Intermolecular attractions

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

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.

Intramolecular and Intermolecular Force The covalent bond is intramolecular. Bond strength 16 kJ/mol Bond strength = 431 kJ/mol

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

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

PhET simulation ons

1. van der Waals forces van der waals forces: 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.

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

Creation of Instantaneous Dipoles Two molecules approaching each other Collision creates instantaneous dipoles Instantaneous dipoles induce temporary dipoles in other molecules induced dipole       +               induced dipole 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).

Creation of Instantaneous Dipoles

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.

Actual B.P. of SiH ° C Actual B.P. of Ar -186 ° C Si H H HH SiH 4 vs. Ar Mass of SiH4 ~32 amu Mass of Ar ~40 amu Based on mass and mass alone which one would you expect to have a higher boiling point and thus higher intermolecular forces? Answer: Argon 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 Ar

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

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.

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.

H 2 S vs. H 2 O HH S HH O Mass = 18 amu Mass = 34 amu B.P. = 100 ° C B.P. = -60 ° C H 2 S and H 2 O have similar structures. H 2 S has more mass then H 2 O. Why then does water boil at a higher temperature then hydrogen sulfide? Lets compare what is different between the two. H – S EN diff.: 0.4 H – O EN diff.: 1.4 Electronegativity values: H=2.1, S=2.5, O=3.5 Both polar, but one is MORE polar then the other. What can you say about polarity and intermolecular forces? In general the more polar a bond is the higher the dipole-dipole intermolecular attraction and thus a higher M.P./B.P.

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.

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.

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

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

H-bond in hydrogen fluoride 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. F H F H F H F H F H F H Note there is only one H-bond per molecule

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

Difference in Boiling Points of NH 3, H 2 O, and HF The order of the strength of individual hydrogen bond is HF > H 2 O > NH 3 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 NH 3 have only one per molecule.

Hydrogen Bonds: In water, ammonia & ammonia solution

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

Wk11Obj1 AND Wk11Obj2 H-C vsCl-ClvsH-F Electronegativity Values: 2.1 to to to 4.0 Difference: Polarity of bond: Polarnon-polar polar Type of intermolecular force: Dipole-dipole van der waals H-bonding Inter forces/B.P./M.P.: lowvery low high

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

H H O H H S Which compound will have the higher boiling point? A.) H 2 O B.) H 2 S What is the strongest intermolecular force the molecule will allow? A.) van der waals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van der waals B.) dipole-dipole C.) hydrogen bonding Wk11Obj1 AND Wk11Obj2

H N H H H P H H Which compound will have the higher boiling point? A.) NH 3 B.) PH 3 What is the strongest intermolecular force the molecule will allow? A.) van der waals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van der waals B.) dipole-dipole C.) hydrogen bonding Wk11Obj1 AND Wk11Obj2

H C H H H C H H OH C H H H O H H C Which compound will have the higher boiling point? A.) CH 3 OCH 3 B.) CH 3 CH 2 OH What is the strongest intermolecular force the molecule will allow? A.) van der waals B.) dipole-dipole C.) hydrogen bonding What is the strongest intermolecular force the molecule will allow? A.) van der waals B.) dipole-dipole C.) hydrogen bonding Wk11Obj1 AND Wk11Obj2

H C H H H C H H OH C H H H C H H C H H H C H H C HO Which compound will have the higher boiling point? A.) CH 3 CHO B.) CH 3 CH 2 CH 3 C.) CH 3 CH 2 OH Wk11Obj1 AND Wk11Obj2

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).

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

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

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