Instantaneous Dipole Induced Dipole Attractions Dispersion Forces 6

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Instantaneous Dipole - Induced Dipole Attractions (Dispersion Forces) 6 S Ho Ching Man (26)

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

 • Introduction • Properties of Dispersion Forces • Factors affecting the strength of

• 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

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

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

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?

What is Dispersion Forces?

 • The instantaneous dipole arises from constant movement of electrons. • Induced dipoles

• 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

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

• 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

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

Ans:

Ans:

Properties : • occur in ALL molecules (both polar and non -polar) • result

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

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

Ans: • Since size of I 2 > Br 2 > Cl 2, •

Ans: • 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.

Strength • Increase with surface area • larger surface in contact • closer interaction

Strength • Increase with surface area • larger surface in contact • closer interaction between different molecules ÞHigher chance of forming induced dipoles

Example Both are non-polar Same no. of electrons (isomer) Pentane (C 5 H 12)

Example Both are non-polar Same no. of electrons (isomer) Pentane (C 5 H 12) Boiling point: 36. 1°C 2, 2 -dimethylpropane (C 5 H 12) Boiling point: 9. 5°C

Ans: pentane molecules 2, 2 -dimethylpropane molecules pentane molecule is cylindrical => larger surface

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

Check-point Two Source : Wikipedia HCl HBr HI dipole moment 1. 05 0. 82

Check-point Two Source : Wikipedia HCl HBr HI dipole moment 1. 05 0. 82 0. 38 boiling point/o. C -85 -66. 4 -34 Boiling point: HI > HBr > HCl

Ans: polarity : HCl > HBr > HI Boiling point : HI > HBr

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.

Importance of Dispersion Force • Affect boiling points of particles * larger atoms Increasing

Importance of Dispersion Force • Affect boiling points of particles * larger atoms Increasing molecular size 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 -153 degrees C Radon -62 degrees C

 • Almost entirely responsible for the attraction between non-polar molecules like bromine, butane

• 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

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

• 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

Reference • http: //en. wikipedia. org/wiki/Main_Page • http: //www. chem. purdue. edu/gchelp/liquids/disperse. html •

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/miscell enous/helpfile/Liquidand. Solid/London. Dispersion. htm • http: //chemsite. lsrhs. net/bonding/London. Dispersion. html