Lecture Three Flashcards
What two arrangements are possible for permanent dipole moments?
Head to tail
Side by side (favoured if molecules not too fat)
What is the formula for the head to tail arrangement?
U= u1u2/2 pi epsilon 0 r^3
What is the formula given for the energy of fixed parallel arrangement of dipoles?
U=u1u2(1-3cos20)/2 pi epsilon 0 r^3
What are dipole dipole forces responsible for and what happens if heat is applied?
For ordered arrangement of polar solvents. Weak at higher temps the arrangements are broken up- system becomes disordered.
Define polarisability?
When an electric field distorts a molecule and forms an induced dipole moment. It is proportional to the electrical field strength.
What is the proportionality constant?
It is the molecular polarisability and reflects the ease of distortion of the electron distribution in the molecule by applied electric field
How can a non polar molecule have an induced dipole?
If an ion approaches a non-polar molecule it can induce a dipole moment.
The dipole moment depends on the polarisability of the non polar molecule
What is the energy equation for the induced dipole?
U=-1/2 x Z^2 ae ^2/ 4 pi epsilon 0 r^4
Alternative way for a non polar molecule to have an induced dipole?
If a polar molecule is in contact with a non-polar molecule
What is the energy equation for induced dipole using a polar molecule?
U=u^2a/4 pi epsilon 0 r^6
What are London dispersion forces and how do they occur?
Attractive forces that exist between all molecules polar or non-polar
Due to perpetual flux of electron clouds in the molecule- induced dipoles on neighbouring molecule, causing net induced-dipole-induced-dipole attraction
Are dispersion forces truly weakest force?
Yes but dipole dipole force can only occur if molecule is arranged correct (thermal vibrations can disrupt) way where dispersion occur no matter what
Why are mp higher for solids
Because dipole dipole locked in and can’t rotate
Which atoms are hydrogen bonds primarily present?
N,O,F,Cl (very electronegative)
