C6.3 Intermolecular Forces

Question 1

3 types of intermolecular forces

Answer 1

induced dipole-dipole interactions (London forces)
permanent dipole-dipole interactions
hydrogen bonding

Question 2

intermolecular forces definition

Answer 2

weak interactions between dipoles of different molecules

Question 3

what are london forces?

Answer 3

• weak intermolecular forces that exist between all molecules (polar or non polar)
• act between induced dipoles in all different directions
• result from interactions of electrons between molecules

Question 4

origins of induced dipole-dipole interactions

how they move

Answer 4

1. movement of electrons produces a changing dipole in a molecule
2. any instant, instantaneous dipole exists, with position constantly moving
3. instantaneous dipole induces dipole on neighbouring molecule
4. induced dipole induces further dipoles, attract each other, due to opposing dipoles

Question 5

how long do induced dipoles last?

Answer 5

they are temporary, in next instant may disappear, only for whole process to take place amongst other molecules

Question 6

how strong are london forces/ what affects their strength?

Answer 6

more electrons in each molecule = larger the instantaneous and induced dipoles, greater the london forces, stronger sttractive forces between molecules
- more electrons = larger induced dipoles = more energy to overcome intermolecular forces = higher boiling point

Question 7

van der waals forces

Answer 7

term for permanent and induced dipole-dipole interactions, people thought it was for just induced dipole-dipole interactions

Question 8

permanent dipole-dipole interactions

Answer 8

act between the permanent dipoles in different polar molecules
polar molecules have induced and permanent dipole-dipole interactions
- extra energy needed to break permanent dipole-dipole interactions = higher b.p
-stronger than induced dipole-dipole interactions

Question 9

what are simple molecular substances?

Answer 9

made up of simple molecules
- which are small units containing a definite number of atoms with a definite molecular formula (Ne, H2, H20,CO2 etc..)

Question 10

simple molecules form a structure of?

Answer 10

in solid state, they form a simple molecular lattice, where:
- molecules held in place by weak intermolecular forces
- atoms in each molecule bonded by strong covalnet bonds

Question 11

what are the properties of simple molecular substances?

only polar and mp and bp

Answer 11

low mp and bp ( all are covalently bonded and can be solidified into simple molecular lattices by reducing their temps = in simple molec. lattice, weak intermolecular forces can be broken even at low temps)
- when a simple molec. lattice broken during melting, only intermolecular forces break nt covalent
-** covalent simple mol. structures can be polar or non polar** non-polar = easier to predict = when simple molec. compound added to non polar, intermolec. forces form between molecules and solvent, interactions weaken interm. forces between simple molec. lattice, intermolec. forces break and compound dissolves

Question 12

solubility of non polar simple molecular substances and non polar solvents

Answer 12

when simple molec. substance added to polar solvent, little interaction bteween molecules in lattice and solvent, therefore intermolecular forces are too strong to be broken down
= non polar simple molecular substances normally dissolve in non polar solvents

Question 13

solubility of polar simple molecular substances

basic on what the solubility is

Answer 13

polar covalent substances can dissolve in polar solvent molecules, as the polar solute and solvent molecules attract each other, similar process to dissolving ionic compounds
eg. sugar issolving in water, sugar has many O-H bonds, attracting and bonding with the water molecules
extends to gas and liquids too

Question 14

what does the solubility of the polar molecules depend on?

Answer 14

depends on the strength of dipole and can be hard to predict
some may contain polar and non polar parts in their structure (ethanol) and dissolve in polar and non polar solvents
some biological molecules have hydrophilic and hydrophobic heads
hydrophillic = polar = contain electronegative atoms (usually o2) = interacts with wtaer
hydrophobic = non polar = comprised of carbon usually = doesnt interact with water

Question 15

electrical conductivity in simple molecular substances

Answer 15

no mobile charged atoms = nothing to complete electrical circuit = **non-conductors of electricity **