SHAPES OF MOLECULES AND INTERMOLECULAR FORCES Flashcards
The electron pair repulsion theory
Electron pairs surrounding central atom determines the molecule / ion shape
Pairs repel each other so they are arranged as far as possible
Arrangement minimises repulsion, so hold atoms in definite shape
Different number of electron pairs = different shape
What 2 factors determines the shape of a molecule / ion
Bonding regions
Lone pairs
Lone pair
Lone pair slightly closer to central atom
Occupies more space than a bonded atom because they repel more strongly than a bonding pair
How does a lone pair affect the bond angle?
Reduces the bond angle by 2.5 per lone pair
What is the bond angle of a tetrahedral molecule
- 5
e. g. CH4
What is the bond angle of a pyramidal molecule
107
e.g. NH3
What is the bond angle of a non linear molecule
104.5
Non-linear
A molecule with 2 bonding regions
Bond angle : 180
linear
e.g. CO2
A molecule with 3 bonding regions
Bond angle: 120
Trigonal planar
e.g. BF3
A molecule with 4 bonding regions
Bond angle: 109.5
Tetrahedral
e.g. CH4
A molecule with 6 bonding regions
Bond angle: 90
Octahedral
e.g. SF6
Examples of tetrahedral molecules
CH4
Ammonium ions
SO4^2-
Examples of trigonal planar molecules
CO3 ^2-
NO3 ^-
What is meant by electronegativity
The attarcted of a bonded atom for the pair of electrons in a covalent bond
What scale is used to compare electrongetavitivties
Pauling scale
What is the most electronegative element?
Fluorine
Electronegativity trend
Electronegativity increases upwards and across towards fluorine
Group 1 is the least electronegative elements
What kind of bond is formed where there is a large difference in electronegativities?
Ionic rather than Covalent
Non polar bond
Bonded electron pair shared equally between bonded atoms
When atoms are the same element / have similar electrongetiavity
Pure covalent bond
Bonded atoms are the same element
Polar covalent bond
Bonded electron pair shared unequally
Different atoms with different electronegativity.
More electronegative has greater attraction for bonded pair electrons
Polar bonds are polarised with…
Dipoles
Permanent dipole
Dipole in a polar covalent bond which doesn’t change
What can polar bonds do?
H2O and CO2
Polar bonds may reinforce one another to produce larger dipole over whole molecule or cancel out
H2O:
Polar
OH bonds have permanent dipoles
Act in opposite directions but don’t exactly oppose each other
Overall O end is delta neg and H end is delta positive
CO2: Non-polar C=O have permanent dipole Act in opposite directions and exactly oppose each other Overall dipoles cancel
Ionic lattices dissolving in polar solvents
Water molecules attract +/- ions
Ionic lattice breaks down as dissolves
Water molecules surround ions
+ ions attracted towards delta negative oxygen of water
- ions attracted to delta positive hydrogen of water
What are intermolecular forces
Weak interactions between dipoles of different molcules
Induced dipole-dipole interactions (London forces)
Exist between all molecules, only temporary
Movement electrons produces changing dipole in any molecule, at any instant an instantaneous dipole will exist but position shifts constantly
Instantaneous dipole induces a dipole on neighbouring molecule, which induces dipoles on further molecules, they then attract one another.
More electrons in each molecule
Larger instantaneous and induced dipoles
Greater induced dipole - dipole interactions
Stronger attractive forces
(Explains increased bp of noble gases)
Permanent dipole - dipole interactions
Act between permanent dipoles in polar molecules
Mean boiling point of polar molecules is much greater:
Have both london and permanent dipole-dipole interactions
Extra energy needed to break additional permanent interactions
So bp is higher
Why is the mean boiling point of polar molecules high
Mean boiling point of polar molecules is much greater:
Have both london and permanent dipole-dipole interactions
Extra energy needed to break additional permanent interactions
So bp is higher
What are simple molecules?
Simple molecules contain only a few atoms held together by covalent bonds.
e.g CO2
What do simple molecules form in a solid state?
Simple molecular lattices
Held together by weak intermolecular forces
But atoms within molecules bonded strongly with covalent bonds
The bp and mp of of simple molecular substances
Low mp/bp
Only weak intermolecular forces break
Not strong covalent bonds
Nonpolar simple molecules are soluble in…
Nonpolar solvents
Why are nonpolar simple molecules soluble in nonpolar solvents
Nonpolar simple molecules tend to be soluble in non-polar solvents (hexane) because intermolecular forces form between molecules and solvent, weakening intermolecular forces in simple lattice, so they break and the compound dissolves
Simple molecules are insoluble in…
Polar solvents
Why are simple molecules insoluble in polar solvents?
Tend to be insoluble in polar solvents (water) because of little interaction between molecules in lattice and solvent molecules, intermolecular bonding in solvent too strong to be broken
What determines the solubility of polar simple molecular substances?
Depends on strength of dipole
May dissolve because polar solute / solvent molecules can attract each other
Some with part polar and part non-polar dissolve
Do simple molecular structures conduct electricity?
No
No mobile charged particles within structure
So nothing to complete an electrical circuit
How many lone pairs does Nitrogen, Oxygen and Fluorine have?
Nitrogen:
Three bonds and 1 lone pair
Oxygen
Two bonds and 2 lone pairs
Fluorine (and all halogens):
One bond and 3 lone pairs
Where is hydrogen bonding found?
Found in molecules containing Electronegative atom with lone pair electrons (O,N,F) attached to a hydrogen atom
Why does ice float?
Solid is less dense than liquid water
Hydrogen bonds hold water molecules apart in open lattice structure, water molecules in ice is held further apart than in water, so ice is less dense that liquid water and floats
2 lone pairs on oxygen and 2 hydrogens, so each molecule can form 4 bonds = open tetrahedral lattice full of holes
