intermolecular forces why do some solids dissolve in water but others do not? why are some...
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Intermolecular Forces
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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 forcesIntermolecular forces
Questions
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Intermolecular Forces(IMFs)Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water (intra)
• Generally, intermolecular forces are much weaker than intramolecular forces.
Intermolecular force• Ion-dipole• dipole - dipole • H-bonding • London dispersion forces
Intramolecular force• Ionic• covalent (network solids)• metallic
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Properties of Liquids
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
Strong intermolecular
forces
High surface tension
Capillary ActionCapillary Action: : Spontaneous rising Spontaneous rising
of a liquid in a of a liquid in a narrow tube.narrow tube.
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Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
attracted to glass
attracted to each other
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Properties of WaterSurface Tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
The stronger the IMFs the molecules have with each other, the higher is its surface tension.
This is because of the cohesive forces inside the liquid (Polar-Polar forces) become much greater than the adhesive forces between the liquid (polar) and the air (non-polar) molecules outside.
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Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong intermolecular
forces
High viscosity
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• Nonpolar Cl2
(Electronegativity
difference is zero)
• Polar HCl (Electronegativity difference
is not zero)
• Ionic NaCl
Bond Polarity
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Types of Intermolecular Forces
1. Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
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Types of Intermolecular Forces
2. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
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Types of Intermolecular Forces3. Hydrogen Bond (strongest)
The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. IT IS NOT A BOND.
A H…B A H…Aor
A & B are N, O, or F
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Hydrogen Bond
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Why is the hydrogen bond considered a “special” dipole-dipole interaction?
Decreasing molar massDecreasing boiling point
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Types of Intermolecular Forces4. Dispersion Forces – van der Walls forces/London forces (weakest)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
Non-polar molecules do not have dipoles like polar molecules.So London dispersion force exists in non polar molecule.
Dispersion forces usually increase with molar mass.
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Intermolecular Forces4. Dispersion Forces Continued
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.
Polarizability increases with:
• greater number of electrons
• more diffuse electron cloud
Dispersion forces usually increase with molar mass.
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SO
O
What type(s) of intermolecular forces exist between each of the following molecules?
HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
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Types of IMF• The hydrogen bonds in water explain its relatively high boiling
point, considering that it is a small molecule. The H-bonds hold the water molecules together as a liquid, so you have to heat it a lot before it will change to a gas. Compare boiling points of these molecules:
Molecule IMF (s) present Molar Mass (g/mol)
Boiling Point (oC)
CH4 16.05 - 164
HCl 36.46 - 85
H2O 18.02 100
London Disp.
London Disp.
Dipole-DipoleLondon Disp./Dipole-Dipole/Hydrogen Bonding
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Water, H20
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Which is related to…
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And Glycerine (Glycerol)
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Compare the three structures
Which of these molecules has the greatest polarity?
Which will exhibit the strongest intermolecular forces?
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Ethanol. Polar or Non-Polar?
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Methanol. Polar or Non-Polar?
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Methane. Polar or Non-Polar?
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Chloro-Methane. Polar or Non-Polar?
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Sulfur Hexafluoride. Polar or Non-Polar?
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Nitrogen Trihidride, aka Ammonia. Polar or Non-Polar?