UNIT: BONDINGTIER 5
-Determine if a molecule is polar or nonpolar
-Evaluate the type of intermolecular
forces that occurs between covalent molecules based on the structural features of the molecules
-Evaluate how the intermolecular forces affect the boiling points of substances
3
Polar Molecules
A polar molecule contains polar bonds has a separation of positive and
negative charge called a dipole, indicated with + and –
has dipoles that do not cancel + –
• •
H–Cl H—N—H dipole
H dipoles do not cancel
p. 380
Ammonia has a trigonal pyramidal shape. The one unshared pair of electrons give that side of the molecule a partial negative charge and the other side a partial positive side.
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Nonpolar Molecules
A nonpolar molecule contains nonpolar bonds
Cl–Cl H–H
or has a symmetrical arrangement of polar bonds
p. 380
Boron trifluoride has a trigonal planar shape with each bond having a 120o angle and no unshared electrons so it is nonpolar. If the shape of any molecule is trigonal planar and the central atom is surrounded by the same three atoms, then it will always be nonpolar
Determining Molecular Polarity
Determine the polarity of the H2O molecule.
Solution: The four electron groups of oxygen are bonded to two H atoms. Thus the H2O molecule has a net dipole, which makes it a polar molecule.
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Carbon dioxide is
linear and has symmetry so it
is nonpolar
Water has a bent shape with two unshared pair of electron on oxygen so it is polar
Methane (CH4) is nonpolar because the carbon is surrounded by the same atom on all sides. However for CH3Cl, CH2Cl2 & CHCl3 have the chlorine and hydrogen attached to the carbon in various ratios, they are polar. Even though they are all have a tetrahedral shape, they are not all nonpolar. However the rule is if the molecule is tetrahedral where the central atom is surrounded by the same for atoms, it will be nonpolar.
Evaluate the type of intermolecular forces that occurs between covalent molecules based on
the structural features of the molecules
TUTORIAL ON INTERMOLECULAR FORCES:
http://www.youtube.com/watch?v=90q7xl3ndJ8&safe=active
Intra vs. Inter molecular forces Intra-
strong forces (ionic or covalent) that hold the atoms in a molecule together takes 464 kJ/mol to break the H-O bonds within a water molecule
responsible for chemical properties
Inter- weak forces that holds molecules to
one another takes only 19 kJ/mol to break the bonds between water molecules
the strength of the intermolecular forces determines the physical properties of the substance melting, boiling, reacting, solubility, conductivity, volatility
3 main “types” of intermolecular forces1. temporary/induced/
instantaneous dipole-dipole forces called Van der Waals’
2. permanent dipole-dipole forces (polar molecules)
3. a stronger type of dipole-dipole bonding called hydrogen bonding
strength increases
VAN DER WAAL FORCES also known as London Dispersion
Forces even nonpolar molecules have forces
that hold them together the distribution of electrons around an
individual atom, at a given instant in time, may not be perfectly symmetrical this can produce temporary/instantaneous
dipoles (polar molecule) this can then induce a nearby molecule to
be polar and therefore a very weak attraction between the two molecules
the more electrons in an atom, the greater the Van der Waals’ forces
Sticky secret. Tiny hairs on geckos' feet help maximize
contact with surfaces, allowing van der Waals forces to go to work.
Dipole-Dipole Forces attractive forces between the
positive end of one polar molecule and the negative end of another polar molecule
must be in close proximity for the dipole-dipole forces to be significant
the more polar the molecule, the greater the dipole-dipole force
stronger than van der Waals’ forces
HYDROGEN BONDING a specific type of dipole-dipole type
interactions stronger than other dipole-dipole and
Van der Waals’ the hydrogen (H) in a molecule is
bonded to a small, highly electronegative element (usually an N, O or F atom) on another molecule
H-NOF
Evaluate how the intermolecular forces affect the boiling points of substances
TUTORIAL:
http://www.youtube.com/watch?v=S4-lAq2IH-8&safe=active
Intermolecular forces affect on boiling point (4.3.2)
the greater polarity of a molecule, the higher the boiling point In HF, H is 2.1 and F is 4.0, difference of
1.9 In HCl, H is 2.1 and Cl is 3.0, difference of
0.9
BP is 20°C BP is -85°C
H2O vs. H2S? In H2O, H is 2.1 and O is 3.5, difference of 1.4 water molecules can hydrogen bond to each other
BP is 100°C
In H2S, H is 2.1 and Cl is 2.5, only a difference of 0.4 H2S can only dipole-dipole bonding to each other
BP is -60°C
H-NOF ?
H-NOF ?yes
H-NOF ?no
CH3OCH3 vs. CH3CH2OH ?
-24°C 78°C only dipole – dipole has hydrogen bonding
H-NOF ?
NH3 vs. PH3 ?
-33°C -87°C has hydrogen bonding only dipole – dipole
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