Intermolecular Forces & Phase Changes
Identifying IMFs, their trends, phase transitions and diagrams, heating curves, and crystalline solids.
Identify the main IMF
METHODIonic solid → ionic attraction; ionic dissolved → ion-dipole. Polar molecule with H on F/O/N → hydrogen bonding; other polar → dipole-dipole; nonpolar → London dispersion (present in all). Geometry sets polarity: symmetric polar bonds cancel (nonpolar); a lone pair on the central atom breaks symmetry (polar).
IMF strength & properties
CORE RULEIonic > hydrogen bonding > dipole-dipole > dispersion (at similar size). But a big polarizable nonpolar molecule can beat a small polar one (I₂ > HCl). Stronger IMF → higher boiling point, viscosity, surface tension; lower vapor pressure.
Phase transitions
REFERENCE| Transition | Name | Heat |
|---|---|---|
| solid → liquid | melting | endo |
| liquid → solid | freezing | exo |
| liquid → gas | vaporization | endo |
| gas → liquid | condensation | exo |
| solid → gas | sublimation | endo |
| gas → solid | deposition | exo |
Temperature is flat during a phase change
CORE RULEDuring a phase change T stays constant: heat changes potential energy (breaking/forming IMFs), not kinetic. Phase change: Q = ΔH × moles (ΔH_fus or ΔH_vap). Within one phase: Q = m·s·ΔT.
Water heating curve (per mole/gram)
METHODUp to 5 steps: warm ice (s = 2.09 J/g·°C) → melt at 0 °C (6.02 kJ/mol) → warm water (4.18) → boil at 100 °C (40.7 kJ/mol) → warm steam (1.84). Sum only the segments the path crosses; convert mass → moles (÷ 18.015) for the ΔH steps.
Non-water substances
WATCH OUTUse the water defaults (ΔH_fus 6.02, ΔH_vap 40.7 kJ/mol) ONLY for water. For any other substance, take its own ΔH_fus, ΔH_vap, and specific heats from the problem; all constants must match the named substance.
Phase diagram
CORE RULERegions: solid (low T, high P), liquid (middle), gas (high T, low P). Lines = two phases coexist. Triple point: all three coexist. Critical point: end of the liquid-gas line (supercritical beyond). Normal boiling point: liquid-gas line at 1 atm.
Crystalline solids
REFERENCE| Type | Bonding | Properties |
|---|---|---|
| Ionic | ion attractions | hard, brittle; conducts only molten/dissolved; high MP |
| Metallic | electron sea | shiny, malleable; conducts; variable MP |
| Covalent network | 3D covalent | very hard; insulator; very high MP |
| Molecular | IMFs | soft; insulator; low MP |
Unit-cell density
CORE RULEρ = n·M / (N_A·a³), n = atoms per cell (SC 1, BCC 2, FCC 4). Convert edge a from pm to cm first (1 pm = 10⁻¹⁰ cm) for ρ in g/cm³. Rearrange to solve for a, M, or n.
Atomic radius by cell type
REFERENCE| Cell | Atoms/cell | Radius |
|---|---|---|
| SC | 1 | r = a/2 |
| BCC | 2 | r = a√3/4 |
| FCC | 4 | r = a√2/4 |
Identify the element
CORE RULEAfter computing M from density + cell data, match within ±0.5 g/mol of the tabulated mass AND confirm the crystal structure. Near-mass elements need the cell-type check: Co (58.93, hcp) and Ni (58.69, fcc) differ by only ~0.24 g/mol, so structure is what distinguishes them.