Predicting Structures (Edexcel AS Chemistry): Revision Note
Exam code: 8CH0
Predicting Structure & Bonding
Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility
Characteristics of Different Compound Structure Types Table
Property | Giant ionic | Giant metallic | Simple covalent | Giant covalent |
|---|---|---|---|---|
Melting and boiling points | high | moderately high to high | low | very high |
Electrical conductivity | only when molten or in solution | when solid or liquid | do not conduct | do not conduct (except graphite) |
Solubility | usually soluble in water | insoluble, but some may react | usually insoluble unless polar | insoluble |
Hardness | hard, brittle | hard, malleable | soft | very hard (diamond and SiO2) or soft (graphite) |
Physical state at room temperature | solid | solid | solid, liquid or gas | solid |
Forces | electrostatic attraction between ions | delocalised sea of electrons attracting positive ions | weak intermolecular forces between molecules and covalent bonds within a molecule | electrons in covalent bonds between atoms |
Particles | ions | positive ions in a sea of electrons | small molecules | atoms |
Examples | NaCl | copper | Br2 | graphite, silicon(IV) oxide |
Predicting Physical Properties
Worked Example
Bonding & structure
The table below shows the physical properties of substances X, Y and Z
Substance | Melting point / °C | Electrical conductivity when molten | Solubility in water |
|---|---|---|---|
X | 839 | good | soluble |
Y | 95 | very poor | almost insoluble |
Z | 1389 | good | insoluble |
Which one of the following statements about X, Y and Z is completely true?
Statement 1: X has a giant ionic structure, Y has a giant molecular structure, Z is a metal
Statement 2: X is a metal, Y has a simple molecular structure, Z has a giant molecular structure
Statement 3: X is a metal, Y has a simple molecular structure, Z has a giant ionic structure
Statement 4: X has a giant ionic structure, Y has a simple molecular structure, Z is a metal
Answer
The correct answer is Statement 4
The relatively high melting point, solubility in water and electrical conductivity when molten suggest that X is a giant ionic structure.
The low melting point of Y suggests that little energy is needed to break the lattice which corresponds to a simple molecular structure. This is further supported by the low electrical conductivity and its being almost insoluble in water.
Compound Z has a very high melting point which is characteristic of a giant structure; it conducts electricity when molten, so it is not giant covalent, and as it is insoluble in water it is most likely a giant metallic lattice.
Worked Example
Bonding & structure
Compound X has the following properties:
Melting point 1450 °C; electrical conductivity poor as a solid and poor when molten.
What is the most probable structure of X?
A. Network covalent
B. Polar covalent molecule
C. Ionic lattice
D. Metallic lattice
Answer:
The correct option is A
A high melting point is characteristic of a giant structure, which could be metallic, ionic or covalent
The poor conductivity as a liquid and solid would match a giant covalent or network covalent structure
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