C6- Shapes of Molecules and Intermolecular forces Flashcards
Electron Pair repulsion theory
Each electron region takes up a position to minimise repulsion
Molecular shape
2 Areas of electron density
180
Linear
Molecular shape
3 Areas of electron density
Trigonal planar
120
Molecular shape
6 areas of electron density
Octahedral
90
Molecular shape
5 areas of electron density
Trigonal bipyramidal
Two bonds at 180
other 3 in a plane at 120
Molecular shape
4 areas of electron density
depends on number of lone pairs present on central atom
lone pairs repel more than bonded pairs
2.5 degrees lost per lone pair gained
Molecular shapes
4 AED 0 lone pairs
Tetrahedral
109.5
Molecular shapes
4 AED 1 Lone pair
Pyramidal
107
Molecular shapes
4 AED 2 Lone pairs
Non Linear
104.5
Molecular shape
wedge/ dash
wedge= towards
dash= away
Electronegativity
ability of an atom to attract a pair of electrons in its own covalent bonds
Pauling scale
Compare electronegativity of atoms of different elements
Large value= Very electronegative
As nuclear charge increases, atomic radius decreases
Most electronegative atoms
F O N Cl
( and group 7)
Polar bond
Asymmetric distribution of charge across the bond
Polar covalent bond
Delta= Slight charge
Non Polar bond
Pure covalent bond
Bonded electron pair is shared between bonded atoms
Usually:
- Atoms are the same e.g. Br2
- Atoms have the same/ similar electronegativity values
Dipole
Permanent dipole
Separation of opposite charges
Polar molecule
There is a difference in electronegativities/ the bonds are polar
Molecule is not symmetrical therefore dipole do not cancel out
Polar solvents and solubility
Polar solvent molecules attract ions
Ionic lattice breaks down as it dissolves
Solution surrounds ions
Permanent Dipole- Dipole interactions
Permanent dipoles attract one and other
Molecules must be polar
London Forces
’ induced dipole-dipole interactions ‘
ALL molecules
Movement of electrons produces changing dipoles
At one moment instantaneous dipole exists, position shifting
Instantaneous dipole induces dipole on neighbouring molecule
Induced dipole induces further dipoles, attract one and other
Strength of london forces
’ induced dipole-dipole interactions’
More Electrons=
Larger instantaneous and induced dipoles
Greater the induced dipole-dipole interactions
stronger attractive forces between molecules
Explain how an instantaneous dipole formed
Fluctuation in electron density in electron density around a molecule creates an instantaneous dipole in a molecule
The instantaneous dipole induces a dipole in a neighbouring molecule
Why does I have a higher boiling point than Br
group 7 = F, Cl, Br, I, At
I2 molecules have more electrons
Form stronger London dispersion forces
Require more energy to overcome
Why does HCl have a higher melting point than Cl2
HCl is a polar molecule, so forms permanent dipole forces
These are stronger than London dispersion forces formed by non polar molecules like Cl2
HCl requires more energy to overcome forces
H bonding
Specific stronger example of a permanent dipole dipole interaction
Small electronegative atom with lone pair interacts with a H atom which is bonded at 180 to N, O or F
Criteria for drawing H bonds
Draw all lone pairs
Draw all partial charges
H bond from atom to lone pair, drawn at 180
Simple molecular substance
Low melting point and boiling point
In a simple molecular lattice (solid) weak intermolecular forces can be overcome by energy present at low temperatures
Covalent bonds are not overcome
Non polar simple molecular substance in non polar solvent
Soluble
Intermolecular forces form between molecules and the solvent
Intermolecular forces overcome and compound dissolves
Non polar simple molecular substance in a polar solvent
Insoluble
Little interaction between molecules in lattice and solvent molecules
The intermolecular bonding within polar solvent is too strong to be broken
Simple molecular substances
Solubility
Polar
–> dissolves similar to an ionic compound
Solubility depends on strength of dipole
Why does water have a relativley high melting point and boiling point
H bonds are extra forces over and above London forces
More energy needed to break than with just London forces
–> appreciable amount
H bonds broken upon boiling
Why does ice float in water
Hydrogen bonds hold water molecules apart in an open lattice structure
Water molecules are further apart in ice than in water
Ice is less dense than water, so it floats
Ionic compound
Solubility
Charged ions can interact with the polar solvent
Breaking down the ionic lattice.
The polar molecules attracting and surrounding the ions
exam technique for 3 marker on name and explain the molecular shape
Name the shape
State number of Bonding pairs and lone pairs
Electrons repel as far apart as possible AND lone pairs repel more than bonding pairs
