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Intermolecular Forces

This article discusses the different types of intermolecular forces (IMFs) and their role in controlling physical properties such as boiling point, melting point, solubility, viscosity, and surface tension. The three main IMFs - dipole-dipole forces, hydrogen bonding, and London dispersion forces - are explained in detail. The importance of polarity and hydrogen bonding in dissolving substances is highlighted, along with the concept of "like dissolves like." Additionally, the article covers the phenomenon of hydrogen bonding and its significance in various substances, such as water and DNA base pairs. The role of London dispersion forces in determining boiling points and the refining of crude oil is also discussed. Practice problems are included to test understanding.

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Intermolecular Forces

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  1. Intermolecular Forces Forces between molecules

  2. Intermolecular Forces (IMFs) • Different molecules have different forces that act between them. • These forces attracting the separate molecules together control many physical properties • Boiling Point • Melting Point • Solubility • Viscosity • Surface Tension

  3. Three Main IMFs • Dipole-dipole forces • Hydrogen bonding • London dispersion forces (LDFs)

  4. Dipole-Dipole Forces • Electronegativity – the ability of an atom in a compound to attract electrons to itself. • Fluorine has the highest electronegativity • The electrons in a compound spend more time around the most electronegative atoms than the other atoms.

  5. Electronegativity • If the electrons are spending more time around the one particular atom, how will that atom be different from the other atoms? • It will have a more negative charge than the other atoms in the compound. • Does not have a full negative charge but a partial negative charge (δ - , lower case delta)

  6. Hydrofluoric Acid (HF) The red end (fluorine) has a partial negative charge.

  7. Polar Molecules • One end of the molecule is positive while the other end of the molecule is negative. • This difference in charge is called a “dipole”

  8. What effect does this have? • How does this change the way two molecules interact? • The positive end of the molecule is attracted to the negative end of a different molecule.

  9. How to spot dipole-dipole forces? • Look for molecular shapes that have uneven placements of atoms. • Bent • Trigonal pyramidal • Anything that has more than one type of atom around the outside

  10. Is Carbon Dioxide Polar?

  11. Is carbon tetrafluoride polar?

  12. Is Water Polar?

  13. Why is Polarity Important • Things that are polar or have charges dissolve in things that are polar. • Things that are nonpolar dissolve in things that are nonpolar. • “Like dissolves Like” • Opposing types do not dissolve in each other.

  14. A Bio Reminder • Hydrophilic – “water loving” – polar • Hydrophobic – “water fearing” - nonpolar

  15. Dissolution Process • How are strong ionic bonds broken in water? • Dissolving_NaCl-Electrolyte.exe • The polar nature of water creates attractions between the water and ionic compound.

  16. Hydrogen Bonding • Special case of dipole-dipole forces. • The difference in electronegativity between some atoms and hydrogen is so strong that it creates a very strong dipole

  17. What elements can do this? • Which elements have the strongest electronegativity? • Anything with an N-H bond, O-H bond, or F-H bond will have hydrogen bonding.

  18. Hydrogen Bonding is Very Important It is the reason why ice floats.

  19. Hydrogen Bonding is Very Important

  20. DNA Base Pairs

  21. London Dispersion Forces (LDFs) • Often called “induced dipoles” • A momentary change in where the electrons are in one molecule, “induces” a dipole in another molecule.

  22. LDFs • The more electrons you have in a molecule, the more likely you are to have momentary imbalances in charges. • The more electrons in an atom, the stronger the London Dispersion Forces. • Any molecule can have London Dispersion Forces.

  23. LDFs • This explains why the boiling point goes up as you move down a column. • Hydrogen telluride has more electrons than hydrogen sulfide • Hydrogen telluride has stronger LDFs • Hydrogen telluride has a higher boiling point

  24. Crude Oil • Crude oil is refined based on differences in LDFs. • Longer carbon chains have higher boiling points • Have larger number of electrons

  25. IMF Comparison • LDFs are the weakest • Dipole-dipole are in the middle • Hydrogen bonding is the strongest.

  26. Practice Problems • List all of the intermolecular forces acting on two phosphorus trichloride molecules • Figure out the formula • Draw a Lewis structure • Figure out the molecular geometry • Check to see what IMFs it has.

  27. Practice Problem • Explain why ammonia (-33.4ºC)has a higher boiling point than phosphine, PH3 (-87.8ºC). Justify your answer. • Figure out ammonia’s formula • Draw Lewis structures for both compounds • Figure out the molecular geometry for both compounds • Figure out what IMFs each compound has. • Compare the two compounds.

  28. Practice Problems • Explain why ammonia (-33.4ºC)has a higher boiling point than phosphine, PH3 (-87.8ºC). Justify your answer.

  29. Practice Problem • Hexane (C6H14) is a liquid at room temperature. Its Lewis structure has each carbon connected to another in a long chain. Will sodium chloride dissolve in hexane? Justify your answer.

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