1 / 25

Properties of Covalent (Molecular) Substances

Properties of Covalent (Molecular) Substances. Properties of Covalent (Molecular) Substances. Poor conductors of heat & electricity in any phase. No charged particles! Low melting & boiling points: easy to pull molecules apart from each other. Majority of solids are soft

edie
Download Presentation

Properties of Covalent (Molecular) Substances

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Properties of Covalent (Molecular) Substances

  2. Properties of Covalent (Molecular) Substances • Poor conductors of heat & electricity in any phase. No charged particles! • Low melting & boiling points: easy to pull molecules apart from each other. • Majority of solids are soft • Low Hf and Hv compared to ionic & metallic substances • High vapor pressure compared to ionic & metallic substances

  3. Properties • Depend on strength of forces between “particles” or separate units. • In covalent substances, the units are molecules.

  4. Intermolecular Forces • Forces between molecules = intermolecular forces • 3 kinds: What are they?

  5. Intermolecular Forces • Dispersion forces occur between nonpolar molecules. (Van der Waals) • Dipole-dipole forces occur between polar molecules. • Hydrogen bonding occurs between molecules with an H-F, H-O, or H-N bond. Memory Jogger Intermolecular forces determine phase!

  6. Weakest intermolecular force = dispersion force. Occurs between nonpolar molecules. 1) Monatomic molecules: He, Ne, Ar, Kr, etc. • Diatomics with 2 atoms of the same element. 3) Very symmetric molecules. Dispersion forces are stronger for larger molecules.

  7. Dipole-dipole forces occur between molecules with a permanent charge separation, i.e., polar molecules. HCl and HBr are examples of polar diatomic molecules.

  8. Hydrogen bonding influences properties!

  9. Molecular Polarity • For diatomics, the molecular polarity is the same as the bond polarity. • For molecules with 3 or more atoms, the polarity depends on the types of bonds and the shape of the molecule!

  10. Molecular Polarity • Depends on 2 factors. • Type of bonds in the molecule • Arrangement of bonds or shape of molecule

  11. For larger molecules, look at the kind & arrangement of bonds to determine overall polarity of molecule.

  12. Hydrogen bonding occurs between molecules containing an H-F, H-O, or H-N bond. It is the strongest intermolecular force.

  13. Properties of Covalent (Molecular) Substances • Poor conductors of heat & electricity in any phase. No charged particles! • Low melting & boiling points: easy to pull molecules apart from each other. • Majority of solids are soft • Low Hf and Hv compared to ionic & metallic substances • High vapor pressure compared to ionic & metallic substances

  14. Melting pt, Boiling pt, Hf and Hv and vapor pressure depend on how hard it is to pull the particles apart. Weak intermolecular forces – it’s easy to pull them apart. Strong intermolecular forces – it’s hard.

  15. Which substance has the strongest intermolecular forces? The weakest? Water Ether

  16. Polarity of Molecules • A molecule may contain polar bonds, but not be polar! Depends on the geometry of the molecule. • If molecule is symmetric, the “pull” of one polar bond is offset by the “pull” of another polar bond. • It’s a tug-of-war that no one can win!

  17. Symmetric Molecules • Contain at least one mirror plane.

  18. Symmetric vs. Asymmetric • CO2 is nonpolar. • Can’t tell the ends apart. • H2O is bent. • Electron cloud is lopsided. • H2O is polar.

  19. Polarity of CO2? Draw an arrow along each bond pointing to the more electronegative atom. If the arrows cancel out, the molecule is NONPOLAR.

  20. Polarity of H2O? The green arrows do not cancel out, so water is polar!

  21. Symmetry of Larger Molecules CF4 is fairly symmetric. Overall, it is nonpolar. The arrows cancel out.

  22. Use the structural formula to predict Molecular Polarity! H H  C  H H  

  23. Ethane = C2H6 Ethene = C2H4 Ethyne = C2H2 These molecules are symmetric and the electron cloud is the same on both ends. Overall, they are nonpolar.

  24. Molecular Polarity • If you know the shape, you can use the arrow technique to determine the polarity. • So how do you get the shape?

More Related