Nitrogen makes up most of the air yet barely reacts with anything. Locked into ammonia it becomes a base, the ammonium ion and the fertilisers that grow half the world's food.
Nitrogen is unreactive because the N≡N triple bond is very strong (944 kJ mol−1) and the molecule is non-polar, so it does not attract electrophiles or nucleophiles. Ammonia is a Brønsted–Lowry base: the lone pair on nitrogen accepts a proton, forming the ammonium ion, NH4+, by a dative bond. NH3 is pyramidal (107°) and NH4+ tetrahedral (109.5°). Ammonium salts react with warm alkali to release ammonia, which turns damp red litmus blue. Ammonium salts are used as fertilisers; their value depends on the percentage of nitrogen by mass.
Compare the N2 molecule with NH3 in the simulator, then add H+ to ammonia and watch the shape change.
Neutralise ammonia with different acids in the simulator, then warm an ammonium salt with sodium hydroxide and test the gas.
| Reaction | Equation |
|---|---|
| forming a salt | 2NH3 + H2SO4 → (NH4)2SO4 |
| releasing ammonia | NH4Cl + NaOH → NaCl + NH3 + H2O |
| ionic form | NH4+ + OH− → NH3 + H2O |
Nitrogen is unreactive because of its strong triple bond and lack of polarity, not because it is small or a gas. When calculating percentage of nitrogen, count every nitrogen atom: NH4NO3 has two, (NH4)2SO4 has two, urea has two.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Why is nitrogen gas very unreactive?
Ammonia acts as a Brønsted–Lowry base because:
What is the shape of the ammonium ion?
An unknown salt is warmed with NaOH(aq); a gas turns damp red litmus blue. The salt contains:
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What is the percentage by mass of nitrogen in ammonium sulfate, (NH4)2SO4? (Ar: H 1.0, N 14.0, O 16.0, S 32.1)
Which fertiliser has the highest percentage of nitrogen by mass?
What volume of ammonia at room conditions is released when 5.35 g of NH4Cl (Mr 53.5) is heated with excess NaOH?
Which describes the bonding in the ammonium ion?
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