Intermolecular Forces Forces between molecules Intermolecular Forces (IMFs) пЃ¬ Different molecules have different forces that act between them. пЃ¬ These forces attracting the separate molecules together control many physical properties пЃ¬ пЃ¬ пЃ¬ пЃ¬ пЃ¬ Boiling Point Melting Point Solubility Viscosity Surface Tension Three Main IMFs пЃ¬ Dipole-dipole forces пЃ¬ Hydrogen bonding пЃ¬ London dispersion forces (LDFs) Dipole-Dipole Forces пЃ¬ Electronegativity вЂ“ the ability of an atom in a compound to attract electrons to itself. пЃ¬ Fluorine has the highest electronegativity пЃ¬ The electrons in a compound spend more time around the most electronegative atoms than the other atoms. Electronegativity пЃ¬ 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. пЃ¬ Does not have a full negative charge but a partial negative charge (Оґ - , lower case delta) пЃ¬ Hydrofluoric Acid (HF) The red end (fluorine) has a partial negative charge. Polar Molecules пЃ¬ One end of the molecule is positive while the other end of the molecule is negative. пЃ¬ This difference in charge is called a вЂњdipoleвЂќ What effect does this have? пЃ¬ How does this change the way two molecules interact? пЃ¬ The positive end of the molecule is attracted to the negative end of a different molecule. How to spot dipole-dipole forces? пЃ¬ Look for molecular shapes that have uneven placements of atoms. Bent пЃ¬ Trigonal pyramidal пЃ¬ Anything that has more than one type of atom around the outside пЃ¬ Is Carbon Dioxide Polar? Is carbon tetrafluoride polar? Is Water Polar? Why is Polarity Important пЃ¬ Things that are polar or have charges dissolve in things that are polar. пЃ¬ Things that are nonpolar dissolve in things that are nonpolar. вЂњLike dissolves LikeвЂќ пЃ¬ Opposing types do not dissolve in each other. пЃ¬ A Bio Reminder пЃ¬ Hydrophilic вЂ“ вЂњwater lovingвЂќ вЂ“ polar пЃ¬ Hydrophobic вЂ“ вЂњwater fearingвЂќ - nonpolar Dissolution Process пЃ¬ How are strong ionic bonds broken in water? пЃ¬ Dissolving_NaCl-Electrolyte.exe пЃ¬ The polar nature of water creates attractions between the water and ionic compound. Hydrogen Bonding пЃ¬ Special case of dipole-dipole forces. пЃ¬ The difference in electronegativity between some atoms and hydrogen is so strong that it creates a very strong dipole What elements can do this? пЃ¬ Which elements have the strongest electronegativity? пЃ¬ Anything with an N-H bond, O-H bond, or F-H bond will have hydrogen bonding. Hydrogen Bonding is Very Important It is the reason why ice floats. Hydrogen Bonding is Very Important DNA Base Pairs London Dispersion Forces (LDFs) пЃ¬ пЃ¬ Often called вЂњinduced dipolesвЂќ A momentary change in where the electrons are in one molecule, вЂњinducesвЂќ a dipole in another molecule. LDFs пЃ¬ The more electrons you have in a molecule, the more likely you are to have momentary imbalances in charges. пЃ¬ The more electrons in an atom, the stronger the London Dispersion Forces. пЃ¬ Any molecule can have London Dispersion Forces. LDFs пЃ¬ пЃ¬ пЃ¬ пЃ¬ 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 Crude Oil пЃ¬ Crude oil is refined based on differences in LDFs. пЃ¬ Longer carbon chains have higher boiling points пЃ¬ Have larger number of electrons IMF Comparison пЃ¬ LDFs are the weakest пЃ¬ Dipole-dipole are in the middle пЃ¬ Hydrogen bonding is the strongest. Practice Problems пЃ¬ List all of the intermolecular forces acting on two phosphorus trichloride molecules Figure out the formula пЃ¬ Draw a Lewis structure пЃ¬ Figure out the molecular geometry пЃ¬ Check to see what IMFs it has. пЃ¬ Practice Problem пЃ¬ Explain why ammonia (-33.4ВєC)has a higher boiling point than phosphine, PH3 (-87.8ВєC). Justify your answer. Figure out ammoniaвЂ™s formula пЃ¬ Draw Lewis structures for both compounds пЃ¬ Figure out the molecular geometry for both compounds пЃ¬ Figure out what IMFs each compound has. пЃ¬ Compare the two compounds. пЃ¬ Practice Problems пЃ¬ Explain why ammonia (-33.4ВєC)has a higher boiling point than phosphine, PH3 (-87.8ВєC). Justify your answer. Practice Problem пЃ¬ 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.