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6S Ho Ching Man (26) Tam Wing See (32) Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)

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Presentation on theme: "6S Ho Ching Man (26) Tam Wing See (32) Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)"— Presentation transcript:

1 6S Ho Ching Man (26) Tam Wing See (32) Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)


3 Introduction Properties of Dispersion Forces Factors affecting the strength of Dispersion Forces Application & Importance of Dispersion Forces

4 Introduction – what is instantaneous dipole? Electrons (e - ) move at very HIGH speed randomly leads to uneven distribution of e - density

5 Fluctuating & Temporary uneven distribution of e - produce a rapidly varying instantaneous dipole

6 Permanent dipole What is induced dipole? An induced dipole is a temporary dipole that is created due to the influence of neighbouring dipole (which can be a permanent or an instantaneous dipole).

7 What is Dispersion Forces?

8 The instantaneous dipole arises from constant movement of electrons. Induced dipoles in neighbouring atoms or molecules

9 Evidence of presence of Dispersion Force 1. liquefaction of noble gases at low temperatures to form liquids presence of attractive forces between non polar atoms E.g. Xe(g)  Xe(l)

10 2. Non-ideal behaviour of gas Cause slight variations in the actual pressure and volume of a gas compared to the ideal gas law predictions. At high temperature + low pressure No significant impact on the properties of the gas Properties closer to Ideal gas

11 Check-point One Arrange the order of the following compounds according to their boiling points. PropaneButanePentane

12 Ans:

13 Properties : occur in ALL molecules (both polar and non-polar) result for very short moment only (no permanent dipole)

14 Strength increase with atomic/molecular size larger, more dispersed electron cloud, greater ease of distortion of electron cloud >> higher polarizability Which of the following has a higher boiling point: Cl 2, Br 2, or I 2 ? Example

15 Since size of I 2 > Br 2 > Cl 2, Non-polar, only dispersion force exists between the molecules dispersion forces are largest in I 2 and smallest in Cl 2, so I 2 should have the highest boiling point. At room temperature, Cl 2 is a gas, Br 2 a liquid and I 2 a solid. Ans:

16 Strength Increase with surface area larger surface in contact closer interaction between different molecules  Higher chance of forming induced dipoles

17 Example Pentane (C 5 H 12 ) 2,2-dimethylpropane (C 5 H 12 ) Boiling point: 36.1°C Boiling point: 9.5°C Both are non-polar Same no. of electrons (isomer)

18 pentane molecule is cylindrical => larger surface in contact so Dispersion Forces is more effective 2,2-dimethylpropane molecules pentane molecules Ans:

19 Check-point Two HClHBrHI dipole moment1.050.820.38 boiling point/ o C-85-66.4-34 Boiling point: HI > HBr > HCl Source : Wikipedia

20 Therefore, dispersion force is dominant factor in this case. in this case. Ans: Although HCl is more polar, the effect of dispersion forces outweights that of dipole-dipole forces. The strength of dispersion forces increases with molecular size/mass. Thus, b.p. increases with molecular size/mass polarity : HCl > HBr > HI Boiling point : HI > HBr > HCl

21 Importance of Dispersion Force Affect boiling points of particles * larger atoms  stronger Dispersion  increase energy required to pull them apart  boiling point increases Boiling points of different ideal gases Helium-269 degrees C Neon-246 degrees C Argon-186 degrees C Krypton-153degrees C Radon-62 degrees C Increasing molecular size

22 Almost entirely responsible for the attraction between non-polar molecules like bromine, butane etc. For example, CH 4 (non-polar) => 100% dispersion force in VDW HCl(polar) => 81%dispersion force & 15% dipole - dipole attraction in VDW Affect (but don’t oppose) the motion of nanomechanical parts that roll or slide over one another.

23 Application of Dispersion Force Liquefaction of gases Low temp  low K.E of molecules High pressure  distance between molecules decreases Stonger dispersion forces Gas  Liquid

24 Control properties of numerous ceramics( 製陶業 ) that contain intergranular films The relationship of dispersion forces and intergranular films is critical in many materials problems e.g control physical properties

25 Reference enous/helpfile/LiquidandSolid/LondonDispersion.htm enous/helpfile/LiquidandSolid/LondonDispersion.htm


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