Phases and Forces

PHASES AND FORCES
Phases
Gases
· Distance between molecules is greatest (low density)
· No appreciable interaction b/w particles
· Easily compressed with indefinite shape and volume
Liquids
· Molecules closer together with very little empty space (higher density than gases)
· Less easily compressed
· Held together more strongly by intermolecular forces
· Molecules can move past one another freely
· Assumes shape of container
· Surface tension: the amount of energy required to stretch or increase the surface of a liquid by a unit area
o Molecules of a liquid are pulled downwards and sideways, and not upwards, which causes the surface to tighten
o The greater the intermolecular forces, the greater the surface tension
· Capillary Action: surface tension pulls liquid upward against the force of gravity. Adhesion is greater than cohesion.
o Adhesion: attraction between unlike molecules (glass tube and water)
o Cohesion: attraction between like molecules (water and water)
· Viscosity: The measure of a fluid’s resistance to flow. (Greater viscosity, slower liquid flow).
o Decrease in temperature increases viscosity
o Stronger intermolecular forces causes greater viscosity
Solids
· Rigid form – no translational motion
· Almost completely incompressible (rigid volume and shape)
· Amorphous solids: a solid that lacks a regular 3D arrangement
o Ex: a liquid that cools quickly and does not have time to align itself in an orderly fashion
o Glass is an amorphous solid
Intermolecular Forces
· Attractive forces between molecules
· Strength of intermolecular forces influences phase of matter
o Stronger intermolecular forces cause smaller distances between molecules
· At lower temperature, intermolecular forces have greater impact
o At these temperatures kinetic energy is decreased and molecules move slower
· B.P. and M.P. of substance reflect strength of intermolecular forces
o The greater the strength of attraction, the more energy required to break apart molecules
Types of Intermolecular Forces (Van Der Waals)
Ion-Dipole (40-600 kJ/mol, 2nd strongest interaction)
· Attraction between ions and polar molecules
· Strength depends on charge and size of ion, strength of dipole moment, and size of molecule
o Cations are usually stronger because they tend to have a more concentrated charge than anions
· EX: NaCl + H2O
Dipole-Dipole (5-25 kJ/mol, 3rd strongest interaction)
· Attraction between polar molecules
· The larger the dipole moment, the greater the force
· EX: HCl + HCl
Hydrogen Bonding (strongest interaction)
· A particularly strong type of dipole-dipole interaction
· Between the H of one molecule and the electronegative N, F, or O of another
· The H—F is the strongest individual hydrogen bond because Fluorine is the most electronegative element
· The greater the presence of hydrogen bonds, the stronger the total intermolecular force, and the greater the boiling point of a molecule
· Causes great surface tension and viscosity
Dipole-Induced Dipole (2-10 kJ/mol, 4th strongest interaction)
· Induced dipole is the separation of positive and negative charges in a nonpolar molecule or atom due to the close proximity of an ion or polar molecule
· Polarizability is the ease with which the electron distribution in an atom or molecule can be distorted
o The more diffuse the electron cloud, the greater tendency it has to be polarized
Dispersion Forces
· Attractive force that arises as a result of temporary induced dipoles
· Usually increase with molar mass (electron cloud bigger, and therefore, more diffuse)
o Melting point and boiling point increase
Properties of Water
· The Hydrogen bonds and asymmetrical polar structure of water cause its atypical behavior
· High specific heat
o Can absorb or release a great amount of heat with little change in temperature
o This phenomena helps moderate the climate
· Solid Form is less dense than liquid form
o The structure of the solid form spaces particles farther apart than particles in the liquid phase
Phase Changes
· Usually occur when heat is added or removed
o The stronger the intermolecular forces, the more energy is required (heat) to pull apart molecules
o Therefore, phase changes to a more gaseous phase are more difficult with molecules having strong attraction
o Phase changes in the opposite direction are easier with molecules having strong attraction
· Sublimation: the immediate transition from the solid to gas phase
· Deposition: the immediate transition from vapor to solid
· Critical Temperature (Tc): highest temperature at which substance can be a liquid
· Critical Pressure (Pc): greatest pressure at which substance can be a liquid
· Supercooling: situation in which a liquid can be temporarily cooled to below its freezing point. This occurs when heat is removed very rapidly.
· Heat of “Phase Change”: the amount of energy needed to support that phase change
o Ex: heat of vaporization= the amount of energy needed to turn liquid to a gas
o ΔHsub = ΔHfus + ΔHvap
Phase Diagrams




General Phase diagram
Water Phase Diagram

Questions:
Which of the following statements are false?
1. Liquids with large intermolecular forces tend to have very low boiling points2. Liquids with large intermolecular forces tend to have considerable surface tension3. When a substance changes from a solid to a liquid, the molecules remain intact4. The hardness of diamond is due to strong dipole-dipole attraction
2 and 4
1 and 4
1 and 3
3 and 4
2 and 3
Answer: 2. If a liquid had strong intermolecular forces it would require more energy to pull apart the molecules. That would cause the boiling point to be higher, and therefore, statement one is false. Surface tension is caused by the pull of strong intermolecular forces on the surface of a liquid, as statement two says correctly. Statement three is correct because phase changes are purely physical changes and do not distort the chemical structure of the molecule or atom. A diamond is made strictly of carbon. Carbon as an atom does not have a dipole moment and therefore the interactions within a diamond can not be dipole-dipole and cannot cause the hardness of the diamond. They in fact are dispersion forces.
The triple point of compound X occurs at a temperature of:
29°C
80°C
-10°C
23°C
50°C
Answer: 4. The triple point of a phase diagram is the point at which a substance is in equilibrium with all three phases. The point at which the three phases meet is the triple point and is at 23°C.

Which of the following hydrocarbons has the highest boiling point?
C2H6
C3H8
C4H10
These are all gases and do not boil
CH4
Answer: 3. Butane, C4H10, has the highest boiling point because it has the greatest molar mass. Statement four is incorrect because all forms of matter, even gasses, have boiling points.