Chapter 10 The sBlock Elements Solutions
Question - 11 : - Berylliumand magnesium do not give colour to flame whereas other alkaline earth metalsdo so. Why?
Answer - 11 : -
Whenan alkaline earth metal is heated, the valence electrons get excited to ahigher energy level. When this excited electron comes back to its lower energylevel, it radiates energy, which belongs to the visible region. Hence, thecolour is observed. In Be and Mg, the electrons are strongly bound. The energyrequired to excite these electrons is very high. Therefore, when the electronreverts back to its original position, the energy released does not fall in thevisible region. Hence, no colour in the flame is seen.
Question - 12 : - Discussthe various reactions that occur in the Solvay process.
Answer - 12 : -
Solvayprocess is used to prepare sodium carbonate.
Whencarbon dioxide gas is bubbled through a brine solution saturated with ammonia,sodium hydrogen carbonate is formed. This sodium hydrogen carbonate is thenconverted to sodium carbonate.
Step 1: Brine solution issaturated with ammonia.
Thisammoniated brine is filtered to remove any impurity.
Step 2: Carbon dioxide isreacted with this ammoniated brine to result in the formation of insolublesodium hydrogen carbonate.
NH3+H2O+CO2→NH4HCO3NaCl+NH4HCO3→NaHCO3+NH4Cl
Step 3: The solutioncontaining crystals of NaHCO3 is filtered toobtain NaHCO3.
Step 4: NaHCO3 is heated stronglyto convert it into NaHCO3.
Step 5: To recover ammonia, thefiltrate (after removing NaHCO3) is mixed with Ca(OH)2 and heated.
Ca(OH)2+2NH4Cl→2NH3+2H2O+CaCl2
Theoverall reaction taking place in Solvay process is 
Question - 13 : - Potassiumcarbonate cannot be prepared by Solvay process. Why?
Answer - 13 : -
Solvayprocess cannot be used to prepare potassium carbonate. This is because unlikesodium bicarbonate, potassium bicarbonate is fairly soluble in water and doesnot precipitate out.
Question - 14 : - Why is Li2CO3 decomposed at alower temperature whereas Na2CO3 at highertemperature?
Answer - 14 : -
Aswe move down the alkali metal group, the electropositive character increases.This causes an increase in the stability of alkali carbonates. However, lithiumcarbonate is not so stable to heat. This is because lithium carbonate iscovalent. Lithium ion, being very small in size, polarizes a large carbonate ion,leading to the formation of more stable lithium oxide.
Therefore,lithium carbonate decomposes at a low temperature while a stable sodiumcarbonate decomposes at a high temperature.
Question - 15 : - Comparethe solubility and thermal stability of the following compounds of the alkalimetals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates (c)Sulphates.
Answer - 15 : -
(i) Nitrates
Thermal stability
Nitrates of alkali metals, except LiNO3, decompose on strongheating to form nitrites.
LiNO3, on decomposition, givesoxide.
Similarto lithium nitrate, alkaline earth metal nitrates also decompose to giveoxides.
Aswe move down group 1 and group 2, the thermal stability of nitrate increases.
Solubility
Nitratesof both group 1 and group 2 metals are soluble in water.
(ii) Carbonates
Thermal stability
Thecarbonates of alkali metals are stable towards heat. However, carbonate oflithium, when heated, decomposes to form lithium oxide. The carbonates ofalkaline earth metals also decompose on heating to form oxide and carbondioxide.
Carbonates of alkali metals are soluble inwater with the exception of Li2CO3. Also, the solubilityincreases as we move down the group.
Carbonatesof alkaline earth metals are insoluble in water.
(iii) Sulphates
Thermal stability
Sulphatesof both group 1 and group 2 metals are stable towards heat.
Solubility
Sulphatesof alkali metals are soluble in water. However, sulphates of alkaline earthmetals show varied trends.
BeSO4 Fairly soluble
MgSO4 Soluble
CaSO4 Sparingly soluble
SrSO4 Insoluble
BaSO4 Insoluble
Inother words, while moving down the alkaline earth metals, the solubility oftheir sulphates decreases.
Question - 16 : - Startingwith sodium chloride how would you proceed to prepare (i) sodium metal (ii)sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?
Answer - 16 : -
(a) Sodium can beextracted from sodium chloride by Downs process.
This process involves the electrolysis offused NaCl (40%) and CaCl2 (60 %) at a temperature of 1123 K inDowns cell.
Steelis the cathode and a block of graphite acts as the anode. Metallic Na and Caare formed at cathode. Molten sodium is taken out of the cell and collectedover kerosene.
(ii) Sodium hydroxide canbe prepared by the electrolysis of sodium chloride. This is calledCastner–Kellner process. In this process, the brine solution is electrolysedusing a carbon anode and a mercury cathode.
Thesodium metal, which is discharged at cathode, combines with mercury to form anamalgam.
(iii) Sodium peroxide
First,NaCl is electrolysed to result in the formation of Na metal (Downs process).
This sodium metal is then heated onaluminium trays in air (free of CO2) to form its peroxide.
(iv) Sodium carbonate isprepared by Solvay process. Sodium hydrogen carbonate is precipitated in areaction of sodium chloride and ammonium hydrogen carbonate.
Thesesodium hydrogen carbonate crystals are heated to give sodium carbonate.
Question - 17 : - Whathappens when (i) magnesium is burnt in air (ii) quick lime is heated withsilica (iii) chlorine reacts with slaked lime (iv) calcium nitrate is heated ?
Answer - 17 : -
(i) Magnesium burns inair with a dazzling light to form MgO and Mg3N2.
(ii) Quick lime (CaO) combineswith silica (SiO2) to form slag.
(iii) When chloride is added toslaked lime, it gives bleaching powder.
(iv) Calcium nitrate, onheating, decomposes to give calcium oxide.
Question - 18 : - Describetwo important uses of each of the following: (i) caustic soda (ii) sodiumcarbonate (iii) quicklime.
Answer - 18 : -
(i) Uses of caustic soda
(a) It is used in soapindustry.
(b) It is used as areagent in laboratory.
(ii) Uses of sodiumcarbonate
(a) It is generally used inglass and soap industry.
(b) It is used as awater softener.
(iii) Uses of quick lime
(a) It is used as a startingmaterial for obtaining slaked lime.
(b) It is used in the manufacture of glass and cement.
Question - 19 : - Draw the structure of (i) BeCl2 (vapour) (ii) BeCl2 (solid).
Answer - 19 : -
(a) Structure of BeCl2 (solid)
BeCl2 exists as a polymerin condensed (solid) phase.
In the vapour state, BeCl2 exists as a monomerwith a linear structure.
Question - 20 : - Thehydroxides and carbonates of sodium and potassium are easily soluble in waterwhile the corresponding salts of magnesium and calcium are sparingly soluble inwater. Explain.
Answer - 20 : -
Theatomic size of sodium and potassium is larger than that of magnesium andcalcium. Thus, the lattice energies of carbonates and hydroxides formed bycalcium and magnesium are much more than those of sodium and potassium. Hence,carbonates and hydroxides of sodium and potassium dissolve readily in waterwhereas those of calcium and magnesium are only sparingly soluble.