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Solids and Liquids Study Guide

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Updated on Sep 25, 2011

Introduction

Unlike gases, solids and liquids are condensed states that contain strong intermolecular forces. These forces result in the properties of solids and liquids.

Intermolecular Forces

Intermolecular forces are the attractive forces that hold molecules and ions together. These forces should not be confused with the intramolecular forces that hold the atoms together in a covalent molecule. Intermolecular forces are grouped into four classifications, each supporting the existence of the condensed states of matter: solids and liquids. In addition, these forces can also explain the nonideal behavior of certain gases.

Ion-ion forces are the strongest of the forces and exist between cations and anions in a crystalline structure. A large amount of thermal energy is needed to break up these ions from their orderly, solid structure to a disorderly, liquid structure.

Dipole-dipole forces are the forces created by the permanent dipole movement of a polar molecule. For example, acetone ((CH3)2C =O) has electronegative oxygen that causes a shift in the electron density toward the oxygen. This distribution of electrons leads to the oxygen having a partial negative charge (δ –) and the adjacent carbon having a partial negative charge (δ –). The charges orient themselves like a magnet, with positive to negative ends.

Solids and Liquids

Ion-dipole forces are a combination of the partial charges of a dipole and the charge of an ion. When table salt (NaCl) dissolves in water, an ion-dipole bond is formed between the sodium and chloride ions and the polar water. Coulomb's Law also explains iondipole forces.

Solids and Liquids

Hydrogen bonding is a special intermolecular force that occurs between a hydrogen atom in a very polar bond (N-H, O-H, F-H) and an electronegative nitrogen, oxygen, or fluorine atom in a molecule. Water's molecules are partially held together by hydrogen bonding.

Solids and Liquids

van der Waals forces, also called dispersion forces, occur when small, temporary dipoles are formed because of the random motion of electrons. Because electrons are not stationary but constantly in motion, they have the probability of not being equidistant from each other, thus having a balanced atom or molecule. These induced dipoles are weak, attractive forces that occur in all types of matter and exist only momentarily before another induced dipole is formed. The strength of van der Waals forces is related to an atom's polarizability (ease of electron movement). Generally, more polarizable atoms are larger (electrons are farther from the nucleus in a larger orbital and can move easier).

Example:

Compounds Intermolecular forces present
Methane (CH4) van der Waals only (nonpolar molecule)
Lithium bromide Ion-ion (ionic compound)
  van der Waals
Methyl alcohol (CH3OH) Dipole-dipole (polar molecule)
  Hydrogen bonding (hydrogen bonded to oxygen)
  van der Waals
Carbon dioxide (CO2) van der Waals only (linear, nonpolar molecule)

Intermolecular forces can also exist between different compounds:

Compounds Intermolecular forces present
HBr and NO3 Dipole-dipole
  Ion-dipole
  van der Waals
NO2 and NH3 Dipole-dipole
  Hydrogen bonding
  van der Waals

Intermolecular forces can also be used to predict the relative boiling and melting point of a compound. When comparing similar compounds, the one with the greater intermolecular forces has a higher boiling and melting point.

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