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?