Electronegativity and Intermolecular forces

Define electronegativity

Relative attraction an atom has for a shared pair of electrons

Pure covalent bond (non-polar covalent)

Atoms share a pair of electrons equally

Polar covalent bond

Atoms share pair of electrons unequally

Why are electrons shared equally in pure covalent bonding?

Difference in electronegativity values is so small that electrons are not pulled significantly closer to either atom.

Why electrons shared unequally in polar covalent bonding

Difference in electronegativity values is significant so electrons are pulled closer to more electronegative atom.

What is significance of a molecule having polar covalent bonds?

•Electrons pulled closer to more electronegative atom.

•More electronegative atom gets a negative dipole.
•Less electronegative atom acquires a positive dipole.

Predicting type of bonding in compound

Pure covalent bond: 0-0.39

Polar covalent bond: 0.4-1.69

Ionic bonding: 1.7+

What is a polar molecule

A molecule that has an uneven distribution of charge, one end is partially positive and other end is partially negative.

Non-polar molecule

Molecule that has an even distribution of charge, no overall positive or negative end.

When is a molecule polar

1. Polar covalent bonds

2. Not symmetrical

When is a molecule is non-polar

1. If bonds are pure covalent

2. If bonds are polar covalent BUT symmetrical

How is it that molecules can have polar covalent bonding but still be non-polar molecules?

If symmetrical, negative and positive coincide and there is no overall positive or negative end.

Symmetrical molecule shapes

Linear, Trigonal planar, tetrahedral

Non symmetrical molecule shapes

pyramidal and v-shaped

Distinguish between intramolecular bonds and intermolecular forces

Intramolecular bonds- bond inside compound

Intermolecular forces- Attraction between one molecule and another.

What are Van der Waals forces

Intermolecular forces between all molecules

Types of Van der Waal forces/intermolecular forces

London dispersion forces- exists between all molecules

Dipole dipole forces- Exists between polar molecules

Ion dipole forces

How are london dispersion forces caused

• All molecules form temporary dipoles due to the movement of electrons

.•Attractions form between negative dipole of one molecule and positive dipole of another molecule

Order of strength of forces from weak to strong

1. London dispersion forces- attraction between temporary dipoles

2. Dipole Dipole forces- attraction between dipoles are permanent

3. Ion dipole force

How are dipole dipole forces caused

• Polar molecules have permanent dipoles

• Attractions between negative dipole of one molecule and positive dipole of another

What is Hydrogen bonding

Particular type of dipole dipole force between positive hydrogen dipole in one molecule and negative nitrogen, oxygen or fluorine dipole in another

Three effects of Hydrogen bonding

1. Explains why H2O, HF and NH3 have higher boiling points than other hydrides

2. Explains H2O higher boiling point than other molecules with similar molecular mass

3. Explains surface tension on water

Ion dipole forces

Exists between an ion and polar molecule

How do intermolecular forces effect boiling points

The stronger the intermolecular forces, the more energy required for substance to boil

When comparing boiling points of substances, check in this order:

1st: Type of intermolecular force

2nd: AR/MR- larger molecules have stronger london dispersion forces

3rd: how polar the bonds are- More polar the bonds, stronger the dipole dipole forces/hydrogen bonding.

Diatomic molecules (Have No Fear Of Ice Cold Beer)

Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine

Solubility of Ionic compounds in water (polar solvent)

Very soluble in water

Why do ionic compounds dissolve in polar solvent

• Ion dipole forces form between partial negative oxygen in polar water and positive ion in ionic compound

•Ion dipole forces form between partial positive hydrogen inn polar water and negative ion in ionic compound

•Crystal lattice pulled apart.

Why dont ionic compounds dissolve in non polar solvents

Ion dipole forces cannot form and crystal lattice cannot be pulled apart because no permanent dipoles are present.

Explain why ammonia is readily soluble in water?

Water and ammonia are polar.
Hydrogen bonds form between partially negative nitrogen in ammonia and partially positive hydrogen in water

Hydrogen bonds form between partially negative oxygen in water and partially positive hydrogen in ammonia.

Would you expect iodine to be soluble in water?

Iodine is sparingly solulble in water
Iodine is non polar, water is polar
Any intermolecular forces that form are too weak

Name an organic solvent iodine is readily soluble in. Explain why this is the case

Cyclohexane
Iodine is non polar, cyclohexane is non polar

London dispersion forces form

Evidence for intermolecular forces

Boiling points and solublilites can only be explained in terms of intermolecular forces.

How real gases deviate from ideal gas behavior is explained in terms of intermolecular forces

Surface tension of water is explained in terms of hydrogen bonding.