OZ7: Hydrogen Bonding

Question 1

Why do molecules with hydrogen bonding have higher energy than those with instantaneous dipole-induced dipole bonds?

Answer 1

Hydrogen bonding is the strongest type of intermolecular bonding

Question 2

What are the 3 requirements for hydrogen bonding?

Answer 2

• Large dipole between a hydrogen atom and a highly electronegative atom
• A second small hydrogen atom
• A lone pair of electrons the non hydrogen atom in the dipole

Question 3

What 3 atoms are most commonly the highly electronegative non hydrogen atom bonded to hydrogen?

Answer 3

• Nitrogen
• Oxygen
• Flourine

Question 4

Why does the non hydrogen atom bonded to the hydrogen atom in the dipole need to be highly electronegative?

Answer 4

To draw bonding electrons away from the hydrogen atoms in their bonds, polarising the molecule

Question 5

Why does the second hydrogen atom need to be small?

Answer 5

• To have a high positive charge density

- To get close to the highly electronegative atom

Question 6

Why does the non hydrogen atom bonded to the hydrogen atom in the dipole need to have a lone pair?

Answer 6

So the second hydrogen atom can form a bond with this pair

Question 7

Give 5 substances that undergo hydrogen bonding.

Answer 7

• Water
• Ammonia
• Hydrogen fluoride
• Alcohols (from the -OH group)
• Amines (from the -NH group)

Question 8

How many hydrogen bonds per molecule can water form? What does this mean?

Answer 8

4

- Means water has a high boiling point

Question 9

Why can water form 4 hydrogen bonds per molecule?

Answer 9

• The oxygen atoms posses 2 lone pairs of electrons

- Water has twice as many hydrogen atoms as oxygen atoms

Question 10

How does hydrogen fluoride undergo hydrogen bonding?

Answer 10

• The hydrogen atoms have a strong positive charge as they are bonded to the highly electronegative fluorine atom
• This positive charge lines up with another fluorine atom’s lone pair
• The hydrogen and fluorine atoms can get very close and therefore attract strongly because the H atom is so small

Question 11

Why does HF have a lower boiling point than water?

Answer 11

• Fluorine has 3 lone pairs

- But HF only has as many H atoms as F atoms so only 1/3 of the available lone pairs are used

Question 12

Why does ammonia not form that many hydrogen bonds?

Answer 12

• Because there is only 1 lone pair on nitrogen

- So only 1 of the 3 hydrogen atoms can form hydrogen bonds

Question 13

Why is ice less dense than water?

Answer 13

• Ice crystals have an open structure
• Four groups around each oxygen atom maximises the hydrogen bonding between the molecules
• There are more gaps in the lattice structure not filled with molecules

Question 14

Why do substances with hydrogen bonding have a high viscosity?

Answer 14

• Hydrogen bonds between molecules are strong
• They cannot constantly break and reform
• So molecules cannot easily flow past each other

Question 15

Why do substances with hydrogen bonding have a high solubility?

Answer 15

• They can form hydrogen bonds with water molecules

- Allows them to mix and dissolve