Oil-based chemistry

Although fractional distillation of crude oil is an important part of the oil industry, it is dealt with in Part 1A of the specification and so is not relevant here (unless you are doing the whole of Module 1 together).

Hydrocarbons (carbon and hydrogen only) can be broken down into smaller units by cracking. This is particularly useful to convert the less-useful fractions of crude oil into more useful substances. It is done by heating the alkanes and passing them over a hot catalyst. This is an example of a thermal decomposition reaction.

When cracking takes place, a saturated molecule (alkane) becomes a smaller alkane and an unsaturated alkene. It is impossible to predict exactly which combination of alkane and alkene will be produced in each case but, just like in any other chemical reaction, the numbers of each kind of atom is maintained.

As an example, C₁₀H₂₂ could break down into C₅H₁₀ and C₅H₁₂

but just as easily, it could form C₄H₈ and C₆H₁₄ or C₆H₁₂ and C₄H₁₀.

Notice how in each case, the total number of carbon atoms is 10 and the total number of hydrogen atoms is 22. The products of cracking can be separated by fractional distillation.

The saturated products (alkanes) are usually used as fuels while the unsaturated products (alkenes) can be used to make polymers and other chemicals. One non-polymerisation use is to convert alkenes into alcohols by heating with steam in the presence of a catalyst.

Polymerisation looks to be quite complicated but try to remember it in simple terms first. Many small, alkene-like molecules (monomers) join together to become one large molecule (polymer). Think of two people facing each other, holding both holding the other’s right hand in their own left hand. Now have the same picture with lots of couples close together but still in their own pair. Now have each pair release one hand but keep the other together with their partner. The free hands can be used to join onto another pair. Eventually, one huge long chain is formed…..a polymer.

Ethene itself makes poly(ethene), propene makes poly(propene). Chloroethene makes poly(chloroethene) and styrene makes poly(styrene), even though nowadays it ought to be phenylethene making poly(phenylethene). Notice that although the polymers are saturated, they still have “-ene” in their name. This is because the name, “poly(ethene)” should be taken as meaning, “the polymer which is made from ethene”.

Polymers have many useful applications:
Low-density poly(ethene) is used to make plastic bags.
Rain gutters can be made from poly(chloroethene).
Dental polymers are used in tooth fillings.

New polymers are being developed.
Some recent innovations are “memory foam” which is used for mattresses. It moulds to the shape of your body as it gets warm and allows you to sleep more comfortably with proper support. Hydrogel wound dressings allow a wound to heal in controlled moist conditions.

The disadvantage of polymers is that many are non-biodegradable. This means that they will not rot down by bacteria in the environment and so care must be taken in their disposal. 

Vegetable Oils

Plants are able to use the Sun’s energy to create complicated molecules for carbon dioxide and water. Among these complicated molecules are vegetable oils. These oils are useful foodstuffs and are also being used increasingly as fuels.

The oils can be extracted by crushing the seeds of a plant like sunflower or oilseed rape. The oils from a plant like lavender can be extracted by steam distillation. This allows the oils to be released from the plant without being exposed to a high temperature that might cause them to break down.

Oils that have carbon-carbon double bonds in them are said to be unsaturated oils. The amount of unsaturation can be determined by reacting a measured amount of the oil with bromine. The bromine is added slowly and the red-brown colour fades. More bromine is added until the colour no longer fades because all of the double bonds have reacted. The more bromine that is needed per gram of oil, the more unsaturated it is. This can also be done with iodine and gives rise to the “iodine number” as a way of classifying oils.

We often see vegetable oil as a means of frying food. Oils have higher boiling points than water so frying food cooks it at a higher temperature that boiling it in water. Sometimes this just makes it cook more quickly but it can also make different reaction happen. As an example, bacon goes crispy and brown when it is fried but stays soft and pink when it is boiled. Compare chips and boiled potato!

Cooking food with oil can have health implications because some of the oil is absorbed into the food and, since it has a high energy content, it can cause a weight increase. We often spread butter on our bread. This is a saturated fat and can lead to high cholesterol levels. This is generally considered to be unhealthy. Vegetable oils are mostly unsaturated and less dangerous in this way.

However, the unsaturated oils are usually liquid at room temperature and not suitable for spreading on bread. Partial hydrogenation can make the oils a little more solid but still keep most of their healthy properties. Hydrogenation is carried out by heating the oil with hydrogen and a nickel catalyst.

Oil and water do not mix! If you shake olive oil and vinegar you get a gloopy mess that eventually separates out into two layers. However, if you add a little egg yolk, the oil and vinegar mix together to form an emulsion. The egg yolk is said to be an emulsifier.

Emulsifiers are one kind of substance that is added to food. Other types of food additive include colourings, anti-oxidants, preservatives and flavourings. Those additives that are considered safe to use in the EC are given an “E-number”.

If a vegetable oil is to be used as a fuel, any unwanted chemicals must be removed but this is not a difficult step. Vegetable oils from your local chipshop can be treated in this way to make biodiesel. This can either be used in place of ordinary diesel or, more commonly at the moment, alongside ordinary diesel fuel in a mixture. Ethanol is another biofuel. It can be made by fermenting sugar that has been grown as sugar cane. This can grow quite quickly, especially in warm climates.

Biofuels are often said to be “carbon neutral”. This means that overall, there is no carbon dioxide released into the atmosphere. Some people mistakenly believe that this means that burning a biofuel doesn’t release carbon dioxide at all. The important point is that there is no overall release of carbon dioxide, once you take into account the carbon dioxide that was absorbed while the plant was growing.

The Earth

The Earth is about 12 800 km in diameter. The deepest drilled holes are only about 12 km so we have only just scratched the surface.

The Earth is made up of four main layers. On the outside there is the crust. That is the bit we walk on! Underneath this thin layer there is the mantle. This is almost solid but can flow in a very sticky way. It is like very thick treacle. Inside that there is the core. The outer core is liquid and the inner core is solid. The core is made of iron and nickel.

Alfred Wegener is famous as the man who invented the theory continental drift. He noticed that some plants and animals (both living and fossilised) were found on either side of some great ocean divides. It suggested that at one time the land masses had been joined together. He also noticed that some of the continents seemed to fit into one another like jigsaw pieces.

He suggested that the continents had all started together as one land mass (called Pangaea). This split into Laurasia and Godwanaland and then these gradually moved to form the continents we know today. The time scale for this is that Laurasia and Godwanaland split from each other about 150 million years ago. The continents move at about the same speed as your fingernail grows.

The continental plates move on the thermal convection currents in the mantle. The driving force for these currents is the radioactive decay that takes place deep within the Earth. Where these plates collide we can see mountain building (such as the Andes of South America) if the plates are of similar density. If one plate is more dense than the other, it will dive beneath it. This can cause volcanoes to form. In the centre of the oceans, there are ridges where new rock is forming as the plates move apart. Magma leaks out from the mantle. It is worth remembering that the main reason that Wegener’s theory was ignored for so long was that he couldn’t explain the mechanism by which the plates moved.

Ever since the Earth was formed, the atmosphere has been evolving. It is widely believed that the earliest atmosphere on Earth was carbon dioxide, methane, ammonia and steam. As the earth cooled, the steam condensed to form liquid water. Much of the carbon dioxide became dissolved in these oceans. At some stage, life began. Early life was very simple and without any cellular structure. Once life had started, it began to produce oxygen. This was a pollutant. As the concentration of oxygen grew, it became impossible for life to continue without the protection of cells. Methane reacted with oxygen to produce more carbon dioxide. Ammonia reacted with oxygen to produce nitrogen gas. Sea creatures in particular stored carbon in the form of calcium carbonate in their shells. Once they died, these hard body parts sank to the bottom of the sea where they formed limestone, taking more carbon out of the system. Even the soft bodyparts could store carbon if they were trapped in anaerobic conditions to form oil and natural gas. Once celled plants and animals developed, life could become more complicated. The final piece in the jigsaw was to have some oxygen react to form ozone. This ozone layer was able to protect the ever more complicated animals and plants from the devastating rays of the Sun.

Nowadays, we have about 78% nitrogen and 21% oxygen in the atmosphere. The remainder is mostly argon although there is a small amount of carbon dioxide and a small but variable amount of water vapour. In cities there are also some traces of pollutant gases such as sulfur dioxide and nitrogen oxides but, even though these are a local problem, they are actually only a very tiny percentage of the whole atmosphere.

We can use the gases of today’s atmosphere. Nitrogen’s unreactivity makes it good to protect substances from oxygen (crisp packets contain nitrogen, oil tankers can be purged of the oil/air mixture so that there is less chance of a spark causing a fire). Oxygen can be used to help people with breathing difficulties. It can also be mixed with flammable gases such as acetylene to produce a flame that is hot enough to cut steel. Argon is extremely unreactive and is used in incandescent (the traditional kind) of lightbulb. Neon is used in electric discharge tubes to produce a neon light. This is the ind of lights seen in city centres such as in Leicester Square in London. Helium is mixed with oxygen so that deep-sea divers are better protected from the “bends”.

The gases are sorted out by the process of fractional distillation. The air is first filtered and then cooled down to very low temperatures (about -200^oC). The liquid air is then slowly allowed to warm up and the various gases distil off.

The Carbon Cycle

Carbon passes through many stages as it is passed from one form to another. As you breathe out (exhale) some carbon dioxide, this could be absorbed by a plant to take part in photosynthesis to make a sugar or starch. When the plant dies, it would be released as carbon dioxide again as the bacteria break it down. Carbon dioxide can dissolve in the oceans and it can be absorbed from there into the shell of sea creatures. When they die, it can be trapped in the form of calcium carbonate and go on to make limestone. The soft parts of the bodies might be trapped and begin the long process of forming oil and natural gas. When the oil is burned, carbon dioxide is released again. Volcanic activity can heat limestone and this can also release carbon dioxide.

This complicated network of possibilities is known as the carbon cycle.