Chapter 5: Structure and Properties of Materials

What are the properties of ionic compounds?

1. High melting and boiling points

2. Conducts electricity in both molten and aqueous state

3. Does not conduct electricity in solid state

4. Most ionic compounds are soluble in water but insoluble in organic solvents

Why does sodium chloride have high melting and boiling points?

Ions in sodium chloride have a giant ionic lattice structure which strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is needed to overcome these strong forces of attraction.

Does magnesium sulfate conduct electricity in molten and aqueous state?

Yes. Magnesium sulfate contains ions that are mobile and carry charges.

Why don’t ionic compounds conduct electricity in solid state?

Ions can only vibrate about their fixed positions. Ions are not mobile and there are no delocalised electrons.

What are the differences in elements, compounds, and mixtures?

1. Composition: elements only have one element, compounds are made out of two or more elements chemically combined, mixtures are made out of two or more elements and/or compounds that are not chemically combined.

2. Purity: elements and compounds are pure, mixtures are impure

3. Melting and boiling points: elements are compounds have a fixed b.p. but mixtures melt over a range of temperatures

4. Compounds have different properties from their constituent elements but mixtures usually have similar elements to its constituent substances

5. Separation methods: elements cannot be broken down into simpler substances, compounds can be separated using chemical methods (e.g. thermal decomposition, electrolysis), mixtures can be separated by physical methods (e.g. filtration, distillation)

What are the properties of simple molecules?

1. Low melting and boiling points

2. Does not conduct electricity in any state

3. Insoluble in water and soluble in organic solvents

Why does carbon dioxide have a low melting and boiling point?

Carbon dioxide exists as simple molecules with weak intermolecular forces of attraction. A small amount of energy is required to overcome these forces of attraction.

Why doesn’t oxygen conduct electricity?

Oxygen have neutral molecules which do not have mobile ions or delocalised electrons to carry charges

What are the properties of diamond?

1. High melting and boiling point

2. Does not conduct electricity in all states

3. Hardest natural substance

4. Insoluble in water and organic solvents

What are the properties of silicon dioxide

1. High melting and boiling point

2. Does not conduct electricity in all states

3. Hard but not as hard as diamond

4. Insoluble in water and organic solvents

What are the properties of graphite?

1. High melting and boiling points

2. Soft and slippery

3. Conducts electricity

4. Insoluble in water and organic solvent

Describe the structure of diamond.

Diamond is has a giant covalent structure. Each carbon atom forms covalent bonds with 4 other carbon atoms, forming a 3D tetrahedral structure.

Describe the structure of graphite.

Graphite has a giant covalent structure. Each carbon atoms forms covalent bonds with 3 other carbon atoms, forming a 2D layered structure of hexagonal rings of carbon atoms.

Describe the structure of silicon dioxide.

Silicon dioxide has a giant covalent structure. Large number of silicon and oxygen atoms are joined together by strong covalent bonds in a regular arrangement.

Why does diamond, silicon dioxide and graphite have high melting and boiling points?

Large amount of energy is needed to break strong covalent bonds between the atoms

Why can graphite be used as a lubricant?

Layers of carbon atoms can slide over one another because a small amount of energy is needed to overcome weak intermolecular forces of attractions between the layers.

Can diamond and silicon dioxide be used as a lubricant? Why or why not?

Diamond and silicon dioxide are hard. Atoms cannot slide over one another because all the toms are bonded together by strong covalent bonds.

Can diamond conduct electricity? Why or why not?

Diamond cannot conduct electricity. Diamond does not have delocalised electrons and mobile ions to carry charges.

Can graphite conduct electricity? Why or why not?

Graphite can conduct electricity. Each carbon only used 3 out of 4 valence electrons for bonding. There is one unbonded delocalised and mobile electron per carbon to conduct electricity.

What are the properties of macromolecules?

1. Most have higher melting and boiling points than simple covalent substances but lower than giant covalent substances

2. No fixed boiling and melting point

3. Does not conduct electricity in all states

4. Insoluble in water and soluble in organic solvents

Why does starch have a higher boiling point and melting point than nitrogen?

Starch has a larger number of intermolecular forces, so a larger amount of energy is needed to overcome all the intermolecular forces of attraction compared to nitrogen, which exists as simple molecules.

Why does graphite have a higher boiling point than carbohydrates?

A large amount of energy is needed to overcome the strong covalent bonds in graphite while a smaller amount of energy is needed to overcome the weak intermolecular forces of attraction in carbohydrates.

Why does starch have no fixed melting and boiling point?

Starch consists if molecules of a range of sizes

Why can’t protein conduct electricity?

All valence electrons in protein are used for bonding and there are no mobile electrons and ions to carry charges.

What are the properties of pure metals?

1. High melting and boiling point

2. Fixed melting and boiling point

3. Good conductor of heat and electricity in solid and molten state

4. Malleable and ductile

What are the properties of alloys?

1. High melting and boiling point

2. Melts and boils over a range of temperatures

3. Good conductor of heat and electricity in solid and molten state

4. Less malleable and ductile, strong and harder than the pure metal they are made from

Why are alloys and metals good conductors of heat?

The sea of delocalised electrons transfers thermal energy efficiently and can carry charges to conduct electricity as they are free to move.

Why do metals and alloys have high melting and boiling points?

They have a giant metallic lattice structure with strong electrostatic forces between positive ions and sea of delocalised electrons which requires a large amount of energy to overcome

Why do alloys have no fixed melting and boiling point?

Alloys are impure substances

Why are metals suitable for making jewellery?

Metals are malleable and ductile. They have a regular lattice arrangement of atoms of the same size. When a force is applied, layers of atoms can slide over one another easily.

Why isn’t stainless steel suitable in making fine jewellery compared to iron.

Stainless steel is an alloy and alloys are less malleable and ductile. They are also strong and harder than pure metals. Atoms of different elements are of different sized, disrupting the regular lattice arrangement of atoms. A larger force is required for the layers to slide over one another.