Intermolecular attractions

1. Intermolecular attractions

2. Thus far… • Bonding covered so far involved intramolecular bonding, i.e. bonding within a molecule or within an ionic crystal lattice. • This is the bonding that holds a molecule together. Now we’re going to talk about intermolecular forces or attractions between two separate molecules. This is the “bonding” that holds to TWO molecules together

3. Intermolecular Force • Intermolecular forces: • The attractive forces holding two separate molecules together • much weaker than intramolecular forces (i.e. bonds) • When a substance melts or boils the intermolecular forces are broken (not the covalent bonds). • Melting and boiling point is a relative indicator of the strength of intermolecular force in a substance.

4. Intramolecular and Intermolecular Force • Bond strength = 431 kJ/mol The covalent bond is intramolecular. • Bond strength16 kJ/mol

5. Kinds of Intermolecular Forces • 3 types of intermolecular forces: 1. van der Waals forces 2. Dipole-dipole forces 3. Hydrogen bond

6. Dipole: seperation of positive and negative charges. • In the case of atoms that separation is the negative electrons being separated from the positive nucleus/protons

7. PhET simulation • http://phet.colorado.edu/en/simulation/balloons

8. 1. van der Waals forces • van derwaalsforces: • are temporary induced dipoles between compounds and atoms. • found in all substances. • weakest of all intermolecular forces. • In nonpolar compounds, it is the ONLY intermolecular force of attraction.

9. Creation of a dipole for van der Waals forces • It is possible for two adjacent neutral molecules to affect each other.

10.  +  +     +    +  +   Creation of Instantaneous Dipoles • Polarization of molecules could also results from one instance to another simply by chance or collisions between them (in the case of liquids and gases). Two molecules approaching each other Collision creates instantaneous dipoles Instantaneous dipoles induce temporary dipoles in other molecules induced dipole induced dipole

11. Creation of Instantaneous Dipoles

12. Dependence of van der Waals force on Mass • Inert gases and the halogens: Notice, boiling point increases going down the group. Similarly molecular weight increases as you go down a group. Using the information that boiling point is an indicator of intermolecular forces, what can you say about the relationship between molecular weight and intermolecular forces? The greater the mass the greater the intermolecular forces.

13. SiH4 vs. Ar Mass of SiH4 ~32 amu Mass of Ar ~40 amu H Ar H Si H Actual B.P. of Ar -186 ° C Actual B.P. of SiH4 -112 ° C H This is opposite of the trend we saw before. What then can account for the difference in B.P./intermolecular forces? Answer: Shape, but more specifically surface area of atom/molecule Based on mass and mass alone which one would you expect to have a higher boiling point and thus higher intermolecular forces? Answer: Argon

14. Van der Waals Forces: A Summary Van derwaals forces depends on: • molar mass • The greater the mass the greater the van derwaals forces. • Shape • The greater the surface area the greater the van derwaals forces.

15. 2. Dipole-dipole force of attraction • Dipole-dipole forces exist between neutral polar molecules • Neutral polar molecules have a permanent dipole vs. a temporary one. • Since the dipoles are permanent, dipole-dipole forces are stronger than van der Waals forces between substances of comparable molar mass.

16. Nature of dipole-dipole force • The dipoles align themselves as shown—oppositely charged ends adjacent to one another. There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble.

17. H2S vs. H2O Mass = 18 amu Mass = 34 amu O S B.P. = 100 ° C B.P. = -60 ° C Electronegativity values: H=2.1, S=2.5, O=3.5 H2S and H2O have similar structures. H2S has more mass then H2O. Why then does water boil at a higher temperature then hydrogen sulfide? Lets compare what is different between the two. H H H H H – S EN diff.: 0.4 H – O EN diff.: 1.4 In general the more polar a bond is the higher the dipole-dipole intermolecular attraction and thus a higher M.P./B.P. Both polar, but one is MORE polar then the other. What can you say about polarity and intermolecular forces?

18. Dipole-dipole force: A Summary • To conclude then, polar substances contain dipole-dipole force. In general, there is a direct relationship between polarity and dipole-dipole force and therefore, boiling point.

19. Nature of H-bond • Hydrogen bonding: • special case of dipole-dipole forces. • covalent bond between Hydrogen and either N, O, or F. • the more polar the stronger the H-bond. • the other molecule must contain a lone pair of electrons. • the higher number of hydrogen bonds per molecule, the stronger the intermolecular force • H-bonds are stronger then the other two intermolecular forces.

20. 3. Hydrogen bond Period 2 hydrides of Groups V, VI and VII: Except for Carbon, everything in period 2 has higher B.P.s then the other hydrides in their group

21. H-bond in ammonia • Note there is only one H-bond per molecule

22. H F H H F F H-bond in hydrogen fluoride F F H F H H • Note there is only one H-bond per molecule Steric reasons prevent fluorine from forming more than one H-bond. • It is too small to accommodate more than two hydrogen atoms around it without the hydrogens themselves repelling each other and destabilizing the bonds causing them to break on their own accord.

23. H-bond in Water • Note there are two H-bond per molecule on average

24. Difference in Boiling Points of NH3, H2O, and HF • The order of the strength of individual hydrogen bond is • HF > H2O > NH3 • Because that is the order of the polarity of the three molecules. (HF is the most polar and then water followed by ammonia, because fluorine is most electronegative followed by oxygen and then nitrogen.) • The boiling point of HF therefore is higher than that of ammonia, but not than that of water. • Even though oxygen is less electronegative than fluorine and has a weaker individual hydrogen bond, the intermolecular force in water is the strongest (and therefore it has the highest boiling point) because water, on average, has 2 hydrogen bonds per molecule while both HF and NH3 have only one per molecule.

25. Hydrogen Bonds: In water, ammonia & ammonia solution

26. 3. Hydrogen bond Summary • A bond between a hydrogen bonded to N, O or F and lone pair in the atom hydrogen is bonded to. • the more electronegative the element bonded to hydrogen the stronger the H-bond • the more the number of hydrogen bonds per molecule, the stronger the intermolecular force • strongest of the intermolecular forces

27. Wk11Obj1 AND Wk11Obj2 H-C vsCl-ClvsH-F Electronegativity Values: 2.1 to 2.53.0 to 3.02.1 to 4.0 Difference: 0.4 0.01.9 Polarity of bond: Polarnon-polar polar Type of intermolecular force: Dipole-dipolevan derwaalsH-bonding Inter forces/B.P./M.P.: lowvery lowhigh

28. Wk11Obj1 AND Wk11Obj2 H – F H – Cl What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding Which compound will have the higher boiling point? A.) H-F B.) H-Cl

29. Wk11Obj1 AND Wk11Obj2 O S H H H H What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding Which compound will have the higher boiling point? A.) H2O B.) H2S

30. Wk11Obj1 AND Wk11Obj2 H H H H N P H H What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding Which compound will have the higher boiling point? A.) NH3 B.) PH3

31. Wk11Obj1 AND Wk11Obj2 H H H O H C C H H H H What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van derwaals B.) dipole-dipole C.) hydrogen bonding H C H O C Which compound will have the higher boiling point? A.) CH3OCH3 B.) CH3CH2OH H H

32. Wk11Obj1 AND Wk11Obj2 H H H H H H H O H C H C C C C H H H H H H H H O C C Which compound will have the higher boiling point? A.) CH3CHO B.) CH3CH2CH3 C.) CH3CH2OH H

33. Summary • Intermolecular forces arise from compositional (identities and properties of the atoms themselves) and structural features of the molecules of the substance. 1. Van der Waals forces • Forces of attraction between temporary dipoles • found in all substances; the bigger the mass stronger the van der Waals forces (everything else being the same) • in nonpolar compounds the only intermolecular force found • between molecules of comparable mass, long, non-spherical molecules have stronger van der Waals forces than spherical molecules • Long molecules are more easily polarized and the degree of polarization is also greater leading to a stronger van der Waals forces. • The difference in boiling point brought about by this structural difference between molecules becomes relevant when considering the boiling points of different isomers of organic compounds, especially different structural isomers (See Core Organic Chemistry: Isomerism).

34. Summary • 2. Dipole-dipole forces • Forces of attraction between permanent dipoles • found in polar molecules • the bigger the dipole moment stronger the dipole-dipole forces (everything else being equal, of course) • 3. Hydrogen bond • A bond between a hydrogen bonded to N, O or F and lone pair in the atom hydrogen is bonded to. • the more electronegative the element bonded to hydrogen the stronger the H-bond • the more the number of hydrogen bonds per molecule, the stronger the intermolecular force • strongest of the intermolecular forces

35. Are the molecules polar? No Yes Everything else being equal, higher boiling point Do they have different structural features? No Yes Bigger mass has stronger van One with more atoms or long molecules der Waals forces and therefore has stronger van der Waals forces higher boiling point & therefore higher boiling point Examples: Examples: _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ Dichotomous key for determining the difference in boiling points (van der Waals forces) (Dipole-dipole forces or H-bond) (See next slide)

36. Dipole-dipole forces or H-bond Do the molecules have hydrogen covalently bonded to N or O or F? No Yes Everything else being equal,higher boiling point. Difference in polarities? No Yes Is there a difference in mass? Is polarities the only difference? No Yes No Yes Similar boiling One with bigger One with the larger points mass has higher dipole moment will have boiling point higher boiling point Examples: Examples: Examples: Examples: ____________________ _______________________ _______________________ ______________________ ____________________ _______________________ _______________________ ______________________ ____________________ _______________________ _______________________ ______________________ (Dipole-dipole forces) (Hydrogen bond) (See next slide) Depends!