- Determine if a molecule is polar or nonpolar -Evaluate the type of intermolecular forces that occurs between covalent molecules based on the structural.

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

Determine if a molecule is polar or nonpolar

TUTORIAL :

http://www.youtube.com/watch?v=uYtwU0uRK7o

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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

INTERMOLECULAR FORCESTOPIC 4.3between

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

1. van der Waals’ forces

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

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

CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?

only van der Waals’low BP

dipole-dipole medium BP

hydrogen bonding

highest BP