chem bonding: forces of attraction Flashcards
what are intermolecular forces of attraction?
weak attractive forces that exist between simple covalent molecules
what are the 3 types of intermolecular forces of attraction?
instantaneous dipole-induced dipole (id-id)
permanent dipole-permanent dipole (pd-pd)
hydrogen bonding
what are id-id attractions?
the weakest of the 3 when all factors are kept constant and exists in non-polar molecules where electron distribution is averagely and uniformly constant
how does number of electrons/electron cloud size affect strength of id-id forces?
larger electron clouds are more easily polarised and distorted, and there is greater ease in the formation of id-id, leading to greater strength of id-id attractions
how does surface area of molecules affect strength of id-id attractions?
elongated straight chain molecules have larger surface area of contact between adjacent molecules and more extensive id-id attractions
what are pd-pd attractions?
more dominant as compared to id-id when factors are kept constant and exists in polar molecules
what conditions must be met for pd-pd attractions to exist?
must contain electronegative atom or halogen
what are hydrogen bonds?
a type of pd-pd attraction that exists between a H atom to a highly electronegative atom and lone pair
criteria for hydrogen bonding?
- . highly polarised H-X bond (when H is covalently bonded to F,O or N)
- at least one lone pair on a nearby highly electronegative atom
how does electronegativity affect strength of hydrogen bond?
greater difference in electronegativity results in more substantial dipole charges at the end of the H-X bond, leading to greater dipole moment and more polar bond
highly electronegative F,O and N atoms attract bonding electrons toward itself and H+ is a bare proton, allowing it to more strongly attract lone pair of electrons
why do certain hydrides have abmormally high mp and bp?
hydrogen bonding is stronger than the id-id forces present in other hydrides, more energy is required to overcome the stronger hydrogen bonding in NH3, H2O and HF, hence their mps and bps are higher than other hydrides present in the corresponding groups
what causes the differences in mp and bp of NH3, H2O and HF?
extensiveness of hydrogen bonding (no. of H atoms & no. of lone pairs) and difference in electronegativity between H and F,O,N
• H2O has the highest bp because it has more extensive hydrogen bonding as it forms an average of 2 hydrogen bonds per molecule
• the greater the difference im electronegativity, the greater the partial charges and stronger the hydrogen bonds (difference decreases from H-F > H-O > H-N)
why is there dimerisation of carboxylic cids RCO2H?
when dissolved in non-polar solvents while in vapour state, the relative molecular mass of carboxylic acids double in mass.
