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BU CHEM 108 - Intermolecular Forces

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CHEM 108 1st Edition Lecture 2Intermolecular ForcesIntermolecular attractive forces affect physical properties of substances:1. Ion-ion interactions:Strongest kind of intermolecular force, this is because the particles in an ionic compound have opposite charges so their force of attraction is much greater. These forces are electrostatic, and are described by coulomb’s law E= (Q1Q2)/d• Energy (E) of interaction is directly proportional to the charges of the ions (Q1,Q2), andinversely proportional to the distance (d) between them. Attractive force will increase as charge on ion increases, and decrease as ionic radius increases.2. Ion-dipole interactions:Interaction between an ion and the partial charge of a molecule with a permanent dipole. These interactions cause molecules to dissolve in water. The sphere of hydration is when the polar molecules of water surround the ions, this is what makes the compound dissociate into ions.3. Dipole-dipole interactions:Interaction of two molecules that have permanent dipole moment, i.e. polar molecules. Permanent dipole moments happen due to the difference in electronegativities in two atoms in a compound. For example in H2O oxygen is highly electronegative so the electrons spend more time in the oxygen’s orbitals making it slightly negative and making hydrogen slightly positive. Occurs only when polar molecules interact with polar molecules.4. Hydrogen bonds Special class of dipole-dipole interactions due to strength, and it requires H atom covalently bonded to strongly electronegative atom.Example: F, O, NThese are the strongest dipole-dipole interactions and are the reason why water has remarkable characteristics. 5. Dispersion forces (London forces)These forces are attractive forces between non-polar molecules, it works because of the instantaneous dipole on one molecule which induces dipole on another molecule. The instantaneous dipole is caused due to momentary changes in the electron distribution in an atom. These forces exist in all molecules, regardless of polarity.Factors affecting strength of dispersion:a. Size of Atoms/Molecules: » Larger atoms/molecules more polarizable than small atoms/molecules. » Dispersion increases with polarizability. b. Shape of Molecules: » Increased surface area = increased interactions between molecules. » Linear molecules have higher dispersion than branched molecules ofsimilar molecular weight.6. Dipole-induced dipole interactionsInduced dipole is when a polar molecule repels or attracts the electrons of another atom inducing a temporary dipole in that atom. Occurs when polar molecules interact with non-polar molecules.7. Van der Waals Forces include: dipole-dipole, London dispersion, and hydrogen bonding forces- Polarizability- the ease with which the electron cloud in an atom can be distorted, inducing a temporary dipole. o Larger atoms have a higher Polarizability due to them having more electrons and those electrons being farther from the nucleus. Magnitude of Intermolecular forces:The strength of the intermolecular forces are in order of 1 being the strongest to 7 being the weakest.1. Ionic Bond: 500-3000 kj/mol2. Covalent Bond: 150-1000 kj/mol3. Ion-dipole: 40-600 kj/mol4. Hydrogen Bond: 4-50 kj/mol5. Dipole-Dipole: 5-20 kj/mol6. Dipole- induced dipole: 1-20 kj/mol7. London Dispersion: 0.1-40 kj/molAn increase in intermolecular force means:• increase boiling point • decrease vapor pressure • increase melting point • increase surface tension • increase


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