Unit 2.1 - Structure and Bonding

Question 1

When are ionic bonds formed?

Answer 1

When a metals and non-metals react, and electrons are transferred from the metal atoms to the non-metal atoms

Question 2

When are covalent bonds formed?

Answer 2

When two non-metal atoms share a pair of electrons

Question 3

Describe and explain the melting and boiling points of ionic substances

Answer 3

Ionic substances have high melting and boiling points, as they form giant ionic structures (or lattices), so a lot of energy is required to separate the positive and negative ions due to the strong electrostatic forces

Question 4

Describe and explain the melting and boiling points of simple covalent substances

Answer 4

Simple covalent substances have lower melting points as, despite strong covalent bonds within molecules, they have weak inter-molecular forces between molecules, so not much energy is required to melt them

Question 5

Describe and explain electrical conductivity of ionic substances

Answer 5

Solid ionic substances do not conduct electricity, as the ions are held firmly in place and cannot move to form an electric current. When the substance is molten or dissolved, it can conduct, as the ions can move to create a flow of charge

Question 6

Describe and explain electrical conductivity of simple covalent substances

Answer 6

Simple covalent substances do not conduct electricity as they have no free electrons

Question 7

Explain why some ionic substances have higher melting/boiling points than others

Answer 7

The greater the charge on the ions, the stronger the forces holding them together, and the higher the melting/boiling point (e.g. MgO > NaCl)

Question 8

Describe and explain the melting/boiling points of giant covalent substances (e.g. graphite, diamond)

Answer 8

Giant covalent substances have very high melting/boiling points as their lattice structure mean they are, in effect, one large molecule, so a lot of energy is required to break the strong covalent bonds and melt the substance

Question 9

Describe and explain the electrical conductivity of giant covalent substances

Answer 9

This is variable; diamond does not conduct electricity as it has no free electrons, whereas graphite, graphene and carbon nanotubes do have free electrons and thus can conduct electricity

Question 10

Explain (in terms of structure) why metals are malleable and ductile

Answer 10

Malleable (can be bent and shaped without breaking) - metals consist of layers of ions which can slide over each other; ductile (can be drawn out into wires, useful for electrical cables) - also due to ions sliding over each other

Question 11

How are metallic bonds formed?

Answer 11

Metals form giant structures in which electrons in the outer shells are free to move; the metallic bond is between the positive metal ion and these free/delocalised electrons

Question 12

Why are metals good conductors of electricity and heat?

Answer 12

Delocalised electrons from outer shells of metal atoms are free to move about the metal, causing good conductivity

Question 13

Why do different metals have different melting/boiling points?

Answer 13

The more outer shell electrons a metal has (e.g. Aluminium - 3), the more delocalised electrons there are, so the stronger the bond and the higher the melting/boiling point.

Question 14

Describe the uses of carbon allotropes and link them to structure and properties.

Answer 14

• Fullerenes - cage-like structures can be used to deliver drugs to certain parts of the body (as the cage structure is impermeable).
• Carbon Nano-Tubes - layers of graphite rolled into tube-like shapes; each carbon atom bonded to three others, so high melting point, sea of free electrons, so tube can conduct electricity.

Question 15

Describe properties and uses of nano-silver particles.

Answer 15

Have anti-bacterial, anti-viral and anti-fungal properties - can be used in plastics, antiseptic sprays, refrigerator linings, deodorants, etc.

Question 16

Describe properties and uses of nano-scale titanium dioxide particles.

Answer 16

• Absorb and reflect UV light - used in sunscreen, as they are transparent and therefore more appealing to customers.
• Catalyse breakdown of dirt in presence of UV light, and cause water to spread out in a thin film rather than forming droplets - used in self-cleaning windows.

Question 17

Explain the reasons for the concern over the use of nano-scale particles.

Answer 17

• Long term effects are as yet unknown - exposure over many years may cause health problems.
• Some concern that nano-silver (in deodorants) and nano-titanium-dioxide (in sunscreen) are applied to skin and can be easily absorbed into body.

Question 18

Name and describe five common smart materials.

Answer 18

Thermochromatic pigments - change colour with change in temperature.
Photochromatic pigments - change colour with change in light intensity.
Polymer gels/Hydrogels - can absorb/expel water and swell/shrink with changes in pH or temperature.
Shape memory alloys - regain shape when heated
Shape memory polymers - regain shape when heated.

Question 19

Give a use of each type of smart material.

Answer 19

T.c. pigments - mugs which change colour to indicate when tea or coffee is safe to drink.
P.c. pigments - photochromatic lenses for glasses, so the glasses act as sunglasses on sunny days but normal glasses indoors.
Hydrogels - hair gel/nappies
S.m. alloys - frames of glasses or car bodies
S.m. polymers - sport equipment (helmets)/car bumpers.

Question 20

What is a smart material?

Answer 20

A material whose properties can changes reversibly in response to a change in its surroundings.