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2 Forces of attraction There exist between Molecules of gases and liquids Forces of attraction. Some forces may be strong other forces may be weak.

3 Forces of attraction -determine ease/difficulty for matter to change states -example: determine boiling point & melting point

4 Forces of Attraction 2 types of forces of attraction: intermolecular - between molecules intramolecular – between atoms WITHIN a molecule -involve the formation of -involve the formation of real chemical bonds

Intermolecular Forces of Attraction Intramolecular forces -stronger than intermolecular forces Intermolecular forces -known as van der Waals forces

6 Intermolecular Forces of Attraction 3 types of intermolecular forces: 1. London Dispersion force (sometimes called induced dipole force*) 2. Dipole-Dipole force 3. Hydrogen bond

7 Intermolecular forces vary in strength from one another: DispersionWeakest Dipole-dipoleStronger Hydrogen bondThe Strongest

8 Intermolecular forces hold molecules together in the Solid & Liquid phase & Allow gases to condense on cooling. They also prevent instantaneous They also prevent instantaneous evaporation from occurring. evaporation from occurring.

9 Dispersion Forces Dispersion forces - also called LONDON Dispersion or induced dipole - also called LONDON Dispersion or induced dipole - result from temporary shift in electron density - result from temporary shift in electron density within the molecules electron cloud (creates a dipole) within the molecules electron cloud (creates a dipole) -most non-polar compounds have dispersion forces Formation of Temporary Dipoles Non-polar molecules ???

10 The larger the molecule the more easily δ charges develop

11 Dispersion Forces Non-polar compounds = molecules without permanent charge distribution within the molecule Molecules that have an even electron distribution Are typically: 1) single atoms, 2) molecules of the same element, 3) hydrocarbons 4) symmetrical molecules For example: Ne, Kr, H2, O2, N2, S8, C3H8, CCl4, CO2

12 Dispersion forces Induced Dipole forces become stronger the larger the atom or molecule. One might expect the boiling pt. of compounds to be low for compounds that interact by this type of force. Why? The minimal force holding these Molecules together Allow them to move readily from the liquid Phase into the gas phase. Or even from the solid phase to gas phase - like I 2

13 Smaller molecule Larger molecule CH 4 C 8 H 18

14 Dispersion forces Trend for non-polar compounds is… increasing boiling point with increasing molecular weight (this trend is also true with melting pts/freezing pts; as MW increases so does mp/fp) Let’s look at some bp data… Low MW Low BP Higher MW Higher BP Low dispersion force higher dispersion force

15 Dispersion forces Which member of each pair should have the higher B.P. (and thus the stronger Induced dipole) N 2, O 2 O 2 – Higher MW B.P: O C / N C larger molecule larger molecule Ne, Kr Kr - Higher MW B.P: Kr C / Ne C Larger Atom Larger Atom F 2, Cl 2 Cl 2 – Higher MW B.P: Cl C / F C Larger Molecule Larger Molecule C 3 H 8 (propane), C 4 H 10 (butane) Butane (larger) B.P: C 4 H C / C 3 H C M.P: C C M.P: C C

16 Dipole- Dipole Forces -occurs in polar molecules -have permanent dipoles -one area is always positive and another part of the molecule is always negative An example of a Polar molecule is HCl -stronger than London dispersion forces

17 Hydrogen Bond Forces - It is the strongest intermolecular force - ONLY exists when a Hydrogen atom is present in the molecule AND it’s bonded to … in the molecule AND it’s bonded to … Either a N, O, or F atom also present in the Either a N, O, or F atom also present in the same molecule. same molecule.

18 Hydrogen bonds In H-bonds the Hydrogen atom is ALWAYS the Positive Dipole. O, F, or N is ALWAYS the Negative dipole As one molecule approaches another the positive Hydrogen aligns with the negative O, F, or N. +- HF + -

19 Hydrogen bonds All Hydrogen bonds are stronger than either Dispersion or dipole-dipole interactions. F>O>>N Some hydrogen bonds are stronger than others. F to H is strongest O to H is mid strength N to H is weakest

20 The Hydrogen Bond Examples of H-bonds are: Water, Methyl or Ethyl Alcohol, HF, NH3

21 Water Alignment during formation of the H-Bond Positive aligns to negative

22 Hydrogen Bonds Compounds that form Hydrogen bonds take a lot more energy to move molecules from the liquid phase into the gas phase. Molecules of similar molecular weight without the Ability to form H-bonds have higher vapor pressures And thus lower B.P. Compound Molar Mass B.P. deg C H20H20H20H NH CH H-bondH-bondNon-polar

23 dec. MW B.P These compounds form H-Bonds If F forms stronger H bonds than O why is the BP of Water higher than HF? (Hint: Consider the Mol. Structure) Theoretical B.P Of Water -75