U1S3 intermolecular forces

Question 1

What are van der waals forces

Answer 1

• electrons in charge clouds are always moving really quickly
• at any particular moment the atoms are likely to be more to one side than the other
• creates a temporary dipole which can cause another temporary dipole in opposite direction of neighbouring atom
• two dipoles are then attracted to eachother
• dipoles are being created and destroyed all the time as electrons are constantly moving

Question 2

What leads to stronger van der waals forces

Answer 2

• larger molecules as have larger electron clouds
• long, straight molecules can lie closer than branched so forces are stronger

Question 3

What are permanent dipole-dipole forces

Answer 3

• in substances made of molecules with permanent dipoles, there are weak electrostatic forces of attraction between dipoles of neighbouring molecules

Question 4

What is hydrogen bonding

Answer 4

• occurs when hydrogen is covalently bonded to flourine, nitrogen or oxygen
• F, N and O are very electronegative so draw bonding electrons away from hydrogen
• bond so polarised and hydrogen has such a high charge density that hydrogen atoms form weak bonds with lone pairs of electrons on F, N or O atoms on other molecules

Question 5

Why is ice less dense than water

Answer 5

• as water cools to form ice, molecules make more hydrogen bonds and arrange themselves in a regular lattice structure
• hydrogen bonds are relatively long so average distance between H2O molecules is greater in ice than in liquid water