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ChemistryMediumClass 11

Group 1 — Alkali metals properties, reactions with water

s-block Elements

5

JEE Qs

8%

Hard

60

min

Pay close attention to anomalous behavior of lithium and the distinct types of oxides formed by different alkali metals when reacting with air/oxygen.

🧮 Key Formulas

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g) (Exothermic reaction, M = Li, Na, K, Rb, Cs)
4Li(s) + O₂(g) → 2Li₂O(s) (Lithium forms normal oxide)
2Na(s) + O₂(g) → Na₂O₂(s) (Sodium forms peroxide)
M(s) + O₂(g) → MO₂(s) (M = K, Rb, Cs form superoxides)
2M(s) + X₂(g) → 2MX(s) (Reaction with halogens)
2M(s) + H₂(g) → 2MH(s) (Reaction with hydrogen to form ionic hydrides)
M(s) + (x+y)NH₃(l) → [M(NH₃)ₓ]⁺ + [e(NH₃)y]⁻ (Solvation in liquid ammonia)

✅ Key Points for JEE

  • 1Alkali metals are highly electropositive and have low ionization enthalpies, hence they readily lose one electron to form M⁺ ions.
  • 2Reactivity with water increases down the group (Li < Na < K < Rb < Cs) due to decreasing ionization enthalpy and increasing atomic size, making electron loss easier.
  • 3Each alkali metal exhibits a characteristic flame colour: Li (crimson red), Na (golden yellow), K (lilac/pale violet), Rb (red-violet), Cs (blue).
  • 4They are strong reducing agents; their reducing power generally increases down the group, but Li is the strongest reducing agent in aqueous solution due to its high hydration enthalpy.
  • 5Alkali metals react differently with oxygen: Li forms oxide (Li₂O), Na forms peroxide (Na₂O₂), while K, Rb, Cs form superoxides (KO₂, RbO₂, CsO₂).

⚠️ Common Mistakes

  • ✕Confusing the type of oxide formed by different alkali metals with oxygen (e.g., assuming all form normal oxides).
  • ✕Incorrectly stating the trend of reactivity with water (e.g., thinking Lithium is most reactive due to its small size, instead of recognizing the trend is increasing down the group).
  • ✕Forgetting or mixing up the characteristic flame colors for different alkali metals.

📝 Practice Questions

See all

Q49. List - I List - II Reaction Type of redox reaction (A) Li (I) 589.2 Match List - I with List - II. Choose the correct answer from the options (B) Na (II) 455.5 (C) Rb (III) 670.8 (D) Cs (IV) 780.0 given below : (1) (A)-(II), (B)-(IV), (C)-(III), (D)-(I) (2) (A)-(I), (B)-(IV), (C)-(III), (D)-(II) (3) (A)-(IV), (B)-(II), (C)-(I), (D)-(III) (4) (A)-(III), (B)-(I), (C)-(IV), (D)-(II)

2024·MCQMedium

Q40.Ion having highest hydration enthalpy among the given alkaline earth metal ions is: (1) Be2+ (2) Sr2+ (3) Ba2+ (4) Ca2+

2023·MCQEasy

Q32.Match List I with List II (A) Slaked lime (I) NaOH (B) Dead burnt plaster (II) Ca ( OH ) 2 (C) Caustic soda (III) Na2CO3 · 10H2O (D) Washing soda (IV) CaSO4 Choose the correct answer form the options given below: (1) (A) - I, (B) - IV, (C) - II, (D) - III (2) (A) - III, (B) - IV, (C) - II, (D) - I (3) (A) - II, (B) - IV, (C) - I, (D) - III (4) (A) - III, (B) - II, (C) -IV, (D) -I

2023·MCQEasy

Q33.Choose the correct statement(s): A. Beryllium oxide is purely acidic in nature. B. Beryllium carbonate is kept in the atmosphere of CO2. C. Beryllium sulphate is readily soluble in water. D. Beryllium shows anomalous behavior. Choose the correct answer from the options given below: JEE Main 2023 (01 Feb Shift 1) JEE Main Previous Year Paper (1) A, B and C only (2) B,C and D only (3) A and B only (4) A only

2023·MCQMedium

Q40.The correct order of bond enthalpy ( kJ mol–1 ) is: (1) Si −Si > C −C > Sn −Sn > Ge −Ge (2) Si −Si > C −C > Ge −Ge > Sn −Sn (3) C −C > Si −Si > Sn −Sn > Ge −Ge (4) C −C > Si −Si > Ge −Ge > Sn −Sn

2023·Assertion ReasoningMedium

Q37. O −O bond length in H2O2 is X than the O −O bond length in F2O2 . The O −H bond length in H2O2 is Y ¯¯ than that of the O −F bond in F2O2 . Choose the correct option for X and Y from the given below. ¯¯ (1) X - shorter, Y− shorter (2) X -shorter, Y -longer (3) X -longer, Y -longer (4) X-longer, Y - shorter

2023·Assertion ReasoningMedium

NCERT Chapters

  • Class 11 Chemistry Ch 10: The s-Block Elements