Nitrogen and ammonia
Nitrogen and ammonia
- Nitrogen gas, $\text{N}_2$, fills the air but reacts with very little.
- Ammonia is a base, thanks to a lone pair on its nitrogen.
- That lone pair also builds the ammonium ion.
Why nitrogen is unreactive
- The two N atoms are joined by a triple bond with a very high bond energy — hard to break.
- The molecule is symmetrical and non-polar — nothing pulls other molecules towards it.
Practice
Nitrogen gas is unreactive because it has:
The N≡N triple bond is very strong, and the symmetrical molecule is non-polar — both make it unreactive.
Ammonia and the ammonium ion
- Ammonia is a base because the lone pair on nitrogen can accept a proton:
$$\text{NH}_3 + \text{H}^{+} \rightarrow \text{NH}_4^{+}$$
- The new N–H bond is a coordinate bond (both electrons from N). The $\text{NH}_4^{+}$ ion has four identical N–H bonds and is tetrahedral.
Practice
Ammonia acts as a base because:
A Brønsted–Lowry base accepts a proton; ammonia's nitrogen lone pair does exactly that.
Practice
The bond formed when NH₃ accepts H⁺ is a coordinate bond because:
Nitrogen supplies both electrons of the new N–H bond, making it a coordinate (dative) bond.
Testing for an ammonium salt
- Warm an ammonium salt with a base (NaOH) to push out ammonia:
$$\text{NH}_4\text{Cl} + \text{NaOH} \rightarrow \text{NaCl} + \text{NH}_3 + \text{H}_2\text{O}$$
- The sharp smell, and damp red litmus turning blue, confirms an ammonium salt.
Practice
The test for an ammonium salt is to warm it with NaOH and look for:
A base displaces ammonia from the salt; the ammonia smells sharp and turns damp red litmus blue.
You've got it
Key idea
- $\text{N}_2$ is unreactive: a strong triple bond + a non-polar molecule
- ammonia is a base — its lone pair accepts a proton → $\text{NH}_4^{+}$ (a coordinate bond, tetrahedral)
- test for an ammonium salt: warm with NaOH → ammonia (sharp smell, red litmus → blue)