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Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)

Chemistry Project. Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces). 6S    Ho Ching Man  (26)          Tam Wing See (32). Content. Introduction Properties of Dispersion Forces Factors affecting the strength of Dispersion Forces

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Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)

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

  2. Content Introduction Properties of Dispersion Forces Factors affecting the strength of Dispersion Forces Application & Importance of Dispersion Forces

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

  4. Fluctuating & Temporary uneven distribution of • e- produce a rapidly varying instantaneous dipole

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

  6. What is Dispersion Forces?

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

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

  9. 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

  10. Check-point One • Arrange the order of the following compounds according to their boiling points. Propane Butane Pentane

  11. Ans:

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

  13. Strength Example 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: Cl2, Br2, or I2?

  14. Ans: Since size of I2 > Br2 > Cl2, Non-polar, only dispersion force exists between the molecules dispersion forces are largest in I2 and smallest in Cl2, so I2 should have the highest boiling point. At room temperature, Cl2 is a gas, Br2 a liquid and I2 a solid.

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

  16. 2,2-dimethylpropane (C5H12) Pentane (C5H12) Boiling point: 36.1°C Boiling point: 9.5°C Both are non-polar Same no. of electrons (isomer) Example

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

  18. Source :  Wikipedia Check-point Two Boiling point: HI > HBr > HCl WHY??

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

  20. Importance of Dispersion Force Increasing molecular size • Affect boiling points of particles * larger atoms  stronger Dispersion  increase energy required to pull them apart  boiling point increases

  21. Almost entirely responsible for the attraction between non-polar molecules like bromine, butane etc. For example,  CH4(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.

  22. 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

  23. 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

  24. Reference http://en.wikipedia.org/wiki/Main_Page http://www.chem.purdue.edu/gchelp/liquids/disperse.html http://www.cartage.org.lb/en/themes/sciences/chemistry/miscellenous/helpfile/LiquidandSolid/LondonDispersion.htm http://chemsite.lsrhs.net/bonding/LondonDispersion.html

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