Solubility: Molecular Polarity

How do molecules stay together?

Two main types of forces that hold molecules together exist: 1) Intramolecular forces: (Intra = “within”). Bonds found inside molecules. Ex: Covalent and ionic bonds

Two main types of forces that hold molecules together exist: 1) Intramolecular forces: (Intra = “within”). 2) Intermolecular forces: (Inter = “between”). Bonds found between molecules. Also called van der Waals forces. Ex: London forces, Dipole-dipole forces

I I The covalent intramolecular bond in I 2 is very strong. I I I I I I I I I I There are weaker intermolecular forces which hold covalent molecules together as a solid or liquid.

Types of Intramolecular Bonds Ionic – transfer of e- from one atom to another Covalent - sharing of e- between atoms a) nonpolar covalent – equal sharing of e- b) polar covalent – unequal sharing of e- - Polar bonds happen when the two atoms attached to the bond have very different electronegativities.

Determining Polarity Electronegativity Difference Type of Bond 0-0.3Nonpolar covalent Polar covalent 2.0 or greaterIonic

Electronegativity of Oxygen = 3.44 Electronegativity of hydrogen = 2.22 Difference = 1.22 Electronegativity Difference Type of Bond 0-0.3Nonpolar covalent Polar covalent 2.0 or greaterIonic

Electronegativity of Oxygen = 3.44 Electronegativity of hydrogen = 2.22 Difference = 1.22 Polar covalent bonds between O and H. Oxygen has a higher EN (stronger attraction for the e-). Electrons are still shared, but spend more time around the oxygen.

Polar covalent bonds One end of the water molecule will be partially negative, while the other end will be partially positive. This separation in charge is called a dipole.

EN of C = 2.5 EN of F = 4.0 Difference = 1.5 POLAR COVALENT BOND

Nonpolar Bond (no dipole) vs. Polar Bond (dipole) +- +

Showing Polarity of a Bond Dipole moment

Determining Polarity Without Knowing the exact EN Polarity depends on the shape of the molecule Ex. CO 2 (nonpolar) and CH 2 O (polar)

Steps: 1)Draw the Lewis dot structure. 2)Analyze each bond to see which atom the electrons in the bond are more attracted to (more EN). 3)Draw your dipole moments. 4)If the dipole moments cancel out = nonpolar If they do not = polar.

Carbon dioxide has polar bonds, but because they cancel out, it is a non-polar molecule. CH 2 O has two non-polar bonds, but it has a net dipole moment, so it is a polar molecule.

Try: NH 3 Net dipole moment This is a polar molecule (has a dipole).

Try: Carbon tetrafluoride This is a non-polar molecule (has no dipole). All moments cancel each other out – no net moment. CF 4

Easy Tips: If there are lone pairs on the central molecule – the molecule is polar. If the molecule is asymmetrical – then it is polar. If the molecule is symmetrical, and does not have lone pairs – it is non-polar. Symmetrical – no lone pairs, therefore, non-polar.

You try: are the following molecules, polar, or non- polar? Draw them out and show the dipole moments. If polar, show which atom is partially negative, and which is partially positive. CHCl 3 O 2 SO 3 CH 4 AsI 3 C 2 H 6

Homework: 1) Sulfur dioxide 2) Nitrogen dioxide (-1 charge) 3) 4) PH 3 5) HCN Are the following molecules, polar, or non-polar? Draw them out and show the dipole moments. If polar, show which atom is partially negative, and which is partially positive.

POLAR Bear

How do intermolecular (Van der Waals) forces form? They form when dipoles exist. They determine whether a compound is solid, liquid or gas at a given temperature (affects the melting and boiling points of a substance)

There are two types of dipoles: -Temporary dipoles -Permanent dipoles There are three types of intermolecular forces: -Dipole-dipole forces -Hydrogen bonds -London Dispersion Forces

1) Permanent dipoles - When 2 atoms in a molecule have very different electronegativities, a permanent dipole is present.

Dipole-dipole force Dipole-dipole forces form between molecules with permanent dipoles.

Another type of intermolecular force that forms between permanent dipoles: hydrogen bonds. Hydrogen bonds form between molecules containing a H-N, H-O, or H-F bond. - It is the strongest type of intermolecular force.

Hydrogen bond

What type of intermolecular force is found between these polar molecules? PH 3 NH 3 OF 3 OH -

Temporary dipoles (induced dipoles): Electrons are always moving around the atom, so the electron distribution in an atom may become unsymmetrical, causing a dipole. +2 e-e- e-e- He  

+2 e-e- e-e- He The random dipole created in one atom, can affect the electron distribution in a nearby atoms. Polarization occurs, and a temporary dipole forms. +2 e-e- e-e- He   The weak attractive force between the (+) side of one molecule and the (-) side of another molecule is called London Dispersion Forces  

+2 e-e- e-e- He London Forces are short lived and temporary, because the electrons are always moving. +2 e-e- e-e- He   Polarization = the process of repelling a nearby atom’s electrons.  

London Forces: -They are present in every molecule. Including covalent and ionic molecules. -They are very weak -The greater the number of electrons, the stronger the London Force. So the greater the atomic number, the stronger the London Force.

Questions… 1)What happens to the strength of the London forces between two identical atoms going: a)Down a group? b)left to right across a period?

What type of intermolecular force?

Strongest bond  Weakest bond Strongest Weakest Covalent bond Ionic bond Hydrogen bond Dipole-dipole force London dispersion force

Melting point: The temperature when a solid becomes a liquid. Boiling point: The temperature when a liquid becomes a gas. The stronger the intermolecular bonds, the higher the melting and boiling points.

Which has a higher boiling point? HCl or F 2 ?

Homework: P181: #77 P202: #12 P203: #13, 14, 16