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Gail Amort- Larson

Cornered By Mike Baldwin Read sections 12.3, 12.4, 12.5 Practice PS #5 is posted 1 2 Last time Ion – ion attraction Intermolecular forces (IMF) Ion is attracted by several other ionswith opposite charge Forces that hold molecules together in the condensed phases Usually strong forces Typically 400 – 4000 kJ/mol Ionic attractions i–n 4–0kJ/mol I–dipole 6kJ/mol Electrostatic attraction (opposite charges) Dipoaletractions F = – k q1q 2 (F is positive b/c q is negative) r2 2 Dipole–dipole attractions 5 – 40 kJ/mol k = constant Hydroge bonding 4–J/mol q , q charges separated by a distance r. 1 2 London dispersion forces (LDF) 0.05 – 40 kJ/mol Stronger force more energy to break intermolecular forces holding molecules together  highe mrelting point (boiling point) 3 4 Which has a higher mp, NaCl or MgO? Ionic radii effect based on ionic radii trends (unit 1) Look at the relative forces for the 2 molecules r = cation ranion q1q 2 r + (99 pm) > r 2+(72 pm) F = – k 2 Na Mg r rCl–(181 pm) > r O2– (140 pm) Relative Charge effect + – You are expected to know the trends, not actual NaCl Na + Cl |1 (–1)| = 1 values for the radii. MgO Mg +2+ O –2 |2 (–2)| = 4 So NaCl> rMgO Ions in MgO have charges of 2 whereas ions in NaCl have charges of 1. So r is smaller in MgO leading to a stronger forces. F > F MgO NaCl FMgO > F NaCl Stronger forces in MgO Overall, both effects contribute to stronger forces in MgO  expect higher mp Experimental mp MgO (2852C) > NaCl (801C) 5 6 Ion – dipole attraction Arrange Na , Cs and Mg in order of solvation energy Attraction between the charge of the ion and an opposite dipole Charge effect greater charge  s
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