3.1.3 intermolecular forces

Question 1

what is the difference between intermolecular and intramolecular forces?

Answer 1

INTER = between molecules
INTRA = in a molecule (ionic,covalent,metallic)

Question 2

describe van der waals forces

Answer 2

temporary dipoles; weakest forces
present in all atoms + mols; due to e-
cause particles to be attracted to each other

Question 3

explain how van der waals forces arise

Answer 3

e- constant movement induces a temporary dipole in molecule
this induces a temp dipole in neighbouring molecule
due to electrostatic attraction between delta +ive and -i’ve charges
VDW ONLY ACTIVE WHEN NEAR EACH OTHER

Question 4

explain the affect of branched chain on VDW forces

Answer 4

straight chain mols = more VDW + higher b.p; mols can lie closer together
branched chain mols = less VDW; lower b.p; can’t be as close together

Question 5

describe permanent dipole-dipole forces

Answer 5

large difference in electronegativity between atoms create permanent dipole
e- more closely attracted to 1 atom
creates delta +ive and -i’ve end of mol
have weak electrostatic forces between opposite charges on nearby mols

Question 6

explain polar molecules interaction with electrostatically charged rods

Answer 6

next to a jet of water - move towards rod
polar liquids = permanent dipoles; rod can be -ively or +ivey charged and mol will turn to have opp charges facing it
more polar = greater deflection

Question 7

describe hydrogen bonding

Answer 7

strongest force
only happens when there are covalent bonds between a molecule and F,N,O
they are the most electronegative elements
draw e- away from H mol

Question 8

explain how hydrogen bonds are formed

Answer 8

H atoms form weak bonds w/ lone pairs on F, N, or O
= bc bond is very polarised; H atom will have +ive charge; can attract to -ive e-
= H also have v. high charge density

Question 9

how does hydrogen bonding affect substances

Answer 9

have higher b.p and m.p
= lots of E to break strong bonds
makes ice less dense than water
= when water cools mols make more H bonds
= arrange themselves into regular lattice
= H bonds relatively long; distance between mols in ice is greater than in water

Question 10

explain the trend in boiling points of group 7 hydrides

Answer 10

compounds w/ H and halides
as you go down
= polarity of molecule decreases; weaker permanent dipoles
= number of electrons in mols decreases; increases strength VDW

Question 11

why does HI have a higher b.p than HBr and HCl

Answer 11

although HCl has greater difference in electronegativity between atoms, the higher no. of e- overcomes that (stronger VDW)