[II] Chemistry - C2

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  • Created by: Olivia
  • Created on: 04-03-14 17:01

Using materials

Rubber is used for car tyres because it is springy, or elastic. Plastics keep their shape when moulded, which makes themuseful for making washing up bowls. Fibres are used to weave cloth for clothes because they are strong when they are stretched. Each material has properties that make it suitable for the job it is doing.

Properties describe how a material behaves. MELTING POINT is the temperature at which a solid turns into a liquid, TENSILE STRENGTH is the force needed to break a material when it is being stretched (in tension), COMPRESSIVE STRENGTH is the force needed to crush a material when it is being squeezed, STIFFNESS is the force needed to bend a material, HARDNESS is a comparison of two materials where one can or cannot scratch the other, and DENSITY is the mass of a given volume of the material.

Many measurements of a property need to be taken because readings vary from the true value. The mean of a set of measurements should give a result that is close to the true value. The smallest to the largest of the measurements is the range. Any measurements that appear unusual or very different from the rest of the results are called outliers and should be checked to find out why they are different.

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Properties and units

Some properties depend on the size and shape of the material being tested. For instance the strength and stiffness of a material depends on its cross-sectional area and is measured in pascals.

Density is the mass contained in a specific volume of the material and is measured in g/cm³ or kg/m³. An object made out of a material with a high density will feel heavier than an object of the same size made of a lower density material.

Errors in measurements produce variation in data. Unless there is a repeated error in the readings such as using a ruler with the end broken off, the true value will fall in the range of the measurements. Outliers can be discarded if it can be shown that there was a particular error in the measurement. 

A bullet proof vest has to stop a bullet which is moving very fast. The fibres in the vest have to be very strong but the vest must not be too heavy to wear for a long time. At the moment, Kevlar fibres have the best set of properties to do this job well.

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Plastics, rubbers and fibres

A manufacturer is producing a new smart phone. The choice of material for the casing is very important. It must be possible to make it into the shape suggested by the designers. It has to be strong enough to protect the electronics inside. It must also wear well so that it keeps its good looks for a long time. It must not make the phone too heavy, and it must be relatively cheap. 

Most things that we buy are made from a number of materials, each with properties that are needed for the job they do. Engineers have a choice of materials for each job and they must compare data on the properties of the material to make their decisions

Rubbers have greater elasticity than other materials - they bounce back when a force is removed. Different rubbers also have different amounts of compressive strength and hardness. For example, the tyres used for racing cars on wet tracks are hard, but those used when the track is dry are soft.

Some materials can be drawn into long, thin filaments which have a greater tensile strength than the original material. Many filaments can be spun together to make fibres and the fibres can then be woven into cloth. Ropes are made by winding fibres together. The strength of a rope depends on the material and the number of fibres wound together.

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Natural and synthetic materials

Everything around us, every living thing, every rock, every pool of water, is made up of chemicals. Many of those chemicals are useful to us.

Metals and shiny, MALLEABLE and conduct electricity. Ceramics include clay, glass and cement, and are hard and strong. Polymers are large molecules that can be made into rubbers, plastics and fibres. Many of the materials we use are mixtures of chemicals. Bronze is a mixture of copper and tin, and concrete is a mixture of cement and sand.

Cotton, paper, silk and wool all come from living organisms - cotton and paper are made from plants and silk and wool are obtained from animals. They are NATURAL MATERIALS which need little processing to be useful. These materials are all polymer.s. Other natural materials come from the rocks in the Earth's crust, for example limestone is turned into cement and haematite is a source of iron.

Many of the materials we use today are synthetic materials. They have been manufactured by chemical reactions which join simple cheicals together to make new materials. The raw materials that provide the simple chemicals often come from the Earth. The most important raw material is crude oil found in rocks. Polyethene is an example of a synthetic material made from simple chemicals found in crude oil.

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What is crude oil?

Synthetic materials have taken the place of some natural materials in our modern world. This is because synthetic materialscan be designed to provide the properties needed for a particular purpose. There may be a shortage of natural materials and the raw materials for making synthetic materials are often cheaper and available in greater amounts. Synthetic materials can be made in whatever quantity is needed.

For many uses, neoprene and silicone rubbers have taken the place of natural rubber, which is obtained as latex sap from rubber trees. Nylon was invented as a synthetic substitute for silk produced by silk larvae. Bakelite was the first synthetic plastic and replaced wood for many uses.

CRUDE OIL or "petroleum", is a mixture of thousands of different compounds. Most of these compounds are hydrocarbons. Hydrocarbons are compounds of just carbon and hydrogen. 

Most of the hydrocarbons in crude oil are used as fuels. When the fuel burns the carbon and hydrogen atoms join up with oxygen atoms and are rearranged into carbon dioxide and water molecules. The number of each type of atom in the products is the same as in the reactions. 

Nearly 90% of crude oil is used as fuel. Less than 3% is used in the chemical synthesis (manufacture) of other chemicals.

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Carbon chains

There are so many different hydrocarbon molecules in crude oil because carbon atoms can join together in chains with hydrogen atoms attached. Crude oil is made up of hydrocarbons with up to 100 carbon atoms. As it is a mixture, the actual composition of crude oil varies from place to place. Some sources of crude oil have more of the smaller molecules, some have more of the larger molecules.

A group of hydrocarbons known as the ALKANES is a series of compounds with similar properties including methane, ethane, propane and butane. All members of the alkane series have the formula CnH2n+2, where n is any whole number. Alkanes are found in fuels. Octane is part of petrol.

When hydrocarbons burn, the hydrogen atoms always join up with oxygen to form water. The carbon atoms join up with oxygen to form carbon dioxide or carbon monoxide, or sometimes the carbon atoms are left on their own as particulate soot. 

A small percentage of the substances in crude oil, mainly the smaller hydrocarbon molecules, is used to make a huge range of other chemicals in the petrochemical industry. For example, a hydrocarbon molecule called ethene, can be combined with water to form ethanol. This is the first stage in making manly more complex molecules.

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Separating hydrocarbons

Crude il is separated by a process called FRACTIONAL DISTILLATION. Crude oil is heated to about 400oC when all the hydrocarbons in it are turned into gases. The gas is passed into a tall tower, called a fractional distillation column, which is cooler towards the top. The gases rise up the column, cool and then condense to liquids at their boiling points. Some have high boiling points so condense low down in the column. Others with lower boiling points rise up the column before they condense. Some stay as gases even at the top of the column. At seven of eight points up the column, substances are collected and piped off. Each substance is a mixture of hydrocarbons with similar boiling points called a FRACTION.

The hydrocarbons in each fraction have boiling points within a particular range of temperatures. The boiling points are similar because the hydrocarbon molecules are a similar size - they have a similar number of carbon and hydrogen atoms. For example, the top fraction (the petroleum gases) all have boiling points below room temperature and have one to four carbon atoms, and thee boil between about 40oC and 170oC. This fraction provides petrol for fuel but also the chemicals used for synthesis in the petrochemical industry.

In an oil refinery, there are a lot of distillation columns and the different fractions are separated a number of times before the final products are ready for use.

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Investigating boiling points

To investigate how boiling points of hydrocarbons vary with the size of the molecule, all other possible factors that could affect the measurement must be kept constant. If a graph was drawn showing the boiling point data for members of the alkane series of hydrocarbons (where the carbons form a chain) - a positive correlation will be shown. 

There are attractive forces between molecules which hold them together. Force is needed to separate the molecules. When heated, the molecules gain energy until at the boiling point they have enough energy to overcome the forces holding them together and the molecules become a gas. The graph shows that as the size of the molecule increases, the force between the molecules increases.

A POLYMER is a very large molecule. It is made by joining together lots of similar small molecules, called monomers, like putting together the links of a chain. The process of making a polymer is called POLYMERISATION. 

A polymer has a chain of thousands of carbon atoms, each joined to the next. Polyethene is the simplest polymer and is made up of carbon and hydrogen atoms. 

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Joining molecules and changing monomers

The monomer used to make polyethene is the hydrocarbon called ethene. Whene ethene polymerises, the carbon atoms of one molecule join up to the carbon atoms of the next. Between 20,000 and 200,000 ethene molecules can join together to form each polymer molecule.

The reaction can be summarised by:

many ethene molecules ---> a polyethene molecule made up of many similar repeated units

Different momomer molecules produce different polymrs. Nylon is also a polymer but is made from joining two different monomers together in a chain.

Today we have a huge variety of polymers made from different starting monomers. Propane is a hydrocarbon similar to ethene and forms the polymer polypropene. It is similar to polyethene but with short branches made up of a carbon and three hydrogen atoms at regular intervals along the chain.

Other polymers are made by replacing one or more of the hydrogen atoms in the monomer with other atoms or groups of atoms. Today there are hundreds of different polymers that are manufactured starting from different monomers. Each polymer has its own property and uses. 

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Better materials

Fifty years ago, water buckets were made of iron. They were difficult and expensive to make, heavy to carry and soon rusty holes caused leaks. Today, buckets are made out of polypropene, a synthetic polymer. Polypropane buckets are tough, strong, light and last for many years. Also polypropane can be easily moulded into complex shapes when it is being made.

Many of the materials that have been in use for thousands of years such as wool, silk, cotton, leather, wood and rubber are natural polymers. These materials are still used for a lot of purposes. However, modern synthetic polymers have replaced them, and metals and ceramics, for many uses.

An important reason why synthetic polymers have been chosen for a purpose is because they are cheaper and are available in larger quantities than the natural material that they replace. This is why nylon replaced silk and wool for stockings in the 1940s. Synthetic polymers have many other properties that make them useful and also their properties can be adjusted to make them suit the job they have to do.

A synthetic polymer called PET has replaced glass for bottled water and soft drinks. Like glass, PET is clear. PET is a strong polymer which does not shatter like glass. It has a much lower density than glass so lorries can carry many more full PET bottles than glass bottles.

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Choosing the right material

When sailing boats carried trade goods across the ocean their sales were made of a cotton cloth. Now racing yachts use sails made of a variety of synthetic polymer fibres including nylon and Kevlar. The information below shows a comparison of the properties of cotton, nylon and Kevlar and explains why Kevlar is the material chosen by many sailors. Kevar does have some disadvantages, however, as the ultraviolet (UV) light in sunlight causes it to break down more rapidly than some materials. Scientists are continuing to search for better materials for many different applications.

Sailcloth material Tensile strength (MPa) Density (g/cm)

Cotton 225 1.54

Nylon 616 1.14

Kevlar 3400 1.45

Some polymers are soft, waxy and melt at low temperatures, while others are hard and strong and do not melt even at high temperatures. All polymers have very large molecules, but if we could look at polymers with a powerful microscope we would see they are not all the same.

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Polymer properties

If you hold two magnets close together and then let go, they will snap together. If you hold them further apart and then do the same thing they may not move at all. There is a force between magnets that is stronger the closer they are together. The same happens with polymer molecules, although they are not magnets. There are small forces between molecules that attract them to each other. The closer the molecules are to each other, the stronger the force needed to pull them apart.

When we heat substances we give the molecules energy. The stronger the force holding the molecules together, the more energy is needed to separate them and the higher the melting point of the substance.

The first samples of polyethene were made in the 1930s. It was a waxy material that softened in hot water. In the 1950s a new way was found to make polyethene but the new material was harder, stronger and melted at a temperature above the boiling point of water. It was found that the early type, now called LDPE, had long molecules with branches. The branches kept the molecules apart so that the forces between them were weak. The later version, called HDPE, did not have branches so that the polymer molecules could lie close to each other, making the forces between them stronger. Not only did this explain the differences between the two types of polyethene, but it also showed how the properties of polymers could be adjusted.

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More force and longer molecules

HDPE (high density polyethene) is highly crystalline. This means that there are a lot of areas in the material where there is a regular pattern of molecules lined up alongside each other. Highly crystalline polymers are strong and have higher melting points, although they can be brittle. Other polymers have been designed so that their molecules can line up even more regularly and close together.

Other ways of increasing the force of attraction between molecules are also used. Polymers with atoms such as nitrogen, fluorine, chlorine and oxygen on the chains also have greater forces between the molecules, which means that higher temperatures are needed to soften them. Nylon and PVC are examples of these types of polymers.

Changing the properties of a polymer means changing the forces between the molecules. One way of doing this is to make the molecules longer. The longer the molecules, the greater the force needed to separate them. UHMWPE is a form of polyethene with very long molecules. It has chains with half a million carbon atoms joined together. UHMWPE is a strong and wears so well it is used for artificial hip joints and chopping boards.

The more crystalline a polymer, the stronger it is. Crystallinity can be increased by reducing the number of branches on the main polymer chain. 

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Harder or softer?

Sometimes we need a polymer to be softer and more flexible. PVC is strong and rigid and is used for window frames and gutters on buildings. When it is used in clothing it is modified. A plasticiser is added. Plasticisers are small molecules that fit between the polymer molecules, keeping them apart and weakening the forces between them. Plasticised PVC is still hard-wearing but is much more suitable for waterproof clothes. 

Most polymers soften when heated to a certain temperature and can then be moulded into shape. They are THERMOPLASTIC. If the polymer molecules are joined together by CROSS-LINKS then the material becomes rigid, strong and does not soften at all. It is called a THERMOSETTING material. Cross-linking locks molecules together so they cannot melt but may leave empty space in the structure so the material is like a sponge. In natural rubber, sulfur atoms cross-link the molecules. This makes the rubber tough and elastic. Cross-linked polymers are used for electric plugs and sockets because they can withstand high temperatures.

The more crystalline a polymer, the stronger it is. Crystallinity can be increased by reducing the number of branches on the main polymer chain. It can also be improved by making the polymer chains themselves as flat and as rigid as possible so that they can line up neatly very close together. This is what makes Kevlar a strong polymer and suitable for making bullet proof vests and high-performance sails.

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When a polymer is turned into a fibre, the heated material is drawn through a tiny hole. This makes the polymer molecules line up and become more crystalline, giving the fibre a higher tensile strength than the original plastic.

Electron microscopes can see structures inside the nucleus of a cell, but atoms are a thousand times smaller. Objects the size of atoms and small molecules are called "nanoscale". The first machine for producing images of atoms was built by IBM in the 1980s. It was a type of electron microscope called a scanning tunnelling microscope (STM). 

Victorian engineers built things on a massive scale - ships, bridges, steam engines. Beginning with large lumps of iron or steel which they cut to shape, they made sure things fitted together properly by measuring the pieces to an accuracy of 1mm. In the 1950s and 1960s, when the microcomputer age began, microtechnology engineers had to learn to make electronic components on pieces of silicon that were 1/1000 mm big.

In 1990, the scientists at IBM found that the scanning tunnelling microscope could be used not only to see atoms but to move atoms. They succeeded in moving xenon atoms to form the company's logo. They had invented NANOTECHNOLOGY.

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Measuring smallness

A nanometre is one-thousandth of a micrometre, or one-millionth of a millimetre. 

1,000 millimetres = 1 metre 

1,000,000 micrometres = 1 metre

1,000,000,000 nanometres = 1 metre

Ten billion atoms in a row would measure about 1 metre. Nanotechnology is about building structures up to a thousand atoms across, that is 100nm, but many structures are smaller than this. 

Nanoparticles are bits of material containing about a thousand atoms, or they are tubes or sheets that are just a few nanometres thick. We did not realise it until recently, but nanoparticles are formed naturally. For example, the soot from a candle and the exhaust from vehicles contain nanoparticles of carbon. Astronomers first found nanoparticles in space when tiny pieces of carbon containing just 60 atoms arranged in a football shape were discovered. Now these "bucky balls" are being made in laboratories, as carbon nanotubes. 

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Nanoparticles of materials such as gold have been found to be very effective catalysts. Reactions take place on the surface of the nanopartcles. Nanoparticles have much more surface on which the reactions can take place compared to larger particles. 

A 1cm cube of gold has a surface area of 6cm. If the cube is cut in half it gains two new surfaces and has a total surface area of 8cm. As it is cut into smaller and smaller pieces the surface area gets bigger and bigger. The surface area of a small amount of gold in the form of nanoparticles is huge and provides very many sites for reactions to take place.

Since Roman times gold has been added to molten glass to give it a red colour. The glass is found in the stained glass windows in old churches. Now it has been discovered that only gold nanoparticles produce the red colour, so the Romans were doing nanotechnology 2000 years ago. The small size of the particles means that they scatter light in a different way to larger pieces of gold. 

Graphite is a soft form of carbon used in pencils but it is made up of sheets of carbon atoms which are strong. Scientists take the individual sheets, one atom layer thick, called graphene and roll hem up to make carbon nanotubes. They can form single tubes or tubes can be stacked inside of each other.

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Making use of nanoparticles

Socks containing silver nanoparticles will never get smelly. It seems there is no limit to the possible uses of nanoparticles. Already they have found their way into medicines, cosmetics, paints and clothes, are are used to strengthen materials and make them more hard-wearing. It is the special properties of nanoparticles that make them so useful.

The silver nanoparticles are added to the fibres before they are knitted into socks, or woven into the cloth for any other type of clothing. Silver and silver compounds have been used in medicines for a long time, because they are known to be toxic to bacteria. Silver nanoparticles are very effective at killing the bateria that get into dirty socks without harming humans. The antibacterial properties of silver nanoparticles are also made use of in wound dressings and in plastics for food containers and packaging.

Titanium oxide is a white compound that has been used in sunscreens for a long time. It forms a white film on the skin and absorbs UV light. A new sunscreen containing nanoparticles of titanium oxide is transparent and so looks a lot cleaner on the skin. The nanoparticles also absorb UV light, but let visible light pass through. In many of the uses of nanoparticles they are mixed with another material, such as metal, ceramic or a plastic. This makes the combined material, called a COMPOSITE, stronger and more hard-wearing. Nanoparticles are added to rubber for use in tyres and tennis balls make the material more hard-wearing. 

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Staying safe with nanoparticles

Some people fear that nanotechnology will produce tiny machines - nanorobots - that could replicate themselves and run wild, consume the Earth and turn it into a grey goo. The idea is fiction but nevertheless there are some real worries about the effects of nanoparticles that are in use today.

The socks contain silver nanoparticles that kill the bacteria that make feet smelly. When the socks are put in the washing machine, some of the silver gets washed out. The silver nanoparticles find their way to sewage treatment plants, where bacteria are used to break down the waste. The silver nanoparticles may kill these useful bacteria and could stop the sewage works from doing its job. If silver nanoparticles are used in other antibacterial cloths as well as socks, and if they escape in the environment, they could kill a lot of useful microorganisms.

Nanoparticles are used in a variety of cosmetics and sunscreens. They are made from materials that have been in use for years and tests have shown that they are not harmful to the skin. However, the nanoparticles of these materials are so small that they can slip through tiny pores in the skin and be absorbed into the blood where they could be carried to the organs in the body. It is not known whether some of these substances are harmful to tissues.

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Staying safe with nanoparticles

There is a lot of research into the uses of nanotechnology but little so far into the possible harmful effects of nanoparticles. One fear is that breathing carbon nanotubes into the lungs could cause diseases in the same way that tiny fibres of asbestos do. Asbestos was a widely used material until it was shown that it caused lung disease. We need to find out how hazardout nanoparticles are so we can assess whether they pose too great a risk to health to be used.

Hazard and risk are differet things. A hazard is a possible danger while the risk is the chance of that danger occuring.

Some people think that as nanoparticles occur naturally, for example in soot and volcanic dust, they are no danger because we have evolved to live with them. Other people disagree because scientists are now making nanoparticles which do not occur naturally and can have very different properties from the same materials in bulk.

Can the nanoparticles added to coatings on windows and paintwork escape into the air to be breathed in? No one knows if carbon nanotubes might clump together in the brain or liver or what effect they may have on normal reactions in cells. Some environmental groups want more control of new nanotechnology and proof that nanoparticles are safe for health and the environment before they are released for general use. 

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