GCSE OCR Gateway Organic Chemistry C6.2i Allotropes of Carbon

GCSE OCR Gateway Organic Chemistry C6.2i  

Summary

Common misconceptions

Carbon chemistry is the basis of life on Earth. Organic chemistry is the basis of many of the materials we produce. Organic compounds are covalent in nature and react in a predictable pattern. Crude oil forms the basis of many useful by-products.

CM6.2i To be able to represent three-dimensional shapes in two dimensions and vice versa when looking at chemical structures, e.g. allotropes of carbon.

Allotropes of carbon

Allotropes are different structural forms of the same element in the same physical state. 

Carbon has several allotropes in its solid state: Diamond, Graphite , graphene, fullerenes and nanotubes.

Diamonds

Graphite

Buckminsterfullerene C₆₀ (dissolved in benzene)

flakes of graphene welded together

Allotropes of carbon are based on the ability of carbon to demonstrate three dimensionality. 

Each carbon atom can form up to four covalent bonds with itself or with other elements. 

When carbon forms four covalent bonds with itself, it forms a diamond structure.  Each bond points to the corner of a regular tetrahedron as in the diagram below.

The covalent bonds are very strong they require massive amounts of energy to break them all and so when combined in the giant diamond structure give diamond its incredible hardness and strength. 

Diamond is the hardest known material. Hence it is used in drill bits.  It will easily cut glass. 

As all the outer shell electrons of the carbon atoms are used in the four bonds there are no delocalised electrons to allow for the conduction of electricity so carbon as diamond is an insulator. 

Breaking these strong covalent bonds as I have said requires massive amounts of energy and so the material is insoluble in water. 

On the other hand carbon atoms in graphite are bonded differently.  Unlike in carbon there are only three bonds per carbon atom n graphite.

Each bond is a very strong covalent bond but as there are only three per carbon atom then the carbon atoms are arranged differently to diamond.

In graphite the carbon atoms are arranged in layers.  The diagram below shows how this happens:

As a result each carbon atom has a delocalised electron to contribute to the conduction of electricity in graphite.  Graphite is good conductor of electricity.

The layers repel each other too and are loosely bonded to each other so that they are able to slide over each other.  Hence, graphite is a smooth and slippery substance and is used in lubricants.  For the same reason, it is also found in pencil “lead” where it is baked with varying amounts of clay to give different degrees of softness to the “lead”.

Graphene is related to graphite since it is composed of a single layer of carbon atoms like those found in graphite.

Fullerenes on the other hand and single wall nano tubes are formed as if the grapheme layer of carbon atoms had been rolled into a tube (SWNT) or folded up into a ball (fulllerenes)

You can see this from the diagram below:

This final chart summarises the properties of the carbon allotropes

GCSE OCR Gateway Chemistry C6.1k-n Recycling

GCSE OCR Gateway Chemistry C6.1k-n Recycling

C6.1k To be able to describe the basic principles in carrying out a life-cycle assessment of a material or product

C6.1l To be able to interpret data from a life-cycle assessment of a material or product

C6.1m To be able to describe a process where a material or product is recycled for a different use, and explain why this is viable

C6.1n To evaluate factors that affect decisions on recycling

What is Life Cycle Assessment (LCA)?

Life Cycle Assessment (LCA) is the detailed analysis that provides the information you need to make the most environmentally friendly decisions throughout product design.

The analysis looks at a product’s entire life, which encompasses

ore extraction,

material production,

manufacturing,

product use,

end-of-life disposal,

and all of the transportation that occurs between these stages. 

A typical life cycle assessment of a chemical product

Life cycle of a plastic bottle

Take as an example the life cycle of a plastic bottle. 

This example of life cycle analysis particularly focuses on the product use and end of life disposal.

Here is the problem with plastic bottles 480 billion were sold in 2016 worldwide a million bottles a minute and 110billion were made by Coca Cola.  































What do we do with the bottle once we have drunk the coke?  you seethe plastic bottle hangs around for a long time if you just chuck it into a ditch.































What better use can we make of the valuable resources contained in all those billions of bottles thrown away each year across the world?  What can we do to prevent them ending up in the sea.  

You can see from the illustration above how the life cycle of the plastic bottle pans out.

After manufacture of the plastic bottles from PET (polyester), their distribution and use what happens to the plastic waste is the crucial question being asked today.

A) Bottles can be chipped and incinerated leading to production of further Carbon dioxide (CO₂) and toxic oxides of nitrogen (NOx)

B) Bottles can be taken to land fill where they take ages to decompose to residue creating volumes of methane (CH₄)

C) If bottles are recycled by being melted down (Bottles made of PET are thermoplastic) and remoulded then energy can be conserved. 

D) Most PET bottles cannot be reused as bottles but must be melted down and then remoulded into some other product.

E) Finally, and this is not shown on the chart above, the plastic polyesters can be hydrolysed back to their respective monomers and then these monomers purified and reused to make new plastic product.  This approach is energy intensive however and not commercially viable at present.

Viable recycling

Viable recycling involves collecting, cleaning and melting down the product plastic into a melt form where it can be remoulded into a new different product. 

This is viable if the new product has a profitable market and can be produced at a lower energy cost than an equivalent product made from scratch and at a price that is also competitive in the market. 

It is these factors of energy cost and marketability that determine whether the product is recyclable not just reusable.

Other factors also come into play such as the extent to which the recycling process contaminates the environment with toxic materials.

You can find out more about plastic bottle recycling here:

GCSE OCR Gateway Chemistry C6.1p Mitigation of Iron Corrosion

C6.1q To be able to explain how mitigation of corrosion is achieved by creating a physical barrier to oxygen and water and by sacrificial protection

How to stop iron rusting

Protective methods

One obvious way of protecting iron and steel from rusting is to coat the metal with a material that water and air cannot penetrate.

Traditionally paint such as Hammerite has been used fairly successfully to protect iron from rusting.  

Similarly, oiling the iron gives a more flexible coating that water cannot penetrate. 

These traditional methods work but are not suitable for all conditions particularly when iron is exposed to sea water a much more vigorous and corrosive environment. 

So how can iron and steel be protected from corrosion especially since most boats and ships are steel hulled?

The answer is in what’s called sacrificial protection.

Sacrificial methods

Because corrosion is an oxidative process connecting iron to a more reactive metal will protect it from rusting.

What happens is that the more reactive metal will oxidise instead of the iron it protects

Typically, magnesium and zinc are used to protect iron .  Its called sacrificial since the other more reactive metal is eventually consumed and sacrificed to protect the iron,

You can see blocks of magnesium bolted to ships hull for this very purpose

You can also set up an interesting and colourful experiment to show sacrificial protection happening. 

You need to make up a hot agar solution containing traces of both potassium hexacyano ferrate(III) (K₃Fe(CN)₆)  and the acid base indicator phenolphthalein.

Pour this solution into test tubes that contain iron nails, one on its own, another wrapped in copper wire, another wrapped in zinc plate and a fourth wrapped in magnesium ribbon. 

Here is the set up:

And in photo:

The blue colour is due to the formation of Prussian Blue a distinctive blue colour that shows the presence of iron (III) (Fe³⁺) ions in rust. 

The magenta colour shows how the magnesium or other reactive metal is protecting iron from corrosion.  The magnesium has reacted to form an alkaline solution hence the phenolphthalein has turned magenta. 

The next photo shows how a boat hull is being protected from corrosion using blocks of magnesium or zinc bolted to the hull. 

The other photo shows a corroded zinc block on a ships hull.