CHEM 1412 Concept Reviews: Liquids, Solids, & Intermolecular Forces

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Intermolecular Forces:  The forces that exist between molecules (or atoms) which are largely responsible for many physical properties and how molecules arrange themselves in the structure of a substance.  The three main types of intermolecular forces are Hydrogen Bonding, Dipole-Dipole Forces, and London Dispersion Forces.

 

London Dispersion Forces:  The forces between molecules which are caused by instantaneous (temporary) dipoles caused by the random motion of electrons in a molecule and/or induced dipoles due to the collision of molecules and even the electrostatic forces between molecules that don’t necessarily collide but come close to one another.  The strength of London dispersion forces increases with increased polarizability.  In general, the larger and/or longer the molecule (or atom), the more polarizable it is.

In this picture, the temporary dipoles of two atoms create an attraction between the atoms,

causing the two atoms to “stick” together.

 

 

Dipole-Dipole Forces:  The intermolecular forces due to the electrostatic attraction between the partially positive end of one polar molecule (permanent dipole) and the partially negative end of a neighboring polar molecule.  Since the dipoles in polar molecules are permanent rather than instantaneous and temporary, the intermolecular forces in these molecules tend to be significantly stronger than those in molecules of comparable size that only have London dispersion forces.

Hydrogen Bonding:  A powerful intermolecular force which is due to the uniquely strong attractive force that occurs between a hydrogen atom that is bonded to a highly electronegative atom (N, O, or F) and a lone pair of electrons on a highly electronegative atom from a different molecule (or at least from a different place on the same molecule).

Hydrogen Bonding in Water

Ion-Dipole Forces:  The strongest of the intermolecular forces which occurs when an ionic compound is mixed with a polar compound (particularly one with hydrogen bonding). 

 

 

Summary of Intermolecular Forces

Type of Intermolecular Force	Present in…	Strength of this Type of Intermolecular Force
London Dispersion Forces	ALL molecules and atoms	Depends on the size and polarizability of the atom or molecule.  These forces can be VERY significant for larger molecules.
Dipole-Dipole Forces	polar molecules	Depends on the magnitude of the dipole moment (i.e. how large the partial positive and negative are).  These forces are generally stronger than dispersion forces.
Hydrogen Bonding	molecules with H-N, H-O, or H-F bonds	VERY strong intermolecular force.  It is essentially an extreme form of dipole-dipole force.
Ion-Dipole Forces	solutions where an ionic compound is dissolved in a polar protic solvent	The STRONGEST intermolecular force which can be more than twice as strong as hydrogen bonding.

 

 

Properties Affected by Intermolecular Forces

Viscosity:  The resistance of a liquid to flow.  The greater the viscosity, the slower the liquid flows.  The two main factors that affect viscosity are the strength of intermolecular forces and the shapes of molecules.  The stronger the intermolecular forces, the higher the viscosity.  The longer the molecules, the more “entangled” they become and therefore the higher the viscosity.

 

Surface Tension:  the energy required to increase the surface area of a liquid.  The stronger the intermolecular forces, the higher the surface tension.

 

Specific Heat:  The amount of energy required to raise the temperature of 1 gram of a substance 1˚C.  Stronger intermolecular forces cause specific heat to increase since the molecules require more energy to break free from the intermolecular forces which prevent them from moving faster and attaining a higher temperature.

 

Boiling Point:  The temperature at which a substance transitions from the liquid state to the gaseous state.  To accomplish this, the intermolecular forces of the molecules in the liquid state must be overcome by the kinetic energy of the particles.  The stronger the intermolecular forces, the more difficult it is to overcome those forces, and therefore the higher the temperature (i.e.-average kinetic energy) that is needed to bring about the phase change.  Boiling point is also impacted heavily by pressure with lower boiling points corresponding to lower pressures.

 

Vapor Pressure:  For any liquid, the highest energy molecules escape into the gas phase by evaporation.  These molecules can later condense and move back into the liquid phase.  Eventually these two processes reach dynamic equilibrium, which is the state at which the two opposing processes are occurring at equal rates such that there is no net change to the system.  The pressure exerted by the vapor over a liquid in dynamic equilibrium is called vapor pressure.  The stronger the intermolecular forces, the more difficult it is for molecules to escape into the gaseous state and the easier it is for the gaseous molecules to condense, therefore, the stronger the intermolecular forces in a liquid, the lower the vapor pressure will be.

 

 

Adhesion vs. Cohesion

Adhesive Forces:  Forces of attraction between molecules of one substance and molecules of another.

Cohesive Forces:  Forces of attraction between molecules of the same substance.

Capillary Action: the ability of a liquid to move against gravity up a narrow tube.  For this phenomenon to occur, the surface material of the tube must have stronger adhesive forces to the liquid molecules than the cohesive forces the liquid molecules have holding each other together.

 

 

Principles Regarding Vapor Pressure

Heat (Enthalpy) of Vaporization (ΔH_vap):  the amount of heat required to vaporize one mole of a liquid to gas.

Dynamic Equilibrium: Equilibrium achieved when both the forward and reverse rates of a process are occurring at equal speeds, resulting in a balanced state.

Le Chatlier’s Principle:  When a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturbance and return to a state of equilibrium.

Boiling Point:  The point at which the vapor pressure is equal to external pressure.  Normal boiling point refers to the boiling point when the external pressure is exactly 1 atm.

 

 

The Clausius-Clapeyron Equation

Linear Form	Two-Point Form
$\ln P_{vap}=-\frac{\Delta H_{vap}}{R}\left(\frac{1}{T}\right)+\ln B$	$\ln\frac{P_2}{P_1}=\frac{\Delta H_{vap}}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right)$
In the linear form Pvap is the vapor pressure, ΔHvap is the heat of vaporization, T is the temperature, R is the gas constant (8.314 J/mol*K), and β is the Clausius-Clapeyron equation constant.	In the two-point form, P1 represents a vapor pressure that occurs at temperature T1.  P2 represents the vapor pressure at temperature T2.  ΔHvap is the heat of vaporization and R is the gas constant (8.314 J/mol*K)

 

Heating Curve		Phase Change Diagram
(Heat can either increase temperature or change the phase but not both at the same time.)		Triple Point:  where all 3 phases are at equilibrium. Critical Point:  Temp. beyond which there is no liquid phase.

 

Fusion:  another name for melting, which is the transition of a substance from its solid state to its liquid state.

 

 

Crystalline Solids: Unit Cells and Basic Structures

Cubic Cell Name	Simple Cubic	Body-centered Cubic	Face-Centered Cubic
Structure of Unit Cell
Number of Atoms in a Unit Cell	$\left(\frac{1}{8}\times8\right)=1$	$\left(\frac{1}{8}\times8\right)+1=2$	$\left(\frac{1}{8}\times8\right)+\left(\frac{1}{2}\times6\right)=4$
Formula for the Length of a Side of the Unit Cell (in terms of “r”, the radius of the atom)	$l=2r$	$l=\frac{4r}{\sqrt{3}}$	$l=\frac{4r}{\sqrt{2}}$
Volume of the Unit Cell	$V=l^3=\left(2r\right)^3$	$V=l^3=\left(\frac{4r}{\sqrt{3}}\right)^3$	$V=l^3=\left(\frac{4r}{\sqrt{2}}\right)^3$
Coordination Number	6	8	12
Packing Efficiency (fraction of volume occupied)	52%	68%	74%

Coordination Number:  the number of atoms with which each atom in the structure is in direct contact.

 

 

Types and Properties of Crystalline Solids