9.1.1 Hydrocarbons and crude oil

A hydrocarbon contains carbon and hydrogen only

1. Only two elements are present, and any third element rules the compound out.
2. Methane, ${CH}_4$, is the simplest hydrocarbon.
3. Ethanol contains oxygen as well, so it is not a hydrocarbon.
4. The carbon atoms are joined to one another in chains or in rings.
5. Carbon forms four bonds, which is what allows those long chains to exist.

Crude oil is a complex mixture of hydrocarbons

1. Crude oil holds a very large number of different hydrocarbons mixed together.
2. The substances are not chemically joined, so they keep their own properties.
3. Their molecules differ in the length of the carbon chain they contain.
4. Some of the molecules have their carbon atoms arranged in rings rather than chains.
5. A mixture can be separated by physical means, which is what makes crude oil useful.

Crude oil formed over millions of years

1. Crude oil formed from the remains of ancient organisms buried in mud on the sea floor.
2. Heat and pressure over millions of years turned those remains into oil.
3. It is therefore a fossil fuel, like natural gas and coal.
4. New oil forms far more slowly than the rate at which it is taken out of the ground.
5. Crude oil is a finite resource, so the supply will eventually run out.

Crude oil supplies fuels and feedstock

1. Most of the crude oil taken from the ground is burned as fuel.
2. Petrol, diesel oil and kerosene all come from crude oil.
3. The rest is used as feedstock, the raw material for the petrochemical industry.
4. That industry makes polymers, solvents, detergents and medicines from oil molecules.
5. Burning oil as fuel uses it up, while using it as feedstock turns it into lasting products.

9.1.2 Fractional distillation of crude oil and its fractions

Fractional distillation separates crude oil by boiling point

1. Crude oil is heated until most of it has turned into a vapour.
2. The vapour passes into a tall fractionating column that is hot at the bottom and cool at the top.
3. A vapour rises until it reaches a level cool enough for it to condense.
4. The liquid that collects at each level is drawn off as a fraction.
5. Each fraction is still a mixture, but of hydrocarbons with similar boiling points.

Chain length decides where a fraction condenses

1. A longer carbon chain gives a molecule with stronger attractions to its neighbours.
2. Stronger attractions mean a higher boiling point.
3. Long-chain hydrocarbons therefore condense low down, where the column is hottest.
4. Short-chain hydrocarbons stay as vapour longer and condense near the top.
5. Molecules that never condense leave the column at the top as gases.

The fractions and what each is used for

1. Gases are drawn off at the top and used for domestic heating and cooking.
2. Petrol is used as fuel for cars.
3. Kerosene is used as fuel for aircraft.
4. Diesel oil is used as fuel for some cars and for trains.
5. Fuel oil is used for large ships and in some power stations, and bitumen surfaces roads and roofs.

Matching a fraction to its job

1. A fuel for a car engine must vaporise easily, which suits the short chains in petrol.
2. A fuel for a ship's engine can be thicker and harder to ignite, so fuel oil serves.
3. Bitumen barely flows at all, which is exactly what a road surface needs.
4. The property that matters is set by the chain length in that fraction.
5. Demand for the fractions does not match the amounts the column produces, which is why some are in short supply.

9.1.3 The alkanes as a homologous series

The fractions differ in the size of their molecules

1. A hydrocarbon in a light fraction has few carbon and hydrogen atoms in its molecule.
2. A hydrocarbon in a heavy fraction has many more of both.
3. Every other difference between the fractions follows from that difference in size.
4. Boiling point rises as the molecules get larger.
5. Viscosity rises too, so the heavier fractions flow less easily.

Ease of ignition falls as the molecules get larger

1. A short-chain hydrocarbon vaporises readily and catches light easily.
2. A long-chain hydrocarbon is harder to vaporise, so it is harder to ignite.
3. Petrol ignites far more readily than fuel oil for this reason.
4. Ease of ignition therefore runs opposite to boiling point and viscosity.
5. A fuel is chosen partly on how easily it needs to ignite in the engine that burns it.

Most of these hydrocarbons are alkanes

1. The hydrocarbons in the fractions are mostly members of the alkane family.
2. An alkane has its carbon atoms joined by single bonds only.
3. A molecule with only single bonds between carbons is saturated.
4. The general formula for an alkane is $C_n{H}_{2n+2}$.
5. Methane, ethane, propane and butane are the first four members.

A homologous series shares a general formula

1. Members of a homologous series all fit the same general formula.
2. Each member differs from the next by ${CH}_2$ in its molecular formula.
3. Their physical properties change gradually along the series, as boiling point does.
4. Their chemical properties are similar, because the molecules are built the same way.
5. The alkanes are one such series, and there are others built round different groups.

Using the pattern to predict a member

1. The next alkane after butane is $C_5{H}_{12}$, found by adding ${CH}_2$.
2. The general formula gives the same answer, since $n=5$ gives $2n+2=12$.
3. Its boiling point lies above butane's, because its chain is longer.
4. It reacts in the same way as the others, since its chemistry is that of an alkane.
5. A prediction of this kind uses the series rather than a recalled fact.

9.1.4 Complete and incomplete combustion

Complete combustion gives carbon dioxide and water

1. Burning a hydrocarbon in a plentiful supply of oxygen is complete combustion.
2. Every carbon atom ends up in carbon dioxide and every hydrogen atom in water.
3. The general pattern is: $$hydrocarbon+oxygen\to carbon\; dioxide+water$$
4. Methane burning completely gives: $${CH}_4+2O_2\to {CO}_2+2H_{2}O$$
5. Energy is given out, which is the reason hydrocarbons are burned at all.

Incomplete combustion happens in a limited supply of oxygen

1. With too little oxygen there is not enough to convert every carbon atom fully.
2. Some carbon is only partly oxidised, giving carbon monoxide, $CO$.
3. Some carbon is not oxidised at all, and appears as soot.
4. Water is still formed, because hydrogen is oxidised first.
5. Less energy is released than from complete combustion of the same fuel.

Carbon monoxide is toxic

1. Carbon monoxide is colourless and odourless, so it gives no warning of its presence.
2. Breathed in, it binds to the haemoglobin in red blood cells.
3. Haemoglobin that has bound carbon monoxide can no longer carry oxygen.
4. The blood therefore delivers less oxygen to the body's cells.
5. The effects range from headache and drowsiness to unconsciousness and death.

Problems caused in appliances that burn fuels

1. A boiler or heater starved of air produces carbon monoxide indoors.
2. Soot builds up inside the appliance and in the flue.
3. A soot deposit blocks the airway, which makes the oxygen supply worse still.
4. Soot that glows in the flame makes it yellow, which is a visible warning sign.
5. Regular servicing keeps the air supply clear and the combustion complete.

9.1.5 Pollutants from burning fuels

Sulfur impurities give sulfur dioxide

1. Crude oil contains small amounts of sulfur mixed in with the hydrocarbons.
2. That sulfur passes into the fuels made from the oil.
3. Burning the fuel oxidises the sulfur: $$S+O_2\to {SO}_2$$
4. Sulfur dioxide therefore leaves the chimney or exhaust with the other waste gases.
5. Removing the sulfur from the fuel before it is burned prevents this.

Sulfur dioxide dissolves to give acid rain

1. Sulfur dioxide rises into the atmosphere and dissolves in water droplets.
2. The solution formed is acidic, so the rain that falls has a lower pH than normal.
3. Acid rain can fall a long way from where the gas was released.
4. Rain is always slightly acidic from dissolved carbon dioxide, so acid rain is more acidic than that.
5. The damage it causes depends on how much the pH has fallen.

The damage acid rain causes

1. Acid rain makes lakes and rivers more acidic, which kills fish and other water life.
2. It damages trees, both directly and by harming the soil they grow in.
3. It reacts with limestone and marble, eating away buildings and statues.
4. It corrodes metal structures such as bridges and railings.
5. Soil chemistry is altered, which affects the crops that can be grown.

Oxides of nitrogen form in hot engines

1. Air is about $78\%$ nitrogen and $21\%$ oxygen, and both are drawn into an engine.
2. Nitrogen is normally unreactive, so the two pass through unchanged at ordinary temperatures.
3. Inside a running engine the temperature is high enough for them to react.
4. The two elements combine directly: $$N_2+O_2\to 2NO$$
5. The oxides of nitrogen formed are pollutants and contribute to acid rain as well.

9.1.6 Hydrogen as a fuel and non-renewable fossil fuels

Petrol and hydrogen burn to different products

1. Petrol is a hydrocarbon fuel, so burning it gives carbon dioxide and water.
2. Hydrogen burns to give water alone: $$2H_2+O_2\to 2H_{2}O$$
3. No carbon is present in hydrogen, so no carbon dioxide can form.
4. No carbon monoxide or soot is possible either, whatever the air supply.
5. That difference in products is the starting point for comparing the two fuels.

The advantages of hydrogen

1. Burning hydrogen produces no carbon dioxide, so it adds nothing to the greenhouse effect at the point of use.
2. It produces none of the carbon-based pollutants, though a hot engine still makes oxides of nitrogen from the air.
3. Hydrogen can be made from water, which is not a finite resource in the way crude oil is.
4. It releases a large amount of energy per gram compared with petrol.
5. A hydrogen vehicle can be refuelled in minutes, unlike a battery that must recharge.

The disadvantages of hydrogen

1. Hydrogen is a gas, so a large volume is needed unless it is compressed or cooled.
2. Storing it under high pressure makes the tank heavy and costly.
3. It is highly flammable, and a leak mixes with air to form an explosive mixture.
4. There are very few filling stations, so a vehicle cannot be refuelled everywhere.
5. Most hydrogen is currently made from natural gas, which releases carbon dioxide elsewhere.

Petrol, kerosene, diesel and methane are non-renewable

1. Petrol, kerosene and diesel oil are all obtained from crude oil.
2. Methane is the main constituent of natural gas.
3. All four are fossil fuels, formed from the remains of organisms over millions of years.
4. They form far more slowly than they are used, so they are non-renewable.
5. A non-renewable fuel cannot be replaced once it has been burned.

9.1.7 Cracking of larger alkanes

Cracking breaks large alkanes into smaller molecules

1. A large, saturated hydrocarbon molecule is broken into smaller ones.
2. The large molecule comes from one of the heavier fractions of crude oil.
3. Breaking it produces a smaller alkane and at least one alkene.
4. Decane can crack to give octane and ethene: $$C_{10}{H}_{22}\to C_8{H}_{18}+C_2{H}_4$$
5. The atoms are only rearranged, so the equation balances on both sides.

Some of the products are unsaturated

1. There are not enough hydrogen atoms to make every product a saturated molecule.
2. One product therefore forms a carbon to carbon double bond.
3. A molecule with such a double bond is unsaturated, and is an alkene.
4. Ethene is the alkene produced most often, and it is the most useful.
5. More than one alkene can form, and hydrogen is sometimes a product as well.

Why cracking is necessary

1. Fractional distillation gives a fixed proportion of each fraction from the oil.
2. Demand for petrol and other short-chain fuels is far higher than that proportion.
3. The heavier fractions are produced in greater amounts than anyone needs.
4. Cracking converts that surplus into the shorter molecules that are in demand.
5. It also produces the alkenes used to make polymers, which distillation cannot supply.

The conditions used

1. The heavy fraction is first heated until it vaporises.
2. The vapour is passed over a hot catalyst, which breaks the molecules up.
3. A high temperature alone will also crack the molecules without a catalyst.
4. Cracking is a thermal decomposition, since heat breaks one substance into others.
5. The products are separated afterwards, since a mixture is always formed.