bonding and structure Qs

Question 1

Explain why the bond angle in water H₂O is 104.5 degrees and not 109.5 degrees like SiH₄.

Answer 1

because water has lone pair electrons which repel more than bonding pairs

Question 2

what is the shape of ammonia and why does it form this shape

Answer 2

• ammonia has three bonding pairs and one lone pair
• electron pairs repel each other, they try to get into a position of maximum seperation or minimum repulsion
• lone pairs repel more strongly than bonding pairs, causing the bond angle between bonding pairs to decrease
• so with three bonding pairs and one lone pair ammonia has a trigonal pyramidal shape with a bond angle of 107degrees

Question 3

Why does H2O have a much higher boiling point than H2S

Answer 3

• H2S forms London forces. these are weak intermolecular forces and easy to break
• H2O forms hydrogen bonds. these are much stronger than london forces and are difficult to break
• to form hydrogen bonds there must be a hydrogen atom bound tightly to a highly electronegative element such as oxygen, nitrogen or fluorine
• the positively charged hydrogen must then bond to a lone pair of electrons on either the fluorine, the oxygen or the nitrogen atom

Question 4

why is CCl4 not polar when C-Cl bond is polar

Answer 4

• CCl4 is tetrahedral and therefore has rotational symmetry
• all the bonds are facing in opposing directions, overall, the molecule is non polar