States of Matter
States of Matter
Questions here ask you to explain the properties of solids, liquids and gases in terms of the particles present and the forces between them, and to compare crystal structures such as sodium chloride, diamond, graphite, ice and metals.
From gas to solid
The state depends on the balance between thermal kinetic energy and the strength of intermolecular attractions. Heating a solid supplies energy to overcome some attractions (melting) and then all of them (boiling); the temperature stays constant during a phase change because the energy goes into breaking forces, not raising kinetic energy. Vapour pressure rises with temperature, and a liquid boils when its vapour pressure equals the external pressure. Hydrogen bonding explains water's unusually high melting and boiling points, high specific heat capacity and high enthalpy of vaporisation.
Crystal structures
Sodium chloride is a 6:6 ionic lattice, melting high and conducting when molten. Diamond has each carbon tetrahedrally bonded to four others, so it is extremely hard and non-conducting. Graphite has layers of trigonal carbons with one delocalised electron each: it conducts in-plane and cleaves between layers, giving lubricity. Ice has an open hydrogen-bonded lattice, which is why it is less dense than liquid water. Metals are close-packed cations in a sea of electrons.
Worked example
How much energy is needed to vaporise 100 g of water at 100 degrees C? Moles of water = 100 / 18.0 = 5.56 mol. Using the enthalpy of vaporisation of water, 40.7 kJ mol-1: energy = 5.56 x 40.7 = 226 kJ. The reasoning matters more than the number: the calculation assumes boiling at 100 degrees C and uses the molar enthalpy of vaporisation, so the answer is in kJ, not kJ mol-1, and no temperature change term is included because the water is already at its boiling point.
Traps
Saying ice is denser than water; confusing melting (some forces overcome) with boiling (all overcome); assuming all covalent substances are simple molecular; forgetting that dissolving an ionic solid needs energy to break the lattice but gains hydration energy; and mixing up sublimation with evaporation.
Key ideas
- State depends on kinetic energy versus intermolecular attraction.
- Temperature is constant during a phase change.
- Boiling occurs when vapour pressure equals external pressure.
- Ice is less dense because of its open hydrogen-bonded structure.
- Graphite conducts along layers; diamond does not.