Intermolecular Forces

1. Intermolecular Forces Mr. Shields Regents Chemistry U07 L01

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 It’s these forces of attraction that ultimately determine How easy or difficult it is for Gases to coalesce into Liquids and liquids to solidify into solids. These forces of attraction will, for example, determine a liquids boiling point.

4. Forces of Attraction What are the forces of attraction we’re talking about? For gases and liquids these forces are of two types: intermolecular - betweenmolecules intramolecular - within the molecule Intramolecular forces involve the formation of true chemical bonds

5. Intermolecular Forces of Attraction Intramolecular forces of attraction are much stronger Than intermolecular forces - on the order of 30 – 400x stronger Intermolecular forces of attraction are also known as van der Waals forces So how many types of intermolecular or van der Waals forces of attraction exist? Let’s take a look …

6. Intermolecular Forces of Attraction There are three types of intermolecular forces: 1. London Dispersion force (sometimes called induced dipole force*) 2. Dipole-Dipole force 3. Hydrogen bond NYS Regents also refers (incorrectly) to induced dipole IMF as “Van Der Waal’s”

7. Intermolecular Forces of Attraction As we mentined before Intermolecular forces are much weaker than intramolecular (i.e. chemical bond) forces And… Intermolecular forces also vary in strength from one another: Dispersion Weakest (<1 Kcal) Dipole-dipole Stronger (2-5 Kcal) Hydrogen bond The Strongest (12-16 Kcal) (for comparison, a true covalent bond is about 400Kcal)

8. Intermolecular Forces of Attraction Intermolecular forces hold molecules together in the Solid & Liquid phase & Allow gases to condense on cooling. They also prevent instantaneous evaporation from occurring. Strong Attractive forces between molecules also make Vaporization (L  G) phase changes more difficult. Why?

9. ??? Formation of Temporary Dipoles Non-polar molecules Dispersion Forces Dispersion forces, the weakest force, is sometimes called LONDON Dispersion Forces or induced dipole forces - They result when there is a momentary shift in electron density within the molecules electron cloud This intermolecular force is the predominant intermolecular force for non-polar compounds

10. Non-polar compounds are molecules that do not have a permanent charge distribution within the Molecule. How does this shift happen?

11. δ -δ+ δ -δ+ δ- δ+ δ -δ+

12. The larger the molecule the more easily δcharges develop

13. Dispersion Forces As we said before Non-polar compounds are molecules that do not have a 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

14. Dispersion forces Induced Dipole forces become stronger the larger the Atom or Molecule. - larger electron clouds are easier to deform 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 I2

15. CH4 C8H18 Smaller molecule Larger molecule

16. Low MW Low BP Higher MW Higher BP Low dispersion force higher dispersion force Dispersion forces If Molecules have High BP’s they will haveLow VP (and conversely Low Bp’s mean High VP) Since dispersion forces increase with increasing Molecular weight the Trend for non-polar compounds is… increasing BP with increasing MW . (this trend is also true with melting pts/freezing pts; as MW inc so does mp/fp) Let’s look at some bp data…

17. Dispersion forces Which member of each pair should have the higher B.P. (and thus the stronger Induced dipole) N2, O2 O2 – Higher MW B.P: O2 -183.0 C / N2 -195.6 C larger molecule Ne, Kr Kr - Higher MW B.P: Kr -152.9 C / Ne -245.9 C Larger Atom F2, Cl2 Cl2 – Higher MW B.P: Cl2 -34.6 C / F2 -187 C Larger Molecule C3H8 (propane), C4H10 (butane) Butane (larger) B.P: C4H10 -0.5 C / C3H8 -42.1 C M.P: -134.4 C -187.7 C

18. Dipole- Dipole Forces Molecules that have permanent dipoles are said to be Polar. What does it mean to be a polar molecule? One area is always positive and another part of the Molecule is always negative An example of a Polar molecule is HCl

19. Dipole-Dipole Forces Dipole- Dipole forces are stronger than London dispersion forces (<1 Kcal vs. 2-5 Kcal) - In this intermolecular force Dipoles are always present as opposed to constantly forming and reforming dipoles in weaker Dispersion force molecules

20. Hydrogen Bond Forces The hydrogen bond is a special form of the Dipole-Dipole force, its not really a true chemical bond. - It is the strongest intermolecular force - much stronger than either Dipole-Dipole Or Dispersion forces. However It can ONLY exist when a Hydrogen atom is present in the molecule AND it’s bonded to … - Either aN, O,orFatom also present in the same molecule.

21. + - Hydrogen bonds In all compounds that form 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. H F + -

22. Hydrogen bonds All Hydrogen bonds are stronger than either Dispersion or dipole-dipole interactions. However, among molecules with Hydrogen bonds there are some that Are stronger than others. How do we determine what H-bonds Are the strongest? Strength of H-bond: F>O>>N

23. The Hydrogen Bond Examples of compounds that can form H-bonds are: Water, Methyl or Ethyl Alcohol, HF, NH3

24. Water Alignment during formation of the H-Bond Positive aligns to negative Water molecules in ice align to form a repeating hexagon

25. 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 havehigher vapor pressures And thuslower B.P. H-bond H-bond Non-polar

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