intermolecular forces
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Questions. 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? The answers have to do with …. Intermolecular forces. Generall y, inter molecular forces are much weaker than intra molecular forces. - PowerPoint PPT PresentationTRANSCRIPT
Intermolecular forcesIntermolecular forces
• 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?
• The answers have to do with …
Questions
Intermolecular ForcesIntermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Generally, intermolecular forces are much weaker than intramolecular forces.
“Measure” of intermolecular force - boiling point- melting point- Surface tension- evaporation rate
There are three main types of IMFs
•London Dispersion Forces (IMF)
•Dipole-Dipole Forces (DD)
•Hydrogen Bonding Forces (HB)
Types Of Intermolecular Forces:
London Dispersion Forces•The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors.Fritz London
1900-1954
•London forces increase with the size of the molecules.
London Dispersion ForcesLondon Dispersion Forces• Non-polar molecules do not have dipoles
like polar molecules. How, then, can non-polar compounds form solids or liquids?
• London forces are attractive forces caused 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
• The resulting tiny dipoles cause attractions between molecules
London forcesLondon forces
Instantaneous dipole: Induced dipole:
Eventually electrons shift so that tiny
dipoles form
A dipole forms in one atom or molecule,
inducing a dipole in the other
London Dispersion ForcesLondon Dispersion Forces
Dipole-Dipole ForcesAttractive forces between polar molecules
Orientation of Polar Molecules in a Solid
Hydrogen BondThe hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond in one molecule and an electronegative O, N, or F atom.
Hydrogen BondingHydrogen BondingBonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen
Hydrogen bonding between ammonia and water
Maximum Density40C
Ice is less dense than water
Density of Water
Water is a Unique Substance
The hexagonal structure of ice
The H-bonding abilitiy of the
water molecule
Forces & Properties
• Bigger size = Stronger force = higher melting and boiling point because more energy is needed to separate the molecules
Ex. H2 Vs. Cl2
• Heavier the molecule = stronger force• Ex. F2 Vs. Cl2
Molecular shape and boiling pointLonger molecule = stronger force
C5H12 (straight structure) Vs. C5H8 (branched structure)
Properties of LiquidsSurface 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(liquid beads up)
=
(polar molecules, (polar molecules, liquid metals). liquid metals).
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
Figure 12.20 Shape of water or mercury meniscus in glass
adhesive forces
stronger cohesive forces
• This is why oil and water will not mix! Oil is nonpolar, and water is polar.
• The two will repel each other, and so you can not dissolve one in the other
Like Dissolves Like
•Polar substances dissolves Polar substances
•Non-Polar Dissolves Non-polar sbubstances
Bond Polarity• “Like Dissolves Like”
– Polar dissolves Polar– Nonpolar dissolves
Nonpolar