CIE A Level Chemistry复习笔记2.4.1 Nitrogen and its Compounds

• Nitrogen is a diatomic molecule and the main unreactive gas in air
• 78% of air is nitrogen gas
• The lack of reactivity of nitrogen gas can be explained by looking at its intramolecular bonds
• Intramolecular bonds are the bonds within a molecule

Bonding in nitrogen

• The electron configuration of a nitrogen atom is 1s² 2s² 2p³
• To achieve a full outer shell of electrons, it needs to gain three electrons
• Nitrogen atoms therefore form a triple covalent bond between two nitrogen atoms in which they share three electrons with each other

The diagram shows a triple covalent bond between two nitrogen atoms to achieve a full outer shell of electrons

• The bond enthalpy of the nitrogen triple bond is 1000 kJ mol^-1
• This means that 1000 kJ of energy is needed to break one mole of N₂ triple bond
• As it is so difficult to break the nitrogen triple bond, nitrogen and oxygen gas in air will not react with each other
• Only under extreme conditions will nitrogen gas react (eg. during a thunderstorm)

Polarity of nitrogen

• The electrons in a nitrogen molecule are shared equally between the two nitrogen atoms
• Therefore, nitrogen molecules are nonpolar molecules

Since the electronegativity of the two nitrogen atoms is the same, the will pull the electrons towards them equally so overall the molecule is nonpolar

• Due to the lack of polarity, nitrogen gas is not attracted to or likely to react with other molecules the way polar molecules would

• Ammonia is a compound of nitrogen and will turn damp red litmus paper blue as it is an alkaline gas
• Ammonia is made on a large scale in industry using the Haber process:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Basicity of ammonia

• Ammonia can act as a Brønsted–Lowry base by accepting a proton (H+) using the lone pair of electrons on the nitrogen atom to form an ammonium ion:

NH₃(aq) + H⁺(aq) →  NH₄⁺(aq)

• In an aqueous solution of ammonia, an equilibrium mixture is established

NH₃(aq) + H₂O(l) ⇌  NH₄⁺(aq) + OH^-(aq)

• Since the position of the equilibrium lies well over to the left the ammonia solution is only weakly alkaline
• There is a higher concentration of ammonia molecules than hydroxide ions in solution
• Ammonia is therefore a weak base

Structure & formation of ammonium ion

• The ammonium ion is formed by an acid-base reaction of ammonia with water:

NH₃(aq) + H₂O(l) ⇌  NH₄⁺(aq) + OH^-(aq)

• The nitrogen in ammonia is covalently bonded to three hydrogen atoms and has one lone pair of electrons causing the ammonia molecule to have a pyramidal shape

Ammonia has a pyramidal shape due to its lone pair of electrons

• The nitrogen atom in ammonia uses its lone pair of electrons to form a dative bond with a proton to form the ammonium ion
• The ammonium ion has a tetrahedral shape in which all bonds have the same length

The tetrahedral shape of the ammonium ion

Preparation of ammonia gas from an ammonium salt

• Ammonia gas can be prepared from an ammonium salt and a base in an acid-base reaction:

• Ammonium chloride (NH₄Cl) and calcium hydroxide (Ca(OH)₂) are mixed together and then heated
• NH₄⁺acts as an acid (proton donor) and OH^- acts as a base (proton acceptor)
• This acid-base reaction can be used to test if an unknown solution contains ammonium ions
• If the unknown solution does contain ammonium ions, it will react with calcium hydroxide to form ammonia gas
• This ammonia gas will turn damp red litmus paper blue

The diagram shows the apparatus set up for the preparation of ammonia gas from an ammonium salt and calcium hydroxide