Ozone Story

• the ability of an atom to attract the bonding electrons in a covalent bond is called electronegativity 


  it is measured using the pauling scale
• fluorine (4.0) has the highest electronegativity, followed by oxygen
• electronegativity increases across periods and decreases down groups 


  the least electronegative elements have a value of around 0.7
• when two atoms with different electronegativities are joined together, they share their electron pair unequally
• non-polar molecules do not have any permanent dipoles because all the bonds are non-polar or the dipoles cancel out
• when there is a difference in electronegativity, the bonding electrons will be pulled towards the electronegative atom and there will be a charge across the bond (polar)


  the difference in electronegativity causes a dipole
• polar bonds don’t always make polar molecules 


  a more complicated molecule may have several polar bonds. if they are arranged in opposite directions, they will cancel each other out so the molecule is non polar overall

  if they point in roughly the same direction, the molecule will be polar
• there are three types of intermolecular bonds

  • instantaneous dipole bonds
  • permanent dipole bonds
  • hydrogen bonds
• Instantaneous dipole bonds


  Cause all atoms and molecules to be attracted to each other
• Electrons in charge clouds

  • Always moving really quickly
  • At any particular moment, the electrons are likely to be more to one side than the other
  • At this moment, it would have a temporary dipole
• Temporary dipole induction


  1. Dipole can induce another temporary dipole in the opposite direction on a neighbouring atom

  2. They are then attracted to each other
• Domino effect of dipole induction

  The second dipole can induce a third, like a domino effect
• Because the electrons are constantly moving, the dipoles are being created and destroyed all the time
• Even though the dipoles are changing, the effect is still the same
• intermolecular bonds in organic molecules depend on the shape


  -the longer the carbon chain, the stronger the instantaneous dipole bonds because there is more molecular surface contact and more electrons to interact

  -branched chain alkanes can’t pack closely together and their molecular surface contact is small compared to straight chain alkanes
• the heavier the atom or molecule, the stronger the intermolecular bonds


  larger molecules have larger electron clouds, so stronger instantaneous dipole bonds

  • for a liquid to boil, the intermolecular bonds need to be overcome. you need more energy to overcome stronger intermolecular bonds so liquids with stronger instantaneous dipole induced dipole bonds will have higher boiling points
• as you go down the group, the instantaneous dipole induced dipole forces increase because the atomic size increases
• polar molecules also have permanent dipole bonds
• hydrogen bonding is the strongest intermolecular force

  this only happens when hydrogen is covalently bonded to fluorine, nitrogen or oxygen


  they are very electronegative so draw the bonding electrons away from the hydrogen atom

  the bond is polarised so polarised and hydrogen has such a high charge density as it is so small that the hydrogen atoms form weak bonds with lone pairs of electrons
• hydrogen bonds explain why ice is less dense than water


  in ice, the water molecules are arranged so that there is the max number of hydrogen bonds. the lattice structure formed wasted lots of space

  as the ice melts, some of the hydrogen bonds are broken and the lattice breaks down allowing molecules to fill the spaces
• substances that form hydrogen bonds have high melting and boiling points because a lot of energy is required to overcome the intermolecular bonds

  hydrides of oxygen,nitrogen and fluorine generally have the highest boiling points compared to any other hydrides in their group
• some compounds have multiple types of intermolecular bonds


  for example, butan-1-ol and butan-2-ol have the same molecular formula and form one hydrogen bond per molecule . this difference is due to the strength of instantaneous dipole induced dipole bonds between the molecules
• particles must collide to react but only react when conditions are right

  • they must collide in the right direction and facing eachother the right way
  • they collide with at least a certain amount of kinetic energy
• the minimum amount of kinetic energy particles need to react is called the activation enthalpy or activation energy 


  reactions with high activation energy need to be given extra energy to react by heating them
• enthalpy profile diagram

  
• molecules in a gas don’t all have the same amount of energy

  
• increasing the temperature makes the reactions faster. the particles on average will have more kinetic energy and move faster 


  so a greater proportion of molecules will have the activation energy and be able to react so pushes the boltzmann distribution curve to the right

  
• there are four things affecting the reaction rate

  • increasing concentration
  • increasing pressure
  • catalysts
  • increasing temperate
• increasing concentration 


  means the particles are closer together on average. if they’re closer, they’ll collide more often. more collisions mean more chances to react
• increasing pressure speeds up reactions 


  same reason as concentration
• catalysts can speed up reactions


  they lower the activation enthalpy by providing a different way for the bonds to be broken and remade. if activation enthalpy is lower, more particles will have enough energy to react
• increasing temperature 


  more energy so more likely to react when they collide. and because they’re moving faster, they collide more frequently
• you can use experiments to find out the rate of a reaction. you can either monitor the loss of a reactant or the formation of a product at regular intervals throughout a reaction.


  the methods you can use

  • measuring the volume of a gas produced
  • measuring the loss of a mass as a gas is produced
  • measuring the change in ph
  • measuring a temperature change
  • taking samples at regular intervals and analysing them by titrations
• homogenous catalysts are in the same state are the reactants

  they increase the rate of reaction by providing an alternative reaction pathway with a lower activation enthalpy. the catalyst is chemically unchanged at the end of the reaction
• homogenous catalysts wor by forming intermediate compounds with the reactants. the products are then formed from them

  its enthalpy profile will have two humps on it
• haloalkanes are alkanes with halogen atoms
• the boiling points of the haloalkane increase down the group


  as you go down the group, the atomic radius and the number of electron shells they have decreases

  this leads to stronger instantaneous dipole induced dipole forces between molecules

  you have to put in more energy to overcome them
• the carbon-halogen bond in haloalkanes is polar


  fluorine,chlorine and bromine are much more electronegative than carbon

  the carbon is electron deficient and it can be attacked by a nucleophile (electron pair donor)

  OH-, NH3 and H2O are all nucleophiles that can react with haloalkanes
• haloalkanes can undergo nucleophilic substitution

  
• water can act as a nucleophile too 


  it forms an intermediate with an oxygen that has three bonds. this is unstable so on O-H bond breaks