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critical point
critical pressure
critical temperature
dipole-dipole force (VER)
dispersion force (VER)
freezing point
gas
hydrogen bonding (VER)
intermolecular force (VER)
ion-dipole force (VER)
ion-ion attraction (VER)
lattice structure
liquid
Londong force (VER)
melting point (VER)
meniscus
miscible
phase diagram (VER)
polarizable
solid
sphere of hydration
sublimation
supercritical fluid
surface tension
triple point
vapor pressure
viscosity
Liquids and solids are formed by many atoms, ions, or molecules held closely together. In
liquids the particles (atoms, ions, or molecules) can move freely but remain adjacent to the
other particles. In solids not only are the particles adjacent, but the motion of the particles is
limited. In gases (Chapter 8) the particles move freely and independently; that is, without
attaching to other particles.
The atoms, ions, or molecules are held together by intermolecular forces. All these forces
are based on the attraction of a positive charge to a negative charge. The magnitude of the
force (U) is expressed by Coulomb's law
U = k(Q1Q2 / d)
(Equation 9.2)
where Q is the charge, d is the distance between the charges, and k is a proportionality
constant.
The strongest intermolecular force is ion-ion attraction. This is the intermolecular force in
ionic compounds (salts). The force is stronger for ions with higher charges (greater Q) or
with smaller distances between ions. The distance between ions depends on the size of the
ions and the arrangement of ions, known as the lattice structure. Substances with ionic
forces of attraction exist in the solid state at room temperature.
>> Explore
: Lattice Energy Tutorial (MUITO INTERESSANTE)
Polar molecules have partial charges and are held to each other by dipole-dipole
intermolecular forces. Because these are only partial charges, unlike the full charges of
ions, these forces are much weaker. Thus substances with dipole-dipole forces of attraction
tend to be gases (occasionally liquids) at room temperature.
When ionic compounds are mixed with polar compounds, they may dissolve. The many
small dipoles of the solvent may be more attractive than the other ions of the salt, creating
ion-dipole forces. Dissolution will only occur if the ion-ion attraction is sufficiently weak and
the ion-dipole forces are sufficiently strong. Because there is both a full and a partial charge
in ion-dipole forces, these forces are stronger than dipole-dipole forces, but not as strong
as ion-ion forces.>
Another special case of dipole-dipole forces is hydrogen bonding. Hydrogen bonds occur
between molecules that have a hydrogen covalently bonded with an oxygen, nitrogen, or
fluorine atom. Oxygen, nitrogen, and fluorine are the most electronegative atoms, which
therefore attract most of the electrons of the covalent bond. When the other end of that
covalent bond is a hydrogen, the electronegative atom is pulling all electrons associated
with hydrogen away from it. Hydrogen acts almost as a positive charge. This high charge
makes hydrogen bonds a very strong type of dipole-dipole force. Substances which
hydrogen bond are typically liquids at room temperature.
>> Explore
: Hydrogen Bonding in Water Tutorial (MUITO INTERESSANTE)
Nonpolar molecules are attracted to each other by temporary dipoles, which are called
London or dispersion forces. Deforming the electron clouds surrounding the molecules
creates temporary dipoles. With more electrons the electron cloud is more easily and more
greatly deformed; thus the dipoles are larger and the attraction greater. Molecules with
easily deformed electron clouds are called polarizable. Molecules with London forces are
normally gases at room temperature.
>> Explore
: Intermolecular Forces Tutorial (MUITO INTERESSANTE)
Forces of attraction can be overcome by increasing temperature and thus the kinetic
energy (movement) of the particles. Consequently, when the temperature is sufficient to
overcome some of the attraction, solids will melt. That temperature is called the melting
point. In the reverse direction, energy is removed so that a liquid becomes a solid. This
temperature is called the freezing point. The value of the melting point and freezing point is
the same. The stronger the force of attraction, the higher the temperature must be to reach
the melting point.
Similarly, kinetic energy allows particles on the surface of a liquid to escape the attractive
forces and become a vapor. In a closed system, particles both escape to form vapor
(gaseous state) and are captured by the liquid. When the number of particles escaping the
liquid and the number of particles being captured by the liquid are the same, the system is
in equilibrium. The pressure of the gaseous particles in such a system is called the vapor
pressure. The vapor pressure depends only on the identity of the substance (attractive
forces) and temperature. Substances with stronger attractive forces allow fewer particles to
escape and thus have lower vapor pressures. An increase in temperature allows more
particles to escape and increases the vapor pressure. When the vapor pressure is the
same as the atmospheric pressure, any molecule can escape the liquid and the liquid boils.
This temperature is called the boiling point.
Forces of attraction determine the solubility of substances. The forces of attraction between
the solute and solvent must be stronger than those between the solute and itself for a
substance to be soluble. This often occurs when the forces of attraction are similar, so
polar substances dissolve in polar substances and nonpolar substances dissolve in
nonpolar substances. In the case of ionic compounds, the solute-solvent forces can
overcome the ion-ion forces with numbers. Since there are usually many more solvent
molecules than solute molecules, the weaker ion-dipole forces can sometimes overcome
the ion-ion forces. When water is the solvent, six waters typically surround each ion. These
water molecules are called the sphere of hydration. When two liquids dissolve in each other
in all proportions, the solutions are called miscible.
In addition to temperature, pressure can change the physical state of a substance. A
sufficient increase in pressure can force a gas to become a liquid and a liquid to become a
solid. The graph of physical states at various pressures and temperatures is called a phase
diagram. The phase diagram is used to determine the physical state of a substance for
specific conditions of pressure and temperature, and how the physical state changes with
pressure and temperature.
>> Explore
: Phase Diagrams Tutorial (MUITO INTERESSANTE)
A typical phase diagram has solids at high pressures and low temperatures, gases at low
pressures and high temperatures, and liquids in the middle. These physical states are
divided with lines. A point on the line dividing liquids and gases is a pressure and
temperature where both liquid and gas exist, and the substance boils. The line between
liquids and solids represents the conditions where substances melt (or freeze). At low
pressures and temperatures, there is a line dividing solids and gases that are the
conditions for sublimation (turning a solid to a gas without passing through the liquid
phase). At the triple point all three lines meet and all three phases exist. Unlike the melting
point line, the boiling point line actually comes to an end at what is known as the critical
point. Above this pressure (critical pressure) and temperature (critical temperature), the
substance exists as a supercritical fluid. A supercritical fluid has most of the properties of a
gas but solvates like a liquid.