C1 - Carbon Chemistry (OCR Gateway Science B)

Atoms have a positive nucleus with orbiting electrons:

• Atoms are really tiny. They're too small to see, even with a microscope. They have a nucleus which is positively charged, and electrons which are negatively chared. The electrons move around the nucleus in layers known as shellls.
• Atoms can form bonds to make molecules or compounds. It's the electrons that are involved in making bonds. Sometimes an atom loses or gains one or more electrons and this gives it a charge (positive if it loses an electron and negative if it gains one)/
• Charged stoms are known as ions. If a positive ion meets a negative ion they'll be attracted to one another and join together. This is called an ionic bond.
• The other main type of bond is called covalent bond - atoms in a covalent bond share a pair of electrons.

Formulas to know:

• Carbon dioxide - CO2
• Hydrogen - H2
• Water - H2O
• Oxygen - O2

• Carbon monoxide - CO
• Hydrochloric acid - HCl
• Calcium chloride - CACl2
• Magnesium chloride - MgCl2
• Sodium carbonate - Na2CO3
• Calcium carbonate - CaCO3
• Sulfuric acid - H2SO4
• Magnesium sulfate - MgSO4

Chemical changes are shown using chemical equations:

• One way to show a chemical reaction is to write a word equation. It's not as quick as using chemical symbols and you can't tell straight away what's happened to each of the atoms, but it's dead easy.
• eg: methane + oxygen -> carbon dioxide + water
• The molecules on the left-hand side of the equation are called the reactants (because they react with each other)
• The molecules on the right hand side are called the products (because they've been produced by the reactants)

Symbol equations show the atoms on both sides:

• Chemical changes can be shown in a kind of shorthand using symbol equations. Symbol equations just show the formulas of the reactants and products...
• magnesium + oxygen -> magnesium oxide // 2Mg + O2 -> 2MgO

Additives make food last longer and look and taste better:

Additives are added to lots of our foods to improve their flavour, colour or to make them last longer:

• Food colours make food look more appealing.
• Flavour enhancers bring out the taste and smell of food without adding a taste of their own.
• Antioxidants help to preserve food.
• Emulsifiers help oil and water blend together in food like salad cream and ice cream.

Emulsifiers help oil and water mix:

• You can mix an oil with water to make an emulsion.
• Emulsions are made up of droplets of one liquid suspended in another liquid.
• Oil and water naturall seperate into two layers with the oil floating on top of the water - they don't 'want' to mix. Emulsifiers help stop the two liquids in an emulsion from seperating out.
• Mayonnaise, low-fat spread and ice-cream are foods which contain emulsifiers.
• Emulsifiers are molecules with one part that's attracted to water and another part that's attracted to oil or fat.

• The head is attracted to water and is called hydrophillic, thet tail that is attracted to oil is called hydrophobic.
• The hydrophilic end of each emulsifier molecule bonds to water molecules.
• The hydrophobic end of each emulsifier molecule bonds to oil molecules.
• When you shake oil and water together with a bit of emulsifier the oil forms droplets, surrounded by a coating of emulsifier... with the hydrophilic bit facing outwards.
• Other oil droplets are repelled by the hydrophillic bit of the emulsifier, while water molecules latch on. So the emulsion won't seperate out.

Some foods have to be cooked:

There are loads of different ways to cook food, eg: boiling, steaming, frying, grilling and cooking in an oven or microwave.

• Many foods have a better taste and texture when cooked.
• Some foods are easier to digest once they've been cooked (eg: potatoes, flour)
• The high temperatures involved in cooking also kill off microbes that cause disease - this is very important with meat.
• Some foods are poisonous when raw, and must be cooked to make them edible, eg: red kidney beans contain a poison that's only destroyed by at least 10 minutes of boiling (and 2 hours of cooking in total)

Cooking causes chemical changes:

Cooking food produces new substances. That means a chemical change has taken place. Once cooked, you can't change it back: the cooking process is irreversable.

• EG: EGGS AND MEAT: Eggs and meat are good sourcces of protein. Protein molecules change shape when you heat them. The energy from cooking breaks some of the chemical bonds in the protein and this allows the molecules to take a different shape. This gives the food a more edible texture. The change is irreversible. It's called denaturing.
• EG: POTATOES: Potatoes are plants, so each potato cell is surrounded by a rigid cell wall made of cellulose. Humans can't digest cellulose, so it's difficult for us to get to the contents of the cells. Cooking the potato ruptures (breaks down) the cell walls. it also makes the starch grains inside the cell swell up and spread out. these changes make the potato softer and more flexible, and much easier to digest.

Baking powder undergoes a chemical change when heated:

• When you heat baking powder, it undergoes thermal decomposition.
• Thermal decomposition is when a substance breaks down into simpler substances when heated. Many thermal decompositions are helped along by a catalyst. (Thermal decomposition is different from a lot of reactions you'll come across, since they're only one substance to start with)
• Baking powder contains the chemical sodium hydrogencarbonate

• EG: EGGS AND MEAT: Eggs and meat are good sourcces of protein. Protein molecules change shape when you heat them. The energy from cooking breaks some of the chemical bonds in the protein and this allows the molecules to take a different shape. This gives the food a more edible texture. The change is irreversible. It's called denaturing.
• EG: POTATOES: Potatoes are plants, so each potato cell is surrounded by a rigid cell wall made of cellulose. Humans can't digest cellulose, so it's difficult for us to get to the contents of the cells. Cooking the potato ruptures (breaks down) the cell walls. it also makes the starch grains inside the cell swell up and spread out. these changes make the potato softer and more flexible, and much easier to digest.

Baking powder undergoes a chemical change when heated:

• When you heat baking powder, it undergoes thermal decomposition.
• Thermal decomposition is when a substance breaks down into simpler substances when heated. Many thermal decompositions are helped along by a catalyst. (Thermal decomposition is different from a lot of reactions you'll come across, since they're only one substance to start with)
• Baking powder contains the chemical sodium hydrogencarbonate

• The equation for sodium hydrogencarbonate's thermal decomposition is: sodium hydrogencarbonate -> sodium carbonate + carbon dioxide + water, 2NaHCO3 -> Na2CO3 + CO2 + H2O
• Baking powder is used in baking cades - the carbon dioxide produced makes the cake rise.
• You can check that it is actually carbon dioxide that has been formed using a chemical test: Carbon dioxide can be detected using limewater - CO2 turns limewater cloudly when it's bubbled through.

Perfumes can be natural or artificial:

• Chemicals that smell nice are used as perfumes and air fresheners. Esters are often used as perfumes as they usually smell quite pleasant.
• Esters are pretty common in nature. Loads of common fruity smellls (like apples) and flowery smells (like jasmine) contain esters.
• Esters can also be manufactured synthetically to use as perfumes or flavourings, eg: there are esters (or combinations of esters) that smell of lavendar, oranges, cinnamon and so on.

Esters are made by esterification:

• You can make an ester by heating a carboxylic acid with an alcohol. (This is an example of esterification.)
• An acid catalyst is usually used (eg: concentrated sulfuric acid.
• acid + alcohol -> ester + water
• method: Mix 10cm3 of carboxylic solid such as an ethanoic acid with 10cm3 of an alcohol such as ethanol. Add 1cm3 of concentrated sulfuric acid to this mixture and warm gently for about 5 minutes. Tip the mixture into 150cm3 of sodium carbonate solution (to neutralise the acids) and smell carefully (by wafting the smell towards your nose). The fruit-smelling product is the ester.

Perfumes need certain properties:

You can't use any old chemical with a smell as a perfume. You need a substance with certain properties:

• Easily evaporates - the perfume particles won't reach your nose and you won't be able to smell it
• Non-toxic - it mustn't seep through your skin and poison you.
• Doesn't react with water -  it would react with the water in sweat.
• Doesn't irritate the skin - you couldn't apply it directly to your neck or wrists. If you slash on any old substance you risk burning your skin
• Insoluble in water - if it was soluble in water it would wash off everytime you got wet.

New perfumes and cosmetics have to be tested:

• Companies are always developing new cosmetic products to sell to us. Before they're released to the shops, they need to be tested thoroughly to make sure they're safe to use.
• But some tests are carried out using animals, which is contraversial.

• People have different opinions about whether it's okay to test cosmetic products on animals: > Some think it's worth testing cosmetic products on animals first to check they won't damage humans. > Others claim that it's wrong to cause suffering to animals just to test the safety of a cosmetic - especially when the results of the animal tests might not be conclusive.
• Because of the concerns about animal welfare, testing cosmetics on animals has now been banned in the EU (except for a few tests which are still allowed).

States of matter - depend on the forces between particles:

All stuff is made of particles (molecules, ions or atoms) that are constantly moving, and the forces between these particles can be weak or strong, depending on whether it's a solid, liquid or gas.

SOLIDS:

• There are strong forces of attraction between particles, which holds them in fixed positions in a very regular lattice arrangement.
• The particles don't move from their positions so all solids keep a definite shape and volume, and don't flow like liquids.
• The particles vibrate about their positions - the hotter the solid becomes, the more they vibrate (causing solids to expand slightly when heated).

If you heat the solid (give the particles more energy) eventually the solid will melt and become liquid...

LIQUIDS

• There is some forces of attraction between the particles. They're free to move past eachother but they do tend to stick together.
• Liquids don't keep a definite shape and will flow to fill the bottom of a container. But they do keep the same volume.
• The particles are constantly moving with random motion. The hotter the liquid gets, the faster they move. This causes liquids to expand slightly when heated.
• If you now heat the liquid, eventually it will boil and become a gas...

GASES

• There's next to no force of attraction between the particles - they're free to move. They travel in straight lines and only interact when they collide.
• Gases don't keep a definite shape or volume and will always fill any container. When particles bounce of the walls of a container they exert a pressure on the walls.
• The particles move constantly with random motion. The hotter the gas gets the faster they move. Gases either expand when heated, or their pressure increases.

How we smell stuff - votality's the key:

• When a liquid is heated, the heat energy goes to the particles, which makes them move faster.
• Some particles move faster than others.
• Fast-moving particles at the surface will overcome the forces of attraction from the other particles to escape. This is called its volatility.
• How easily a liquid evaporates is called its volatility.

• So the evaporated particles are now drifting about in the air, the smell receptors in your nose pick up the chemical then you smell it.
• Perfumes need to be quite volatile so they can evaporate enough for you to smell them. The particles in liquid perfumes only have very weakk attraction between them. It's easy for the particles to overcome this and escape - so you only need a very little heat energy to make the perfume evaporate.

A solution is a mixture of solvent and solute:

When you add a solid (the solute) to a liquid (the solvent) the bonds holding the solute molecules togehther sometimes break and the molecules then mix with the molecules in the liquid - forming a solution. This is called dissolving. Whether the bonds break depends on how strong the attractions are between the molecules witin each substance and how strong the attractions are between the two substances.

• Solution - a mixture of a solute and a solvent that does not seperate out.
• Solute - the substance being dissolved
• Solvent - the liquid its being dissolved into
• Soluble - it will dissolve
• Insoluble - it will not dissolve
• Solubility - measure of how much will dissolve.

Nail varnish is insoluble in water:

Nail varnish doesn't dissolve in water. This is for two reasons:

• The molecules of nail varnish are strongly attracted to each other. This attraction is stronger than the attraction between the nail varnish molecules and the water molecules.
• The molecules of water are strongly attracted to eachother. This attraction is stronger than the attraction between the water molecules and the nail varnish molecules.

because the two substances are more attracted to themselves than eachother, they don't form a solution.

but soluble in acetone:

• Nail varnish dissolves in acetone - more commonly known as nail varnish remover. This is because the attraction between acetone molecules and nail varnish molecules is stronger than the attraction holding the two substances together.
• So the solubility of a substance depends on the solvent used.

Pigments give paints their colours:

• Paint usually contains the following parts: solvent, binding material and pigment.
• The pigment gives the paint its colour.
• The binding medium is a liguid that carries the pigment parts and holds them together. When the binding medium goes solid it sticks the pigments to the surface you've painted.
• The solvent is the stuff that thins the paint and makes it easier to spread.

Paints are colloids:

• A colloid consists of really tiny particles of one kind of stuff dispersed in (mixed in with) another kind of stuff. They're mixed in, but not dissolved.
• The particles can be bits of solid, droplets of liquid or bubbles of gas.
• Colloids don't seperate out because the particles are so small. They don't settle out at the bottom.
• A paint is a colloid where particles of a pigment (usually a solid) are dispersed through a liquid.

Some paints are water-based and some are oil-based:

When you're painting, you usually apply paint in a thin layer. The paint dries as the solvent evaporates. A thin layer dries quicker than a thick layer. Depending on what type of job you're doing, you might choose a water-based or oil-based paint.

WATER BASED:

• Emulsion paints are water-based. The solvent used in these paints is water, and the binding medium is usually an acrylic or 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 emulsion paint dries quite quickl.
• Emulsion paints are fast drying and don't produce harmful fumes - so they're ideal for painting things like inside walls.

OIL-BASED:

• Traditional gloss paints and artists' oil paints are oil based. This time. the binding material is oil, and the solvent is an organic compound that'll dissolve in oil.

Some paints are water-based and some are oil-based:

When you're painting, you usually apply paint in a thin layer. The paint dries as the solvent evaporates. A thin layer dries quicker than a thick layer. Depending on what type of job you're doing, you might choose a water-based or oil-based paint.

WATER BASED:

• Emulsion paints are water-based. The solvent used in these paints is water, and the binding medium is usually an acrylic or 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 emulsion paint dries quite quickl.
• Emulsion paints are fast drying and don't produce harmful fumes - so they're ideal for painting things like inside walls.

OIL-BASED:

• Traditional gloss paints and artists' oil paints are oil based. This time. the binding material is oil, and the solvent is an organic compound that'll dissolve in oil.

• Oil paints dry in two stages. First the solvent evaporates, and then the oil is oxidised by oxygen in the air, before it turns solid. So they tend to take longer to dry than water-based paints.
• Oil paints are glossy, waterproof and hard-wearing, but the solvvents used to make them often produce harmful fumes. They're best used for painting things  like outside doors and metalwork.

Thermochromic pigments change colour when heated:

• Thermochromic pigments change colour or become transparent when heaten or cooled.
• Different pigments change colour forat different temperatures, so a mixture of different pigments can be used to make a colour-coded temperature scale. These are used to make basic thermomenters that you stick on your forehead to take your temperature.

There are lots of other uses for thermochromatic pigments:

• Thermochromatic pigmentst are used in electric kettles that change colour as the water boils.
• Baby products, like bath toys and baby spoons often have them added as a safety feature - you can tell at a glance if the baby's bath water or food is too hot.
• They're used on drinks mugs to warn you when the contents are too hot to drink.
• Most mood rings make use of thermochromic pigments - the middle of the ring contains heat sensitive pigments that change colour depending on the temperature of your finger.

You can mix thermochromatic pigments with paints too:

• Thermochromic pigments can be mixed with acrylic paint, giving a wide range of colour changes. For example, mixing a blue thermochromic pigment that loses its color above 27*c with a yellow acrylic paint would give a paint that's green below 27*c.
• These paints are used on novelty mugs. For example mugs have a design that changes colour when a hot drink's poured into them. Other mugs use a thermochromic pigment that becomes transparent when heated. A picture underneath the paint is only visible when a hot drink is poured in.

Phosphorescent pigments glow in the dark:

• Phosphorescent pigments absorb natural or artificial light and store energy in their molecules. This energy is released as light over a period of time - from a few seconds to a couple of hours.
• An obvious one is a watch or clock with glow-in-the-dark hands.
• Other uses include traffic signs, emergency exit signs, toys and novelty decorations.
• Glow-in-the-dark watches used to be made with radioactive paints. These aints would glow for years without needing to be 'charged up' by putting them in the light. However, they weren't safe and would give atomic radiation, unlike phosphorescent pigments.

Plastics are long-chain molecules called polymers:

• Polymers are formed when lots of small molecules called monomers join together. This reaction is called polymerisation - and it usually needs high pressure and a catalyst.
• Plastics are polymers. They're usually carbon based and their monomers are often alkenes.

Additional polymers are made from unsaturated monomers:

• The monomers that make up additional polymers have a double covalent bond.
• Molecules iwth at least one double covalent bond between carbon atoms are called unsaturated compounds. Molecules with no double bond between 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 addition polymerisation.

Forces between molecules determine the properties of plastics:

• Strong covalent bonds hold the atoms together in polymer chains. But it's the forces between the different chains that deterime the properties of the plastics.

WEAK FORCES - If the plastic is made up of long chains that are held together by weak intermolecular forces, then the chains will be free to slide over each other. This means that the plastic can be stretched easily, and will have a low melting point.

STRONG FORCES - Some plastics have stronger bonds between the polymer chains - these might be covalent bonds between the chains, or cross-linking bridges. These plastics have high melting points are rigid and can't be stretched, as the crosslinks hold the chains firmly together.

Polymers' properties decide what they're used for:

Different polymers have different physical properties - some are stronger, some are stretchier, some are more easily moulded, and so on. These different physical properties make them suited for different uses:

• Strong, rigid polymers such as high density polyethene are used to make plastic milk bottles.
• Light, stretchable polymers such as low density polyethene are used for plastic bags and squeezy bottles. Low density polyethethene has a low melting point, so it's not good for anything which will get hot.
• PVC is strong and durable, and it can be made either rigid or stretchy. The rigid kind is used to make window frames and piping. The stretchy kind is used to make synthetic leather.
• Polystyrene foam is used in packaging to protect breakable things, and it's used to make disposable coffee cups (the trapped air in the foam makes it a brilliant thermal insulator).

Polymers are often used to make clothes:

• Nylone is a synthetic polymer often used to make clothes. Fabrics made from nylon are not waterproof on their own, but can be coated with polyurethane to make tough, hard-wearing and waterproof outdoor clothing which also helps keep UV light out.

• One big problem is that the polyurethane coating doesn't let water vapour pass through it. So if you get a bit hot or do a bit of exercise, sweat condenses on the inside. This makes skin and clothes wet and uncomfortable - the material isn't breathable.
• Some fabrics, eg: GORE-TEX products, have all the useful properties of nylon/polyurethane ones, but are also breathable. If you sweat in breathable material, water vapour can escape - so no condensation.

• GORE-TEX fabrics are made by laminating a thin film of plastic called expanded PTFE onto a layer of another fabric, such as polyester or nylon. This makes the PTFE sturdier.
• The PTFE film has tiny holes which let water vapour through - so it's breathable. But it's waterproof since the holes aren't big enough to let big water droplets though and PTFE repels liquid water.
• This material is great for the outside - you can hike without getting rained on or sweating.

Non-biodegradable plastics cause disposable problems:

• Most polymers aren't biodegradable - they're not broken down by micro-organisms, so they don't rot. This property is useful until it's time to get rid of your plastic.

• It's difficult to get rid of plastics - if you bury them in a landfill site, they'll still be there years later. Landfill sites fill up quickly, and they're a waste of land. And a waste of plstic.
• When plastics are burnt, some of them release gases such as acidic sulfur dioxide and poisonous hydrogen chloride and hydrogen cyanide.
• The best thing is to reuse plastics as many times as possible and then recycle them if you can. Sorting out lots of different plastics for recycling is difficult and expensive though.
• Chemists are working on a variety of ideas to produce polymers that biodegrade or dissolve - that way any plastic that is thrown away breaks down or dissolves rather than sitting there in landfill for ages.

Hydrocarbons only contain hydrogen and carbon atoms:

• A hydrocarbon in any compound that is formed from carbon and hydrogen atoms only. So C10H22 (decane, an alkane) is a hydrocarbon, but CH3COOC3H7 (an ester) is not - it's got oxygen atoms in it.
• Hydrocarbons are really useful chemicals - fuels like petrol and diesel are hydrocarbons and lots of plastics are made from hydrocarbons too.

Covalent bonds hold atoms in a molecule together:

• All the atoms in hydrocarbon molecules 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 - which is an atom's main aim in life.
• Each covalent bond provides one extra shared electron for each atom. And each atom involved has to make enough covalent bonds to fill up its outer shell. So carbon atoms always want to make a total of 4 bonds, while hydrogen atoms only want to make 1.

Alkanes have all C-C single bonds:

• Alkanes are the simplest type of hydrocarbon you can get. They're just chains of carbon atoms with two or three hydrogen atoms attached to each one three if the carbon's at the end of the chain, two if it's in the middle).
• Alkanes are saturated compounds - this means they contain only single covalent bonds between their carbon atoms. (They don't have any double bonds that can open up and join onto things.)
• You can tell the difference between an alkane and an alkene by adding the substance to bromine water - an alkane won't decolourise the bromine water.
• Alkanes won't form polymers - same reason again, no double bonds to open up.

• The first four alkanes are methane (natural gas), ethane, propane and butane.
• Methane - CH4
• Ethane - C2H6
• Propane - C3H8
• Butane C4H10

Covalent bonds can be single or double bonds:

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

Alkenes have C=C double bond:

• Alkenes are hydrocarbons with one or more double bonds between carbon atoms.
• They're unsaturated compunds. An unsaturated compound is just one that contains at least one double covalent bond.
• Their double bonds can open up and join onto things. This makes alkenes much more reactive than alkanes - they can form polymers by opening up their double bonds to 'hold hands' in a long chain.

• The first three alkenes are:
• Ethene (C2H4)
• Propene (C3H6)
• Butene (C4H8)

Alkenes react with bromine water:

• Bromine water is a bright orange solution that contains bromine, Br2.
• It's really reactive - if there are any double bonds around, they'll spring open and react with the bromine. When this happens, the orange colour disappears from the solution - the bromine water is decolourised.

• You can use this to test whether what you've got is an alkene or not. You just have to take a sample of your hydrocarbon, mix it with bromine water, and shake:
• If it's a saturated compound, like an alkane, no reaction will hapen and it'll stay bright orange. 
• If it's an alkene an additional reaction will take place. The bromine will add to the double bond, making a colourless dibromo compound - so the bromine water is decolourised.

Crude oil is seperated into different hydrocarbon fractions:

• Crude oil is formed from the buried remains of plants and animals - it's a fossil fuel. Over millions of years, with high temperature and pressure, the remains turn to crude oil which can be drilled up.
• Crude oil is a mixture of lots of different hydrocarbons. Remember that hydrocarbons are chains of carbon atoms (eg: alkanes and alkenes) of various lengths.
• The different compounds in crude oil are seperated by fractional distillation. The oil is heated until most of it has turned into gas. the gases enter a fractionating column (and the liquid bit, bitumen, is drained off at the bottom.). In the column, there's a temperature gradient (ie: it's hot at the bottom and gets gradually cooler as you go up).
• The longer hydrocarbons have high boiling points. They turn back into liquids and drain out of the column early on, when they're near the bottom. The shorter hydrocarbons have lower boiling points. They turn to liquid and drain out much later on, near to the top of the column where it's cooler.
• You end up with the crude oil mixture seperated out into different fractions. Each fraction contains a mixture of hydrocarbons with similar boiling points.

Hydrocarbon properties change as the chain gets longer:

As the size of the hydrogen molecule increases:

• The boiling point increases.
• It gets less flamable.
• It gets more viscous (doesn't flow easily).
• It gets less volatile (doesn't evaporate as easily).

That's how distillation works - you can seperate out the random mixture of all kinds of hydrocarbons into groups (fractions) that have similar chain lengths and so similar properties. Then you can use them for various useful things like powering vehicles, heating houses and making roads. It works because on of those properties that each group has in common is the boiling point.

It's all down to the bonds in and between hydrocarbons:

• There are two important types of bond in crude oil: the strong covalent bond between the carbons and hydrogens within each hydrocarbon molecule and the intermolecular forces of attraction between hydrocarbon molecules in the mixture.
• When the crude oil mixture is heated, the molecules are supplied with extra energy.
• This makes the molecules move about more. Eventually a molecule might have enough energy to overcome the intermolecular fores that keep it with the other molecules.
• It can now go off as a gas.
• The covalent bonds holding each molecule together are much stronger than the intermolecular forces as they don't break, that's why you end up with lots of little molecules.
• The intermolecular forces  of attraction break a lot more easily in small molecules than they do in bigger molecules. That;s because they are much stronger between big molecules than they are between small molecules.
• That's why big molecules have higher boiling points than small molecules do.

Cracking is splitting up a long-chain hydrocarbons:

• Cracking turns long alkanes molecules into smaller alkane and alkene molecules (which are much more useful).
• It's a form of thermal decomposition, which is when one substance breaks down into at least two new ones when you heat it. This means breaking strong covalent bonds, so you need lots of heat and a catalyst.
• A lot of the longer molecules produced from fractional distillation are cracked into smaller ones because there's more demand for products like petrol and kkerosene (jet fuel) than for deisel or lubricating oil.
• Cracking also produces lots of alkene molecules, which can be used to make polymers (mostly plastics).

Conditions needed for cracking: hot, plus a catalyst:

• Vaporised hydrocarbons are passed over powdered catalyst at about 400*c - 700*c.
• Aluminium oxide is the catalyst used. The long-chain molecules split apart or 'crack' on the surface of the bits of catalyst.

Crude oil provides important fuels for modern life:

• Crude oil provides the energy needed to do lots of vital things - generating electricity, heating homes etc.
• Oil provides the fuel for most modern transport - cars, trains, planes etc. It also provides the raw materials needed to make various chemicals, including plastics.
• As Earth's population increases, and as countries like India and China become more developed, more fossil fuels are burned to provide electricity - both for increased home use and to run manufacturing industries.

But it will run out eventually:

• Crude oil supplies are limited and non-renweable. New reserves are sometimes found, and new technology means we can get to oil that was once too difficult to extract. But one day we will run out.
• Alternatives include nuclear or wind power to generate electricity, ethanol to power cars, and solar energy to heat water.
• Some people think we should stop using oil for fuel (where we have alternatives) and keep it for makings plastics and other chemicals. This could lead to conflict for resources between the fuel and chemical industries.

Oil can cause political and enviromental problems:

POLITICAL-

• As stocks of oil get used up, the price of oil will rise and plastics and fuels will get more expensive. Countries with big stocks of oil might start keeping more of it for their own use, rather than selling it.
• The countries with the most oil and natural gas will have power over the other countries - they can choose who they do/don't supply. This could cause political conflicts between countries, or even wars.
• It'll get harder for countries without lots of oil and gas, like the UK, to get hold of it. We might have to depend on politically unstable countries for our supplies, and then we could be cut off at any time.

ENVIROMENTAL-

• Oil tanker crashes (and problems with oil rigs) can lead to huge amounts of crude oil being realeased into the sea. Oil floats on water and the action of waves and tides spreads it out into big oil slicks.

• Oil covers sea birds' feathers and stops them being waterproof. Water then soaks into their downy feathers, and they die of cold. Also, birds can't fly when their feathers are matted with oil.
• Detergents are often used to clean up oil slicks. They break the oil into tiny droplets, making it easier to disperse. But some detergents harm wildlife - they can be toxic to marine creatures like fish and shellfish.

There's lots to consider when choosing the best fuel:

• Energy value (ie: amount of energy) 
• Availability
• Storage
• Cost
• Toxicity
• Ease of use
• Pollution

Complete combustion happenswhen there's plent of oxygen:

• The complete combustion of any hydrocarbon in oxygen will produce only carbon dioxide and water as waste products, which are both quite clean and non-poisonous.
• hydrocarbon + oxygen -> carbon dioxide + water (+ energy)
• Many gas heaters release these waste gases into room, which is okay. As long as the gas heater is working properly and the room is well ventilated there's no problem.
• This reaction, when there's plenty of oxygen, is known as complete combustion. It releases lots of energy and only produces those two harmless waste products.
• When there's plenty of oxygen and combustion is complete, the gas burns with a clean blue flame.
• You need to give a balanced symbol equation for the complete combustion of a simple hydrocarbon when you're given its molecular formula, eg: CH4 + 2O2 -> 2H2O + CO2 (+energy)
• The water pump draws gases from the burning hexane through the appartus. Water collects inside the cooled U-tube and you can show that it's water by checking its boiling point. The limewater turns miky, showing that carbon dioxide is present.

Incomplete combustion of hydrocarbons is NOT safe:

• If there isn't enough oxygen the combustion will be incomplete. This gives carbon monoxide and carbon as waste products, and produces a yellow fame.
• hydrocarbon + oxygen -> carbon dioxide + water + carbon monoxide + carbon (+ energy)
• The carbon monoxide is a colourless, odourless and poisonous gas and it's very dangerous. Every year people are killed while they sleep due to faulty gas firers and boilers filling the room with deady carbon monoxide (CO) and nobody realising - this is why it's important to regularly service gas applicances. The black carbon given off produces sooty marks - a clue that the fuel is not burning fully.
• So basically, you want lots of oxygen when you're burning fuel - you get more heat energy given out and you don't get any messy soot or poisonous gases.
• You need to be able to write a balanced symbol equation for incomplete combustion too, eg: 4CH4 + 6O2 -> C + 2CO + CO2 + 8H2O (+ energy)

Incomplete combustion of hydrocarbons is NOT safe:

• If there isn't enough oxygen the combustion will be incomplete. This gives carbon monoxide and carbon as waste products, and produces a yellow fame.
• hydrocarbon + oxygen -> carbon dioxide + water + carbon monoxide + carbon (+ energy)
• The carbon monoxide is a colourless, odourless and poisonous gas and it's very dangerous. Every year people are killed while they sleep due to faulty gas firers and boilers filling the room with deady carbon monoxide (CO) and nobody realising - this is why it's important to regularly service gas applicances. The black carbon given off produces sooty marks - a clue that the fuel is not burning fully.
• So basically, you want lots of oxygen when you're burning fuel - you get more heat energy given out and you don't get any messy soot or poisonous gases.
• You need to be able to write a balanced symbol equation for incomplete combustion too, eg: 4CH4 + 6O2 -> C + 2CO + CO2 + 8H2O (+ energy)

PHASE 1 - Volcanoes gave out steam and CO2:

• 1. The Earth's surface was originally molten for many millions of years. Any atmosphere boiled away.
• 2. Eventually it cooled and a thin crust formed, but volcanoes kept erupting, releasing gases from inside the Earth. This 'degassing' released mainly carbon dioxide, but also steam and ammonia.
• 3. When things eventually settled down, the early atmosphere was mostly CO2 and water vapour (the water vapour later condensed to form the oceans). There was very little oxygen.

PHASE 2 - Green plants evolved and produced oxygen:

• 1. A lot of the early CO2 dissolved into the oceans.
• 2, Green plants evolved over most of the Earth. As they photosynthesised, they removed CO2 and produced O2.
• 3. Thanks to the plants the amount of O2 in the air gradually built up and much of the CO2 eventually got locked up in fossil fuels and sedimentary rocks.
• 4. Nitrogen gas (N2) was put into the atmosphere in two ways - it was formed by ammonia reacting with oxygen, and was released by denitrifying bacteria.

• 5. N2 isn't very reactive. So the amount of N2 in the atmosphere increased, because it was being made but not broken down.

PHASE 3 - Ozone layer allows evolution of complex animals:

• It bulid-up of oxygen in the atmosphere killed off early organisms that couldn't tolerate it.
• But it did allow the evolution of more complex organisms that made use of the oxygen.
• The oxygen also created the oxone layer (O3), which blocked harmful rays from the Sun and enabled even more complex organisms to evolve.
• There is virtually no CO2 left now.

Today's atmosphere is just right for us:

• The present composition of Earth's atmosphere is: 78% nitrogen, 21% oxygen and 0.035% carbon dixode.
• There are also: varying amounts of water vapour and noble gases (mainly argon)

Carbon is constantly being recycled:

Carbon is the key to the greenhouse effect - it exists in the atmosphere as carbon dioxide gas, and is also present in many other greenhouse gases (eg: methane).

• The carbon on Earth moves in a big cycle - the diagram below is a pretty good summary.
• Respiration, combustion and decay of plants and animals add carbon dioxide to the air and remove oxygen.
• Photosynthesis does the opposite - it removes carbon dioxide and adds oxygen.
• These processes should all balance out. However, humans have upset the natural carbon cycle, which has affected the balance of gases in the atmosphere.

Human activity affects the composition of air:

• The human population is increasing. This means there are more people respiring - giving out more carbon dioxide. 
• More people means that more energy is needed for lighting, heating, cooking, transport etc. And people's lifestyles are changing too. More and more countries are becoming industrialised and well off. This means the average energy demand per person is also increasing (since people use more electrical gadgets, cars and travel on planes)/

• This increased energy consumption comes mainly from the burning of fossil fuels, which releases more carbon dioxide.
• More people also means more land is needed to build houses and grow food. This space is often made by chopping down trees - this is called deforestation. But plants are the main things taking carbon dioxide out of the atmosphere (as they photosynthesise) - so fewer plants means less carbon dioxide is taken out of the atmosphere.
• The graph shows hows CO2 levels in the atmosphere have risen over the last 150 years.

Acid rain is caused by sulfur dioxide and oxides of nitrogen:

• When fossil fuels are burned they release mostly CO2 (a big cause of global warming).
• Butthey also release other harmful gases - especially sulfur dioxide and various nitrogen oxide.
• The sulfur dioxide (SO2) comes from sulfur impurities in the 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. (This can happen in the internal combusition engines of cars.)
• When these gases mix with clouds they form dilute sulfuric acid and dilute nitric acid.
• This then falls as acid rain.
• Power station and internal combustion engines in cars are the main causes of acid rain.

Acid rain kills fish, trees and statues;

• Acid rain causes lakes to become acidic and many plants and animals die as a result.
• Acid rain kills trees and damages limestone building and ruins stone statues. It also makes metal corrode.

Acid rain is caused by sulfur dioxide and oxides of nitrogen:

• When fossil fuels are burned they release mostly CO2 (a big cause of global warming).
• Butthey also release other harmful gases - especially sulfur dioxide and various nitrogen oxide.
• The sulfur dioxide (SO2) comes from sulfur impurities in the 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. (This can happen in the internal combusition engines of cars.)
• When these gases mix with clouds they form dilute sulfuric acid and dilute nitric acid.
• This then falls as acid rain.
• Power station and internal combustion engines in cars are the main causes of acid rain.

Acid rain kills fish, trees and statues;

• Acid rain causes lakes to become acidic and many plants and animals die as a result.
• Acid rain kills trees and damages limestone building and ruins stone statues. It also makes metal corrode.

Acid rain is caused by sulfur dioxide and oxides of nitrogen:

• When fossil fuels are burned they release mostly CO2 (a big cause of global warming).
• Butthey also release other harmful gases - especially sulfur dioxide and various nitrogen oxide.
• The sulfur dioxide (SO2) comes from sulfur impurities in the 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. (This can happen in the internal combusition engines of cars.)
• When these gases mix with clouds they form dilute sulfuric acid and dilute nitric acid.
• This then falls as acid rain.
• Power station and internal combustion engines in cars are the main causes of acid rain.

Acid rain kills fish, trees and statues;

• Acid rain causes lakes to become acidic and many plants and animals die as a result.
• Acid rain kills trees and damages limestone building and ruins stone statues. It also makes metal corrode.

Acid rain is caused by sulfur dioxide and oxides of nitrogen:

• When fossil fuels are burned they release mostly CO2 (a big cause of global warming).
• Butthey also release other harmful gases - especially sulfur dioxide and various nitrogen oxide.
• The sulfur dioxide (SO2) comes from sulfur impurities in the 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. (This can happen in the internal combusition engines of cars.)
• When these gases mix with clouds they form dilute sulfuric acid and dilute nitric acid.
• This then falls as acid rain.
• Power station and internal combustion engines in cars are the main causes of acid rain.

Acid rain kills fish, trees and statues;

• Acid rain causes lakes to become acidic and many plants and animals die as a result.
• Acid rain kills trees and damages limestone building and ruins stone statues. It also makes metal corrode.

Acid rain is caused by sulfur dioxide and oxides of nitrogen:

• When fossil fuels are burned they release mostly CO2 (a big cause of global warming).
• Butthey also release other harmful gases - especially sulfur dioxide and various nitrogen oxide.
• The sulfur dioxide (SO2) comes from sulfur impurities in the 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. (This can happen in the internal combusition engines of cars.)
• When these gases mix with clouds they form dilute sulfuric acid and dilute nitric acid.
• This then falls as acid rain.
• Power station and internal combustion engines in cars are the main causes of acid rain.

Acid rain kills fish, trees and statues;

• Acid rain causes lakes to become acidic and many plants and animals die as a result.
• Acid rain kills trees and damages limestone building and ruins stone statues. It also makes metal corrode.

Acid rain is caused by sulfur dioxide and oxides of nitrogen:

• When fossil fuels are burned they release mostly CO2 (a big cause of global warming).
• Butthey also release other harmful gases - especially sulfur dioxide and various nitrogen oxide.
• The sulfur dioxide (SO2) comes from sulfur impurities in the 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. (This can happen in the internal combusition engines of cars.)
• When these gases mix with clouds they form dilute sulfuric acid and dilute nitric acid.
• This then falls as acid rain.
• Power station and internal combustion engines in cars are the main causes of acid rain.

Acid rain kills fish, trees and statues;

• Acid rain causes lakes to become acidic and many plants and animals die as a result.
• Acid rain kills trees and damages limestone building and ruins stone statues. It also makes metal corrode.

Oxides of nitrogen also causes photochemical smog:

• Photochemical smog is a type of air pollution cause by sunlight acting on oxides of nitrogen. These oxides combine with oxygen in the air to produce ozone (O3)
• Ozone can cause breathing difficulties, headaches and tiredness. 
• Don't confuse ground-level ozone with the useful layer high up in the atmosphere)

Carbon monoxide is a poisonous gas:

• Carbon monoxide (CO) can stop your blood doing its proper job of carrying oxygen around the body.
• A lack of oxygen in the blood can lead to fainting, a coma or even death.
• Carbon monoxide is formed when petrol or disel in car engines is burnt without enough oxygen - this is incomplete combustion.

It's important that atmospheric pollution is controlled:

• The build up   of all these pollutants can make life unhealthy and miserable for many humans, animals and plants. The number of cases of respiratory illnesses (eg: asthma) has increased in recent years = especially among young people. Many people blame these on atmospheric pollution so efforts are made to improve this.

• Catalytic converters on motor vehicles reduce the amount of carbon monoxide and nitrogen oxide getting into the atmosphere. The catalyst is normally a mixture of platinum and rhodium.
• It helps unpleasant exhaust gases from the car react to make things that are less immediately dangerous (though more CO2 is still not exactly ideal).
• carbon monoxide (2CO) + nitrogen oxide (2NO) -> nitrogen (N2) + carbon dioxide (2CO2)