Chapter 12: Liquids & Solids Flashcards
Surface tension
Elastic force in the surface of a liquid
The amount of energy required to strech or increase the surface of a liquid by a unit area
Strong intermolecular forces = high surface tension
Capillary action
More pronounced in a cylinder with small diameter
Miniscus forms either a concave (U-shaped) or convex (dome-shaped) curve at the liquid’s surface due to cohesion and adhesion
Cohesion
Attractions between like molecules
Adhesion
(Think tape: sticks to other material)
Attractions between unlike molecules
Viscosity
A measure of a fluid’s resistance to flow
As T increases, viscosity decreases
Vapor pressure
Depends upon magnitude of intermolecular forces
High vapor pressure = volatile substances
Evaporation or vaporization
Change of phase: from liquid to gas
*Initially more rapid than condensation
Condensation
Change of phase: gas to liquid
Can occur when gas strikes liquid surface and becomes trapped by intermolecular forces
*Initially slower than evaporation
Dynamic equilibrium
When a forward process and reverse process occur at the same rate
Both processes going on (dynamic)
Number of gas molecules in the gas phase at any point in time doesn’t change (equilibrium)
Equilibrium vapor pressure
The pressure exerted by the molecules that have escaped to the gas phase, once the pressure has stopped increasing
Boiling point
The temperature at which its vapor pressure equals the external pressure
Melting point
The temperature at which the energies of individual particles enable them to break free of their fixed positions in the solid – allowing them to flow past one another
Stronger intermolecular forces = higher melting point
Crystalline solid
Rigid and long-range internal order/structure of molecules/particles
Lattice structure
Arrangement of particles in a crystalline solid
Depends on the nature and size of the particles involved
Unit cell
The basic repeating structural unit of a crystalline solid
Lattice point
Each particle within a unit cell
Simple/primitive cubic cell
8 lattice points (particles)
1/8 (8 particles) = 1 whole particle in simple cubic cell
Coordination number: 6
Coordination number
The number of atoms/particles surrounding an atom/particle in a crystal lattice
Larger coordination # = more tightly packed cell
Body-centered cubic cell
9 lattice points (8 corners, 1 center)
1/8 (8 corner lattice points) + 1 = 2 whole particles in cell
Coordination number:
8
Face-centered cubic cell
8 corners + 6 sides = 14 particles/lattice points
1/8 (8 corners) + 1/2 (6 sides) = 4 whole particles in cell
Coordination number: 12
Molar heat of vaporization ΔHvap
Amount of heat required to vaporize a mole of substance at its boiling point
kJ/mol
Critical temperature TC
Particular temperature at which a substance in its gas phase cannot be liquefied, no matter how great the applied pressure
Critical pressure CP
The minimum pressure that must be applied to liquefy a substance at its critical temperature
Supercritical fluid
A fluid at a temperature and pressure that exceed critical temperature and pressure
Temperatures above critical temperature = no longer a distinction between gas and liquid phases, there’s simply a fluid (plasma)
Used as solvents in many industrial applications
Fusion
Melting – phase change of solid to liquid
Freezing point
AKA melting point
Temperature at which solid and liquid phases coexist at equilibrium
Normal melting/freezing point
The temperature of melting/freezing at 1 atm
Molar heat of fusion ΔHfus
Energy (kJ/mol) required to melt 1 mole of a solid
*MUCH smaller than ΔHvap because solids & liquids are more alike than liquids & gases
Supercooling
Liquid is temporarily cooled to below its freezing point
Occurs when heat is removed from a liquid so rapidly that the molecules literally have no time to assume the ordered structure of a solid
Unstable - stirring or adding small “seed” crystal solidifies sample quickly
Sublimation
Phase change from solid to gas
Thus, solids can have vapor pressure
Deposition
Phase change – vapor phase to solid phase
Molar enthalpy of sublimation ΔHsub
Amount of energy (kJ/mol) required to sublime 1 mole of a solid
ΔHsub = ΔHvap + ΔHfus
Phase diagram
Summarizes conditions (T & P) at which a substance exists in its various phases
Separated by phase boundary lines
Triple point
The point on a phase diagram at which all three phase boundary lines meet
The only combination of T & P at which all three phases of a substance can be in equilibrium with each other
Critical point
Point at which the liquid-vapor phase boundary line abruptly ends
This corresponids with the critical temperature and pressure (CT and CP)
