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State Board · Class 11 · Chemistry

Lesson 8: Elements of Groups 1 and 2

7 concepts · 56 practice questions with answers · Free to read

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Hydrogen: position, occurrence and isotopes

Link to Std 9–10 : In Std 8–9 hydrogen was the lightest gas; here we see its special place in the periodic table and its isotopes.

From daily life

Hydrogen balloons rise because H₂ is far lighter than air; the Sun and stars are mostly hydrogen.

Simple definition

Hydrogen (1s¹) is the most abundant element in the universe. It resembles group 1 (loses e⁻ to H⁺) and group 17 (gains e⁻ to H⁻), so it has a unique position. Isotopes: protium ¹H, deuterium ²H (D), tritium ³H (T).

Hydrogen isotopesThe three isotopes of hydrogenp¹HProtium99.985 %pn²H (D)Deuterium0.015 %pnn³H (T)TritiumradioactiveHeavy water D₂O: boiling point 374.6 K, density 1.11 g/mLUsed as a neutron moderator in nuclear reactors

In the figure : Particles in the three isotopes.

Rule 1: Position of hydrogen — Like group 1: one valence electron, forms H⁺, acts as a reducing agent. Like group 17: diatomic H₂, forms H⁻ (hydride). But its ionisation enthalpy (1312 kJ/mol) is much higher.

  • Easy : H → H⁺ + e⁻ (group 1).
  • Medium : H + e⁻ → H⁻ (hydride, as in NaH).
  • Exam-type : H₂ and Cl₂ are both diatomic.

Rule 2: Isotopes — Same Z = 1; mass numbers 1, 2, 3 (0, 1, 2 neutrons). Tritium is a β⁻ emitter with half-life ≈ 12.3 years (textbook: 12.4).

  • Easy : Protium 99.985 %, deuterium 0.015 %.
  • Medium : Molar mass of D₂O = 2 × 2 + 16 = 20 (H₂O = 18).
  • Exam-type : T has 1 proton and 2 neutrons.

Rule 3: Heavy water D₂O — D₂O has higher values than H₂O: boiling point 374.6 K, density 1.11 g/mL. Used as a moderator in reactors.

  • Easy : 1 mol D₂O = 20 g, H₂O = 18 g.
  • Medium : 2D₂O → 2D₂ + O₂ (electrolysis).
  • Exam-type : Deuterium reacts more slowly than H (kinetic isotope effect).

Calculation: mass percent of H in H₂O

  • Step 1: M(H₂O) = 2 × 1.008 + 15.999 = 18.015.
  • Step 2: Mass of H = 2.016.
  • Step 3: Percent = 2.016 ÷ 18.015 × 100 = 11.19 %.

CET level (MCQ)

  • CET 1) Neutrons in tritium? 1) 0 2) 1 3) 2 4) 3 — Answer: 3). (3 − 1 = 2)
  • CET 2) The hydride ion is? 1) H⁺ 2) H⁻ 3) H₂ 4) H₃⁺ — Answer: 2).

Real-life use: Heavy water in reactors, tritium in luminous dials, hydrogen in fuel cells.

Remember

H = 1s¹; resembles groups 1 and 17; isotopes ¹H, ²H, ³H; tritium is radioactive.

Common mistake

Calling deuterium radioactive; only tritium is.

CET trap

Tritium has 2 neutrons (A − Z) and only one proton.

Shortcut

Memory clue: "Pro-Deu-Tri : neutrons 0-1-2".

Spot the difference

IsotopeANeutronsNature
Protium10Stable
Deuterium21Stable
Tritium32Radioactive

Practice questions

Easy
  1. 1Why does hydrogen have a unique position?
  2. 2Name the three isotopes.
  3. 3What is heavy water?
Medium
  1. 4Molar mass of D₂O?
  2. 5How many neutrons in tritium?
  3. 6Mass % of H in H₂O?
Challenging
  1. 7Give examples where H forms H⁺ and H⁻.
  2. 8Why is heavy water used in reactors?
Show answers (try them yourself first)
  1. 1It resembles both groups 1 and 17.
  2. 2Protium, deuterium, tritium.
  3. 3D₂O.
  4. 420 g/mol.
  5. 52.
  6. 611.19 %.
  7. 7HCl (H⁺ character), NaH (H⁻).
  8. 8It slows neutrons (moderator).

Dihydrogen: preparation, properties and uses

From daily life

Like cooking gas, H₂ burns and gives only water, so it is a clean fuel.

Simple definition

H₂ is a colourless, odourless, combustible gas and the lightest. Prepared in the lab from Zn + acid; in industry by steam reforming and the water-gas shift.

H2 preparationPreparation of H₂LaboratoryZn + 2HCl → ZnCl₂ + H₂Electrolysis2H₂O → 2H₂ + O₂Industrial: steam reformingCH₄ + H₂O1270 K, NiCO + 3H₂syngasCO + H₂O673 K, Fe₃O₄CO₂ + H₂water-gas shift reaction

In the figure : Methods of preparing H₂.

Rule 1: Laboratory preparation — Zn with dilute acid, or Zn with NaOH.

  • Easy : Zn + 2HCl → ZnCl₂ + H₂.
  • Medium : Zn + 2NaOH → Na₂ZnO₂ + H₂.
  • Exam-type : 13 g Zn (0.2 mol) gives 0.2 × 22.4 = 4.48 L H₂ at STP.

Rule 2: Industrial preparation — CH₄ + H₂O → CO + 3H₂ (1270 K, Ni); CO + H₂O → CO₂ + H₂ (673 K, Fe₃O₄ + Cr₂O₃); electrolysis.

  • Easy : C + H₂O → CO + H₂ (water gas).
  • Medium : 1 mol CH₄ gives 4 mol H₂ in total (3 + 1).
  • Exam-type : Electrolysis of acidified water: H₂ at the cathode.

Rule 3: Properties and uses — 2H₂ + O₂ → 2H₂O; N₂ + 3H₂ ⇌ 2NH₃ (Haber); metal hydrides; reductions. Uses: ammonia, hydrogenation of oils, fuel cells, rocket fuel.

  • Easy : CuO + H₂ → Cu + H₂O (reduction).
  • Medium : 2Na + H₂ → 2NaH (ionic hydride).
  • Exam-type : Vegetable oil + H₂ (Ni) → vanaspati ghee.

Calculation: volume of H₂ from 13 g Zn (STP)

  • Step 1: n(Zn) = 13 ÷ 65 = 0.2 mol.
  • Step 2: Zn : H₂ = 1 : 1 ⇒ n(H₂) = 0.2 mol.
  • Step 3: V = 0.2 × 22.4 = 4.48 L.

CET level (MCQ)

  • CET 1) Water gas is? 1) CO + H₂ 2) CO₂ + H₂ 3) CH₄ + H₂ 4) CO + N₂ — Answer: 1).
  • CET 2) H₂ at STP from 6.5 g Zn + acid (Zn = 65)? 1) 0.56 L 2) 1.12 L 3) 2.24 L 4) 4.48 L — Answer: 3). (0.1 mol.)

Real-life use: Hydrogen fuel-cell cars and buses; vanaspati ghee; fertilisers via the Haber process.

Remember

Reforming: 1270 K, Ni; shift: 673 K, Fe₃O₄; Haber: N₂ + 3H₂ ⇌ 2NH₃.

Common mistake

Writing that H₂ supports combustion; it burns but does not support it.

CET trap

1 mol CH₄ gives 3 mol H₂ (with CO) and 4 mol after the shift; check which stage the question asks about.

Shortcut

Memory clue: "reform 1270, shift 673".

Spot the difference

MethodReactionCondition
LabZn + 2HCldilute acid
ReformingCH₄ + H₂O1270 K, Ni
ShiftCO + H₂O673 K, Fe₃O₄

Practice questions

Easy
  1. 1Name three uses of H₂.
  2. 2How is H₂ made in the lab?
  3. 3What is water gas?
Medium
  1. 4Find the volume of H₂ from 13 g Zn.
  2. 5Write the water-gas shift reaction.
  3. 6How is vanaspati ghee made?
Challenging
  1. 7How many moles of H₂ come from 1 mol CH₄ overall?
  2. 8Why is H₂ a clean fuel?
Show answers (try them yourself first)
  1. 1Ammonia, hydrogenation, fuel cells.
  2. 2Zn + dilute HCl.
  3. 3CO + H₂.
  4. 44.48 L.
  5. 5CO + H₂O → CO₂ + H₂ (673 K, Fe₃O₄).
  6. 6Hydrogenation of oil with H₂ over Ni.
  7. 74 mol.
  8. 8It forms only water.

Groups 1 and 2: electronic configuration and physical properties

From daily life

In a big family the youngest wanders off easily; an alkali metal's lone outer electron leaves easily.

Simple definition

Group 1 (alkali metals) ns¹: Li, Na, K, Rb, Cs. Group 2 (alkaline earth metals) ns²: Be, Mg, Ca, Sr, Ba. Down the group radius rises and IE falls.

Group 1 trendsGroup 1: ionisation enthalpy (kJ/mol) and melting point (K)Li520Na496K419Rb403Cs376IE₁ ↓Li454Na371K336Rb312Cs302m.p. ↓Atomic radius (pm): Li 152 · Na 186 · K 227 · Rb 248 · Cs 265 ↑Hydration enthalpy Li⁺ −506 · Na⁺ −406 · K⁺ −330 kJ/molLi⁺ is most hydrated ⇒ least mobile in water

In the figure : IE and melting point of group 1.

Live figure

Groups 1 and 2: electronic configuration and physical properties

In the app you can move this figure with your finger.

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Flame testFlame test coloursLicrimsonNagolden yellowKlilacCsblueCabrick redSrcrimsonBaapple greenExcited electrons falling back emit visible lightBe and Mg give no colour (excitation needs more energy)

In the figure : Flame test colours.

Rule 1: Group 1 trends — Radius ↑ (152 → 265 pm); IE₁ ↓ (520 → 376); melting point ↓ (454 → 302 K); density generally ↑ (K < Na is an exception).

  • Easy : Li is hardest, Cs softest.
  • Medium : IE₂ is huge (e.g. Na 4562), so only +1 ions form.
  • Exam-type : Hydration enthalpy Li⁺ −506 > Na⁺ −406 > K⁺ −330.

Rule 2: Group 2 trends — Radius: Be 111, Mg 160, Ca 197, Sr 215, Ba 222 pm ↑; IE₁: 899, 737, 590, 549, 503 kJ/mol ↓; smaller, harder, higher melting than group 1.

  • Easy : Ca (197 pm) is smaller than K (227 pm).
  • Medium : IE₃ is very high ⇒ only +2.
  • Exam-type : Group 2 ions hydrate more than group 1.

Rule 3: Flame test — Excited electrons give visible light: Li crimson, Na golden yellow, K lilac, Cs blue; Ca brick red, Sr crimson, Ba green. Be, Mg give none.

  • Easy : Na's yellow is 589 nm (the D line).
  • Medium : Yellow street lamps use sodium vapour.
  • Exam-type : Firework colours: Sr (red), Ba (green).

Why is Li⁺ the least mobile in water?

  • Step 1: Li⁺ is the smallest ⇒ highest charge density.
  • Step 2: It binds more water molecules (hydration −506 kJ/mol).
  • Step 3: The hydrated ion is large ⇒ moves slowly in solution.

CET level (MCQ)

  • CET 1) Lowest IE₁? 1) Li 2) Na 3) K 4) Cs — Answer: 4).
  • CET 2) Which gives a blue flame? 1) Li 2) Na 3) K 4) Cs — Answer: 4).

Real-life use: Sodium vapour lamps, firework colours, flame tests in mineral analysis.

Remember

Group 1: ns¹, +1; group 2: ns², +2; down the group radius ↑, IE ↓; Be, Mg show no flame colour.

Common mistake

Writing that Li⁺ moves fastest; it moves slowest because it is the most hydrated.

CET trap

K is less dense than Na (0.86 < 0.97 g/cm³); density does not always increase.

Shortcut

Memory clue: "Li red, Na yellow, K violet".

Spot the difference

ItemGroup 1Group 2
Configurationns¹ns²
IonM⁺M²⁺
HardnessSoftHarder
IE₁LowerHigher

Practice questions

Easy
  1. 1Write the general configuration of group 1.
  2. 2Why does Cs give a blue flame?
  3. 3Why does IE fall down group 2?
Medium
  1. 4Which has the higher IE₁, Na or Mg?
  2. 5Why is Li⁺ less mobile in water?
  3. 6Why do Be and Mg give no flame colour?
Challenging
  1. 7Why is Na's IE₂ so high?
  2. 8Order of melting points of Li, Na, K.
Show answers (try them yourself first)
  1. 1ns¹.
  2. 2Excited electrons falling back emit blue light.
  3. 3Radius increases, so outer electrons leave more easily.
  4. 4Mg (737 > 496).
  5. 5Greater hydration ⇒ larger ion.
  6. 6Excitation needs more energy.
  7. 7The second electron comes from a stable neon core.
  8. 8Li (454) > Na (371) > K (336).

Chemical properties of alkali and alkaline earth metals

From daily life

Sodium skitters and ignites in water, so it is stored under kerosene.

Simple definition

Metals of both groups are strong reducing agents (Li strongest, E° = −3.04 V) and reactive: oxides in air, hydroxides + H₂ with water.

Rule 1: Reaction with oxygen and water — Li → Li₂O; Na → Na₂O₂ (peroxide); K, Rb, Cs → MO₂ (superoxide). 2M + 2H₂O → 2MOH + H₂; Mg with hot water/steam, Ca with cold water.

  • Easy : 4Li + O₂ → 2Li₂O.
  • Medium : 2Na + 2H₂O → 2NaOH + H₂.
  • Exam-type : Ca + 2H₂O → Ca(OH)₂ + H₂.

Rule 2: Reaction with N₂, halogens and H₂ — Only Li reacts with N₂ in group 1 (Li₃N); group 2: Mg₃N₂, Ca₃N₂. M + X₂ → MX; M + H₂ → MH (ionic hydrides in group 1).

  • Easy : 6Li + N₂ → 2Li₃N.
  • Medium : 3Mg + N₂ → Mg₃N₂.
  • Exam-type : 2Na + Cl₂ → 2NaCl.

Rule 3: Solubility and thermal stability of compounds — Group 2: sulphate solubility falls down the group (BeSO₄ > BaSO₄); carbonate thermal stability rises. Group 1 carbonates are stable (except Li₂CO₃).

  • Easy : CaCO₃ → CaO + CO₂ (≈ 1200 K).
  • Medium : 2NaNO₃ → 2NaNO₂ + O₂; 4LiNO₃ → 2Li₂O + 4NO₂ + O₂.
  • Exam-type : 2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂.

Why does sulphate solubility fall down group 2?

  • Step 1: Solubility depends on hydration enthalpy versus lattice enthalpy.
  • Step 2: Down the group hydration energy falls faster than lattice energy (SO₄²⁻ is large).
  • Step 3: So BaSO₄ is insoluble (used in gravimetric analysis).

CET level (MCQ)

  • CET 1) K burning in air gives? 1) K₂O 2) K₂O₂ 3) KO₂ 4) K₃N — Answer: 3).
  • CET 2) Which group 1 metal reacts with N₂? 1) Na 2) K 3) Li 4) Rb — Answer: 3).

Real-life use: Li makes Li₃N; Mg is used in flash powder; BaSO₄ in barium meals for X-rays.

Remember

Li₂O, Na₂O₂, KO₂; only Li reacts with N₂; group 2 sulphates ↓ soluble; carbonates ↑ stable.

Common mistake

Writing that all group 1 metals form oxides; Na forms peroxide and K superoxide.

CET trap

LiNO₃ decomposes to NO₂; NaNO₃ does not.

Shortcut

Memory clue: "Li-oxide, Na-peroxide, K-super".

Spot the difference

MetalIn air
LiLi₂O
NaNa₂O₂
K, Rb, CsMO₂
MgMgO + Mg₃N₂

Practice questions

Easy
  1. 1Write how Na reacts with water.
  2. 2Why is Na stored in kerosene?
  3. 3Why does only Li react with N₂?
Medium
  1. 4Equation for heating CaCO₃.
  2. 5Why is BaSO₄ insoluble?
  3. 6Difference in heating LiNO₃ and NaNO₃.
Challenging
  1. 7Write the reaction of Mg with steam.
  2. 8Why are group 2 carbonates more stable down the group?
Show answers (try them yourself first)
  1. 12Na + 2H₂O → 2NaOH + H₂.
  2. 2To avoid reaction with air and water.
  3. 3Li⁺ is small, giving high lattice energy (Li₃N).
  4. 4CaCO₃ → CaO + CO₂.
  5. 5Larger cation ⇒ less hydration, greater relative lattice energy.
  6. 6LiNO₃ → Li₂O + NO₂ + O₂; NaNO₃ → NaNO₂ + O₂.
  7. 7Mg + H₂O (steam) → MgO + H₂.
  8. 8A larger cation polarises the carbonate ion less.

Diagonal relationship, uses and biological importance

From daily life

Two pupils seated diagonally may behave alike; diagonal elements in the periodic table resemble each other.

Simple definition

Diagonal relationship: Li and Be of period 2 resemble Mg and Al of period 3 because their ionic potential (charge ÷ radius) is similar. Biological role: Na⁺/K⁺ pump, Mg in chlorophyll, Ca in bones.

DiagonalDiagonal relationshipLiBeBNaMgAlKCaGaLi ↔ MgBe ↔ AlB ↔ SiReason: similar ionic potential (charge ÷ radius)Li, Mg: form nitrides · carbonates decompose on heatingBe, Al: amphoteric oxides · covalent chlorides

In the figure : The Li–Mg and Be–Al pairs.

Rule 1: Similarities of Li and Mg — Both form nitrides with N₂; carbonates decompose to CO₂ on heating; hydroxides are weakly soluble; LiCl and MgCl₂ are deliquescent and soluble in ethanol.

  • Easy : Li₂CO₃ → Li₂O + CO₂.
  • Medium : MgCO₃ → MgO + CO₂.
  • Exam-type : Both give normal oxides, not peroxides or superoxides.

Rule 2: Similarities of Be and Al — Amphoteric oxides and hydroxides; covalent chlorides (Lewis acids); passive in conc. HNO₃; complex ions [Be(OH)₄]²⁻, [Al(OH)₄]⁻.

  • Easy : BeO + 2NaOH + H₂O → Na₂[Be(OH)₄].
  • Medium : Al₂O₃ reacts with both acid and base.
  • Exam-type : BeCl₂ and AlCl₃ form chloride-bridged dimers.

Rule 3: Uses and biological roles — Li: batteries; Na–K: reactor coolant; Mg: alloys; Ca: cement; Na⁺/K⁺: nerve impulses; Mg: photosynthesis; Ca: bones, blood clotting.

  • Easy : Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in.
  • Medium : Mg²⁺ sits at the centre of chlorophyll.
  • Exam-type : Bones: calcium phosphate (hydroxyapatite).

Why a diagonal relationship?

  • Step 1: Ionic potential rises across a period (charge rises).
  • Step 2: It falls down a group (radius rises).
  • Step 3: Moving right and down compensate, giving similar ionic potential ⇒ similar properties.

CET level (MCQ)

  • CET 1) The diagonal partner of Li? 1) Na 2) Mg 3) Al 4) Be — Answer: 2).
  • CET 2) Metal in chlorophyll? 1) Fe 2) Ca 3) Mg 4) Zn — Answer: 3).

Real-life use: Li-ion batteries, light Mg alloys, calcium for bones, and Na/K for nerve signals.

Remember

Li–Mg, Be–Al; due to ionic potential; Na⁺/K⁺ pump 3 : 2; Mg in chlorophyll.

Common mistake

Writing Li–Al; the pairs are Li–Mg and Be–Al.

CET trap

Beryllium compounds are covalent, not ionic; Be²⁺ is tiny and strongly polarising.

Shortcut

Memory clue: "Li-Mg, Be-Al".

Spot the difference

PairSimilarity
Li–MgNitrides, carbonates giving CO₂
Be–AlAmphoteric, covalent chlorides

Practice questions

Easy
  1. 1What is a diagonal relationship?
  2. 2State two similarities of Li and Mg.
  3. 3State one similarity of Be and Al.
Medium
  1. 4Ratio of the Na⁺/K⁺ pump?
  2. 5What is the biological role of Mg?
  3. 6Why is BeCl₂ covalent?
Challenging
  1. 7Write the equation for heating Li₂CO₃.
  2. 8Show with reactions that Al₂O₃ is amphoteric.
Show answers (try them yourself first)
  1. 1Resemblance between diagonal elements.
  2. 2Nitrides; carbonates release CO₂.
  3. 3Amphoteric oxides.
  4. 43 Na⁺ : 2 K⁺.
  5. 5Central atom of chlorophyll.
  6. 6Be²⁺ is small and highly polarising.
  7. 7Li₂CO₃ → Li₂O + CO₂.
  8. 8Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄].

Sodium carbonate, sodium hydroxide and calcium carbonate

From daily life

Washing soda (Na₂CO₃), caustic soda (NaOH) for soap and limestone (CaCO₃) for building.

Simple definition

Na₂CO₃ by the Solvay process; NaOH by chlor-alkali electrolysis; CaCO₃ is limestone, heated to give lime (CaO); cement and plaster of Paris.

Solvay processSolvay process: Na₂CO₃NaCl + NH₃ + H₂O+ CO₂NaHCO₃ ↓low solubilityNa₂CO₃+ H₂O + CO₂heat ~440 KCaCO₃ → CaO + CO₂≈ 1200 KCaO + H₂O → Ca(OH)₂2NH₄Cl + Ca(OH)₂ → CaCl₂+ 2NH₃ + 2H₂O (NH₃ recycled)CO₂ recycled

In the figure : Solvay process: NH₃ and CO₂ are recycled.

Rule 1: Solvay process — NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl; 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. NaHCO₃ is sparingly soluble so it precipitates.

  • Easy : 2 NaCl → 1 Na₂CO₃ (mole ratio 2 : 1).
  • Medium : 58.5 kg NaCl ⇒ 58.5 × 106 ÷ 117 = 53 kg Na₂CO₃ (100 %).
  • Exam-type : NH₃ recovery: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O.

Rule 2: NaOH: chlor-alkali — 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (electrolysis of brine). Cl₂ at the anode, H₂ + NaOH at the cathode.

  • Easy : NaOH in soap, paper and textile industries.
  • Medium : NaOH + HCl → NaCl + H₂O.
  • Exam-type : CO₂ absorber: 2NaOH + CO₂ → Na₂CO₃ + H₂O.

Rule 3: Lime, cement and plaster of Paris — CaCO₃ → CaO + CO₂ (≈ 1200 K). Cement: lime, silica, alumina, iron oxide + 2–3 % gypsum. POP: 2(CaSO₄·2H₂O) → 2(CaSO₄)·H₂O + 3H₂O (393 K).

  • Easy : 1 tonne CaCO₃ ⇒ 560 kg CaO (56 ÷ 100).
  • Medium : Gypsum slows the setting of cement.
  • Exam-type : POP: plaster for a broken bone.

Calculation: lime from 1 tonne limestone

  • Step 1: M(CaCO₃) = 100, M(CaO) = 56.
  • Step 2: 100 g → 56 g ⇒ 1000 kg → 560 kg.
  • Step 3: CO₂ = 440 kg (mass is conserved).

CET level (MCQ)

  • CET 1) In the Solvay process NaHCO₃ precipitates because it is? 1) very soluble 2) sparingly soluble 3) unstable 4) heavy — Answer: 2).
  • CET 2) Formula of POP? 1) CaSO₄·2H₂O 2) CaSO₄·½H₂O 3) CaSO₄ 4) CaCO₃ — Answer: 2).

Real-life use: Glass, soap and paper industries; cement in construction; POP for fractures.

Remember

Solvay: 2NaHCO₃ → Na₂CO₃; chlor-alkali: Cl₂ at the anode; CaCO₃ → CaO + CO₂; POP has ½H₂O.

Common mistake

The Solvay process does not work for K₂CO₃ because KHCO₃ is soluble.

CET trap

Both CO₂ and NH₃ are recycled in the Solvay process; only CaCl₂ is waste.

Shortcut

Memory clue: "Solvay = soda, anode = chlorine, POP = half water".

Spot the difference

CompoundMethodUse
Na₂CO₃SolvayGlass, washing soda
NaOHChlor-alkaliSoap
CaOHeating CaCO₃Cement

Practice questions

Easy
  1. 1Write the main reaction of the Solvay process.
  2. 2How is NaOH manufactured?
  3. 3How is lime made?
Medium
  1. 4How much Na₂CO₃ from 58.5 kg NaCl (100 %)?
  2. 5Why is gypsum added to cement?
  3. 6State the use and formula of POP.
Challenging
  1. 7How much CaO from 1 tonne CaCO₃?
  2. 8How is NH₃ recovered in the Solvay process?
Show answers (try them yourself first)
  1. 1NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl.
  2. 2Electrolysis of brine.
  3. 3By heating CaCO₃.
  4. 453 kg.
  5. 5To slow the setting.
  6. 6Plaster for bones; CaSO₄·½H₂O.
  7. 7560 kg.
  8. 8With NH₄Cl and Ca(OH)₂.

Hydrogen peroxide and lithium aluminium hydride

From daily life

Hair bleach and the fizzing wound cleaner are H₂O₂; it releases oxygen and kills germs.

Simple definition

H₂O₂: oxygen has O.N. −1; both oxidising and reducing; open-book structure. LiAlH₄: a powerful reducing agent used in organic chemistry.

H2O2 structureH₂O₂: open-book structureOOHHO–O 147.5 pmO–H 95 pmH–O–O 94.8°dihedral 111.5° (gas)20-volume H₂O₂: 1 mL ⇒ 20 mL O₂ (STP) ⇒ ≈ 1.79 M ≈ 6 %Both oxidising and reducing agent (O.N. −1)

In the figure : Structure of H₂O₂ and 20-volume strength.

Rule 1: Preparation and decomposition of H₂O₂ — BaO₂ + H₂SO₄ → BaSO₄ + H₂O₂; industrially, auto-oxidation of 2-ethylanthraquinol. 2H₂O₂ → 2H₂O + O₂ (light, heat or metal catalysts); so it is stored in dark/plastic bottles.

  • Easy : 1 mL of 20-volume H₂O₂ gives 20 mL O₂ at STP.
  • Medium : Molarity = 20 ÷ 22400 × 2 × 1000 ≈ 1.79 M.
  • Exam-type : Strength ≈ 1.79 × 34 ÷ 10 ≈ 6.07 %.

Rule 2: Oxidising behaviour — In acid: H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O (E° = +1.78 V).

  • Easy : 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O.
  • Medium : PbS + 4H₂O₂ → PbSO₄ + 4H₂O (restoring old paintings).
  • Exam-type : 2I⁻ + H₂O₂ + 2H⁺ → I₂ + 2H₂O.

Rule 3: Reducing behaviour and LiAlH₄ — H₂O₂ reduces stronger oxidisers: 2MnO₄⁻ + 5H₂O₂ + 6H⁺ → 2Mn²⁺ + 8H₂O + 5O₂. LiAlH₄: 4LiH + AlCl₃ → LiAlH₄ + 3LiCl; LiAlH₄ + 4H₂O → LiOH + Al(OH)₃ + 4H₂.

  • Easy : KMnO₄'s pink colour disappears (reduced by H₂O₂).
  • Medium : LiAlH₄ + aldehyde → primary alcohol.
  • Exam-type : LiAlH₄ is used in dry ether (it reacts violently with water).

Calculation: molarity of 20-volume H₂O₂

  • Step 1: 2H₂O₂ → 2H₂O + O₂; 20 mL O₂ from 1 mL = 20 ÷ 22400 = 8.93 × 10⁻⁴ mol.
  • Step 2: H₂O₂ = 2 × 8.93 × 10⁻⁴ = 1.786 × 10⁻³ mol per mL.
  • Step 3: Molarity = 1.786 M; % = 1.786 × 34 ÷ 10 = 6.07 %.

CET level (MCQ)

  • CET 1) O.N. of O in H₂O₂? 1) −2 2) −1 3) 0 4) +1 — Answer: 2).
  • CET 2) Molarity of 20-volume H₂O₂? 1) 0.89 2) 1.79 3) 3.58 4) 5.36 — Answer: 2). (20 ÷ 11.2 = 1.79.)

Real-life use: Wound cleaning, hair/paper bleaching, rocket propellant; LiAlH₄ for reductions in drug manufacture.

Remember

H₂O₂: O.N. −1, both roles; 20 volume ≈ 1.79 M ≈ 6 %; LiAlH₄ in dry ether.

Common mistake

Calling H₂O₂ only an oxidiser; with strong oxidisers it is a reducer.

CET trap

'x volume' means x mL O₂ per mL at STP; molarity = x ÷ 11.2.

Shortcut

Memory clue: "M = volume ÷ 11.2" (H₂O₂).

Spot the difference

RoleReactionO.N.
OxidisingH₂O₂ + 2I⁻ → …−1 → −2
ReducingH₂O₂ + MnO₄⁻ → …−1 → 0

Practice questions

Easy
  1. 1What is the O.N. in H₂O₂?
  2. 2Why is H₂O₂ stored in dark bottles?
  3. 3State a use of LiAlH₄.
Medium
  1. 4Find the molarity of 20-volume H₂O₂.
  2. 5Write the oxidation of Fe²⁺ by H₂O₂.
  3. 6The role of H₂O₂ in its reaction with KMnO₄.
Challenging
  1. 7Write the O.N. changes in H₂O₂ decomposition.
  2. 8Why is LiAlH₄ not used in water?
Show answers (try them yourself first)
  1. 1−1.
  2. 2Light decomposes it.
  3. 3Reducing aldehydes to alcohols.
  4. 420 ÷ 11.2 = 1.79 M.
  5. 52Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O.
  6. 6Reducing agent.
  7. 7−1 → −2 and −1 → 0 (disproportionation).
  8. 8It reacts violently with water releasing H₂.

Quick revision of the whole lesson

ConceptKey formula / fact
H1s¹; H⁺ and H⁻; ¹H, ²H, ³H
H₂ preparationZn + 2HCl ; CH₄ + H₂O (1270 K, Ni) ; CO + H₂O (673 K)
Group 1/2ns¹ / ns²; radius ↑ IE ↓
In airLi₂O ; Na₂O₂ ; KO₂
DiagonalLi–Mg ; Be–Al
SolvayNaCl + NH₃ + CO₂ + H₂O → NaHCO₃ ; 2NaHCO₃ → Na₂CO₃
Chlor-alkali2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
H₂O₂20 volume = 1.79 M ≈ 6 %
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🚀 Quick revision of the whole lesson

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🎯 Most important points for the exam

  • Position and isotopes of hydrogen.
  • Preparation and uses of H₂.
  • Trends of groups 1 and 2 and flame tests.
  • Oxide types, solubility, thermal stability.
  • Diagonal relationship; biological roles.
  • Na₂CO₃, NaOH, CaCO₃, cement, POP.
  • Properties of H₂O₂; LiAlH₄.
Study mantra

understand the idea → remember the formula → solve the example → solve problems yourself.