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Question 1

Covalent Network Substances

Answer 1

Examples: Carbon in diamond, silicon, silicon dioxide
Composition: few non-metallic elements, and a few compounds of non metals
Melting point: HIGH
Electrical conductivity: solid - no, molten - no
Hardness: VERY, atoms not easily displaced

Question 2

Ionic Substances

Answer 2

High melting point: Lots of energy is
needed to overcome the attraction between ions

Electrical conductivity: solid - no, molten - yes

Hardness: ions are not easily displaced.

Brittleness: If a layer of ions is displaced with respect to neighbouring layers, the repulsion between like charges forces the lattice apart.

Formulas of ionic compounds
* The formula of an ionic compound refers to the relative number of cations and anions in the lattice.
* The relative number of cations and anions must be such that the compound has zero net electrical charge.

Question 3

Solids

Answer 3

Melting: Conversion of Solid to Liquid
* The melting point of a solid substance is the temperature at which the crystal lattice collapses.
* Molar enthalpy change of fusion is the heat energy absorbed when a mole of a solid melts.

Heat energy absorbed on melting = molar enthalpy change of fusion = ∆(fus)H (kJ mol-1)

Heat energy evolved on freezing = molar enthalpy change of crystallization
= -∆fusH (kJ mol-1)

Low melting points -> low enthalpy

transition metals -> high enthalpy

Question 4

Sublimation

Answer 4

the process by which molecules escape directly from the solid to the gas phase.

– ∆subH , is the energy absorbed during sublimation.

Question 5

Phase Diagrams: Critical points

Answer 5

Under a certain temperature and pressure, the interface between the liquid and the vapour phases for a substance disappears

These points occur at a specific critical temperature, Tc , and critical pressure, pc.

• A supercritical fluid exists at a pressure and temperature above the critical point for a substance.

Question 6

Phase Diagrams: Supercritical fluids

Answer 6

• At high pressure, molecules are tightly packed like a liquid. Therefore, its density is close to that of a liquid.
• At high temperature, each molecule has enough kinetic energy to exceed the forces holding molecules together. Therefore, its viscosity is close to that of a gas.
• Supercritical CO2: green solutions for solvent extraction.

Question 7

Polymorphic Forms of Solids

Answer 7

Solid material can often exist under different conditions in more than one form or crystal structure

High-pressure region of the phase diagram for
water, showing several other forms of ice,
besides the familiar hexagonal form (Ice I).

None of these forms are stable below 2 ×
108 kPa, even at -60°C.

Question 8

Allotropes

Answer 8

Similar to polymorphs but for elements