chp 12 and 15 Flashcards
importance of shape of molecule
affects vapour pressure, melting point, boiling point and solubility. shape determines how it interacts with other molecules
VSEPR
uses knowledge of valence shell to predict shape
based on principle that negatively charged electron pairs in valence shell repel each other. meaning that these electrons are arranged as far away from each other as possible
electron pairs in the single covalent bonds repel each other
lone pairs are treated the same as electron pairs in covalent bonds because they still take up space int he atom
double and triple bonds are treated as a single bond
tetrahedral (methane)
electron pairs in the single covalent bonds repel each other
repulsion forces the bonds as far apart from each other as possible, so hydrogens are arranged in a tetrahedral shape at an angle of 109.5º
pyramidal (ammonia)
lone pairs are treated the same as electron pairs in covalent bonds because they still take up space int he atom
3 hydrogen atoms for a pyramidal shape with the nitrogen. the ,one pair occupies more space because it isn’t covalently bonded to a and is therefore closer to the nitrogen. this means that the 3 covalent bonds are pushed closer together making the angle less than 109.5º
trigonal planar (water)
2 lone pairs and 2 covalent bonds
4 electron pairs repel each other and 2 hydrogen atoms for a v shape with the oxygen. because of the two lone pairs the 2 covalent bonds are pushed closer together
angle around central atom less than 109.5º
120º
linear (HF)
fluorine has 3 lone pairs and 1 covalent bond
four pairs form a tetrahedral arrangement due to mutual repulsion
results in a linear model
180º
BH3
BeH2
3 electron pairs, no lone pairs
trigonal planar
2 electron pairs, no lone pairs
linear
strength of intermolecular forces
covalent molecular forces have lower melting and boiling points because forces BETWEEN molecules are weaker. these forces are disturbed when covalent substance change state
vapour pressure
pressure that gaseous molecules exert on the closed container walls when the rates of evaporation and condensation become equal
liquids with stronger intermolecular forces have lower vapour pressure molecules are held together tightly making it harder for them to escape from the surface of the liquid
increasing temp increase vapour pressure. molecules have a higher average kinetic energy that enables them to overcome intermolecular forces. as temperature increases, more particles have enough energy to escape from liquid to gas. increasing pressure
boiling point of a liquid
temperature at which the liquids vapour pressure reaches the atmospheric pressure of the surroundings
electronegativity and polarity
in intermolecular forces, the electrostatic attraction is between positive and negative charges in the molecules
charges result because of uneven electron distribution within molecules
electronegativity is the tendency of an atom in a covalent bond to attract electrons
in a covalent bond atoms are competing for electrons being shared between them
non polar diatomic molecules
when atoms have identical electronegativities, then electrons are shared equally between them. non polar because there is no charges on either end
even distribution of valence electrons between atoms
if electronegativities between atoms are very similar, then it can be considered non-polar
electron density
measure of the probability of an electron being present at a particular location within an atom
polar diatomic molecules
2 atoms with different electronegativities, electrons stay closer to the most electronegative atom because they are more strongly attracted to that atom
polar: imbalanced electron distribution. has two oppositely charged poles
all diatomic with different atoms are polar to some extent, greater difference in electronegativities the greater the polarity
delta
partial charge, adding up partial charges = 0 because charge are due to uneven sharing of electrons, so molecules are still neutral
negative: has larger share of electrons, excess of electrons
positive: lost some of its share of electrons
dipole
separation of positive and negative charges
polyatomic molecules polarity
symmetrical balanced dipoles= non-polar
methane: carbon is more electronegative but individual dipoles cancel each other out. results in a molecule with no overall dipole
in non-symmetrical molecules, individual dipoles don’t cancel each other, creating a net dipole making the molecule polar
dipole dipole forces
only in polar molecules
results from attraction between positive and negative ends of polar molecules
weak since partial charge are small
more polar the molecule, the stronger the force
stronger force the higher the melting and boiling points because force bond molecules together in solid or liquid. higher energy is required to to break stronger bonds
hydrogen bonding
only occurs in molecules in which a hydrogen atom is covalently bonded to a N,O or F
NOF are small and very electronegative so they strongly attract electrons in a covalent bond. creates a large partial positive charge on hydrogen atom, which is attracted to lone pairs of electrons in neighbouring NOF atoms
hydrogen in covalent bond with NOF is is exposed due to lack of electrons and is highly attracted to lone pairs of NOF in other molecules
strong intermolecular bond, x10 than dipole dipole, but 1/10 of ionic
dispersion forces
forces of attraction in non polar substances (existence of intermolecular forces, without them nothing will hold molecules together and they would be gases)
caused by temporary (instantaneous) dipoles in molecules that result in random movement of the electrons surrounding molecule
strength increases as size of molecules increase. larger molecules have more electrons so it is easier to create temporary dipoles. larger molecules have higher melting and boiling points
molecules that form long chains have higher dispersion forces, more contact area to interact with it neighbouring molecules to form stronger forces
how to increase polarity of a molecule
larger difference in electronegativities
high degree of asymmetry in the shape of the molecule causing an imbalance in the bond dipole
requirement of hydrogen bonding
- hydrogen atom covalent bonded to an NOF atom
- lone pair of electron of NOF atom of neighbouring molecules
H bonds are caused by NOF’s small radius and high electronegativities and single electron of hydrogen atom
larger radii reduce concentration of negative charge around them.
hydrogen bonding in water
ice floats because it is less dense
in ice water molecules are held in a crystal lattice. in which molecules are held further part then in liquid water
when ice melts crystal lattice collapses and molecules pack together more tightly
crystal has greater volume, 4 hydrogen bonds to 4 neighbouring molecules
electrostatic attraction
strong in water because of large electronegativity difference meaning larger partial charge
water properties
can dissolve many substances easily high specific heat capacity high latent heat of vaporisation expands on freezing react with CO2 to make glucose and oxygen
