Study guides › C/P › General Chemistry
Phases & Intermolecular Forces
The forces between molecules decide whether a substance is a gas, liquid, or solid and how it behaves at each stage. You will rank boiling points, read phase diagrams and heating curves, and calculate the energy of phase changes.
Study this chapter in PrepMed (free)
What this chapter covers
- Intermolecular Forces & Boiling Point Trends. Ranking compounds by boiling point is one of the most common MCAT tasks, and it depends on the type and strength of IMFs.
- Liquids: Vapor Pressure, Surface Tension & Capillarity. Vapor pressure links IMFs to boiling point, and cohesion versus adhesion explains every surface phenomenon.
- Phase Diagrams. Phase diagrams show which phase is stable at any pressure and temperature, and the MCAT always asks about the triple and critical points.
- Heating Curves & Energy of Phase Changes. Heating-curve problems combine q = mcΔT with q = nΔH, and the plateaus test whether you understand potential energy.
- Solids: Types and Crystal Structure. The kind of particles and bonding in a solid predict its melting point, conductivity, and hardness.
Key points to know
- IMFs act between molecules; boiling breaks IMFs, not covalent bonds
- Dispersion forces exist in all molecules and rise with size and surface area
- Vapor pressure depends on temperature and IMFs only, not on liquid volume
- Boils when vapor pressure = external pressure; normal bp is at 1 atm
- Triple point: all three phases coexist; critical point: liquid and gas merge
- Above the critical point the substance is a supercritical fluid
- Sloped segments use q = mcΔT; plateaus use q = nΔH
- Plateau: temperature constant, potential energy rises, kinetic energy unchanged
Try a practice question
Which molecule can form hydrogen bonds with other molecules of its own kind?
- mol:COC{dimethyl ether}
- mol:CC(=O)C{acetone}
- mol:CCO{ethanol}
- mol:CCC{propane}
Show the answer
C. A molecule needs an H bonded to N, O, or F to donate a hydrogen bond. Only ethanol has an O–H. Ether and acetone can accept hydrogen bonds but cannot donate them, and propane is nonpolar.
Common questions
What does this chapter cover?
Phases & Intermolecular Forces covers Intermolecular Forces & Boiling Point Trends, Liquids: Vapor Pressure, Surface Tension & Capillarity, Phase Diagrams, Heating Curves & Energy of Phase Changes, Solids: Types and Crystal Structure.
What should I know first?
IMFs act between molecules; boiling breaks IMFs, not covalent bonds Dispersion forces exist in all molecules and rise with size and surface area Vapor pressure depends on temperature and IMFs only, not on liquid volume
24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.