Forces Between Molecules
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Transcript of Forces Between Molecules
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Forces Between Molecules
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Bonding model for covalent molecular substances
• Bonding for covalent molecular substances falls into two categories
1. The strong forces of attraction which holds atoms together within molecules
2. The weak forces of attraction between molecules
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Forces between molecules (intermolecular forces)
• So far we have spoken about the forces or bonds between atoms e.g. covalent, ionic and metallic
• Now we will learn about the forces between molecules or compounds are called intermolecular forces
• Inter means between or among
• Internet, interstate, international
• What would Interstellar travel be?
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Intramolecular forces• What would intramolecular forces be?
• Forces within molecules e.g covalent, metallic or ionic
• intra means within
• Intrastate, intranet, intracellular
• Most of the intermolecular forces we look at occur between covalently bonded molecules or covalent molecular substances
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Overview
• All matter is held together by force. • The force between atoms within a
molecule is a chemical or intramolecular force.
• The force between molecules are a physical or intermolecular force.
• These physical forces are what we overcome when a chemical changes its state (e.g. gas liquid).
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What causes intermolecular forces?• Molecules are made up of charged particles:
positive nuclei and negative electrons.
• When one molecule approaches another there is a multitude of forces between the particles in the two molecules.
• Each electron in one molecule is attracted to the nuclei in the other molecule but also repelled by the electrons
in the other molecule.
• The same applies for nuclei
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Types of Intermolecular forces
• The three main types of intermolecular forces are:1. Dipole-dipole attraction occur only btw
polar molecules
2. H bonding – only with Hydrogen and Oxygen, Fluorine and Nitrogen)
3. Dispersion forces
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1. Dipole Dipole attraction or Dipole forces
• Dipole forces occur between two polar molecules
• The dipole force is the electrostatic attraction between the partial positive on one hydrogen on one molecule and the partial negative charge on the sulfur of another molecule.
• One type of intermolecular force is dipole dipole
H Cl
+ –+ –
+ –+ –
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• Draw the molecule and determine its shape using VSPER
• Determine polarity within molecule
• Determine how the molecules would interact
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2. Hydrogen bonding
• Hydrogen bonds are a special case of dipole forces
• Hydrogen bonding occurs between molecules in which hydrogen is bonded to nitrogen, oxygen or fluorine
• It occurs because N, F and O are very electronegative
• This makes the molecule formed very polarised
• The partial positive charge on hydrogen and the lone pair of electrons on N,F and O
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• Hydrogen bonds are stronger than dipole dipole attractions
• We know this because of the difference in melting points of molecules with dipole dipole attractions and hydrogen bonding
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3. Dispersion forces • Dispersion forces, or London forces, are the
weakest of the intermolecular forces.
• Dispersion forces occur between polar and non-polar molecules. They are generally weak and only considered in the absence of stronger intermolecular forces.
• The greater the number of electrons within a molecule, the more significant the dispersion forces btw the molecule.
• They occur between non-polar molecules.
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• Essentially they are the same as dipole forces- an electrostatic attraction between a partial positive charge and a partial negative charge.
• However, the partial charges are more rare in nonpolar molecules. It is perhaps better to think of these interactions as those between a "temporary dipole" and an "induced dipole".
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Stages for Dispersion Forces
1. There are two molecules
2. One molecule has a temporary dipole
3. The other molecule has an induced dipole
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• http://www.youtube.com/watch?v=IgcGuEwHHKY&feature=related
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Strength of intermolecular forces
1. Hydrogen bonds are the strongest intermolecular force
2. Dipole Dipole Interactions
3. Disperson Forces are the weakest intermolecular force
dispersion forces < dipole-dipole interactions < hydrogen bonds
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How do intermolecular forces affect melting point?
• When heat is applied to a solid, the molecules begin to vibrate more and more. The energy possessed by the molecules increases to the point that some intermolecular attractions are overcome.
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How to measure the strength of a chemical bond?
• One measure of the strength of a chemical bond is the energy required to break one of those bonds.
• http://www.youtube.com/watch?v=IgcGuEwHHKY
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Overview• Intermolecular forces (dispersion forces, dipole-
dipole interactions and hydrogen bonds) are much weaker than intramolecular forces (covalent bonds, ionic bonds or metallic bonds)
• dispersion forces are the weakest intermolecular force (one hundredth-one thousandth the strength of a covalent bond), hydrogen bonds are the strongest intermolecular force (about one-tenth the strength of a covalent bond).
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Quiz
1. Which attractions are stronger: intermolecular or intramolecular?
2. Which is stronger a covalent bond or a dipole-dipole attraction?
3. Suggest some ways that the dipoles in London forces are different from the dipoles in dipole-dipole attractions.
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1. Intramolecular are stronger.2. A covalent bond is approximately100x stronger.3. London forces
– Are present in all compounds– Are transient in nature (dipole-dipole are more
permanent).– London forces are weaker