s block elements – Important Questions & Answers
s block elements
Que 1. Why are alkali metals not found in nature?
Ans 1. Due to high chemical reactivity alkali metals do not occur free in nature. They are found in the earth’s crust in the form of halide, sulphate, carbonate, silicate, borate, oxide ores, etc
Que 2. Why are halides of beryllium polymeric?
Ans 2. The halides of Be are electron deficient as their octets are incomplete. Therefore, to complete their octets, the halides polymerize.
Que 3. Arrange the alkaline earth metal carbonates in the decreasing order of thermal stability. Give a reason to support your answer.
Ans 3. BaCO3 > SrCO3 > CaCO3 > MgCO3 > BeCO3
Reason: Thermal stability of alkaline earth metal carbonates increases with cationic size (results in a large lattice enthalpy). All carbonates undergo thermal decomposition to form the metal oxide and carbon dioxide. The thermal stability of alkaline earth metal carbonates increases on moving down the group.
Que 4. Why do hydrides and halides of Be polymerise?
Ans 4. As BeH2 have only four electrons in the valence shell, therefore, they are electron-deficient molecules. To make up for their electron deficiency, each Be atom forms four three-centre two-electron bonds or banana bonds. Thus, it is due to electron deficiency that BeH2 and BeCl2 have polymeric structures.
Que 5. What is the chemical formula of Plaster of Paris?
Ans 5. CuSO4.1/2H2O (Copper sulphate hemihydrate)
Que 6. Which out of MgSO4 or BaSO4 is more soluble in water? Why?
Ans 6. MgSO4
Reason. Generally, the Solubility of an ionic compound depends upon lattice energy and hydration energy of its ions. High the hydration enthalpy more is the solubility. Hydration enthalpy decreases down the group due to an increase in cation charge and hence a lower ionic charge/size ratio. As the size of the Barium ion is much larger than the Magnesium ion, BaSO4 is insoluble in water.
Que 7. Which elements of the alkaline earth metals family do not give characteristic flame colouration?
Ans 7. Be and Mg
Que 8. Complete the following reaction:
i) Mg(NO3)2 —Δ→
ii) LiOH —- Δ→
iii) Na2O + H2O →
iv) 2 Na+ O2 →
Ans 8. i) Mg(NO3)2 —Δ→ 2MgO + 4NO2 + O2
ii) LiOH —- Δ→ Li2O + H2O
iii) Na2O + H2O → Na2CO3
iv) 2 Na + O2 → Na2O2
Que 9. (i) Why is it necessary to remove CO2 continuously during the manufacture of CaO from CaCO3 during the manufacture of CaO from CaCO3?
(ii) Why the temperature of this reaction CaCO3 → CaO + CO2 is maintained at 1270 K.
Ans 9. (i) As the reaction is of reversible nature, CO2 formed must be removed as soon as it is formed so that the reaction does not proceed in the backward direction.
(ii) The temperature shouldn’t be allowed to rise above 1270 K, otherwise, silica (SiO2) present as an impurity in limestone will combine with CaO to form CaSiO3.
Que 10. Arrange LiF, NaF, KF, RbF, and CsF in order of increasing lattice energy.
Ans 10. CsF < RbF < KF < NaF< LiF
s block elements
Que 11. Name the chief factors responsible for anomalous behaviour or lithium.
Ans 11. The anomalous behaviour of lithium is because of the following reasons:
(i) Small size of atom and ion.
(ii) High ionization enthalpy.
(iii) Absence of d-orbitals in its Valence shell.
Que 12. (A) State as to why
(a) Alkali metals are prepared by electrolysis of their fused chlorides.
(B) (a) Sodium is found to be more useful than potassium?
(b) Explain what happens when fused sodium metal reacts with ammonia?
Ans 12. (A) (a) i. Alkali metals are strong reducing agents, hence cannot be extracted by reduction of their oxides and other compounds.
ii. Being highly positive in nature it is not possible to displace them from their salt solution by any other element.
iii. Alkali metals cannot be obtained by the electrolysis of the aqueous solution of their salts because H2 is liberated at the cathode instead of alkali metal. That’s why alkali metals are prepared by electrolysis of their fused chloride,
NaCl → Na+ + Cl–
During electrolysis: At anode: 2Cl– → Cl2 + 2e–
At cathode: 2Na+ + 2e– → 2Na
(B) (a) Sodium is found to be more useful than potassium as it is highly reactive but not as reactive as potassium. Sodium if used.
i. As a coolant in a nuclear reactor.
ii. In the manufacture of tetraethyl lead an anti-knock additive for petrol.
4C2H5Cl + 4Na ——-Pb→ (C2H5)4 Pb + 3Pb + 4NaCl
iii. In sodium vapour discharge lamps.
iv. As a laboratory reagent for organic analysis.
(b) Na + NH3 → NaNH2 + H2
Que 13. What happens when
(i) Magnesium is burnt in the air
(ii) Quick lime is heated with silica
(iii) Chlorine reacts with slaked lime
(iv) Calcium nitrate is heated.
Ans 13. (i) A mixture of magnesium oxide and magnesium nitride is formed
5Mg + O2 + N2 → 2 MgO + Mg3N2
(ii) Calcium silicate is formed.
CaO + SiO2 → CaSiO3
(iii) Calcium oxychloride (bleaching powder) is formed
Ca(OH)2 + Cl2 → CaOCl2 + H2O
(iv) Nitrogen dioxide is evolved.
Ca(NO3)2 → CaO + 2 NO2 + O2
s block elements
Que 14. Ions of an element of group 1 participate in the transmission of nerve signals and the transport of sugars and amino acids into cells. This element imparts a yellow colour to the flame in the flame test and forms an oxide and a peroxide with oxygen. Identify the element and write a chemical reaction to show the formation of its peroxide. Why does the element impart colour to the flame?
Ans 14. The yellow colour flame in the flame test indicates that the alkali metal must be sodium. It reacts with O2 to form a mixture of sodium peroxide, Na2O2 and sodium oxide, Na2O.
4 Na + O2 → 2Na2O
2 Na2O + O2 → 2 Na2O2
2 Na + O2 → Na2O2
Ionisation enthalpy of sodium is low. When sodium metal or its salt is heated in Bunse flame, the flame energy causes an excitation of the outermost electron which on reverting back to its initial position gives out the absorbed energy as visible light. That’s why sodium imparts a yellow colour to the flame.
Que 15. Describe the importance of the following
(i) Limestone
(ii) Cement
(iii) Plaster of Paris.
Ans 15. i) Limestone: It is used
a) As a building material
b) In the manufacture of quick lime
c) In the Solvay process prepare Na2CO3 as it is a source of CO2
d) In metallurgy for the extraction of iron
e) In toothpaste and certain cosmetics
ii) Cement: It is an important building material. It is used in concrete and reinforced concrete, in plastering and in the construction of bridges, dams and buildings.
iii) Plaster of Paris: It is used
a) In making moulds for pottery and ceramics etc.
b) In surgical bandages for setting broken bones of the body
c) For making statues, models, decorative materials, and blackboard chalk.
Que 16. During the extraction of sodium by Down’s process, anhydrous calcium chloride is mixed with sodium chloride and the molten mass is electrolyzed.
Ans 16. Anhydrous CaCl2is mixed with NaCl in order to reduce its melting point from 1074 K to nearly 873 K (Impurities lower the melting point of a solid.) at this reduced temperature
(a) Sodium does not vaporize, and no metallic fog is formed.
(b) Chlorine does not corrode the material of the vessel.
(c) The electrolysis process becomes comparatively cheap since the consumption of electricity is reduced as a lower temperature is needed.
s block elements
Que 17. State why
a) A solution of Na2CO3 is alkaline?
b) Alkali metals are prepared by electrolysis of their fused chlorides
c) Sodium is found to be more useful than potassium?
Ans 17. (a) Sodium carbonate being salt of strong base (NaOH) and weak acid (H2CO3) forms an alkaline solution upon hydrolysis
Na2CO3 + 2H2O → 2NaOH + H2CO3
(b) Since the discharge potential of alkali metals is much higher than that of hydrogen, therefore, when the aqueous solution of an alkali metal chloride is subjected to electrolysis, H2 instead of the alkali metal is produced at the cathode. Therefore, to prepare alkali metals, electrolysis of their fused chlorides is carried out.
(c) Sodium is relatively more abundant than potassium. At the same time, it is also less reactive and its reactions with other substances can be better controlled.
Que 18. Why are potassium and caesium, rather than lithium used in photoelectric cells?
Ans 18. The ionization enthalpy of lithium is quite high. The photons of light are not in a position to eject electrons from the surface of lithium metal. Therefore, the photoelectric effect is not noticed. However, both potassium and caesium have comparatively low ionization enthalpies. This means that the electrons can quite easily be ejected from the surface of these metals when photons of a certain minimum frequency (threshold frequency) strike against their surface
Que 19. Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at a higher temperature?
Ans 19. Li+ ion is very small in size. It is stabilized more by smaller anions such as oxide ions rather than large anions such as carbonate. Therefore, Li2CO3 decomposes into Li2O on mild heating. On the other hand, the Na+ ion is larger in size. It is stabilized more by carbonate ion than oxide ion. Hence, Na2CO3 does not undergo thermal decomposition easily.
Que 20. Explain the significance of sodium, potassium, magnesium, and calcium in biological fluids.
Ans 20. Significance of sodium and potassium:
(i) Sodium ions participate in the transmission of nerve signals.
(ii) Sodium ions also regulate the flow of water across the cell membranes and in the transport of sugars and amino acids into the cells.
(iii) Potassium ions are the most abundant cations within cell fluids, where they activate many enzymes and participate in the oxidation of glucose to produce ATP.
(iv) Potassium ions in combination with sodium ions are responsible for the transmission of nerve signals. (v)The functional features of nerve cells depend upon the sodium-potassium ion gradient that is established in the cell.
Significance of Magnesium and Calcium:
(i) Magnesium ions are concentrated in animal cells and Calcium ions are concentrated in body fluids, outside the cell.
(ii) All enzymes that utilize ATP in phosphate transfer require magnesium ion as a cofactor.
(iii) In green plants magnesium is present in chlorophyll.
(iv) Calcium and magnesium ions are also essential for the transmission of impulses along nerve fibres.
(v) Calcium ions are important in blood clotting and are required to trigger the contraction of muscles.
(vi) Calcium ions also regulate the beating of the heart.
s block elements
