Presentation on theme: "6S Ho Ching Man (26) Tam Wing See (32) Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)"— Presentation transcript:
6S Ho Ching Man (26) Tam Wing See (32) Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces)
Introduction Properties of Dispersion Forces Factors affecting the strength of Dispersion Forces Application & Importance of Dispersion Forces
Introduction – what is instantaneous dipole? Electrons (e - ) move at very HIGH speed randomly leads to uneven distribution of e - density
Fluctuating & Temporary uneven distribution of e - produce a rapidly varying instantaneous dipole
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).
What is Dispersion Forces?
The instantaneous dipole arises from constant movement of electrons. Induced dipoles in neighbouring atoms or molecules
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)
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
Check-point One Arrange the order of the following compounds according to their boiling points. PropaneButanePentane
Properties : occur in ALL molecules (both polar and non-polar) result for very short moment only (no permanent dipole)
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
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:
Strength Increase with surface area larger surface in contact closer interaction between different molecules Higher chance of forming induced dipoles
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)
pentane molecule is cylindrical => larger surface in contact so Dispersion Forces is more effective 2,2-dimethylpropane molecules pentane molecules Ans:
Check-point Two HClHBrHI dipole moment boiling point/ o C Boiling point: HI > HBr > HCl Source : Wikipedia
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
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
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.
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
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