savvy-chemist: GCSE OCR Gateway Chemistry C2.3a-c Carbon Allotropes

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4:

In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification.

I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months.

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least.

That written in italics is for the higher tier paper only.

C2 Elements, Mixtures and Compounds

C2.3 Properties of materials

C2.3a recall that carbon can form four covalent bonds

CM2.3i represent three-dimensional shapes in two dimensions and vice versa when looking at chemical structures, e.g. allotropes of carbon

Carbon is in Group 4(14) of the Periodic Table.

Group 4 elements have four electrons in their outer shells.

Four more electrons would fill the outer-shell of carbon.

Covalent bonds form between carbon and other elements like hydrogen (H), oxygen (O), carbon (C) itself, nitrogen (N) and sulfur (S).

These covalent bonds that form are made up of pairs of electrons shared between carbon and the other element.

The particles that form between carbon and these other atoms are called molecules because they are distinct groups (sometimes very large) of atoms.

Methane (CH₄) is an example of a molecule where carbon and hydrogen atoms share 4 pairs of electrons.

A Lewis or dot and cross diagram illustrates this structure:

A line represents each shared electron pair.

But the molecule is not this shape in three dimensions.

The illustration below shows the position of four covalent bonds around the carbon atom in 3D

The illustration above shows the display formula or simple drawing of methane with the angle between the fours covalent bonds at 90^o

But in 3D the other two illustrations (ball and stick and space-filling models) show the angle between the four bonds to be greater at 109½^o.

The shape of the methane molecule is not a flat cross but a triangular based pyramid or tetrahedron: look at the illustration below:

C2.3b explain that the vast array of natural and synthetic organic compounds occur due to the ability of carbon to form families of similar compounds, chains and rings.

One amazing things about carbon that is not true of any other element is that carbon atoms bond to other carbon atoms so well that chains and rings of carbon atoms exist.

The simplest chain of carbon atoms is C₂H₆ ethane.

The simplest ring of carbon atoms is C₃H₆ cyclopropane.

These molecules have similar molecular formulae but the atoms are arranged very differently.

Ethane looks like this:

Cyclopropane looks like this:

You can see a rotatable 3D model of cyclopropane here

This illustration shows some of the simplest chain hydrocarbons.

And this illustration shows some of the simplest ring carbon molecules

And these are just a small selection of the millions of chain and ring molecules formed because carbon can bond strongly to itself and other nonmetals.

C2.3c explain the properties of diamond, graphite, fullerenes and graphene in terms of their structures and bonding.

CM2.3i represent three-dimensional shapes in two dimensions and vice versa when looking at chemical structures, e.g. allotropes of carbon.

In C2.2d(iii) there is an explanation of the structure and bonding in diamond

Typical giant covalent structures are diamond, graphite, nano–tubes and graphene sheets.

i) Diamond

Four bonds connect each carbon atom to four others in a tetrahedral arrangement.

These inter–atomic bonds are very strong.

The ball and stick model below shows this tetrahedral arrangement of atoms.

The arrangement is repeated continuously throughout a diamond crystal.

Hardness

The result is that diamond is the hardest known naturally occurring material.

Conduction of electricity

It does not conduct electricity because all the atom’s electrons and used in forming covalent bonds with other carbon atoms.

Melting and boiling points

Its melting and boiling points are incredibly high because to melt or boil diamond each individual strong bond has to be broken and that would take a vast amount of energy.

ii) Graphite

Three bonds connect each carbon atom to three others in a sheet of atoms arranged in hexagons.

These inter–atomic bonds are very strong.

But the sheets bond weakly to each other so that the sheets can easily slide over each other.

As with diamond the arrangement is repeated through a crystal of graphite.

The giant structure is evident in this picture below:

Hardness

The weak bonds between the layers of carbon atoms allow these layers to slide over each other making graphite soft and useful in pencil “lead” and in oils as a lubricant.

Conduction of electricity

As only three bonds hold each carbon atom in place in the layers there is one electrons per carbon atom free (or delocalized) that allows for the conduction of electricity.

Melting and boiling points

But its melting and boiling points are very high since again, as in the case of diamond, every strong bond in each layer has to be broken for the material to melt or boil and this would take an incredibly high amount of energy.

iii) Nano tubes and fullerenes

Nano-tubes are tubes of rolled up graphite sheets.

Each carbon atom is bonded to three others using three strong bonds.

The carbon atoms are arranged in hexagons.

Fullerenes are spheres of carbon atoms composed of hexagons and pentagons.

These molecules of carbon C₆₀, C₇₀ etc. have strong bonds within the molecule but weak bonds between the molecules.

The melting and boiling points of fullerenes are low because not much energy is needed to separate one molecule from another due to these weak inter molecular bonds.

For this reason too fullerenes are soluble in organic solvents like petrol. C₆₀ forms a red solution in petrol (octane).

iv) Graphene

Graphene is a single sheet of graphite.

A one atom thick layer.

The carbon atoms are arranged in hexagons and each has three bonds connecting it to three others.

So we would expect grapheme to be very strong as the bonds holding the carbon atoms in the sheet are very strong.

We’d also expect the sheet to conduct electricity since only three bonds hold each atom in place leaving a free electron to form the electric current.