GCSE OCR Gateway Chemistry C2.3 e-f Explaining properties of materials.

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

C2.3d use ideas about energy transfers and the relative strength of chemical bonds and intermolecular forces to explain the different temperatures at which changes of state occur.

C2.3f explain how the bulk properties of materials (ionic compounds; simple molecules; giant covalent structures; polymers and metals) are related to the different types of bonds they contain, their bond strengths in relation to intermolecular forces and the ways in which their bonds are arranged.

More on this topic can be found here

Let’s look at five typical substances:

An ionic substance: sodium chloride mp 808^oC, bp 1465^oC

A simple molecular substance: water mp 0^oC, bp 100^oC

A giant covalent molecular substance: diamond which sublimes above 5000^oC

A polymer: polythene mp 115–135^oC, bp 388–408^oC

A metal: copper mp 1085^oC, bp 2562^oC

You can find a huge list of melting and boiling points of a variety of substances here

Ionic substances: sodium chloride NaCl

When ionic substances melt the positive and negative ions separate.

The regular arrangement of ions breaks down but the ions still stay close to each other, probably in random clusters.

Heating sodium chloride to 808^oC gives the substance enough energy to break the electrostatic attraction of the ions for each other.

If the temperature is raised to 1465^oC then the ions have gained sufficient energy to fly apart from each other, completely overcoming the electrostatic attraction of positive ion for negative ion, and enter the gaseous state. 

The lattice energy represents the strength of all the bonds between all the ions in an ionic substance. 

The Lattice energy of sodium chloride is +787kJ/mol.  This means that the energy needed to take all the ions apart in a mole of sodium chloride is 787kJ.

If the ions have twice the charge (+2 and –2) but the substance has same structure then the lattice energy.

Magnesium oxide (MgO) has the same structure as sodium chloride but each ion has twice the charge so the ionic bond between all the ions is much stronger and its lattice energy is much higher: 3800kJ/mol.

Magnesium oxide then has higher melting and boiling points than sodium chloride.

Magnesium oxide: mp 2852^oC, bp 3600^oC.

This kind of behavior where increased ion charge leads to increases in melting and boiling point is true for those ionic substances that do not decompose on heating such as chlorides and oxides.

But it is not true for those ionic substances that do decompose on heating carbonates or nitrates or sulphates .

Ionic substances are usually hard because the electrostatic forces between the ions are strong. 

Ionic substances usually conduct electricity when molten because the ions have become free to move around and can be attracted to positive and negative electrodes. 

Some ionic substances are soluble in water because the water molecules can bond electrostatically to their ions. 

In other ionic substances the forces between the ions are too strong (as in magnesium oxide) to allow the ions to separate in the presence of water so these ionic substances are insoluble in water.

Simple Molecular substances: water

Simple molecular substances contain two types of bond.

There are very strong covalent bonds between the atoms in the molecule i.e. in water there are strong bonds between the oxygen atom and the two hydrogen atoms.

Then there are much weaker bonds between molecules in the solid state. 

The intermolecular bonds are sometimes called van der Waals forces or London forces. 

When ice is heated the energy given to the substance first breaks the intermolecular bonds. 

We can tell these bonds are weak because the melting point is very low at 0^oC.

If the substance is heated further it will boil at 100^oC because the  intermolecular bonds between the molecules in water are weak.

Many simple molecular substances behave just like water substances such as ethanol, methane, propanone or carbon dioxide.

All have strong covalent bonds between atoms and weak bonds between molecules.

Giant Molecular substances: Diamond

Giant molecular substances have a structure made up of all strong covalent bonds in a network formation. 

Since the covalent bonds between the atoms are all the same strength and very strong it takes a considerable amount of energy to break them.

So if diamond is to melt or boil all the strong covalent bonds between all the carbon atoms in a diamond crystal are going to have to break. 

What in fact happens when diamond is heated is that its atoms separate from the structure at about 5000^oC and the diamond becomes a gas.

Moving from solid to gas without passing through the liquid state is called sublimation. 

You can see good animations of the diamond structure here.

Diamond is hard (hardest natural substance) because of the very strong covalent bonds between the carbon atoms and the arrangement where each atom is held in place by 4 of these strong covalent bonds.  The arrangement of atoms–a tetrahedral arrangement–also lends strength to the substance.

A polymer: polythene mp 115–135^oC, bp 388–408^oC

Polymers are long chain like structures of atoms usually carbon based. 

A polymer has a pattern of repeating atoms–a repeat unit.

Repeat units are based on the molecule used to form the polymer the monomer.

Polythene has ethene C₂H₄ for its monomer.

Join the repeat units at the stars and you build the polymer structure that in part could look like this:

There are strong covalent bonds between the atoms of the polymer but thousands of weak bonds between the polymer chains.

The polymer chains are never identical, some are longer some are shorter than others, so the number of weak bonds between the chains varies and that means the melting and boiling points of different chains will vary.

The amount of crystallinity in the polymer also affects the melting and boiling point. 

More crystallinity probably raises the melting point and boiling point because the polymer chains will be more ordered and closer together so the bonds between them will be stronger.

Overall these two factors mean that the melting and boiling points of polymers are in a range of values and not precise.

A metal: copper mp 1085^oC, bp 2562^oC

Metal melting and boiling points are very precise unlike those of polymers.

Bonding in metals depends on the number of outer shell electrons.

When bonds form between atoms the atoms’ outer shells overlap to form one complete “sea of electrons” or a “cloud of electrons” or sometimes called delocalised electrons.

So the electron cloud bonds together the remaining particles that are positively charged ions. 

The positive ions and sea of electrons are in the form of a 3D lattice or network and are like a climbing frame in the local park.

The sea of electrons allows the metal particles to move around each other whilst remaining bonded.

As result we can easily bend a piece of metal and stamp it and reform it into a new shape.

C2.3e use data to predict states of substances under given conditions.

Substance	Mp (oC)	Bp  (oC)	State at room temperature	State at 100oC	State at 1000oC
Sodium chloride	808	1465	Solid	Solid	liquid
water	0	100	liquid	gas	gas
diamond	5000+	5000+	Solid	Solid	Solid
Magnesium oxide	2852	3600

What are the states of magnesium oxide at the three temperatures?

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.

There are many sites on the web where you can pick up further detail about the allotropes of carbon.  Here are just two links you could follow. 

Allotropes on Youtube here

All about graphene on Youtube here