c13 - Types of Intermolecular Forces States of Matter and...

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Types of Intermolecular Forces States of Matter and KMT In liquids and solids particles are close enough to have attractive forces present. In molecular solids and liquids the forces between molecules, the intermolecular forces are based on various electrostatic attractions that are weaker than the forces between oppositely charged ions Coulombs law: F = q 1 q 2 / r 2
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Intermolecular Forces influence things like: Melting point (m.p.) and boiling point (b.p.) Solubility of gases, liquids and solids in solvents Structure determination as in DNA and proteins
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Types of Intermolecular Forces First a short review: Bonding or intramolecular forces arise form electrostatic attractions between opposite charges. Ionic Cation-anion 400-4000 kJ/mol ex. NaCl Covalent Nuclei-shared e- pair 150-1100 kJ/mol ex.H – H Metallic Cations delocalized e- 75-1000 kJ/ mol ex. Fe Easy to comprehend and understand using Coulombs law: F is proportional to q1 * q2 / r 2 (I will refer to this frequently, be familiar with what the equation means.)
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Intermolecular forces are due to attraction between molecules because of partial charges or attraction of ions and molecules. These two types of forces differ in magnitude, here is why: Back to Coulomb’s law: F is proportional to q1 * q2 / r 2 Bonding forces are relatively strong because of larger charges and shorter distances Intermolecular forces are relatively weaker because they typically involve smaller charges and are further apart.
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How far apart are they? Using Cl 2 as an example Distances b/t nuclei in solid Cl 2 we get two different values Cl – Cl bonded is the shorter and called bond length and half of it is the covalent radius The longer distance b/t two nonbonded Cl’s is the van der Waal distance, half of this is the VDW radius. And is always larger than the covalent radius. VDW radius is half the closest two molecules that are nonbonding can get.
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There are several types of Intermolecular Forces, we will look at 6 different types Ion-Dipole Forces Occurs when an ion and a polar molecule (most often water), a dipole, attract each other. Most important example is when an ionic compound is dissolved in water, NaCl, ions separated b/c of attractions of ions with polar ends of the water molecule. This explains why a solid that melts at 801 C dissolves in room temperature water. The enthalpy of hydration for Na+ is - 405 kJ/mol Cl - is -363 kJ/mol and is just enough to overcome the lattice energy (NaCl lattice energy is about 787 kJ/mol) that holds the ionic compound together.
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Which should have the more negative hydration energy, F - or Cl - ? Why? F - is smaller and water can get closer and thus hold tighter 1. Place the following cations in order from the lowest to the highest hydration energy. a) K
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This note was uploaded on 02/10/2011 for the course CHM 2046 taught by Professor Veige/martin during the Spring '07 term at University of Florida.

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c13 - Types of Intermolecular Forces States of Matter and...

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