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Chemistry 3% exam weight

Group V trends and bonding

Part of the UNEB UACE (Uganda) study roadmap. Chemistry topic chemis-006 of Chemistry.

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🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Group V (N, P, As, Sb, Bi) elements sit in the p-block with five valence electrons and display the trend: electronegativity falls and metallic character rises down the group, while the +3 oxidation state becomes more stable than +5. Nitrogen itself is unusual because it forms strong pπ–pπ multiple bonds and a lone pair on each atom.

Ammonia, NH₃, has trigonal pyramidal geometry (bond angle ≈107°) because the lone pair on nitrogen repels the three N–H bond pairs. NH₃ acts as a weak monobasic base in water:

  • NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, with Kb ≈ 1.8 × 10⁻⁵ at 25 °C.

The ammonium ion NH₄⁺ is identified by warming with NaOH; liberated NH₃ turns damp red litmus paper blue and gives white smoke with HCl fumes.

Nitrite NO₂⁻ vs nitrate NO₃⁻: dilute H₂SO₄ + KI gives a brown I₂ colour with NO₂⁻ but no reaction with NO₃⁻. The brown ring test (FeSO₄ + concentrated H₂SO₄ layered carefully) confirms NO₃⁻ via the brown [Fe(H₂O)₅(NO)]²⁺ complex.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Moving from N down to Bi, atomic radius increases, first ionisation energy falls, and the elements shift from non-metal (N, P) through metalloid (As, Sb) to metal (Bi). Nitrogen is restricted mainly to oxidation states −3 to +5 because it cannot expand its octet; heavier members use d-orbitals and the +3 state dominates as the inert pair effect strengthens.

In NH₃, three bond pairs + one lone pair give sp³ hybridisation with trigonal pyramidal geometry. The lone-pair/bond-pair repulsion compresses the H–N–H angle from the tetrahedral 109.5° to ≈107°.

Nitrogen chemistry in solution

SpeciesReagentObservationEquation (ionic)
NH₄⁺NaOH, warmNH₃ gas; damp red litmus → blue; white smoke with HClNH₄⁺ + OH⁻ → NH₃ + H₂O
NO₂⁻KI + dilute H₂SO₄Brown solution (I₂ liberated)2NO₂⁻ + 2I⁻ + 4H⁺ → I₂ + 2NO + 2H₂O
NO₃⁻FeSO₄ + conc H₂SO₄ (layered)Brown ring at junctionNO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O; NO + Fe²⁺ → [Fe(H₂O)₅(NO)]²⁺

Systematic cation analysis

UNEB UACE expects students to apply the group reagent sequence:

  1. Group I: dilute HCl — Pb²⁺, Ag⁺ (white chlorides).
  2. Group II: H₂S in dilute HCl — Cu²⁺ (black), Pb²⁺ (black, if not removed).
  3. Group III: NH₄Cl/NH₃ then H₂S — Fe³⁺ (black), Al³⁺ (white, amphoteric).
  4. Group IV: H₂S in NH₃ — Zn²⁺ (white).
  5. Group V: (NH₄)₂CO₃ in NH₃ — Ca²⁺, Ba²⁺ (white carbonates).
  6. Soluble group: Mg⁺, Na⁺, K⁺, NH₄⁺ — identified by flame tests and specific reagents.

Tip: Always record the colour of the original solution first; Cu²⁺ blue, Fe³⁺ yellow-brown, Ni²⁺ green — these often replace the need for confirmatory tests.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Eliminating interference in confirmatory tests

Flame-test reliability is undermined by sodium’s intense yellow emission, which masks K⁺ lilac. Always view K⁺ through cobalt blue glass, which absorbs the Na yellow and reveals the lilac. Similarly, when testing for NH₄⁺ with NaOH, warming is essential: cold NaOH merely shifts the equilibrium, while heat drives NH₃ out of solution and over the litmus paper.

The brown ring test fails if too much water is present because the complex [Fe(H₂O)₅(NO)]²⁺ dissociates. Pour concentrated H₂SO₄ slowly down the side of a test tube containing the nitrate solution layered over FeSO₄; the brown ring forms at the acid/water interface where NO is generated and captured by Fe²⁺ before oxidation to NO₂.

Solubility rules as a decision tool

Before adding any reagent, predict the outcome: all nitrates and all Group I/ammonium salts are soluble — so a “white precipitate” with dilute HCl cannot be a nitrate or a sodium salt. Carbonates, phosphates, and hydroxides of Groups II–IV are insoluble unless the cation is Group I or NH₄⁺. This logic lets you eliminate possibilities before reaching for the reagent bottle.

Exam-style application

Paper 2 carries a compulsory qualitative-analysis question worth roughly 8–12 marks. You are typically given a mixture containing 2–3 cations and 1–2 anions and must (a) record observations in a table, (b) state inferences, and (c) write ionic equations for each confirmatory test. Marks are lost for skipping warming, omitting state symbols, or confusing NO₂⁻ with NO₃⁻.

Common mistakes to avoid

  • Reporting “brown fumes” with concentrated H₂SO₄ as proof of NO₃⁻ — those fumes are NO₂ from a nitrite or a reducing agent, not a nitrate.
  • Adding excess NH₃ to a Cu²⁺/Zn²⁺ mixture and assuming both dissolve; Cu²⁺ gives the deep blue [Cu(NH₃)₄]²⁺ but Zn²⁺ only redissolves in excess NH₃ because Zn(OH)₂ is amphoteric.
  • Forgetting that Pb²⁺ appears in both Group I (white PbCl₂) and Group II (black PbS) because PbCl₂ is sparingly soluble in hot water.

Practice prompts

  1. A white solid dissolves in water to give a neutral solution. Addition of NaOH on warming releases a gas that turns damp red litmus blue, and gives white smoke with a glass rod dipped in concentrated HCl. Addition of dilute H₂SO₄ + KI produces a brown colour, but the brown ring test with FeSO₄ is negative. Identify the cation and anion present.
  2. A green solution gives a black precipitate with H₂S in dilute HCl. The filtrate, when made alkaline with NH₃ and saturated with H₂S, yields no further precipitate. What cation is present, and which confirmatory test would you run to confirm it?

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