Year 12

AQA A level Chemistry Nitrogen and Sulfur Notes

Nitrogen & Sulfur

— Chemistry · AS Level (7405) · —
★ Exam Tip: Easy, "maze ka" chapter. Usually only 1–2 MCQs in Paper 1 and they are straightforward. Must memorise: dot-and-cross of NO & NO₂, the ozone-depletion equations, the catalytic-converter equation, eutrophication order, and shapes of SO₂ / SO₃. Ammonia tests/reactions are "most probably not in exam" — just read them once.

1. Nitrogen's Lack of Reactivity

Why is N₂ inert? (that's why packets of chips are filled with N₂ — it won't react with the food!)

N₂ has a N≡N triple bond — very strong & hard to break

N≡N bond energy = 944 kJ mol⁻¹
Note: board writes 944; the MCQ (Q2 below) prints "994". Don't worry about the exact digit — the point is that the bond energy is very high.

When does nitrogen DO react? (examples)

① During lightning ② In internal combustion engines (hot!) ③ Haber process NO and NO₂ (= NOx) NH₃ ④ also: bacteria "fix" nitrogen → NO / NO₂ (nitrogen fixation)
NOx just means "oxides of nitrogen" (NO + x oxygens, x = 1 or 2)

2. Ammonia (NH₃)

Key idea: NH₃ is a weak base — it accepts an H⁺ by dative (coordinate) bonding using its lone pair.

Remember: whenever a base accepts H⁺ it does so through a dative bond. NH₄⁺ in turn can donate that H⁺ to other species (acts as an acid).
N HHH • • 107°
NH₃ — trigonal pyramidal (lone pair squeezes 109.5° → 107°)
N HHHH + 109.5°
NH₄⁺ — tetrahedral (4th bond is the dative one)

Reactions of ammonia

  • Reacts with acids → ammonium salts:
NH₃ + HCl → NH₄Cl
  • NH₃ can be displaced (liberated) from its salt by a strong base:
NH₄Cl + NaOH → NaCl + H₂O + NH₃
Trick: to get NH₃ out of an ammonium salt → react it with a strong base (NaOH, limewater…) & heat. NH₃ just flies off as a gas.

Tests for ammonia gas (NH₃)

Test Observation
Hold damp red litmus paper near turns blue (NH₃ dissolves in the water → alkaline). Not bleached!
Bring a glass rod dipped in conc. HCl near steamy / white fumes of NH₄Cl (NH₃ + HCl → NH₄Cl)
MCQ Q5MCQ · ammonia from ammonium salt

Which reagent, mixed and heated with ammonium sulfate, liberates ammonia? A aq. bromine · B dilute HCl · C limewater · D acidified K₂Cr₂O₇(VI)

NH₃ is displaced from NH₄⁺ salts by a base ⇒ look for the only base/alkali.

A ✗ bromine is an oxidising agent, not a base B ✗ dilute HCl is an acid — acid + NH₃ makes the salt, doesn't free NH₃ C ✓ limewater, Ca(OH)₂ = alkali (base) D ✗ acidified dichromate is an acidic oxidant
(NH₄)₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O + 2NH₃

Answer: C

3. Uses of Nitrogen Compounds

Ammonia NH₃ fertilisers nitric acid HNO₃ TNT (explosives) · detergents · paints · dyes · nylon
nylon → clothes, threads, packaging; TNT → explosions (dhamaka!)

4. Environmental Consequences — Eutrophication

Cause: fertilisers contain nitrates (soluble in water). Rain/irrigation washes them into rivers, ponds, ground water — some water body always gets them. Nitrates promote growth of algae (green layer on the surface).

Nitrates infertiliser washed intowater by rain algae grow fast(algal bloom) Q algae BLOCKsunlight underwater plantsdie (no light) P bacteria decomposeplants, use up O₂ R O₂ falls O₂ depletes → fish die = EUTROPHICATION
order to remember: Q → P → R (algae → plants die → bacteria use O₂)
Term: the explosive growth of algae = algal bloom; the whole chain ending in fish dying = eutrophication.
MCQ Q16MCQ · eutrophication order

Stages: P water plants on lake bed die (no sunlight) · Q excessive growth of algae · R excessive bacterial activity reduces O₂. Order?

Algae first (Q) → they block light → plants die (P) → bacteria decompose them and eat O₂ (R).

A ✗ P→Q→R: plants can't die before the algae block light B ✗ P→R→Q C ✓ Q → P → R D ✗ Q→R→P: bacteria need dead plants first

Answer: C

5. NO and NO₂ — Dot-and-Cross & Free Radicals

These two cause a lot of trouble. Must be able to draw (or memorise) their dot-and-cross diagrams — can come in exam.

NO × ×× ×•×• • •• ••• unpaired e⁻ NO
N=O double bond; N has 1 unpaired e⁻
ONO • •• ••• וו ×× × • •• •••• unpaired e⁻ N uses lone pair → dative bond to O NO₂
O=N→O style; unpaired e⁻ on N
Why free radicals? The leftover unpaired electron on N is neither a lone pair nor a bonding pair ⇒ NO and NO₂ are free radicals (shown as ·NO and ·NO₂ in equations) ⇒ very reactive ⇒ cause problems ①–④ below.
Note: in a dot-and-cross diagram just draw the unpaired electron; no need to write "free radical". In the board's NO₂, one N–O is a double bond and the other uses N's lone pair (dative), leaving the unpaired e⁻ on N.

6. Problem ① — Photochemical Smog (PAN)

NO and NO₂ react with unburnt hydrocarbons to form PAN = peroxyacetyl nitrate (just remember "PAN" — enough!).

H₃CCO OON OO
PAN: CH₃C(=O)–O–O–NO₂
  • PAN is a component of photochemical smog
  • Smog irritates eyes and causes breathing problems

7. Problem ② — Depletion of the Ozone Layer

NO and NO₂ catalyse the depletion of ozone because they are free radicals.

Natural process first: UV hits O₃ → it breaks into O₂ and an O· radical (this absorbs the UV that would harm us). The O· quickly re-bonds → O₃ again. So the layer normally protects us.

O₃(g) —UV→ O₂(g) + O·(g)

But if NOx reaches the ozone layer it starts a chain reaction:

Step 1·NO(g) + O₃(g) → ·NO₂(g) + O₂(g)
Step 2·NO₂(g) + O·(g) → ·NO(g) + O₂(g)
NetO₃(g) + O·(g) → 2O₂(g)
·NO + O₃→ ·NO₂ + O₂ ·NO₂ + O·→ ·NO + O₂ ·NO₂ formed → used in step 2 ·NO REGENERATED → attacks another O₃ …
NO is used in step 1 and regenerated in step 2 ⇒ it never runs out ⇒ one NO molecule can destroy endless O₃
Remember: the two steps can be written in either order (step 2 above step 1 is fine). Learn both equations + net for the exam.
  • NO / NO₂ are homogeneous catalysts: same phase (gas) as reactants; they take part in the reaction and get regenerated.
  • Even a tiny amount of NOx released is disastrous — the catalyst is never used up, so it keeps destroying ozone (a big chain reaction). So NOx must not be released into the atmosphere.
Sir's trick (homogeneous): homogeneous catalyst = takes proper part in the reaction and is regenerated later, same state as reactants.

8. Fixing the NOx Problem — Catalytic Converters

Biggest source of NOx = internal combustion engines (very high T lets N₂ + O₂ react). So fit a catalytic converter in the car exhaust to reduce NO and NO₂ emission:

CO + ½NO → CO₂ + ½N₂ (or CO + NO → CO₂ + ½N₂ …)
Where: CO ← incomplete combustion | NO ← high T in engine | CO₂ = still a greenhouse gas but far less harmful than CO / NOx | N₂ = inert & harmless.
  • Contains platinum, rhodium, palladium as a honeycomb structure → large surface area ↑
  • They catalyse the reaction (it happens by itself but very slowly), turning harmful pollutants into harmless ones.
CO, NO, NO₂ CO₂, N₂ Pt / Rh / Pd honeycomb
honeycomb ⇒ big surface area ⇒ fast catalysis

9. Problem ③ — NOx Catalyse SO₂ → SO₃

NO and NO₂ can also catalyse the formation of SO₃ from SO₂ (again homogeneous catalysts, again regenerated):

Step 1·NO₂(g) + SO₂(g) → SO₃(g) + ·NO(g)
Step 2·NO(g) + ½O₂(g) → ·NO₂(g)
NetSO₂(g) + ½O₂(g) → SO₃(g)
  • NO₂ is used in step 1 and regenerated in step 2 (NO and NO₂ cancel in the net equation).
  • Again the two steps can be interchanged (1 ↔ 2).
Link: SO₂ → SO₃ is the key step of the Contact process (already covered there). Here NOx speeds it up in the atmosphere → more SO₃ → more acid rain.

10. Problem ④ — Acid Rain

SO₃ and NO₂ dissolve in atmospheric water (vapour) to form acids:

SO₃(g) + H₂O(l) → H₂SO₄(aq)
NO₂(g) + H₂O(l) → HNO₃(aq)

Effects of acid rain

  • Washes out nutrients from soil → plant growth prevented.
  • Destroys the waxy layer (waxy cuticle) on leaves → less resistant to pests & disease.
  • Changes the pH of water bodies & soil → kills life. (board says "increases pH" — acid rain actually decreases pH, sir corrected this in the lecture; teacher meant the pH goes down = more acidic)
  • Destroys marble structures (marble = CaCO₃; carbonate + acid → salt + CO₂ + H₂O):
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O(g) + CO₂(g)
  • Corrodes metals (acid + metal → salt + H₂):
H₂SO₄ + Fe → FeSO₄ + H₂
MCQ Q39MCQ · acid rain

Which reaction does not contribute to acid rain? A combustion of fossil fuels · B oxidation of SO₂ → SO₃ catalysed by NO₂ · C reaction of NO with CO in a catalytic converter · D reaction of SO₃ with water

A ✗ fossil fuels release SO₂/NOx ⇒ contributes B ✗ makes SO₃ ⇒ contributes C ✓ CO + NO → CO₂ + N₂ removes NO ⇒ helps, doesn't contribute D ✗ SO₃ + H₂O → H₂SO₄ is acid rain itself

Answer: C

MCQ Q43MCQ · effects of acid rain

Which statement does not describe an effect of acid rain? A erosion of stone buildings · B ozone depletion · C increases corrosion of some metals · D leaching of nutrients/minerals from soil

Ozone depletion is caused by NOx/CFC radical chain reactions (problem ②), not by acid rain itself.

Answer: B

11. Sulfur — Structures of SO₂ and SO₃

Sulfur is in group 16, just like oxygen. Contact process was already done earlier — apart from it, only the shapes of SO₂ and SO₃ are needed.

S× × OO ≈120°? SO₂ — bent / V-shaped
S=O ×2 (double bonds) + 1 lone pair on S
S OOO 120° SO₃ — trigonal planar
3 double bonds, no lone pair on S
  • SO₂ is equivalent to H₂O: S bonds to 2 O and has a lone pair ⇒ bent / V-shaped.
  • SO₃: 3 bonding regions, no lone pair ⇒ trigonal planar, 120°. All S=O bonds are double bonds.
Bond angle of SO₂ — discrepancy! Sir said 104.5° (104 also accepted) in the lecture (by analogy with H₂O); the board label says ≈ 120° (3 electron regions around S). Safest for the exam: write "bent / V-shaped" and know both numbers; real value is close to 119°, i.e. slightly below 120° because the lone pair pushes the bonds closer.
Trick: "SO₂ ≈ water-like (bent, lone pair on top); SO₃ = flat triangle."
MCQ Q23MCQ · Contact process, Kc

SO₃ is made from SO₂ + O₂ in the Contact process. Which condition affects the value of Kc? A adjusting the temperature · B increasing pressure · C removing SO₃ from the equilibrium mixture · D using a catalyst

Kc changes only with temperature. Pressure / concentration changes shift the position but not Kc; a catalyst only changes the rate.

A ✓ temperature B ✗ pressure — position shifts, Kc unchanged C ✗ removing SO₃ — position shifts, Kc unchanged D ✗ catalyst — speeds both directions equally

Answer: A

MCQ Q2MCQ · why N₂ is inert

N₂ is used as an inert atmosphere because it is unreactive. Best explanation? A its molecule contains a triple bond · B the bond energy of the molecule is high (944/994 kJ mol⁻¹) · C bond is very short (0.110 nm) · D three p orbitals half-filled

"Best" explanation of low reactivity = the energy needed to break the bond is huge. A and C are true facts but are only why the energy is high.

Answer: B

✓ Quick Recap — Full Chapter Cheat-Sheet
  • N₂ inert: N≡N, 944 kJ mol⁻¹. N₂ reacts: lightning, engines → NO, NO₂ (NOx); Haber → NH₃; bacteria fixation.
  • NH₃: weak base, dative bond to H⁺; trigonal pyramidal 107° → NH₄⁺ tetrahedral 109.5°.
  • NH₃ + HCl → NH₄Cl · NH₄Cl + NaOH → NaCl + H₂O + NH₃. Tests: damp red litmus → blue; HCl rod → steamy NH₄Cl fumes.
  • Uses: NH₃ → fertilisers, HNO₃ (→ TNT, detergents, paints, dyes, nylon).
  • Eutrophication: nitrates → algae (Q) → block light, plants die (P) → bacteria use O₂ (R) → fish die.
  • NO, NO₂ = free radicals (unpaired e⁻). Problems: ① PAN → photochemical smog; ② ozone depletion (O₃ + ·NO → ·NO₂ + O₂; ·NO₂ + O· → ·NO + O₂; net O₃ + O· → 2O₂); ③ SO₂ → SO₃ (·NO₂ + SO₂ → SO₃ + ·NO; ·NO + ½O₂ → ·NO₂); ④ acid rain (H₂SO₄, HNO₃).
  • Homogeneous catalysts (same phase, regenerated). Fix: catalytic converter (Pt/Rh/Pd honeycomb): CO + ½NO → CO₂ + ½N₂.
  • Acid rain: nutrient washout, waxy layer destroyed, pH ↓, marble (CaCO₃) eroded, metals corroded.
  • SO₂ bent (lone pair; sir said 104.5°, ≈120° on board); SO₃ trigonal planar 120°.
  • Kc depends on temperature only.