5 Permanent dipole A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.
6 Instantaneous dipole An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.
7 Instantaneous dipole An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.
8 Induced dipole An induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which may be a permanent or an instantaneous dipole). Permanent dipole
9 11.2 Van der Waals Forces
10 Van der Waals Forces Van der Waals forces Dipole- Dipole Interaction Dipole- Dipole Interaction Dipole- Induced Dipole Interaction Dipole- Induced Dipole Interaction Instantaneous Dipole- Induced Dipole Interaction Instantaneous Dipole- Induced Dipole Interaction London dispersion forces
11 Dipole-dipole interactions Electrostatic interactions between polar molecules
12 Dipole-dipole interactions In a sample containing many polar molecules A balance of attraction and repulsion holding the molecules together
13 Dipole-induced dipole interactions When a non-polar molecule approaches a polar molecule (with a permanent dipole), a dipole will be induced in the non-polar molecule. Dispersion forces exist among all molecules and contribute most to the overall van der Waals forces.
14 Polarizability : - A measure of how easily the electron cloud of an atom/molecule can be distorted to induce a dipole Polarization
15 In general, size of electron cloud electron cloud is less controlled by positive nuclei extent of electron cloud distortion polarizability stronger dispersion forces
16 Instantaneous dipole-induced dipole interactions 11.2 Van der Waals forces (SB p.277) The instantaneous dipole arises from constant movement of electrons. Induces dipoles in neighbouring atoms or molecules
20 Evidence for the presence of London dispersion forces 1. Condensation of noble gases at low temperatures to form liquids and solids presence of attractive forces between non-polar atoms E.g. Xe(g) Xe(s) H sub = -14.9 kJ mol 1
21 Evidence for the presence of London dispersion forces 2.The non-ideal behaviour of gases van der Waals equation
22 Strength of van der Waals forces 11.2 Van der Waals forces (SB p.279) Much weaker than covalent bonds Less than 10% the strength of covalent bonds van der Waals radius > covalent radius I2I2
23 Q.59 The electron clouds of adjacent iodine molecules would repel each other strongly until the equilibrium van der Waals distance is restored.
24 The strength of van der Waals forces can be estimated by melting point, boiling point, enthalpy change of fusion or enthalpy change of vapourization. Higher m.p./b.p./ H fusion / H vap stronger van der Waals forces
25 Strength of van der Waals forces Depends on three factors (in decreasing order of importance) : - 1. Size of molecule 2. Surface area of molecule 3. Polarity of molecule
26 Size of molecule Size of electron cloud MoleculeBoiling point ( o C) Helium Neon Argon -269 -246 -186 Fluorine Chlorine Bromine -188 -34.7 58.8 Methane Ethane Propane -162 -88.6 -42.2 1. Size of Molecule Polarizability Dispersion forces Rel. molecular mass Sometimes !
27 The van der Waals forces also increase with the surface area of the molecule. 2. Surface area of molecule van der Waals' forces are short-ranged forces Atoms or molecules must come close together for significant induction of dipoles.
28 Pentane (C 5 H 12 ) 2,2-dimethylpropane (C 5 H 12 ) Boiling point: 36.1C Boiling point: 9.5C Both are non-polar Same no. of electrons
29 2,2-dimethylpropane molecules pentane molecules larger contact area smaller contact area rod-shapedspherical in shape
30 Pentane (C 5 H 12 ) Larger contact surface area Higher chance of forming induced dipoles stronger dispersion forces Boiling point = 36.1 C
31 2,2-dimethylpropane (C 5 H 12 ) 2,2-dimethylpropane (C 5 H 12 ) Smaller contact surface area lower chance of forming induced dipoles weaker dispersion forces Boiling point = 9.5 C
32 3. Polarity of molecules For molecules with comparable molecular sizes and shapes, dispersion forces are approximately equal. Polar/polar > polar/non-polar > non-polar/non-polar Then, strength of van der Waals forces depends on the polarity of molecules involved
33 RMM = 58.0, ++ ++ Dipole-dipole forces + Dispersion forces Dispersion forces only b.p. = 50 C b.p. = 0 C
34 Other examples : - 1.Graphite layers of large surface area strong van der Waals forces 2.Polyethene vs ethene (m.p. > 100 C) (m.p. = 169 C)
35 Molecule % contribution to the overall van der Waals' forces Dipole- dipole interaction Dipole- induced dipole interaction Instantaneous dipole- induced dipole interaction C 4 H 10 00100 HCl15481
36 Q.60(a) CH 3 Cl < CH 3 Br < CH 3 I b.p./ C -24.2 3.56 42.4 The strength of dispersion forces increases with molecular size/mass. Thus, b.p. increases with molecular size/mass Although chloromethane is more polar, the effect of dispersion forces outweights that of dipole-dipole forces.
38 Q.60(c) F 2 Cl 2 ClFCH 2 Cl 2 F 2 < ClF < Cl 2 F 2. It is because 1.ClF has a greater molecular size than F 2 and thus has stronger dispersion forces than F 2 2. ClF is polar and its molecules are held by both dipole-dipole forces and dispersion forces.
39 Q.60(c) F 2 CH 2 Cl 2 -188 C -100 C -34.0 C 39.6 C Cl 2 > ClF. It is because 1.Cl 2 has a greater molecular size than ClF and thus has stronger dispersion forces than ClF. 2.Although ClF is polar, the effect of dispersion forces outweights that of dipole-dipole forces.
40 Q.60(c) F 2 CH 2 Cl 2 -188 C -100 C -34.0 C 39.6 C CH 2 Cl 2 > Cl 2. It is because 1.CH 2 Cl 2 has a greater molecular size than Cl 2 and thus has stronger dispersion forces than Cl 2. 2.CH 2 Cl 2 is polar and its molecules are held by both dipole-dipole forces and dispersion forces.
41 Q.60(d) NO < C 2 H 6 RMM 28.0 28.0 b.p./ C -151 -89
42 1 pm = 0.001 nm 1 nm = 10 9 m
43 C 2 H 6 > NO. It is because 1.C 2 H 6 has a greater molecular size and contact surface area than NO and thus has stronger dispersion forces than NO. 2.Although NO is polar, the effect of dispersion forces outweights that of dipole-dipole forces. NO < C 2 H 6 RMM 28.0 28.0 b.p./ C -151 -89
44 The melting of a solid involves the separation of molecules from a regularly packed molecular crystal. Thus, m.p. of a solid depends on 1. The strength of van der Waals forces 2. Packing efficiency of molecules in the crystal lattice
45 Symmetry of molecule Packing efficiency m.p.
82 B.p. as molecular size (dispersion > dipole-dipole) However, H 2 O, HF and NH 3 have abnormally high b.p. There exist unusually strong dipole-dipole forces (H-bond) All are polar
83 Formation of hydrogen bonding When a hydrogen atom is directly bonded to a highly electronegative atom (e.g. fluorine, oxygen and nitrogen), a highly polar bond is formed. 2.1 4.03.53.0
84 Electrostatic attractions exist between this partial positive charge and the These attractions are called hydrogen bonds lone pair electrons on a highly electronegative atom (i.e. fluorine, oxygen or nitrogen) of another molecule.
85 hydrogen bond
86 Formation of hydrogen bonds between H 2 O molecules. hydrogen bond