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Chem 2 - Analyzing Strengths of Intermolecular Forces III
This document discusses factors that affect the strengths of intermolecular forces between molecules. It identifies the main types of intermolecular forces from strongest to weakest as ion-dipole, hydrogen bonding, dipole-dipole, dipole-induced dipole, and dispersion forces. The strength of intermolecular forces depends on properties of the molecules such as their polarity, ability to form hydrogen bonds, molecular mass, and shape. Physical properties like boiling point, vapor pressure, and melting point increase as the strength of intermolecular forces between molecules increases.
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Chem 2 - Analyzing Strengths of Intermolecular Forces III
- 1. Analyzing Strengths of IntermolecularForces (Lecture Pt. 3) By Dr. Shawn P. Shields This work is licensed by Shawn P. Shields-Maxwell, Ph. D., under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License.
- 2. Factors Affecting Strengthsof IM Forces Strengths of intermolecular forces depend on many factors. When comparing sets of molecules, one needs to analyze the type and relative strengths of IM forces present. So how is this done?
- 3. Types of IntermolecularForces and Relative Strengths Ion-Dipole Hydrogen bonding Dipole-dipole Dipole-induced dipole Dispersion forces (or London forces) weakest strongest
- 4. Factors Affecting Strengthsof IM Forces All matter has electrons, so (London) dispersion forces are present in all molecules (ions, etc.). We also need to determine whether other (stronger) forces are present, as well.
- 5. Determining Relative Strengthsof IM Forces Is the molecule polar or nonpolar? (Does the molecule have a permanent dipole?)
- 6. Determining Relative Strengthsof IM Forces Is the molecule polar or nonpolar? (Does the molecule have a permanent dipole?)
- 7. Identifying Hydrogen Bonding IsH-bonding present? (Is there an H directly bonded to F, N, or O present?)
- 8. Identifying Hydrogen Bonding IsH-bonding present? (Is there an H directly bonded to F, N, or O present?) (Don’t be fooled by H bonded to C with a highly electronegative atom “somewhere”.)
- 9. Relative Strengths ofDispersion Forces Compare the presence and number of heavy atoms in the molecules.
- 10. Relative Strengths ofDispersion Forces Neither molecule has a permanent molecular dipole. Heavier atoms contribute to stronger dispersion forces.
- 11. Effect of MolecularShape Extended versus compact structures
- 12. Effect of MolecularShape Extended versus compact structures Extended molecules have more “opportunities” for interaction.
- 13. Effect of MolecularShape Stronger cumulative interactions for extended (spaghetti-like) structures
- 14. IM Forces andPhysical Properties Boiling point increases with increasing strength of IM forces. Vapor pressure decreases with increasing strength of IM forces. (It is harder for molecules to escape to the gas phase.) Melting point increases with increasing strength of IM forces.
- 15. Physical Properties andIM Forces Which has the higher boiling point? Is H-bonding present?
- 16. Physical Properties andIM Forces Which is predicted to have the higher melting temperature?
- 17. Physical Properties andIM Forces Which is predicted to have the higher melting temperature?
- 18. Physical Properties andIM Forces Which is predicted to have the lower vapor pressure?
- 19. Physical Properties andIM Forces Which is predicted to have the lower vapor pressure?
- 20. Example Problems will beposted separately! (Remember, you must be able to determine if the molecule has a permanent molecular dipole in order to analyze IM forces. Please see my review molecular dipole examples if you need help.)