Chemistry unit one


Atoms, Molecules and Compounds

Atoms have a nucleus which is positively charged with electrons which are negatively charged. The electrons move around the nucleus in cellars called shells.Atoms can form bonds to make molecules or compounds. Sometimes an atom loses or gains one or more electrons and this gives it a charge (positive if it loses an electron, negative if it gains one). Charged atoms are known as ions. If a positive and a negative ion meet they will be attracted to one another and form an ionic bond. A covalent bond is where atoms share a pair of electrons.

Formulas to remember:

Carbon dioxide: CO2 Hydrogen: H2 Water: H2O Hydrochloric acid: HCl Carbon Monoxide: CO Oxygen: O2 Sodium Carbonate: Na2CO3 Sulfuric Acid: H2SO4

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Chemical equations

Word equations: Magnesium + Oxygen ---> Magnesium oxide

                      or Methane + Oxygen ---> Carbon dioxide + water

Symbol equations : Magnesium + Oxygen ---> Magnesium oxide

                                   2Mg        +     O2    --->         2MgO

Balanced symbol equations: eg) H2SO4 + NaOH ---> Na2SO4 + 2H2O

                                                   H2SO4 + 2NaOH ---> Na2SO4 + 2H2O

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Additives are added to our foods to improve flavour, colour or shelf life. Food colouring makes food look more appetising, flavour enhancers bring out the taste and smell of a food without adding their own taste, antioxidants preserve food and emulsifiers help oil and water blend together.

Emulsions are made up of lots of droplets of one liquid suspended in another. Oil and water naturally separate and the oil sits on top of the water - Emulsifiers help to stop this. Emulsifiers have a hydrophilic head and a hydrophobic tail.

The hydrophilic end bonds to water molecules and the hydrophobic end bonds to oil molecules. When you shake oil and water together with emulsifier, the oil forms droplets surrounded by a coating of emulsifier - with the hydrophilic bit facing out. Other oil droplets are repelled by the hydrophilic head however the water molecules latch on, meaning the emulsion wont seperate.

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Cooking and chemical change

 Many foods have a better taste and texture when cooked. Some are easier to digest once theyre cooked. The high temperatures involved in cooking also kill off microbes that cause disease. Some foods are poisonous when raw and require cooking to make them edible.

Cooking foods produces new substances. That means a chemical change has taken place. Cooking is an irreversible process. Eggs and meat - Eggs and meat are sources of protein. Protein molecules change shape when heated. The energy from cooking breaks some of the chemical bonds in the proteins allowing the molecule to take a different shape. This gives the food a more edible texture. This is called denaturing enzymes. Potatoes - Each potato cell is surrounded by a cell wall of cellulose which humans can't digest. Cooking the potatoes ruptures the cell walls and makes the starch grain inside the cell wall swell up and spread out. This makes the potato softer.

When you heat baking powder it undergoes thermal decomposition.This is when a substance breaks down into simpler substances when heated. This is helped by a catalyst. The equation for baking powders thermal decomposition -                                                                                  Sodium hydrogencarbonate ---> Sodium carbonate + Carbon dioxide + WaterCarbon dioxide can be detected using limewater - it turns cloudy when CO2 is bubbled through. Carbon dioxide makes cakes rise

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Esters are often used as perfumes. They are common in nature and loads of fruity/flowery smells contain esters. Esters can also be manufactured synthetically to use as perfumes or flavourings, for example there are esters that smell like lavander, oranges or cinnamon.

Esters are made by heating a carboxylic acid with an alcohol. An acid catalyst is usually used (concentrated sulphuric acid). Acid + alcohol ---> ester + water.

Perfumes need to evaporate easily so the perfume particles reach your nose, need to be non toxic so they dont poison you, be non irritable so they dont irritate the skin, not react with water or they would react with sweat and be insoluble in water or they'd wash off in rain etc.

Companies have to test products thoroughly to ensure they're safe for use. Some are tested on animals which is controversial - some people think its worth testing on animals first to ensure no harm comes to humans however others thinkg that the suffering of animals just to test a cosmetic product is wrong. Because of the concerns about animal welfare testing on animals is now banned in the EU.

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Kinetic theory and forces between particles

Everything is made of particles and the force between these particles varies depending on whether it is a solid, liquid or gas. Solids have strong forces of attraction between particles holding them in fixed positions in a very regular lattice arrangement. The particles dont move so solids keep a definite shape and volume. The particles vibrate about their positions - the hotter the solid the more the particles vibrate. This causes solids to expand slightly when heated.                                                                                    Melting solids makes liquids. There is some force of attraction between particles but theyre free to move past eachother. They tend to stick together. Liquids dont have a definite shape but keep the same volume. The particles move in a random motion and move quicker when heated.                    When liquids are boiled gases are made. There is no force of attraction between particles and they travel in straight lines often colliding with one another. They dont have a definite shape or volume and will fill any container.

When a liquid is heated the energy goes to the particles which makes them move faster. Some particles move faster than others meaning fast moving particles at the surface will overcome the forces of attraction from other particles and escape - they evaporate. How easily a liquid evaporates is called its volitility. The evaporated particles drift in the air and the smell receptors in your nose pick up the chemical.

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When a solid is added to a liquid the bonds holding the solid together are broken and the molecules mix with the molecules in the liquid forming a solution. This is called dissolving. Whether or not the bonds break depends on how strong the attractions are between the molecules within each substance an dhow strong the attractions are between the two substances.

 Nail varnish is insoluble in water as the molecules of nail varnish are strongly attracted to eachother. This attraction is stronger than the attraction between the varnish and the water molecules. The molecules of water are also strongly attracted to each other. This attraction is stronger than the attraction between the water and nail varnish molecules. However it is soluble in acetone as the attraction between acetone molecules and nail varnish molecules is stronger than the attraction holding the two substances together. The solubility of a substance depends on the solvent used.

Alcohols and esters can be used as solvents and so can many other things. Some solvents however are very poisonous.

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Pigments give paint their colour. Paint usually contains solvent, binding medium and pigment. The binding medium is a liquid that carries the pigment and holds them together. When the binding medium goes solid it sticks the pigments to the painted surface. The solvent thins the paint and makes it easier to spread.

Paints are colloids. Colloids consist of tiny particles of one kind of stuff dispersed in another kind of stuff. They're mixed in but not dissolved. The particles cn be bits of solid, bubbles of gas or droplets of liquid. Colloids dont separate out as the particles are too small. A paint is a colloid where particles of a pigment are dispersed throughout a liquid.

Emulsion paints are water based. The solvent in these paints is water. The binding medium is usually an acrylic or a vinyl acetate polymer. A water based emulsion dries when the solvent evaporates leaving behind the binder and pigment as a thin solid film. A thin layer of this paint dries quickly. They dont produce harmful fumes so theyre ideal for painting inside walls.

Traditional gloss paint is oil based. The binding medium is oil and the solvent is an organic compound that dissolves oil. Oil paints dry in two stages - first the solvent evaporates and then the oil is oxidised before it turns solid. Oil paints are glossy and waterproof but they produce harmful fumes. theyre better for use outside.

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Special pigments

Thermochromatic pigments change colour when heated. Different pigments change colour at different temperatures, so a mixture of pigments can be used to make a colour coded temperature scale. Thermochromatic pigments can be used in kettles, baby products, mugs or mood rings.

Thermochromatic pigments can be mixed with acrylic paints. For example, mixing a blue thermochromatic pigment that loses its colour above 27 degrees with a yellow paint will give a paint thats green below 27 degrees and yellow above. These paints are used on novelty mugs.

Phosphorescent pigments absorb light and store the energy in their molecules. This energy is released as light over a period of time. Uses include: watches with glow in the dark hands, traffic signs, childrens toys and novelty decorations.

Glow in the dark watches used to be made with radioactive paints which could glow for years without needing to be charged up. However they were unsafe and gave out high doses of atomic radiation.

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Plastics are long chain molecules called polymers. They're formed when monomers join together. This is called polymerisation which usually requires high pressure and a catalyst. The monomers that make up polymers have a double covalent bond. Molecules with at least one double covalent bond between carbon atoms are called unsaturated compounds. Molecules with no double bond betewen carbon atoms are saturated compounds. Lots of unsaturated monomer molecules (alkenes) can open up  their double bonds and join together to form polymer chains. This is called additional polymerisation.

Strong covalent bonds hold the atoms together in polymer chains. But its the forces between the different chains that determine the properties of the plastic.

Weak forces: if the plastic is made up of long chains held together by weak intermolecular forces the chains will be free to slide over each other. This means the plastic can stretch easily and will have a low melting point.

Strong forces: Some plastics have stronger bonds between the polymer chains - these might be covalent bonds or cross linking bridges.These plastics have higher melting points and are rigid and cant be stretched as the cross links hold them together.

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Hydrocarbons - alkanes

A hydrocarbon is any compound that is formed form hydrogen and carbon atoms only. Hydrocarbons are really useful chemicals as fuels like petrol and diesel are hydrocarbons.

All the atoms in a hydrocarbon molecule are held together by covalent bonds. These covalent bonds are very strong. They form when atoms 'share' electrons. This way both atoms get a full outer shell. Each covalent bond provides one extra shared electron for each atom. Each atom involved has to make enough covalent bonds to fill its outer shell. So carbon atoms always want to make a total of 4 bonds while hydrogen atoms only want to make one.

Alkanes are the simplest hydrocarbon. They're just chains of carbon atoms with two or three hydrogen atoms attached to each one. Alkanes are saturated compounds meaning they contain only single covalent bonds. You can tell the difference between an alkane and an alkene by adding the substance to bromine water. an alkane wont decolourise the water.

Alkanes wont form polymers as there are no double bonds to open up. The first four alkanes are methane, ethane, propane and butane.

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Hydrocarbons - alkenes

A single covalent bond is formed when two atoms share a pair of electrons so both can have a full outer shell. Sometimes to fill up thier outer shells two atoms will share two pairs of electrons rather than just one. By doing this the atoms form a double bond. Carbon atoms can do this - each bond still provides one shared electron for each atom but this time there are two bonds between the carbons.

Alkenes have a double bond. Theyre unsaturated compounds meaning they contain at least one double covalent bond. Their double bonds can open up and join on to things making them more reactive than alkanes. They can form polymers by opening up their double bonds to hold hands in a chain. The first three alkenes are ethene propene and butene.

Alkenes react with bromine water. When an alkene is mixed with bromine water the double bonds spring open and react with the bromine decolourising it.

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Fractional distillation

Crude oil is formed from the buried remains of plants and animals - its a fossil fuel. Crude oil is a mixture of lots of different hydrocarbons (chains of carbon atoms of different lengths). The compounds in crude oil are separated by fractional distillation. The oil is heated until most of it has turned to gas. The gases enter a fractionating column (and the liquid, bitumen is drained off at the bottom). In the column there is a temperature gradient. This means that the longer hydrocarbons with higher boiling points turn back into liquids and exit the column eary on and the shorter hydrocarbons with lower boiling points turn back into liquids later on up the column.

You end up with the crude oil mixture separated out into different fractions. Each fraction contains a mix of hydrocarbons with similar boiling points. The products of fractional distillation are (in order of higher boiling point to lower) - Bitumen, Oil, Kerosene (paraffin), Naptha, Petrol and LPG (liquified petroleum gas).

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Hydrocarbon properties - bonds

As the size of a hydrobarbon molecule increases the boiling point increases, it becomes less flamable, it gets more viscous and it gets less volatile.

There are two important types of bond in crude oil - the strong covalent bonds between the carbons and hydrogens within each hydrocarbon molecule and the intermolecular forces of attraction between different hydrocarbon molecules in the mixture. When the crude oil mixture is heated, the molecules have extra energy. this means the molecules can move about more meaning eventually a molecule may have enough energy to overcome the intermolecular forces that keep it with the other molecules. It can then become a gas.

The covalent bonds are much stronger than the intermolecular forces meaning they dont break. This is why you end up with lots of little molecules. The intermolecular forces of attraction break more easily in small molecules as they are stronger in big molecules than they are in small ones. Even though a big molecule can overcome the forces attracting it to another molecule at a few points along its length, its still got lots of other places where the force is strong enough to hold it in place.

This is why bigger hydrocarbon molecules have higher boiling points.

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Cracking turns long alkane molecules into smaller alkane and alkene molecules which are more useful. It is a form of thermal decomposition which means strong covalent bonds have to be broken. This requires lots of heat and a catalyst. A lot of the longer molecules produced by fractional distillation are cracked into smaller ones as theres more demand for products like petrol and kerosene than there is for diesel or lubricating oil. Cracking also produces lots of alkene molecules which can be used to make polymers.

Vaporised hydrocarbons are passed over powdered catalyst at about 400 degrees celcius - 700 degrees celcius. Aluminium oxide is the catalyst used. the long chain molecules split apart or 'crack' on the surface of bits of the catalyst. For example : Paraffin can be turned into octane for fuel and ethene for plastics.

Cracking helps match supply and demand. For example, 13% of crude oil is kerosene but only 8% of this is in demand. The other 5% can be cracked to create more petrol as the demand is high and the percentage in crude oil is low.

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Use of fossil fuels

Crude oil provides the energy needed to do lots of things. It generates electricity and heating, provides fuel for modern transport and provides raw materials for making chemicals including plastics. As the worlds population grows more fuels are being burned to provide energy in places like china and india. However it will run out eventually. The supply is non renewable and is very limited.Its good to start thinking of alternative fuel supplies like using nuclear or wind power for electricity, ethanol to power cars and solar energy to heat water.

Political issues with fuel: The price of oil rises as stocks are used up meaning plastics and fuels will get more expensive. Countries with big stocks will start keeping oil for their own use rather than selling it. The countries with the most oil with haver power over other countries causing political conflict or even war. Countries like the UK will have to rely on other countries for fuel ad could be cut off at any time.

Environmental issues: Oil tanker crashes can lead to crude oil being leaked into the seas, leading to oil slicks. Oil covers sea birds feathers stopping them being waterproof. This means the birds die of cold. Detergents are used to clean up oil slicks. They break the oil into tiny droplets making it easier for them to disperse, But some detergents harm wildlife - they can be toxic to marine creatures.

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Burning fuels

The complete combustion of and hydrocarbon in oxygen will produce only carbon dioxide and water as waste products which are both quite clean.

Hydrocarbon + Oxygen ---> Carbon dioxide + water + energy

Many gas heaters release these waste gases into the room which is fine as long as the gas heater is working properly and the room is well ventilated. This reaction is known as complete combustion. It releases lots of energy and only produces two harmless waste products. When theres plenty of oxygen and combustion is complete the gas burns with a clean blue flame. The balanced symbol equation for this reaction is : CH4 + 2O2 --> 2H2O + CO2 (+energy)

Incomplete combustion is not safe. If there isnt enough oxygen carbon monoxide and carbon will be given off as waste products and a smokey yellow flame will be produced. Carbon monoxide is a colourless, odourless poisonous gas. The balanced symbol equation for this is

4CH4 + 6O2 ---> C + 2CO + CO2 + 8H2O (+ energy)

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The evolution of the atmosphere

Phase 1 - The earths surface was originally molten. Any atmosphere boiled away. Eventually it cooled and a thin crust formed, but volcanoes errupted releasing gases from inside the earth including carbon dioxide steam and ammonia. When things settled down the early atmosphere was mostly CO2 and water vapour (which later condensed to form oceans). There was little oxygen.

Phase 2 - A lot of the CO2 dissolved in the oceans. Green plants evolved over most of the earth and removed CO2 through photosynthetis, producing oxygen. Most of the CO2 eventually got locked up in fossil fuels and sedimentary rocks. Nitrogen gas was put into the atmosphere as it was released by denitrifying bacteria and was formed by ammonia reacting with oxygen. N2 isnt very reactive so the amount in the atmosphere increased ( made but not broken down)

Phase 3 - The build up of oxygen killed up early organisms that couldnt tolerate it. It allowed the evolution of more complex organisms that made use of the oxygen. The oxygen also created the ozone layer which blocked harmful rays from the sun and enabled even more complex organisms to evolve.

Todays atmosphere is 78% nitrogen, 21% oxygen, 0.035% carbon dioxide and has varying amounts of water and noble gases.

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The carbon cycle

The carbon on earth moves in a big cycle - Respiration, combustion and decay of plants and animals add carbon dioxide to the air and remove oxygen. Photosynthesis removes carbon dioxide and adds oxygen. These processes should balance out however humans upset the natural carbon cycle, effecting the balance of gases in the atmosphere.

The human population is increasing. This means more people respire meaning more carbon dioxide is given out. More people means more energy is required meaning more fossil fuels are burnt, releasing yet more carbon dioxide into the atmosphere. The average energy demand per person is also increasing. More people means more land is needed to build houses and grow food. This space is made by chopping down trees meaning there are fewer trees to take carbon dioxide out of the atmosphere.

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Air pollution and acid rain

When fossil fuels burn they release mostly CO2, however they also release other harmful gases - especially sulfur dioxide and various nitrogen oxides. The sulfur dioxide comes from sulfur impurities in fossil fuels. However the nitrogen oxides are created from a reaction between the nitrogen and oxygen in the air caused by the heat of the burning. When these gases mix with clouds they form dilute sulphuric acid and dilute nitric acid. This then falls as acid rain. Power stations and internal combustion engines are the main causes of this.                                        Acid rain causes lakes to become acidic meaning many plants and animals die. Acid rain also kills trees and damages limestone buildings . It also makes metal corrode.                          Photochemical smog is a type of air pollution caused by sunlight acting on oxides of nitrogen. These oxides combine with oxygen in the air to produce ozone. Ozone can cause breathing difficulties, headaches and tiredness.                                                                                       Carbon monoxide can stop your blood doing its proper job of carrying oxygen around the body. A lack of oxygen can lead to fainting, a coma or even death. Carbon monoxide is formed when petrol or diesel is burnt without enough oxygen.                                                                                                                                     The build up of these pollutants is controlled to lessen the risk of people developing respiratory illnesses. Catalytic converters reduce the amount of carbon monoxide and nitrogen oxides released into the atmosphere. The catalyst is usually a mix of platinum and rhodium. I

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