Intermolecular forces, haloalkanes Flashcards
OZ5 OZ 6
what are intermolecular forces?
weak forces holding simple molecules together are called intermolecular forces. they are broken when heat is applied causing state changes.
what are the 3 main types of intermolecular forces?
- Instantaneous dipole induced dipole forces, IDID’s
- Permanent dipole - Permanent dipole forces, PDPD’s
- Hydrogen bonding
whats the criteria for IDID’s?
having electrons, all small molecules and the noble gases have IDID’s
how do IDID’s arise?
IDID’s arise due to the random movement of electrons in a molecule creating a temporary and instantaneous dipole dipole. this will then induce a dipole in the neighbouring molecule.
whats the criteria for a PDPD’s?
2 polar molecules, permanent dipole is attracted to another permanent dipole.
whats the criteria for hydrogen bonding?
H directly bonded to F/O/N on molecule two
whats the maximum number of hydrogen bonds that a water molecule can make?
4
whats the marking criteria for drawing a hydrogen bond diagram?
- bond angle between H and molecule 2 must be 180 degrees
- partial charges must be shown
- lone pairs must be shown on molecule 2
what is a hydrogen bond?
hydrogen bonding is just an extra strong PDPD force. it is unusually strong as F, O,and N all have a very high electronegativity which makes the H unusually partially positive. the lone pair on a F, O and N will attract this partially positive H very strongly.
what are the consequences of intermolecular forces (IMF)?
- the stronger the IMF the more energy is needed to break them so the higher the melting and boiling point
- molecules with the same type of IMF will mix with each other in the liquid state
- the strength of IMF determines the surface tension and viscosity of a liquid
- ice is less dense that water because of the IMF
what does molecules mixing when they have the same IMF show?
how some molecules will mix with water and others won’t
(oil + water)
why is ice less dense than water?
each water molecule creates 4 hydrogen bonds with neighbouring molecules, creating a giant lattice with large spaces in between. these large spaces in between mean the ice is less dense.
can you compare PDPD’s and hydrogen-bonds?
no, since as far as we know all H-bonding is of the same strength and all PDPD’s are the same strength
how can we compare IDID’s?
- size of the molecule, more e- means stronger IDID’s
- shape of molecule, more linear = molecules get closer together = stronger IDID’s
how to determine the bonds and mbp and bpt in a simple molecule?
is there a H directly bonded to F, O, N?
/ yes \ no
H-bonds polar bonds?
+ / yes \n
IDID’s symmetrical? \
| / y \ n IDID’s
high MP IDID’s PDPD’s |
+ BP | + v low
v low mp + bp IDID’s mp
| +
low mp + bp bp
describe the reactivity of haloalkanes?
the relative reactivity depends on the halogen present,
Electronegativity decreases down the group.
which intermolecular bond can you compare when trying to work out the MP + BP of two molecules?
PDPD’s all have the same strengths so cannot be compared.
you can compare IDID’s, there are two factors to be considered.
1. bond length
2. bond polarity
Bond length is more important since the longer the bond the weaker and therefore the easier the electron can be gained.
whats the melting and boiling points of water and why?
relatively high, this is due to the fact water can make 4 hydrogen bonds therefore a lot of kinetic energy is required to break the bonds
how does electronegativity give an uneven distribution of charge which results in polar covalent bonds?
electronegativity is the ability of an atom to pull the bonding electrons towards itself, therefore the atom with the highest electronegativity the stronger it will be able to pull the bonding electrons closer and become partially negative whilst the other atom becomes partially positive
why does electronegativity increase across a period?
each atom has one more proton in its nucleus yet the same number of shells so the nucleus becomes less shielded as the charge becomes more positive.
why does electronegativity decrease down a group?
going down a group each atom has one more electron shell, therefore the nucleus becomes more shielded and the positive charge decreases
how are IDID’s affected by branching?
the branches prevent the molecules becoming close enough to form IDID’s creating weak intermolecular bonding
give an example of a balanced equation haloalkane reacting with hydroxide ions
CH2BrCH3 + OH- => CH3CH2OH
name the top 4 most electronegative elements
F - 4.0
O - 3.5
N, Cl - 3.0
whats the trend in bond polarity of the carbon-halogen bonds
the carbon will be partially charged and therefore attract nucleophiles. the bind formed will be broken by heterolytic fission
describe and explain the experiment used to investigate the reactivity of haloalkanes
- in a test tube add 1cm cubed of a haloalkane
- add a few drops of ethanol
- warm some AgNO3
- add the AgNO3 to the test tube and plug the top with cotton wool, then place into a water bath
- iodine will react first, then bromine then chlorine
describe the boiling points of haloalkanes
IDID’s increase from fluoro to iodo haloalkanes due to the increase in electrons
what experiment shows the reactivity of haloalkanes?
- to 1cm3 of each haloalkane add a few drops of ethanol
- warm some AgNO3 (silver nitrate)
- add to the haloalkane ethanol mix, then plug the top with cotton
- leave in water bath
- Iodine will react first, Bromine second, Chlorine third
