3 - Forces between molecules

Question 1

What are the three intermolecular forces?

Answer 1

• Van Der Waals (induced dipole-dipole)
• Permanent dipole-dipole
• Hydrogen bonding

Question 2

What is the strongest intermolecular force?

Answer 2

Hydrogen bonds

Question 3

What is the weakest intermolecular force?

Answer 3

Van Der Waals

Question 4

Where are Van Der Waals forces found?

Answer 4

Between all molecules and noble gases

Question 5

How do Van Der Waals forces arise?

Answer 5

• Electrons are constantly moving randomly
• Electron density can fluctuate and parts of the molecule may have more electrons on one side resulting in a temporary dipole
• This can cause dipoles to form in neighbouring molecules, called induced dipoles

Question 6

Factors affecting the strength of Van Der Waals forces

Answer 6

Size - Larger atoms / molecules have more electrons and a greater volume of electron density, resulting in stronger temporary dipoles and a higher chance that they will form
Surface area - Atoms / molecules that have a larger SA have stronger induced dipole-dipole forces as more of the electron cloud is exposed for interactions

Question 7

Using Van Der Waals forces, explain the increasing boiling points of the halogens down group 7

Answer 7

• The number of electrons increases down group 7 resulting in the molecules becoming larger
• This explains why Cl₂ is a gas and I₂ is a solid as I₂ is larger and has stronger VdWs forces

Question 8

Using Van Der Waals forces, explain why long chain alkanes have a larger boiling points than spherical shaped branched alkanes

Answer 8

• Long chain alkanes have a larger surface area
• More of the electron cloud is exposed so larger area of contact between molecules for VdWs to form
• Therefore stronger VdWs forces

Question 9

Where are permenant dipole-dipole forces found?

Answer 9

Between polar molecules

Question 10

How do permanent dipole-dipole forces arise?

Answer 10

• They arise from the unequal sharing of electrons in covalent bonds
• Electrostatic forces of attraction between the partial positive end of a polar molecule and the partial negative end of another

Question 11

Using VdWs and permenant dipole-dipole forces, why does methanal (CH₂O) have a higher boiling point than ethane (C₂H₆)?

Answer 11

• Methanal is a polar molecule due to the carbonyl group (C=O) so contains both VdWs and permenant dipole-dipole forces
• However ethane is a non-polar molecule so only contains VdWs
• More energy is required to overcome the stronger IMF in methanal resulting in a high BP, even though the molecules are of a similar size and have the same Mr

Question 12

Polar molecules have stronger intermolecular forces than non-polar molecules of a similar size. Why is this?

Answer 12

Polar molecules contain Van Der Waals forces aswell as permenant dipole-dipole forces whereas non-polar molecules only contain Van Der Waals forces

Question 13

Where is hydrogen bonding found?

Answer 13

Between polar molecules with a H-F, H-O or H-N bond

Question 14

How does hydrogen bonding arise?

Answer 14

• Large difference in electronegativity between H and F, O or N
• Must be a lone pair on the F, O or N atom
• Small size of H atoms allows it to get close to lone pair
• Lone pairs on F, O and N atoms have high electron density
• Positively charged hydrogen is strongly attracted to the negatively charged lone pair, forming a H bond between molecules

Question 15

Why does H₂O have a significantly higher boiling point than the other hydrides?

Answer 15

• H₂O forms hydrogen bonds between them
• The strong IMF created by hydrogen bonding requires a lot of energy to overcome, increasing the boiling point

Question 16

Why is ice less dense than water?

Answer 16

• When water freezes, H₂O molecules arrange into a lattice
• As a result, the molecules in solid ice are further apart from eachother making it less dense