C3 – Unit 2

Properties of substances

Lesson 1

Electrical conductivity

1. Elements

Here, sulphur is being tested. The meter gives a ‘beep’ sound if it conducts. All metallic elements conduct. Most non-metallic elements do not conduct. Graphite a form of carbon does conduct.

2. Covalent compounds

Here, methylated spirits is being tested in a conductivity flask. The meter is set to measure electrical resistance. Here – infinite resistance. Covalent compounds do not conduct electricity.

3. Solid compounds

A similar experiment was carried out on solid compounds, shown is copper sulphate. None conducted.

4. Ionic compounds

Ionic compounds conduct when dissolved in water

 

Lesson 2

Ionic bonding tends to take place in compounds made from metals combined with non-metals.

The metal atom can lose electrons to become a positive ion

eg. Mg 2)8)2 → Mg2+ 2)8)

By losing 2 electrons it’s electron arrangement becomes like that of Neon a noble gas, ie. stable.

The non-metal atom can gain electrons to become a negative ion

eg. Cl 2)8)7 → Cl 2)8)8

By gaining 1 electron it’s electron arrangement becomes like that of the noble gas Argon, ie. stable.

These oppositely charged ions are electrostatically attracted to each other forming an ionic bond.

Investigated conductivity of ionic compounds in more detail.

eg Copper II Chloride

A DC (direct current) supply was used – this is so the products can be predicted at each electrode

A current flowed

Pink copper was formed on the negative electrode

Bubbles were given off at the positive electrode and there was a smell of Chlorine

When an ionic substance conducts a current a chemical change takes place – here the copper chloride was broken down to the elements copper and chlorine.

In solution the ions can move.

Metal ions are positively charged. Copper (Cu2+) ions move to the negative electrode

Non-metal ions are negatively charged. Chloride (Cl) ions move to the positive electrode

Looked at film on molten ionic compounds conducting.

Again the ions can move so the melt conducts. Again a chemical change takes place. For lead bromide lead forms at the negative electrode and bromine at the positive electrode.

 

Lesson 3

Ionic Lattices

Looked at crystals with digital microscopes

The crystals appear as little cubes. The shape of the crystal comes from the arrangement of the ions. They are arranged in an ionic lattice.

This is a two dimensional representation. The oppositely charged ions attract each other – ionic bonds. This bonding is very strong so ionic lattices have high melting points.

 

Lesson 4

Ionic formulae and colour of ions sheet

This was carried out by internet research

Answers –

Name Ionic Formula Colour
Sodium chloride

Na+ Cl

colourless

Sodium sulphate

(Na+)2 SO42-

colourless

Potassium chloride

K+ Cl

colourless

Potassium nitrate

K+ NO3

colourless

Magnesium sulphate

Mg2+ SO42-

colourless

Magnesium nitrate

Mg2+ (NO3)2

colourless

Calcium sulphate

Ca2+ SO42-

colourless

Calcium chloride

Ca2+ (Cl)2

colourless

Calcium nitrate

Ca2+ (NO3)2

colourless

Aluminium sulphate

(Al3+)2 (SO42-)3

colourless

Ammonium sulphate

(NH4+)2 SO42-

colourless

Ammonium nitrate

NH4+ NO3

colourless

Copper II chloride

Cu2+ (Cl)2

blue

Copper II nitrate

Cu2+ (NO3)2

blue

Copper II sulphate

Cu2+ SO42-

blue

Ammonium dichromate

(NH4+)2 Cr2O72-

orange

Potassium dichromate

(K+)2 Cr2O72-

orange

Sodium dichromate

(Na+)2 Cr2O72-

orange

Potassium permanganate

K+ MnO4

purple

Sodium permanganate

Na+ MnO4

purple

Nickel II chloride

Ni2+ (Cl)2

green

Cobalt II sulphate

Co2+ SO42-

pink

The task can also be done using the Display board of ionic compounds and their colour

An ion tends to be coloured if it contains a transition metal.

 

Lesson 5

Coloured ions moving

A U-tube was set up containing a solution of copper II dichromate. Areas containing a colourless electrolyte were left just above the solution separated by cotton wool plugs.

A current was passed through it for several hours

A blue colour from the copper (Cu2+) ions formed at the negative electrode.

An orange colour from the dichromate (Cr2O72-) ions appeared at the positive electrode.

The ions had moved to the electrodes of opposite charge.

 

Lesson 6

Covalent bonding

In covalent compounds electrons are shared to have electron arrangements similar to that of a noble gas.

Covalent compounds are usually formed from non-metallic elements.

 

Orbitals

One way to view the covalent bond is to consider orbital diagrams.

As well as being in energy levels , electrons occupy areas called orbitals. These are areas where there is a 90% chance of finding the electron. Two electrons can occupy an orbital. For hydrogen and helium there is one spherical orbital, for the other elements (at present) there are four sausage shaped orbitals arranged in a tetrahedral shape.

eg

 orbitals

or

orbital

 

Formulae from orbitals

The orbitals that have one electron in them overlap or mix so an orbital with two electrons in it is produced.

eg. methane – carbon hydride

Four Hydrogens are required to fill all the orbitals. The formula is CH4

The carbon now resembles Neon and the Hydrogen resembles helium as far as their orbitals are concerned.

 

Lesson 7

Covalent substances can exist in two different forms –

1. Covalent molecular

All substances show a very weak bonding known as Van der Waals Forces. This bonding is so weak it is usually masked by other bonds. It is the only bonding in the noble gases. It is also the bonding that occurs between molecules.

Structure of water – here showing 5 molecules

Covalent molecular substances have low melting and boiling points due to the weak forces holding the molecules together.

 

 

2. Covalent networks

Structure of diamond a form of carbon

Covalent networks have extremely high melting and boiling points because of the strong covalent bonds linking all the atoms together. Diamond is also extremely hard.

 

Lesson 8

Solubilities and bonding

Sodium Chloride and Salol were stirred in Water and Heptane

Solvent Sodium chloride
Salol
Heptane Insoluble Soluble
Water Soluble Insoluble

Covalent substances (Salol) tend to dissolve in covalent liquids (Heptane) sometimes called non-aqueous solvents.

Ionic substances (Sodium chloride) tend to dissolve in water.

CFE Outcomes

Level 4

Through gaining an understanding of the structure of atoms and how they join, I can begin to connect the properties of substances with their possible structures.

SCN 4-15a

National 4

Atomic structure and bonding related to properties of materials
Covalent bonding as electron sharing and ionic bonding of electron transfer.
Physical properties of substances linked to bonding.

There are two types of compound. Covalent compounds form when non-metal atoms form covalent bonds by sharing their outer electrons. Covalent compounds exist as molecules.
Ionic compounds form when metal atoms join to non-metal atoms by transferring electron(s) from the metal to the non-metal. The resulting charged particles are called ions and an ionic bond is the attraction of the oppositely charged ions.
To be sure of the bonding present in a substance the properties must be tested.
Covalent compounds, made of molecules, have low melting and boiling points. As a result, they can be found in any state at room temperature. Ionic compounds have high melting and boiling points. As a result, they are found in the solid state at room temperature. Only ionic compounds can conduct electricity, they can only do this when molten or in solution.

National 5

Atomic structure and bonding related to properties of materials
Ions. Ionic bonding.
Covalent molecular, covalent network and ionic lattices.
Physical properties of chemicals explained through bonding.
In a covalent bond, the shared pair of electrons is attracted to the nuclei of the two bonded atoms. More than one bond can be formed between atoms leading to double and triple covalent bonds. Covalent substances can form either discrete molecular or giant network structures. Diagrams show how outer electrons are shared to form the covalent bond(s) in a molecule and the shape of simple two-element compounds. Covalent molecular substances have low melting and boiling points due to only weak forces of attraction between molecules being broken. Giant covalent network structures have very high melting and boiling points because the network of strong covalent bonds must be broken.
Ionic bonds are the electrostatic attraction between positive and negative ions. Ionic compounds form lattice structures of oppositely charged ions. Ionic compounds have high melting and boiling points because strong ionic bonds must be broken in order to break down the lattice. Dissolving also breaks down the lattice structure.
Ionic compounds conduct electricity, only when molten or in solution due to the breakdown of the lattice resulting in the ions being free to move. Experimental procedures are required to confirm the type of bonding present in a substance.