Chapter 6: Shapes of molecules and intermolecular forces (6.1, 6.2) Flashcards
what are the three types of intermolecular forces (weakest to strongest)
induced dipole-dipole interactions (London forces)
permanent dipole-dipole interactions
hydrogen bonding
how do induced dipole-dipole interactions (London forces) come about
- movement of electrons movement of electrons produces a changing dipole in a molecule
- at any instant, an instantaneous dipole will exist, but its position is constantly shifting
- the instantaneous dipole induces a dipole on a neighbouring molecule (the positive charge attracts any electrons or the other way around)
- the induced dipole induces further dipoles on neighbouring molecules, which attract one another
how permanent are induced dipoles
they are temporary, can appear and then disappear in an instant of time
electron-pair repulsion theory
The electron pairs surrounding a central atom determine the shape of the molecule or ion
The electron pairs repel one another so that they are arranged as far apart as possible
The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape
Different numbers of electron pair result in different shapes
lone pairs of electrons repel more than bonded pairs of electrons
greater the number of electron pairs, the smaller the bond angle will be in a molecule
4 bonded pairs, 0 lone pairs
tetrahedral
109.5 degree bond angle
3 bonded pairs, 1 lone pair
pyramidal
107 degree bond angle
2 bonded pairs, 2 lone pairs
non linear
104.5 degree bond angle
2 bonding regions
linear
180 degree bond angle
3 bonded pairs, no lone pairs
trigonal planar
120 degree bond angle
6 bonded pairs, no lone pairs
octahedral
90 degree bond angle
factors that effect electronegativity
different nuclear charges (high nuclear charge means high electronegativity)
atoms may be different sizes (low atomic radius means high electronegativity)
one shared pair of electrons may be closer to one nucleus than the other (closer means high electronegativity)
electronegativity trends in the periodic table
electronegativity increases up and along (right) the periodic table
non-polar bonds
the bonded electron pair is shared equally between the bonded atoms
a bond will be non-polar if the bonded atoms are the same or if the bonded atoms have similar electronegativity
non-polar solutes are soluble in non-polar solvents (e.g. hydrocarbon liquids like hexane)
polar bonds
the bonded electron pair is attracted more to one of the bonded atoms than the other because it is more electronegative
polar solutes are soluble in polar solvents (e.g. water)
What effects the strength of induced dipole-dipole interactions?
More electrons, larger instantaneous and induced dipoles, the greater the induced dipole, dipole interaction and the stronger the attractive forces between molecules.
Why is the boiling point of a polar substance larger than a non polar substance with the same number of electrons.
1) Because non-polar molecules only have london forces
2) Polar substances have London forces and permanent dipole-dipole interactions between molecules
3) Extra energy is needed to break additional dipole-dipole interactions
How do elements get their shapes?
The electron pairs repel one another so they are as far apart as possible to minimise repulsion giving them a definite shape.
How much is the bond angle of a molecule reduced with each lone pair?
2.5 degrees
What shape is a carbonate ion?
Trigonal planar
What shape is a nitrate ion?
Trigonal planar
What shape is a sulfate ion?
Tetrahedral
What shape is an ammonium ion ?
Tetrahedral
How are things charged in a polar molecule?
The more electronegative atom is negatively charged (d-) and the other atoms are positively charged (d+)
How do you know if a molecule with more than two different atoms is polar?
It depends on the shape of the molecule, the dipoles may reinforce each other making a larger dipole or they may cancel each other out.
Give an example of where two dipoles cancel each other out to make a non polar molecule?
Carbon dioxide
1) The two C=O have a permanent dipole
2) The two dipoles act in opposite directions because the shape is linear
3) Overall the dipole cancels out and the overall dipole is 0
Explain how water breaks down a sodium chloride lattice (an ionic substance)?
1) Water attracts Na+ and Cl- ions
2) Na+ ions attracted towards oxygen of water molecules and Cl- ions are attracted towards the hydrogen in the water molecules
3) The ionic lattice breaks down and dissolves
How do hydrogen bonds form?
Between molecules containing an electronegative atom with a lone pair of electrons and a hydrogen atom attaches to this electronegative atom.
Why is solid Ice less dense than water?
Hydrogen bonds hold water molecules apart in an open lattice structure, the water molecules are held further apart in Ice than in water and this open lattice makes solid Ice less dense than liquid water and floats
Why does water have a relatively high boiling point?
1) Water has london forces but also hydrogen bonds
2) A reasonably large amount of energy is needed to break the hydrogen bonds in water
3) This gives water a higher boiling point than what would be expected from just London forces.
What are some other anomalous properties in water?
It has a relatively high surface tension this allows insects to walk on pond surfaces.
bond enthalpy of London forces
1-10 kJ/mol
bond enthalpy of permanent dipole-dipole interactions
3-25 kJ/mol
bond enthalpy of hydrogen bonds
10-40 kJ/mol
bond enthalpy of single covalent bonds
150-500 kJ/mol
electronegativity difference in pure covalent bonds
0-0.4
electronegativity difference in polar covalent bonds
0.4-1.8
electronegativity difference in ionic bonds
greater than 1.8
