Questions
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• Why do some solids dissolve in water but others do not?
• Why are some substances gases at room temperature, but others are liquid or solid?
• What gives metals the ability to conduct electricity?
• What makes non-metals brittle? • The answers have to do with …
Intermolecular forces
Questions
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Introduction• There are 2 types of attraction in molecules:
intramolecular bonds & intermolecular forces• We have already looked at intramolecular
bonds (ionic and covalent bonding)• INTRAmolecular forces are the forces of
attraction within a molecule or ionic compound
• These are the forces that hold molecules and compounds together
INTERMOLECULAR FORCES
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INTERMOLECULAR FORCES
• Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule)
• These forces are weaker than intramolecular bonding forces
• There are 3 types of IMF’s (from weakest to strongest) 1) van der Waal’s
2) dipole-dipole3) hydrogen-bonding
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1. Van der Waal’s forces
• Non-polar molecules do not have dipoles like polar molecules.
• How, then, can non-polar compounds form solids or liquids?
• What holds the molecules close to one another when in the liquid or solid state?
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Van der Waal’s forces• Van der Waal’s forces are due to small
temporary dipoles that exist in non-polar molecules
• Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical ie not evenly distributed
• The resulting tiny dipoles cause attractions between neighbouring atoms/molecules
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vanderWaal’s forces
Instantaneous dipole: Induced dipole:
Eventually electrons are situated so that tiny dipoles form
A dipole forms in one atom or molecule, inducing a
dipole in the other
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• What trends do you notice?
:// . . / / /http wps prenhall com wps media objects
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The boiling points (and melting points) of the halogens DECREASES
as their MOLAR MASS increases (ie DOWN the group)
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2. Dipole - Dipole attractions•We have seen that bonds within molecules can have a separation of charge that creates dipole moments
• If these dipoles combine in a non-symmetrical manner, then a polar molecule results.
H Cl
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Polar and non-polar molecules
wps.prenhall.com
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2. Dipole - Dipole attractions• Neighbouring molecules are attracted to one another through dipole-dipole forces of attraction
• The greater the ∆EN of the dipole moment, the stronger the dipole-dipole forces of attraction between molecules, and the higher the boiling point, because it takes more energy to move molecules apart.
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The greater the ∆EN of the dipole moment, the stronger the dipole-dipole forces of attraction
between molecules•
H-Cl EN = 0.8 Strongest dipole
Highest Boiling Point
H-Br EN = 0.7
H-I EN = 0.4 Weakest dipole
Lowest Boiling Point
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Recall: the 3 states of matter
www.chem.ufl.edu
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3. Hydrogen-bonding
• H-bonding is a special type of dipole - dipole attraction that is very strong
• NOTE: it is not a “Bond” but actually an intermolecular force of attraction between neighbouring molecules
• It only occurs between molecules that have an O-H, N-H or F-H bond which are all highly polar
bonds with large EN values
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3. Hydrogen-bonding• Compare the EN for H-Cl and O-H
bond in H2OH-Cl = 2.9-2.1 = 0.8, O-H = 3.5-2.1 = 1.4
• The high EN of N-H, O-H, and H-F bonds cause hydrogen bonding forces to be the strongest IMF (about 5x stronger than normal dipole-dipole forces)
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The hydrogen-bonding attraction between molecules of water account for its high surface tension.
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NOTE: the hydrogen “bond” is between molecules of water – it is not a TRUE
bond!!
www.landfood.ubc.ca/courses
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Water Water has a very high melting and boiling point due to extensive hydrogen bonding. Having two hydrogens it can form two H- bonds per molecule allowing a lattice diamond-like structure to be built up, as shown below in ice. Note that each O atom is “bonded” to 4 H’s either through H-bonding or a covalent bond. This causes the EXPANSION of water when it freezes.
• http://ibchem.com/IB/ibnotes/brief/bon-sl.htm
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www.goiit.com
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H – bonding and boiling pointPredicted and actual boiling points
-200-150-100-50
050
100
Period
Boi
ling
poin
t Group 4Group 5
Group 6
Group 7
2 3 4 5
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Trends in Boiling PointsBoiling points increase down a group (as period
increases) for two reasons: 1) EN tends to increase and 2) size increases. A larger size means greater vanderWaal’s forces.
Boiling points are higher than expected for H2O, HF, and NH3 because these are capable of hydrogen-bonding with neighbouring molecules (high EN), creating very strong intermolecular forces. This makes it more difficult to move molecules apart in the evaporation process, resulting in a high boiling point.
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Ionic-Dipole Attractive Forces• Positive and negative ions that are
present in ionic compounds will be attracted to dipoles such as those of a polar molecule like water
• This “ionic-dipole” attraction between positively and negatively charged centers accounts for the solubility of most ionic compounds in polar solvents
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Ionic – dipole attractive forces allow ionic compounds to
dissolve in a polar solvent like water
faculty.clintoncc.suny.edu
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Ionic-Dipole Attraction