Intermolecular Forces

Topics: Chemical bonding, Intermolecular force, Covalent bond Pages: 29 (8826 words) Published: November 9, 2011
Oakland Schools Chemistry Resource Unit

Intermolecular Forces 
Brook R. Kirouac  David A. Consiglio, Jr.  Southfield‐Lathrup High School Southfield Public Schools   

Bonding: Intermolecular Forces
Content Statements:

C2.2: Chemical Potential Energy Potential energy is stored whenever work must be done to change the distance between two objects. The attraction between the two objects may be gravitational, electrostatic, magnetic, or strong force. Chemical potential energy is the result of electrostatic attractions between atoms. C3.3: Heating Impacts Heating increases the kinetic (translational, rotational, and vibrational) energy of the atoms composing elements and the molecules or ions composing compounds. As the kinetic (translational) energy of the atoms, molecules, or ions increases, the temperature of the matter increases. Heating a sample of a crystalline solid increases the kinetic (vibrational) energy of the atoms, molecules, or ions. When the kinetic (vibrational) energy becomes great enough, the crystalline structure breaks down, and the solid melts. C4.3: Properties of Substances Differences in the physical and chemical properties of substances are explained by the arrangement of the atoms, ions, or molecules of the substances and by the strength of the forces of attraction between the atoms, ions, or molecules. C4.4: Molecular Polarity The forces between molecules depend on the net polarity of the molecule as determined by shape of the molecule and the polarity of the bonds. C5.4: Phase/Change Diagrams Changes of state require a transfer of energy. Water has unusually high-energy changes associated with its changes of state.

Content Expectations: C2.1c: Compare qualitatively the energy changes associated with melting various types of solids in terms of the types of forces between the particles in the solid. C3.3B: Describe melting on a molecular level. C4.3A: Recognize that substances that are solid at room temperature have stronger attractive forces than liquids at room temperature, which have stronger attractive forces than gases at room temperature. C4.3c: Compare the relative strengths of forces between molecules based on the melting point and boiling point of the substances. C4.3d: Compare the strength of the forces of attraction between molecules of different elements. (For example, at room temperature, chlorine is a gas and iodine is a solid.) C4.3f: Identify the elements necessary for hydrogen bonding (N, O, and F). C4.3g: Given the structural formula of a compound, indicate all the intermolecular forces present (dispersion, dipolar, hydrogen bonding). C4.4a: Explain why at room temperature different compounds can exist in different phases. C5.4c: Explain why both the melting point and boiling points for water are significantly higher than other small molecules of comparable mass (e.g., ammonia and methane)

Instructional Background Information: Melting on a Molecular Level: Melting involves the disruption of the crystal lattice of a solid via the absorption of kinetic energy by the molecules in the lattice from their surroundings. As the forces holding the lattice together increase in strength so does the melting point of the solid. Strong Forces: Ionic bonding, metallic bonding, and network-covalent bonding. Strong intermolecular forces result in room temperature solids with high melting and boiling points. Metallic and Network Bonding is difficult to quantify. Ionic Bonding: Ionic bonds are the result of electrostatic attraction between positive and negative ions. Ionic bonding is directly proportional to ionic charge and inversely proportional to ionic size. Weak Forces: Hydrogen bonding, dipole-dipole interactions, London dispersion forces. Hydrogen Bonding: Hydrogen bonding is the unusually strong dipole-dipole interaction that occurs when a highly electronegative atom (N, O, or F) is bonded to a hydrogen atom. This bond nearly strips the hydrogen atom of its electrons leaving,...
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