OZ - bonding and structure *01 *02 Flashcards
what is electronegativity?
the ability of an atom to attract the bonding electrons in a covalent bond
how is electronegativity measured?
using the pauling scale
trend in electronegativity?
increases across periods
decreases down groups
(ignoring noble gases)
what are the most electronegative elements?
nitrogen, oxygen, fluorine, chlorine
electronegativity - non-polar covalent bonds
if both atoms have similar electronegativity the e- will sit midway between the two nuclei and the bond will be non-polar
eg. H2, Cl2
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
electronegativity - polar molecules: what makes it polar?
whether a molecule is polar or not depends on SHAPE and the POLARITY of bonds
a polar molecule has an overall dipole
eg.
HCl is polar
CO2 is non-polar: the polar bonds balance/cancel each other out
CHCl3 is polar: the polar bonds all point in roughly the same direction so overall dipole
what are the three types of intermolecular bonds?
- instantaneous dipole - induced dipole
- permanent dipole - permanent dipole
- hydrogen bonding
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
intermolecular bonds - in organic molecules
depends on shape!
eg. alkanes
- ID-ID bonds between molecules
- the longer the carbon chain the stronger the ID-ID bonds because there is more molecular surface contact and more electrons interact
- so as molecules get longer, harder to separate them - needs more energy to overcome ID-ID bonds
- branched-chain alkanes cannot pack closely together so their molecular surface contact is small compared to straight-chain alkanes (of a similar molecular mass)
- so fewer ID-ID bonds can form
intermolecular bonding - what can affect the strength of the bond?
the heavier the atom the stronger the intermolecular bonds
- not all ID-ID bonds are the same strength.
Larger molecules have large electron clouds so stronger ID-ID bonds
molecules with greater surface area also have stronger ID-ID forces because they have a bigger exposed electron cloud
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)
intermolecular bonds - experiment to find out if molecules in a liquid are polar or non-polar
put an electrostatically charged rod next to a jet of polar liquid (water) the liquid will move towards the rod
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 - compounds with multiple types
- compounds that form H bonds have other types of intermolecular bonds that can affect their properties
eg. butan-1-ol and butan-2-ol
both have one H bonds per molecules BUT:
1-ol b.p = 117degrees while 2-ol b.p = 99degrees
- this difference is due to the strength of the ID-ID bonds between the molecule - 1-ol is less branched than 2-ol so the surface contact is greater
- the molecules can pack closely together allowing it to form stronger ID-ID bonds so 1-ol has a higher b.p
intermolecular bonding - boiling points of halogens
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)
how could experiments show the strengths of different intermolecular bonds
when liquids evaporate they take in heat (endo) and so the temp. around them decreases
- the more easily a substance evaporates the faster its rate of evaporation will be and so the surrounding temp. will also decrease at a faster rate.
- can use this temperature change to investigate how easily a liquid evaporates and so what type of intermolecular bonds its likely to form
