Chemistry unit two


The Earth's structure

The crust is earths thin outer layer of solid rock. It is about 20km deep. The lithosphere includes the crust and the upper part of the mantle and is made up of a jigsaw of tectonic plates. It is cold and rigid and is about 100km thick.                                                              The mantle is the solid section between the crust and the core. Near the crust it is very rigid however the temperature increases deeper into the mantle meaning it becomes less rigid and can flow very slowly (its semi liquid).                                                                                                          The core is just over half of the earths radius. The inner core is solid while the outer core is liquid.
Radioactive decay creates a lot of the heat inside the earth. This heat creates convection currents in the mantle which causes the plates of the lithosphere to move.

Tectonic plates are large chunks of rock which float on the mantle as theyre less dense. The plates move very slowly at a rate of about 2.5cm a year. Volcanoes and earthquakes often occur where the plates meet. It's the movement of the plates against eachother that often causes them.

Scientists use seismic waves to study the earths structure. They're produced by earthquakes or man made explosions. By measuring the time it takes for these waves to travel through the earth scientists can come to conclusions about the earths structure. P waves can travel through solids and liquids whilst s waves can only travel through solids.

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Plate tectonics

People used to be puzzled as to why the coasts of africa and south america fit together and why there are similar animals/fossils in each area. In 1914 alfred wegener hypothesised that Africa and South America had previously been one continent and had split. He found evidence to back this up: there were matching layers in the rocks on each continent and there were similar earth worms living in each. Wegeners theory of continental drift supposed that 300 million years ago the earth was one big continent which split and drifted due to tectonic plates. Wegeners theory explained things that couldn't be confirmed by the land bridge theory (eg the formation of mountains: he concluded this happened when continents smashed into eachother). Wegener claimed that the continents movements could be due to tidal forces and the Earth's rotation however other scientists proved this to be impossible.                                                                                                                                    In the 1960s scientists investigated the mid atlantic ridge which runs the whole length of the atlantic. They found evidence that magma rises up through the sea floor, solidifies and forms underwater mountains which are roughly symmetrical on either side of the ridge. The evidence suggested that the sea floor was spreading at about 10cm per year. The continents are also moving apart because as liquid magma come sout of the gap, iron particles in the rocks tend to align themselves with the earths magnetic field - as it cools they set in position. This was convincing evidence that new sea floor was being created and the continents were moving apart. This supported Wegeners theory.

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Volcanic erruptions

Volcanoes occur when molten rock (magma) from the mantle emerges through the earths crust. Magma rises up through the crust and boils over here it can. When below the surface molten rock is called magma however above ground it's called lava.

The crust at the ocean floor is denser than the crust below the continents meaning when two tectonic plates collide the oceanic plate will be forced under the continental plate. This is called subduction. Oceanic crust also tends to be cooler at the edges of a tectonic plate so the edges sink easily pulling the oceanic crust down. As the oceanic crust is forced down it melts and starts to rise. If this molten rock finds its way to the surface volcanoes form.

Igneous rock is made when any sort of molten rock cools down and solidifies. Lots of rocks on the surface of the earth were formed this way. The type of igneous rock and behaviour of the volcano depends on how quickly magma cools and the composition of the magma. Some volcanoes produce magma that forms iron rich basalt. The lava is runny and the erruption is fairly safe. If the magma is silica rich rhyolite, the erruption is explosive. It produces thick lava which can be blown violently out the top of the volcano.

Geologists study the magma movement below the ground near the volcano in order to predict potential erruptions.

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Types of rock

Sedimentary rocks are formed from layers of sediment in lakes or seas. Over millions of years the layers get buried under more layers and the weight pressing down squeezes out the water. Fluid flowing through the pores deposit natural mineral cement. Limestone is a sedimentary rock formed from seashells. It is formed from calcium carbonate and the shells are mostly crushed. When limestone is heated it thermally decomposes to make calcium oxide and carbon dioxide:                   CaCO3 ---> CaO + CO2

Metamorphic rocks are formed by the action of heat and pressure on sedimentary or even igneous rocks over long periods of time. The mineral structure and texture may be different but the chemical composition if often the same. So long as the rocks dont actually melt they are metamorphic. If they melt and turn to magma theyre gone.

Marble is a metamorphic rock formed from limestone. Very high temperatures and pressures break down the limestone and it reforms as small crystals. This gives marble a more even texture and makes it much harder.

Igneous rocks are formed when magma cools. They contain lots of minerals in randomly arranged interlocking crystals making them very hard. Granite is a very hard igneous rock. It's ideal for steps and buildings.

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Construction materials

Rocks are usually a mix of materials. Ores are minerals which useful materials can be extracted from - for example, aluminium and iron can be extracted from their ores.                                   Glass is made by melting limestone (calcium carbonate), sand (silicon dioxide) and soda (sodium carbonate) until it melts. When the mixture cools it becomes glass.                                                                                                                                               Clay is a mineral formed from weathered and decomposed rock. It's soft when its dug up meaning its easy to mould into bricks, however it can be hardened when fired at very high temperatures. This makes it ideal for building with as bricks can withstand the weight of a lot more bricks on top of them.                                                                                                                                        Clay contains aluminium and silicates. Powdered clay and powdered limestone are roasted in a rotating kiln to make a mixture of calcium and aluminium silicates called cement. When cement is mixed with water a slow chemical reaction takes place causing the cement to gradually set hard. Cement can be mixed with sand, aggregate and water to make concrete. Concrete is a cheap and quick way of constructing buildings. Reinforced concrete is a composite material - its a mix of concrete and a solid steel support.                                                                                  Quarrying uses up land and destroys habitats. It is costly to make quarrying sites look pretty again. Transporting rock causes noise and pollution. The quarrying process produces dust and creates a lot of noise. Mines can collapse and people have drowned in quarry lakes.

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Electrolysis is the process of passing a current through a piece of impure copper to split the pure copper off from the impurities. The copper is immersed in liquid (the electrolyte) which conducts electricity. eLectrolytes are usually free ions dissolved in water. Copper(II) sulphate solution is the electrolyte used in purifying copper - it contains Cu2+ ions. The electrical supply acts as a pump. This is the process:
1) It pulls electrons off copper atoms at the anode, causing them to go into solution as Cu2+ ions.2) It then offers electrons at the cathode to nearby Cu2+ ions to turn them back to copper atoms.   3) The impurities are dropped at the anode as a sludge, whilst pure copper atoms bond to the cathode.                                                                                                                                             The cathode is negative meaning it attracts pure copper and the anode is positive meaning oxidation occurs and the impure copper dissolves away from it. The oxidation reaction is:                 Cu ---> Cu2+ + 2e. The reduction reaction at the cathode is Cu2+ + 2e ---> Cu                                The reduction reaction at the cathode is: Cu2+ + 2e ---> CuIts cheaper to recycle copper than it is to extract new copper from ore. Recycling copper only uses 15% of the energy that'd be used to mine and extract the same amount. But it can be hard to convince people to sort out their metal waste and then recycle it.                                                                                                                                                                         

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An alloy is a mixture of two or more different metals (like brass or bronze). They can be made from a mixture of a metal and a non metal too (like steel). Alloys often have properties which make them more useful than the metals they are made from.                                                                                                                                        Steel is an alloy of iron and carbon, It is harder than iron and is also a lot stronger, as long as the amount of carbon doesn't get higher than about 1%. Iron on its own corrodes quite easily but stee; is less likely to rust. A lot of things are made from steel - such as girders, bridges, engine parts, cutlery, washing machine ships and more.                                                                Brass, Bronze, Solder and Amalgam are all alloys. Brass is an alloy of copper and zinc. Most of the properies of brass are a mixture of those of copper and zinc however brass is much harder. Brass is used for making musical instruments and is used for fixtures and fittings such as screws. Bronze is an alloy of copper and tin. Its much harder and stronger than tin and is more resistant to corrosion. Bronze is used in sculpture, motor bearings, springs and bells. Solder is an alloy of lead and tin. Unlike pure materials it doesnt have a definite melting point but gradually solidifies as it cools. This is useful for soldering things together.                                                                                                                                       Amalgam is an alloy containing mercury. It is used in dentistry for fillings                              Nitinol is the name given to a family of alloys of nickel and titanium that have shape memory.

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Building cars

Iron corrodes easily (It rusts). Rusting only happens when iron is contact with both water and oxygen at once. The chemical reaction that takes place is an oxidation reaction. The iron gains oxygen to form Iron (III) oxide. Water then becomes loosely bonded to the iron oxide to form hydrated iron (III) oxide - rust. (Iron + Oxygen + Water ---> Hydrated iron (III) oxide)                      Rust is a soft crumbly solid which flakes off and allows the iron beneath to rust again; if the water is salty or acidic rusting occurs very quickly. Aluminium however doesn't corrode when wet. This is odd as aluminium is more reactive than iron, however the aluminium reacts very quickly with oxygen in your air to form aluminium oxide, which coats the aluminium and prevents further rusting. Aluminium is a lot less dense than steel so the body of an aluminium car will be lighter meaning a better fuel economy. A car body made of aluminium corrodes less meaning its lifetime is longer. However aluminium is a lot more expensive. Steel is good for the bodywork of a car as it can be made into sheets and welded. Aluminium is strong and has a low density making it good for the engine. Glass is used for windscreens and plastics are used for the interior doors etc as theyre light and hardwearing. Fibres are hardwearing and are used for the seats and floors.                                                                         Recycling cars is important and 85% of the materials in a car must be recyclable.

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Acids and bases

An indicator is a dye which changes colour depending on the pH of a substance. Universal indicator is a mixture of dyes. It changes colour gradually as pH changes. The colour a solution turns can be compared to a chart to estimate its pH. A very strong acid has a pH of 0 whilst a very strong alkali has a pH of 14. a neutral substance has pH 7.

An acid is a substance with a pH less than 7. Acids form H+ ions in water. the pH of an acid is determined by the concentration of H+ ions.

A base is a substance with a pH of more than 7. An alkali is a base that is soluble in water. Alkalis form OH- ions in water.

The reaction between acids and bases  is called neutralisation.

Acid + base ---> Salt + water

(H+ + OH- ---> H2O) This reaction is reversible.

Neutralisation gives a neutral product with a pH of 7.

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Reactions of acids

Some metal oxides/hydroxides dissolve in water. These soluble compounds are alkalis. Even bases that won't dissolve in water still react with acids. So, all metal oxides and hydroxides react with acids to form a salt and water.

Acid + metal (hydr)oxide ---> salt + water

Acids and carbonates produce carbon dioxide.

Acid + carbonate ---> Salt + Water + Carbon dioxide

Acids and ammonia produce ammonium salts

Acid + Ammonia ---> Ammonium salts

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Fertilisers provide plants with essential elements for growth. The three main elements in fertilisers are nitrogen, phosphorus and potassium. These elements are required for plants to live however they may not be present in soil as they've been used up by previous crops. Fertilisers replace these missing elements and provide more of them increasing the crop yield as crops can grow faster and larger. For example, nitrogen added to plant proteins make plants grow faster. Fertilisers must dissolve in water in order for plants to take them in.                                                 If you neutralise a nitric acid with ammonia you get ammonium nitrate. It is a good fertiliser as it has nitrogen from two sources. Ammonium sulfate can also be used as a fertiliser which is made by neutralising sulfuric acid with ammonia. Ammonium phosphate is made in the same way (neutralisation) and so is potassium nitrate (nitric acid and potassium hydroxide).

Fertilisers are useful as they increase crop yield meaning the worlds increasing population can be fed. However they can pollute water supplies if overused causing eutrophication. This is when fertiliser runs off fields due to rain into rivers and streams. The level of nitrates and phosphates increases. Algae in the water use the nutrients to multiply rapidly creating an algae bloom. This blocks off light to the river plants below meaning they can't photosynthesise and die. Aerobic bacteria feed of the dead plants and multiply. As they multiply they use up almost all the oxygen in the water meaning everything in the river dies.

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Preparing fertilisers

You can make most fertilisers using a titration method. To make ammonium nitrate, you fill a burette with nitric acid solution and put it above a flask containing ammonia and a few drops of methyl orange indicator solution. Slowly you add the nitric acid solution until the yellow colour  of the ammonia turns red. You must gently swirl the flask as the acid is added. Methyl orange is red in acids so this colour change means that all the amonia has been neutralised. To get solid ammonium nitrate crystals gently evaporate the solution until only a bit is left. Then leave it to crystalise. The crystals arent pure - they have methyl orange in them. To get pure ammonium nitrate you need to note exactly how much nitric acid it took to neutralise the ammonia and then repeat the titration using the same volume of acid without indicator.

The mass of product you end up with is called the yield of a reaction. The more reactants you start with the higher the actual yield will be. But the percentage yield doesn't depend on the amount of reactants you started with: its a percentage. It is always between 0% and 100%. The predicted yield  = the amount of product you'd get if all the reactant was turned into a product.

percentage yield = (actual yield x 100%) / predicted yield

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The haber process

The haber process takes hydrogen and nitrogen and makes ammonia (NH3). It is a reversible reaction.

N2 + 3H2 ---> 2NH3

The nitrogen is obtained easily from the air. The hydrogen comes from cracking of oil fractions or natural gas. Because the reaction is reversible not all the nitrogen and hydrogen will be turned into ammonia. The N2 and H2 that isnt used is recycled and passed through again so none is wasted. There has to be high pressure of about 200 atmospheres, a temperature of 450 degrees celcius and a catalyst of iron.                                                                                                                                             Higher pressures favour the forward reaction so the pressure is set at 200 atmospheres. This increases the percentage yield of ammonia. Higher temperatures favour the reverse reaction - high temperatures reduce the percentage yield. Lower temperatures mean slower reactions so manufacturers use higher temperatures anyway. 450 degrees is used as a compromise. It gives A reasonable percentage yield.

The iron catalyst makes the reaction quicker getting it to the equilibrium proportions more quickly without effecting the equilibrium position. Without a catalyst the temperature would have to be raised further reducing the % yield.

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Production cost

Production cost depends on many factors. The price of energy is one of these factors. Industry needs to keep its energy bills low - if a reaction needs high temperatures the running costs will be higher. The cost of raw materials is also a factor. This is kept to a minimum by recycling any materials that havent been reacted. This is what happens in the haber process as the % yield is only about 10%. Labour intensive processes are also very expensive. To reduce the amount of staff who need to be paid automation is used. Any savings made however have to be weighed against the initial cost and running cost of machinery.   The cost of equipment depends on the conditions it has to cope with. For example, it costs far more to make something to withstand very high pressures than something that needs to work at atmospheric pressures. Finally, the faster a reaction occurs the better it is in terms of reducing costs. Rates of reaction are increased by the use of catalyst. The increase in production rate has to balance the cost of buying the catalyst in the first place.

Optimum conditions are those that give the lowest production cost per kg of product. This means compromising on speed of reaction or % yield.

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Salt is extracted from underground deposits left millions of years ago when ancient seas evaporated. There are huge deposits of this rock salt under cheshire. Rock salt is a mix of salt and impurities. It can be brought up by being drilled dug or blasted out. It can also be extracted by pumping hot water underground. The salt dissolves and is pumped above ground by the pressure of the water. this is solution mining. Rock salt can be used in its raw form to stop ice forming on roads or it can be used on food or to make chemicals.

Electrolosis of brine gives hydrogen, chlorine and NaOH. The electrodes are made of an inert material so they wont react with any of the products  or the electrolyte. Hydrogen gas is given off at the negative cathode, chlorine gas is given off at the positive anode and sodium hydroxide is formed from the ions left in the solution. The sodium chloride solution contains 4 different ions: Na+, OH-, Cl- and H+. At the cathode two hydrogen ions accept one electron each to become one hydrogen molecule. At the anode, two chloride ions lose one electron each to become one chlorine molecule. Oxidation is the loss of electrons and reduction is the gain of electrons.

The hydrogen gas is used to make ammonia in the haber process and margerine. The chlorine is used to disinfect water, make plastics, solvents or hydrochloric acid and the sodium hydroxide is used to make soap or can react with chlorine to make household bleach.

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