Ionic Lattices (DP IB Chemistry)
Revision Note
Ionic Lattices
What is an ionic lattice?
The ions form a lattice structure, known as an ionic lattice
This is an evenly distributed crystalline structure
Ions in a lattice are arranged in a regular repeating pattern so that positive charges cancel out negative charges
Therefore, the final lattice is overall electrically neutral
What forces hold together an ionic lattice?
The ionic lattice consists of alternating cations and anions
Cations are positively charged ions and anions are negatively charged ions
Therefore, there are strong electrostatic forces of attraction between the oppositely charged ions
Remember: This is one possible definition of ionic bonding
Giant ionic lattice structure diagram
Ionic solids are arranged in lattice structures
Lattice Enthalpy
The lattice dissociation enthalpy (ΔHlattꝋ) is defined as the standard enthalpy change that occurs on the formation of 1 mole of gaseous ions from the solid lattice
Since this is always an endothermic process, the enthalpy change will always have a positive value
The ΔHlattꝋ is always endothermic as energy is always required to break any bonds between the ions in the lattice
NaCl (s) → Na+ (g) + Cl– (g) ΔHlatꝋ = +790 kJ mol -1
Properties of Ionic Compounds
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
Ionic bonding & giant ionic lattice structures
Ionic compounds are strong
The strong electrostatic forces in ionic compounds keep the ions held strongly together
They are brittle as ionic crystals can split apart
Ionic compounds have high melting and boiling points
The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
Melting and boiling points increase with the charge density of the ions due to the greater electrostatic attraction of charges
Mg2+O2– has a higher melting point than Na+Cl–
Ionic compounds are not volatile
Volatility refers to the vapourisation of a chemical
Large amounts of energy are required to overcome the strong electrostatic forces of attraction, which means that ionic compounds are not volatile
Ionic compounds are soluble in water as they can form ion-dipole bonds
Ionic compounds only conduct electricity when molten or in solution
When molten or in solution, the ions can freely move around and conduct electricity
As a solid, the ions are in a fixed position and unable to move around
Table comparing the characteristics of giant ionic lattices with other structure types
| Giant ionic | Giant metallic | Simple covalent | Giant covalent |
|---|---|---|---|---|
Melting / boiling point | High | Moderately high to high | Low | Very high |
Electrical conductivity | Only when molten or in solution | When solid or liquid | Do not conduct electricity | Do not conduct electricity (except graphite) |
Solubility | Soluble | Insoluble but some may react | Usually insoluble unless they are polar | Insoluble |
Hardness | Hard, brittle | Hard, malleable | Soft | Very hard (diamond and silica) or soft (graphite) |
Physical state at room temperature | Solid | Solid | Solid, liquid or gas | Solid |
Forces | Electrostatic attraction between ions | Delocalised electrons attracting positive ions | Weak intermolecular forces 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 |
Worked Example
The table below shows the physical properties of substances X, Y and Z.
Substance | Melting point (oC) | 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:
Compound X has a relatively high melting point, is soluble in water and conducts electricity when molten
This suggests that X has a giant ionic structure
Compound Y has a low melting point which suggests that little energy is needed to break the lattice
This suggests that Y is a simple molecular structure
This is further supported by its low electrical conductivity and it being almost insoluble in water
Compound Z has a very high melting point, which is characteristic of either metallic, giant ionic lattices or giant covalent / molecular lattices
However since it is insoluble in water, compound Z must be a metal
Therefore, the correct answer is Statement 4
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?