Question

Give the reasons of the following questions.
$H_{2}O$ is liquid but $H_{2}S$ is a gas at room temperature
$N{H}_3$ has higher boiling point than $P{H}_3$

Answer 1

• 1) In $H_{2}O$ there is hydrogen bonding between the molecules because oxygen has a high electronegativity(only second to flourine).
  
• So, the hydrogen atoms from a water molecule forms hydrogen bonding with oxygen of neighbouring water molecule resulting in strong intermolecular hydrogen bonding. This is why water is in liquid form at room temperature
  
• While in case of $H_{2}S$, the electronegativity of sulphur is low compared to oxygen so,the hydrogen bonding is negligible in case of $H_{2}S$, and the only binding force is weak Van Der Wals or London force of attraction between the molecules. Therefore $H_{2}S$ is a gas at room temperature.
  
• 2)Although $P{H}_3$ is a larger molecule with greater dispersion forces than ammonia,$N{H}_3$has very polar N-H bonds leading to strong hydrogen bonding. This the dominant intermolecular force and results in a greater attraction between$N{H}_3$ molecules than there is between $P{H}_3$ molecules.
• Thus $N{H}_3$ has higher boiling point than $P{H}_3$.