Chapter 12 Flashcards
Intermolecular Forces
> Forces between particles
Influence physical properties of substance
Relatively weak because they involve smaller particles what are farther apart
Intramolecular Forces
> Force within a particle
Influence chemical properties
Relatively strong because they involve larger charge and are closer together
Types of Intermolecular Forces
- Ionic - cation to anion
- Covalent - nuclei shared e pair
- Metallic - cations delocalized electrons
Forces between Particles: Permanent
Molecules with a permanent dipole moment are POLAR
Forces between Particles: Instantaneous
Non-polar molecules and atoms can have an instantaneous dipole when electron density is condensed in one region at a particular moment
Forces between Particles: Induced
Non-polar molecules and atoms can have an induced dipole when their electron density is displaced because of contact with another charged substance
Types of Intermolecular Forces
Between particles of SAME substance:
- Dipole-Dipole
- Dispersion (London)
- Hydrogen bonding
Between particles of different substances:
- Ion-Dipole
- Ion-induced Dipole
- Dipole-induced Dipole
Dispersion Forces
> Result from instantaneous dipoles on atoms and molecules - occurs naturally
Random motions of electrons within an atom
Example: at any moment, two electrons can be on the same side of an atom, resulting in instantaneous dipole
This can induce a dipole on a second atom of the same kind, and so on
Dispersion Forces: Polarizability
> Strength of dispersion force depends on the polarizability
Tendency for charge separation to occur in a molecule
Particles with increased number of electrons have larger polarizability
Particles with large, spread out geometries have larger polarizability
Dipole-Dipole Forces
> Result from alignment of negative and positive ends of polar molecules
Hydrogen Bonding
> Special type of dipole-dipole force
Interaction of OXYGEN, NITROGEN, or FLOURINE with a hydrogen atom
Strong force because the hydrogen is small and easy to approach
Examples: water, ammonia
Ion-Dipole Force
> Force between an ionic compound and a polar compound
+ ends of molecule attracted to - ions
- ends of molecule attracted to + ions
Phases of Matter
> Gas to liquid, liquid to solid, etc.
> Depends on potential energy of molecules (intermolecular forces) and kinetic energy (temperature)
Properties of Liquids: Surface Tension
> The amount of energy (or work) required to increase the surface area of a liquid
Units = energy per unit area (Jm^-2)
Surface tension decreases as temperature increases
Tension is greater for particles with stronger intermolecular forces
Properties of Liquids: Liquids on Surfaces
> Interaction of liquid with another surface depends on:
1. Cohesive forces (holding liquid together)
2. Adhesive forces (forces between liquid molecules and surface)
A drop of liquid will maintain shape on a surface if cohesive forces are greater than adhesive
Properties of Liquids: Formation of Meniscus
> Concave = adhesive forces with tube are stronger than cohesive
Convex = adhesive forces with tube are weaker than cohesive
Properties of Liquids: Capillary Action
> Stronger adhesive forces cause liquid to rise onto a surface
Properties of Liquids: Viscosity
> Liquid’s resistance to flow
> Stronger intermolecular forces = greater viscosity = less flow
Vapor Pressure
> Molecules evaporate at the surface of a liquid
VP is pressure exerted by a vapor in dynamic equilibrium with its liquid
Closed container: equilibrium exists between evaporating liquid molecules and condensing gas molecules
High vapor pressure = Volatile
VP depends on nature of liquid (weaker IMF, high VP) and temperature (high temp, higher VP)
*Know how to read graph
Clausius-Clapeyron Equation
> Relates heat of vaporization to the measured vapor pressure
ln(P2/P1) = (DeltaHvap/R)(1/T1 - 1/T2)
Plot lnP vs. 1/T to obtain straight line
Slope = -DeltaHvap/R
Vapor Pressure and Boiling Point
> Boiling occurs when pressure exerted by molecules leaving liquid = pressure exerted by molecules of the atmosphere
Temperature at which VP of a liquid is = standard pressure (760mmHg) is the NORMAL BOILING POINT
Phase Diagrams
> O = Triple Point = all phases are in equilibrium
C = critical point = the liquid and gas have same density, supercritical fluid exists
Find normal boiling point at 1atm where liquid and gas meet
Using IMF to predict properties
> Pure substances: IMF can help predict physical properties including boiling point/vapor pressure and viscosity
Take into account total number of electrons
If two substances have same IMF, then the substance with heavier molecular weight will have the strong IMF
Crystal Structures
> Solid structures with plane surfaces, sharp edges and regular geometric shapes
Types: crystal lattices and closest packed structures
Xray diffraction can be used to determine structure
Crystal Lattices
> Contain underlying regular pattern in arrangement of atoms, molecules, or ions
Unit Cell is smallest repeating unit of structure
Simple cubic, body-centered cubic or face-centered cubic
Closest packed structures
> Structures are arranged so particles are in close contact and the volume of the voids is minimal
Hexagonal closest packed (hexagonal unit cell)
Cubic Closest packed (gives face-centered cubic unit cell)
Coordination number = 12
Coordination Number
The number of other particles each particle is in contact with
Number of Atoms per Unit Cell
> Corner particles = 1/8
Face-centered particles = 1/2
Body-centered particles = 1
Simple cubic = 1 atom
FCC = 3
BCC = 2
Xray Diffraction
> Xrays are scattered from electrons in atoms
Pattern of the scatter is related to the distribution of electronic charge in the particles
Can use data to calculate lengths of unit cells, radii and densities
BCC - Length/Radius = 4r = Lroot3
BCC - Density = convert # atoms to grams / Lcm^3
Crystal Structures: Ionic Crystals
> Opposing charges MUST be in contact
Size of ions plays a role in packing particles into crystals
Length/Radius = 2rX + 2rY = L
Density = convert # atoms to grams / Lcm^3
Math Review
Logarithm
logx = n; x = 10^n
Anti-Log
anti-log x = n; n = 10^x
Natural log
lnx = m; x = e^m
anti-lnx = e^x
log(a x b) = log a + log b
log(c/d) = log c - log d
