4. Intermolecular Forces Flashcards
(18 cards)
Define intermolecular force
The bonds that exist between neighbouring simple covalent molecules
What are the types of intermolecular force?
Van Der Waals’
Permanent dipole attractions
Hydrogen bonding
Define van der waals’ force
Attraction between instantaneous and induced dipoles on neighbouring molecules
How do van der waals’ forces occur?
- exist between all simple molecules
- occur because electrons are always moving in a molecule and creating temporary dipoles. These temporary dipoles include temporary dipoles in neighbouring molecules
How does the size of a molecule affect its VdW’s forces?
- strength of VdW attractions are affected by the size of the molecule
- as size of molecule increases, electron cloud size increases with more electrons
- therefore temp dipole at any moment is bigger - stronger attraction
Give examples of where VdW’s forces influence the physical properties of a substance
- increasing boiling point descending group VII
- increasing boiling point when the chain length of an alkane increases
Define permanent dipole-dipole attraction
The attraction between the δ+ end of a permanent dipole on a molecule and the δ- end of the permanent dipole of a neighbouring molecule
Define non-polar molecule
Some molecules and ions contain polar bonds but individual dipoles cancel out and the molecule doesn’t possess a permanent dipole eg CCl4
Define polar molecule
Molecules with polar bonds whose net effect is to make one end of the molecule δ- and the other δ+ giving the molecule a permanent dipole
Define hydrogen bond
The attraction between a lone pair of electrons on a very electronegative atom (N, O or F) in one molecule and a hydrogen atom in a neighbouring molecule, in which the hydrogen atom is covalently bonded to N, O or F
How does the boiling point of water relate to its intermolecular forces?
The boiling points if H2O, HF and NH3 are higher than expected as the H bonds between the molecules of these compounds require a lot of energy to break
Why does ice has a lower density than water?
- Water expands on freezing because the water molecules move apart to maximise the effects of hydrogen bonding in the structure of ice (each H2O molecule is tetrahedrally H bonded to the other 4 H2O molecules)
- ice has an open lattice with H bonds holding the water molecules apart. when ice melts the rigid H bonds collapse, allowing the H2O molecules to move closer
How does the miscibility of liquids relate to their intermolecular forces?
Liquids mix because they form the dame intermolecular forces (like dissolves like) - polar dissolves polar and nonpolar dissolves nonpolar
How does the viscosity of liquids relate to their intermolecular forces?
The greater the intermolecular forces, the greater the viscosity
How does the melting/boiling point of liquids relate to their intermolecular forces?
The higher the point, the more energy is required to break the intermolecular forces
What forces do nonpolar liquids have and give examples of nonpolar liquids
Van der waals
Eg bromine and hexane
What forces do polar liquids have and give examples of polar liquids
Van der waals and dipole interactions
Eg dichloromethane, chloroform CHCl3, acetone C3H6O
What forces do liquids with H bonds have and give examples of these liquids
Van der waals and H bonding
Ammonia NH3, water, ethanol C2H5OH
