Ozone Story Flashcards
Define electronegativity.
Measures the attraction of a bonded atom for the pair of electrons in a covalent bond.
How is electronegativity measured?
Measured using the Pauling scale. It look at atoms within a molecule.
trend in electronegativity?
- increases across periods
- decreases down groups
(ignoring noble gases)
what are the most electronegative elements?
nitrogen, oxygen, fluorine, chlorine
electronegativity - polar covalent bonds
if the two atoms have different electronegativities the bonding e- will be pulled towards the more electronegative atom.
this causes the electrons to be spread unevenly across the bond, and so their will be a charge across the bond. (each atom has a partial charge)
in a polar bond the difference in electronegativity between the two atoms causes a dipole
- a dipole is a difference in charge between the two atoms caused by a shift in electron density in the bond
eg. HCl
the greater the difference in electronegativity….
….the more polar the bond
electronegativity - using the pauling scale
given values
find difference between the values
bonds are polar if difference is more than 0.4
what are the three types of intermolecular bonds?
- instantaneous dipole - induced dipole
- permanent dipole - permanent dipole
- hydrogen bonding
What is a permanent dipole?
A small charge difference across a bond, resulting from a difference in electronegativities of the bonded atoms.
Define an intermolecular force.
An attractive force between neighbouring molecules.
Describe intermolecular bonding: instantaneous dipole - induced dipole
all atoms form ID-ID bonds
electrons in charge clouds are always moving really quickly. At any particular moment, the electrons in an atom are likely to be more to one side than the other - INSTANTANEOUS DIPOLE
this dipole can INDUCE another temporary dipole in the opposite direction on a neighbouring atom
- a second dipole can induce a third
because electrons are constantly moving the dipole are being created and destroyed all the time
- even though the dipoles are changing the overall effect is for the atoms to be attracted to each other
Describe intermolecular bonding: permanent dipole - permanent dipole bonds
the slightly +ve and slightly -ve charges on polar molecules cause WEAK ELECTROSTATIC FORCES of attraction between the molecules
these are the PD-PD bonds
eg. H-Cl—–H-Cl—–H-Cl
+ - + - + -
happen as well as ID-ID (not instead of)
Describe intermolecular bonding - hydrogen bonds
only happens when hydrogen is covalently bonded to Fluorine, Nitrogen or Oxygen
- F, N, O are very electronegative so they draw the bonding electronns away from the hydrogen atom
the bond is so polarised, and hydrogen has such a high charge density (v. small) that the hydrogen atom forms weak bonds with LONE PAIRS on F, N, O atoms of other molecules
water and ammonia both have hydrogen bonding
organic molecules with -OH or -NH groups form hydrogen bonds
intermolecular bonding - why is ice less dense than water?
because of hydrogen bonding:
in ice water molecules are arranged so that there is the maximum number of hydrogen bonds
- the lattice structure formed in this way ‘wastes’ a lot of space
as the ice melts some of the hydrogen bonds are broken and the lattice breaks down
- allow molecules to ‘fill’ the spaces
this causes ice to be less dense than water
intermolecular bonding - how do hydrogen bonds affect how a substance behaves?
- hydrogen bonds are the strongest intermolecular bonds and have a huge affect on the properties of substances
- substances that form hydrogen bonds have high melting and boiling points because a lot of energy is needed to overcome the intermolecular bonds
hydrides of N, O, F usually have the highest boiling points because a lot of energy is needed to break the hydrogen bonds (N, O, F the most electronegative) - substances that form hydrogen bonds are also soluble in water
- can form hydrogen bonds with water molecules, allowing them to mix and dissolve
intermolecular bonding - boiling points of halogens trend?
as you go down the group the boiling points increase
this is because the Mr increases so number of shells of electrons increases and so the atomic/molecular size increases (so stronger ID-ID bonds)
What does PPM stand for?
Parts per million
Amount of gas particles in a sample containing 1 million particles
How can you calculate percentage composition from PPM?
To convert from ppm to %, ÷ by 10,000
What are haloalkanes?
an alkane with at least one halogen atom in place of a hydrogen atom
