The gaseous state
Section: 4 States of Matter | Syllabus: Cambridge AS Level Chemistry (9701)
Origin of Pressure in a Gas
Gas molecules are in constant, random motion. They travel in straight lines until they collide with each other or with the walls of the container.
- When gas molecules collide with the walls of a container, they exert a force on the wall.
- Pressure is defined as force per unit area. The pressure of a gas arises from the collective effect of countless collisions of gas molecules with the container walls.
- Higher temperature → molecules move faster → more frequent and harder collisions → higher pressure.
- Smaller volume → molecules collide with the walls more frequently → higher pressure.
- More molecules → more collisions per unit time → higher pressure.
Figure 4.1: Gas Molecules in a Container
(Box with small circles (gas molecules) moving in random directions shown by arrows. Molecules near the walls shown colliding and bouncing off. Label: "collision with wall → exerts force → contributes to pressure". Show spacing between molecules - mostly empty space.)
Lattice Structures of Crystalline Solids
Solid materials can be classified into four types of lattice structure based on the particles present and the forces holding them together.
Giant Ionic Lattice
In a giant ionic lattice, positive and negative ions are arranged in a regular, repeating 3D pattern. Strong electrostatic attractions act between oppositely charged ions in all directions.
Sodium Chloride (NaCl)
- Each Na⁺ ion is surrounded by 6 Cl⁻ ions, and each Cl⁻ is surrounded by 6 Na⁺ ions - 6:6 coordination.
- The structure extends infinitely in three dimensions - there are no discrete molecules.
Figure 4.2: Giant Ionic Lattice of NaCl
(3D cubic lattice: Na⁺ (small, purple) and Cl⁻ (large, green) alternating. Show one Na⁺ with 6 surrounding Cl⁻ highlighted. Label 6:6 coordination. Show the repeating cubic unit cell.)
Magnesium Oxide (MgO)
- Same NaCl-type structure with Mg²⁺ and O²⁻ ions.
- Higher ionic charges (2+ and 2−) → stronger electrostatic attraction → much higher melting point than NaCl (MgO mp ≈ 2852°C vs NaCl mp ≈ 801°C).
- Smaller ionic radii of Mg²⁺ compared to Na⁺ also contributes to the stronger lattice.
Simple Molecular Lattice
Simple molecular substances consist of discrete molecules held in a lattice by weak intermolecular forces (van der Waals' forces). The covalent bonds within molecules are strong, but the forces between molecules are weak.
Iodine (I₂)
- Discrete I₂ molecules arranged in a regular lattice.
- Held together by id–id (London dispersion) forces between molecules.
- Low melting point - only the weak intermolecular forces need to be overcome on melting, not covalent bonds.
Figure 4.3: Simple Molecular Lattice of Iodine
(Regular array of I₂ dumbbell-shaped molecules. Strong covalent I–I bond within each molecule shown as solid line. Weak London dispersion forces between molecules shown as dotted lines. Label both types of interaction.)
Ice (H₂O)
- Discrete H₂O molecules held in a rigid open hexagonal lattice by hydrogen bonds.
- The open lattice structure means ice is less dense than liquid water (see Chapter 3).
- Higher melting point than iodine because hydrogen bonds are stronger than id–id forces.
Buckminsterfullerene (C₆₀)
- Each C₆₀ molecule consists of 60 carbon atoms arranged in a spherical cage (20 hexagons + 12 pentagons - like a football).
- Carbon atoms within the molecule are joined by covalent bonds.
- C₆₀ molecules are held together in the lattice by id–id forces - making it a simple molecular structure despite the large molecules.
- Relatively low melting point (sublimes at ~600°C) for a carbon allotrope, because only the weak intermolecular forces need to be broken.
- Does not conduct electricity - no delocalised electrons or free ions.
Figure 4.4: Structure of Buckminsterfullerene C₆₀
(Spherical cage of 60 C atoms with alternating hexagonal and pentagonal rings visible. Label: covalent C–C bonds within the molecule; id–id forces between C₆₀ molecules in the lattice. Compare to football shape.)
Giant Molecular (Giant Covalent) Lattice
In a giant molecular lattice, atoms are covalently bonded throughout the entire structure in a continuous 3D network. There are no discrete molecules. Very high melting points because covalent bonds must be broken.
Diamond
- Each carbon atom is bonded to 4 other carbon atoms by strong covalent bonds - sp³ hybridised, tetrahedral arrangement (109.5°).
- The structure is a rigid, continuous 3D network - extremely hard.
- Very high melting point (~3550°C) - many strong covalent bonds must be broken.
- Does not conduct electricity - all four outer electrons are used in bonding; no delocalised electrons.
- Insoluble in water and organic solvents.
Figure 4.5: Giant Covalent Structure of Diamond
(3D tetrahedral lattice: each C atom shown bonded to 4 others in a repeating tetrahedral pattern. Show the repeating unit. Label: sp³ hybridisation, bond angle 109.5°, "no free electrons - non-conductor".)
Graphite
- Each carbon atom is bonded to 3 other carbon atoms in flat hexagonal layers - sp² hybridised, bond angle 120°.
- Each carbon has one unhybridised p electron → these electrons are delocalised across each layer.
- Layers are held together by weak id–id forces - layers can slide over each other easily → graphite is soft and slippery (used as a lubricant).
- Conducts electricity along the layers - delocalised electrons carry charge.
- High melting point (~3600°C) - covalent bonds within layers must be broken.
Figure 4.6: Giant Covalent Structure of Graphite
(Side view: multiple flat hexagonal layers stacked on top of each other with spacing. Top view of one layer: regular hexagonal rings of C atoms with 120° bond angles. Label: covalent bonds within layers (strong), id–id forces between layers (weak), delocalised electrons shown as shading across the layer.)
Silicon(IV) Oxide (SiO₂)
- Each Si atom is covalently bonded to 4 oxygen atoms (tetrahedral arrangement); each O atom is bonded to 2 Si atoms.
- Ratio Si:O = 1:2 → formula SiO₂, but no discrete molecules exist.
- Very high melting point (~1710°C) - strong covalent bonds throughout the structure.
- Hard solid, does not conduct electricity, insoluble in water.
- Structurally analogous to diamond but with alternating Si and O atoms.
Figure 4.7: Giant Covalent Structure of SiO₂
(3D network: Si atoms (large, grey) each bonded to 4 O atoms (small, red). O atoms bridge between Si atoms. Show tetrahedral arrangement around each Si. Label: 4:2 coordination (each Si bonded to 4 O; each O bonded to 2 Si). Compare to diamond structure shape.)
Giant Metallic Lattice
Interactive revision notes, videos and practice questions load below.