Intermolecular forces Flashcards
Why do instantaneous dipoles occur?
Electron density fluctuates over time
What are instantaneous dipoles?
Where the electron density of the molecule is higher on one side than the other, resulting in one end of the molecule having a delta negative charge and the other a delta positive charge
How are induced dipoles generated?
When an instantaneous dipole approaches another molecule, the delta charges on the instantaneous dipole induce changes in the electron density of the other molecule, generating an induced dipole
How will induced dipoles always be aligned?
Induced dipoles will always be aligned in such a way that the interaction with the instantaneous dipole is favourable
What are London forces?
The attraction between instantaneous and induced dipoles
How does the number of electrons in the molecules impact the strength of London forces?
The greater the number of electrons, the greater the strength of London forces
Why does the strength of London forces increase with increasing number of electrons?
The greater the number of electrons, the greater the fluctuation in electron density so the larger the instantaneous and induced dipoles created
What molecules are London forces found between?
London forces are always present between molecules, regardless of any of intermolecular forces also present
How does the shape and size of molecules impact the strength of London forces?
The more points of contact between the molecules, the greater the overall London force
How does increasing chain length impact boiling point?
As the carbon chain increases, the number of points of contact between molecules increases, increasing the strength of the overall London force
Why do unbranched alkanes form strong London forces?
Alkane molecules fit together well and pack closely, meaning there are many points of contact all the way along the chain
How does branching in alkanes affect boiling point and why?
The more branching in the molecules, the fewer points of contact between adjacent molecules (i.e they don’t pack together well). This leads to a decrease in overall London force and a decrease in boiling temperature
Why is the boiling point of alcohols higher than alkanes (with similar sizes)?
Alcohols can form hydrogen bonds between one another
How does enthalpy change of vaporisation indicate the strength of intermolecular bonding?
The greater the enthalpy of vaporisation, the greater the intermolecular forces of attraction
Why is it not always true to say the predominant bonding in alcohols is hydrogen bonding?
Because as chain length increases, London forces eventually predominate over hydrogen bonds
