chem notes week one pt 1.
chem notes week one pt 1. Che 212
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This 23 page Class Notes was uploaded by Darlene Franklin on Friday January 29, 2016. The Class Notes belongs to Che 212 at State University of New York at Oswego taught by Stephany Rugg in Spring 2016. Since its upload, it has received 55 views. For similar materials see General Chem II in Chemistry at State University of New York at Oswego.
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Date Created: 01/29/16
H H Chapter 12.3 H H Intermolecular Forces © 2015 Pearson Education, Inc. Attractive Forces • The particles are attracted to each other by Intermolecular forces. • The strength of the attractive forces depends on the kind(s) of particles. – The stronger the attractive forces between the particles, the more the particles resist Moving. (0 kelvin) • However, no material completely lacks particle motion. © 2015 Pearson Education, Inc. Intermolecular Forces • The strength of the attractions between the particles of a substance determines its State (solid,liquid,and gas) At room temperature, moderate to strong attractive forces result in materials being Solids or Liquids The stronger the attractive forces are, the Higher will be the boiling point of the liquid and the melting point of the solid. © 2015 Pearson Education, Inc. Why Are Molecules Attracted to Each Other? • Intermolecular attractions are due to attractive forces between opposite charges. – (+) ion to (−) ion – (+) end of polar molecule to (−) end of polar molecule – Even nonpolar molecules will have Temporary charges. • Larger charge = stronger attraction • Longer distance = weaker attraction • Intermolecular attractive forces are small relative to the bonding forces between atoms (intramolecular). – smaller charges – over much larger distances © 2015 Pearson Education, Inc. 4 Trends in the Strength of Intermolecular Attraction • The stronger the attractions between the atoms or molecules, the more it will take to separate them. • Boiling a liquid requires adding enough energy to overcome all the attractions between the particles. – But, it does break the bonds. • The higher the normal boiling point of the liquid, the the intermolecular attractive forces. © 2015 Pearson Education, Inc. Types of Intermolecular (Attractive) Forces 1. Temporary polarity in the molecules due to unequal electron distribution leads to attractions called Dispersion Forces 2. Permanent polarity in the molecules due to their structure leads to attractive forces called Dipole-dipole An especially strong dipole– dipole attraction results when H is attached to an extremely Electronegative atom (O, N, F). These are called Hydrogen Bonds (h-bonds) © 2015 Pearson Education, Inc. 1. Dispersion Forces δ- δ+ • Temporary dipole: partial (–) and partial (+) charges • The attractive forces caused by these temporary dipoles are called Dispersion • They are also known as London or Van der Waals forces. – As a temporary dipole is established in one molecule, it induces a dipole in all the Surrounding molecules. • All molecules and atoms will have dispersion forces. He 2e 2n 2p © 2015 Pearson Education, Inc. Effect of Molecular Size on Size of Dispersion Force • The noble gases are all Non-polar atomic elements. • As MW increases, the # of electrons increases. Therefore, the strength of the dispersion forces Increase. • The stronger the attractive forces between the molecules, the higher the boiling point will be. © 2015 Pearson Education, Inc. 8 2. Dipole–Dipole Attractions • Polar molecules have a Permanent dipole. – Bond polarity and molecular geometry (shape) determine the substances polarity – Dipole moment © 2015 Pearson Education, Inc. 9 Clicker: Which of the atoms in the following molecule is the most electronegative? a) Hydrogen b) Carbon c) Oxygen d) All have the same electronegativity © 2015 Pearson Education, Inc. Effect of Dipole–Dipole Attraction on Boiling and Melting Points © 2015 Pearson Education, Inc. 11 Dipole Moment and Boiling Point Relationship © 2015 Pearson Education, Inc. 12 Attractive Forces and Solubility • Solubility depends on the Attractive forces of the solute and solvent molecules. Ethanol water – Like dissolves like. – Miscible liquids will always dissolve in each other. • Polar substances dissolve in Polar solvents. • Nonpolar molecules dissolve in non-polar solvents. • Many molecules have both hydrophilic and hydrophobic parts. – Soap Hydrophobic Hydrophilic © 2015 Pearson Education, Inc. Immiscible Liquids • Pentane, C H , is a non-polar molecule 5 12 • Water is a polar molecule. • The attractive forces between the water molecules are much stronger than the attractions of the water molecules for the pentane molecules. • The result is that the liquids are Immiscible. Grease water © 2015 Pearson Education, Inc. Do the following molecules have dispersion forces of dipole- dipole forces? CCl 4 no CH C3 polar © 2015 Pearson Education, Inc. Hydrogen Bonding: A Dipole–Dipole Interaction • When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it. O,N,F – Ex. O H O • Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes exposed. • The exposed nucleus (proton) acts as a very strong center of Postive charge, attracting all the electron clouds from neighboring molecules. © 2015 Pearson Education, Inc. Hydrogen Bonding in Water and Ethanol © 2015 Pearson Education, Inc. 17 More on Hydrogen Bonding (H Bonds) • Hydrogen bonds are very strong intermolecular attractive forces. – Stronger than dipole–dipole or dispersion forces • But hydrogen bonds are not nearly as strong as chemical bonds. – 2–5% the strength of covalent bonds H bonding is strong © 2015 Pearson Education, Inc. In each pair, which compound has a higher boiling point? Higher HF vs. HCl Molecular weight 19.5 C -85 C © 2015 Pearson Education, Inc. Ion–Dipole Attraction • In a mixture, ions from an ionic compound are attracted to the Dipole of polar molecules. • The strength of the ion–dipole attraction is one of the main factors that determine the solubility of ionic compounds in water. © 2015 Pearson Education, Inc. 20 Size of the Induced Dipole The magnitude of the induced dipole depends on several factors. • Polarizability of the electrons – Volume of the electron cloud Mw= e cloud= Polarirability= Attractions • Shape of the molecule © 2015 Pearson Education, Inc. Summary on Intermolecular Forces • Dispersion forces have the following characteristics: – They are the weakest of the intermolecular attractions. – They are present in all molecules and atoms. – Their magnitude increases with molar mass. • Polar molecules – have dipole–dipole attractive forces. • Hydrogen bonds are – a type of dipole–dipole interaction; – the strongest of the intermolecular attractive forces a pure substance can have; – present when a molecule has H directly bonded to either O, N, or F atoms. • The only example of H bonded to F is HF. • Ion–dipole attractions are – present in mixtures of ionic compounds with polar molecules; – the strong intermolecular attractions; – especially important in aqueous solutions of ionic compounds. © 2015 Pearson Education, Inc. 22 © 2015 Pearson Education, Inc. 23