to steel, it is lighter, corrodes less and is more expensive. If aluminium is used to make a car, it will have a longer lifetime, as aluminium doesn't corrode. Because aluminium is less dense than steel, the car will be lighter, and henceforth, will have a better fuel economy. Below is a diagram depicting the most common locations of corrosion on a steel car.

Rusting is an example of an oxidisation reaction. This is a reaction where oxygen is added to the substance. Rusting needs iron, water, and oxygen, (air). Rusting happens even faster when the water is salty or is acid rain. This is because there are extra ions present, and these act as catalysts and speed up the reaction. Aluminium doesn't react and corrode in air and water; instead, it quickly forms a protective layer of aluminium oxide. This layer prevents any more air or water from coming into contact with the metal. This built-in protection will not flake off. Oxygen is added to the iron in the presence of water:

Iron + Oxygen + Water -> Hydrated Iron(III) Oxide

Most materials used in a car can be recycled. Since 2006, the law states that 85% of a car must be able to be recycled; this will increase to 95% in 2015. Separating all the different materials for recycling can be tricky and time-consuming. However, it saves natural resources, and avoids disposal problems.

Recycling materials means that less quarrying is required, less energy is used to extract them from ores; the limited ore reserves will last longer, (saves natural resources), and disposal problems are reduced.

Recycling the plastics and fibres reduces the amount of crude oil needed to make them, and conserves oil reserves. There are a number of materials in a car that would cause pollution is put into landfill, such as lead in the car battery, so recycling also helps protect the environment.

C2e: Manufacturing Chemicals: Making Ammonia

Ammonia, (NH3), is an alkaline gas made from nitrogen and hydrogen. It can be used to make nitric acid, and fertilisers, (cheap fertilisers are very important in helping to produce enough food for the growing global population). The reaction that makes ammonia is a reversible reaction. So, nitrogen and hydrogen can form ammonia, and ammonia can decompose to make hydrogen and nitrogen. Reversible reactions have a special symbol, (see below), in their equation to show that the reaction can take place in either direction. Below is, as before mentioned, a diagram showing this symbol.

Ammonia is made on a large scale in the Haber process. The reactants are nitrogen, (from the air); hydrogen, (from natural gas or the cracking of crude oil).

Nitrogen + Hydrogen <-> (Reversible) Ammonia; N2(G) + 3H2(G) <-> (Reversible) 2NH3(G)

Optimum conditions aren't used as they would be very expensive to maintain, so a compromise is reached. The nitrogen and hydrogen mixture is under a high pressure of 200 atmospheres. The gases are passed over an iron catalyst at 450 degrees centigrade. Only about 15% of the reactant gases make ammonia. The unreacted gases are recycled. Ammonia is cooled, condensed, and then pumped off as a liquid. The following few images are each diagrams depicting the Haber process. It may seem a little over the top, though this is complex and important stuff; henceforth, a number of diagrams have been included to aid understanding.

The cost of making a new substance depends on the price of energy, (gas and electricity), labour costs, (wages for employees); how quickly the new substance can be made, (cost of catalyst); the cost of starting materials/reactants, and the cost of equipment needed, (such as plants and machinery). Factors that affect the cost of making a new substance include the pressure - the higher the pressure, the higher the plant cost, the temperature - the higher the temperature, the higher the energy cost; the catalysts - catalysts can be expensive to buy, however, production costs are reduced because they increase the rate of reaction; the number of people needed to operate the machinery - automation reduces the wages bill, and finally, the amount of unreacted material that can be recycled - recycling reduces costs. You need to be able to interpret data percentage yield in reversible reactions. You may also be asked to interpret data on other industrial processes, in terms of rate, percentage yield, and cost.

Economic considerations determine the conditions used in the manufacturing of chemicals. The percentage yield achieved must be high enough to produce enough daily product, (a low percentage yield, providing the reaction can be repeated many times with recycled starting materials). The rate of reaction must be high enough to produce enough daily yield of product. The optimum conditions should be used to give the most economical reaction, (this could mean a slower reaction, or a lower percentage yield at a lower cost).

Bearing reference to the Haber process, it is important that the maximum amount of ammonia is made in the shortest possible time, at a reasonable cost. This requires a compromise. For the Haber process: a low temperature increases the yield, but the reaction is too slow. A high pressure increases yield, though becomes more expensive as the yield increases. A catalyst increases the rate of reaction, but doesn't alter the percentage yield. So, a compromise is reached: a temperature of 450 degrees centigrade; a pressure of 200 atmospheres, and a catalyst of iron. This gives a reaction with an acceptable percentage yield.

C2f: Acids and Bases

Indicators are chemicals that change colour to show changes in pH. Some indicators, such as litmus, have only two colours; others, examples including universal indicator, have a range of colours over different pH values. Acids are substances with a pH of less than 7. Bases are the oxides and hydroxides of metals with a pH of greater than 7. Acids turn litmus indicator red and bases turn litmus indicator blue. Soluble bases, (chemicals with a pH greater than 7 and that dissolve in water), are called alkalis. You can find the pH of a solution by using universal indicator. You can add a few drops of universal indicator to the solution and compare the resulting colour against a pH colour chart, (see below).

Metal oxides and metal hydroxides are bases. When they are added to acids in the correct amounts, they can cancel each other out. This is called neutralisation, because the resulting solution has a neutral pH of 7.

Acid + Base -> Salt and Water

As an acid is added to an alkali, the pH of the solution decreases, because the acid neutralises the acid to reach pH 7. As an alkali is added to a acid, the pH of the solution increases because the alkali neutralises the acid to reach pH 7. Acids can also be neutralised by carbonates to produce a salt, water and carbon dioxide gas.

Acid + Carbonate -> Salt + Water + Carbon Dioxide

The first name of a salt comes from the name of the base or carbonate used, for example, sodium hydroxide will make a sodium salt, copper oxide will make a copper salt; calcium carbonate will make a calcium salt, and ammonia will make an ammonium salt.

The second name of a salt comes from the acid used, for example, hydrochloric acid will produce a chloride salt, sulfuric acid will produce a sulfate salt; nitric acid will produce a nitrate salt, and phosphoric acid will produce a phosphate salt.

To illustrate this, neutralising potassium hydroxide with nitric acid will produce potassium nitrate.

Alkalis in a solution contain hydroxide ions, OH-(aq). Acids in solution contain hydrogen ions, H+(aq). The pH of a solution is a measure of the concentration of H+ ions. Neutralisation can be described using the ionic equation: H+(aq) + OH-(aq) -> H2O(l). You should be able to construct word equations and balanced symbol equations for producing salts.

C2g: Fertilisers and Crop Yield

Fertilisers are chemicals that give plants essential chemical elements needed for growth. Fertilisers make crops grow bigger and faster, and increase the crop yield. As world populations rise, fertilisers can increase food supply, but can also cause problems such as the death of animals in waterways. This is known as eutrophication. The three main essential elements found in fertilisers are nitrogen, (N), phosphorous, (P), and potassium, (K). Urea can also be used as a fertiliser. Fertilisers must be soluble in water, so that they can be taken in by the roots of plants in solution.

Fertilisers increase crop yield by replacing essential elements in the soil, that have been used up by another crop, and providing nitrogen as soluble nitrates, which are used by the plant to make protein for growth.

Some fertilisers can be manufactured by neutralising an acid with an alkali. Ammonium sulfate - neutralise sulfuric acid with ammonia; Ammonium nitrate - neutralise nitric acid with ammonia; Ammonium phosphate - neutralise phosphoric acid with ammonia; Potassium nitrate - neutralise nitric acid with potassium hydroxide. You should be able to label the apparatus needed to make a fertiliser by neutralisation; burette; measuring cylinder; funnel. A fertiliser, such as potassium nitrate, can be made by producing a salt from neutralisation.

Eutrophication is when the overuse of fertilisers changes the ecosystem in lakes, rivers and streams. Fertilisers used by farmers may be washed into lakes and rivers, (run-off). This increases the levels of nitrates and phosphates in the water, and more simple algae grow. The algal bloom bocks off sunlight to other plants, causing them to die and rot. Aerobic bacteria feed on the dead organisms and increase in number. They quickly use up the oxygen until nearly all the oxygen is removed, (none is being replaced as the dead plants cannot photosynthesise). There isn't enough oxygen to support the larger organisms, such as fish and other aquatic animals, so they suffocate. Below is a diagram depicting this scenario.

C2h: Chemicals from the Sea: Sodium Chloride

Sodium chloride, or table salt, is used as a food preservative and flavouring. However, it is also useful as a raw material in the chemical industry. It is an important source of chlorine and sodium hydroxide. Sodium chloride can be removed from the sea or mined from salt deposits. It is mined in Cheshire as a solid, (rock salt). This has lead to subsidence in some parts of Cheshire. It is also mined by solution mining for the chemical industry.

When concentrated sodium chloride solution is electrolysed, the electrodes must be made from inert materials, as the products are very reactive. This process forms sodium hydroxide in the solution, hydrogen at the cathode, (negative electrode); chlorine at the anode, (positive electrode). You can test for chlorine using damp litmus paper; if chlorine is present, it will bleach the litmus paper.

There are many uses for the products of electrolysis of sodium chloride. Sodium hydroxide is used to make soap; hydrogen is used in the manufacture of margarine; chlorine is used to sterilise water, make solvents and plastics, such as PVC; chlorine and sodium hydroxide are reacted together to make household bleach. Below is a diagram depicting the electrolysis of sodium chloride.

Brine, (NaCl(aq)) contains Na+, Cl-, OH- and H+ ions. The large scale electrolysis of brine happens as part of the chloro-alkali industry. This is a global market, that generates great profits. Hydrogen is made by reduction at the cathode: 2H+ + 2e- -> H2; reduction is a gain of electrons. Chlorine is made by oxidisation at the anode: 2Cl- - 2e- -> Cl2; oxidisation is a loss of electrons.

Sodium, (Na+) and hydroxide, (OH-) ions remain in solution. This makes the third product of sodium hydroxide.

End of C2

This Has Been a GrumblingGrenade Tackk, (@HarbingerOfNonsense), Thank You Very Much For Your Time.

All content on this Tackk is from the OCR Gateway GCSE Chemistry B Revision Guide.

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