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NCERT Class 11 Chemistry Chapter 10 The s-Block Elements
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The s-Block Elements
Chapter: 10
| Part – II |
1. What are the common physical features of alkali metals?
Ans: The common physical features of alkali metals are:
(i) They are quite soft and can be cut easily. Sodium metal can be easily cut using a knife.
(ii) The metallic bonding present in alkali metals is quite weak. Therefore, they have low melting and boiling points.
(iii) They are light coloured and are mostly silvery-white in appearance.
(iv) Alkali metals and their salts impart a characteristic colour to flames. This is because the heat from the flame excites the electron present in the
The common chemical features of alkali metals are:
(i) They react with water to form respective oxides or hydroxides. As we move down the group, the reaction becomes more and more spontaneous.
(ii) They are strong reducing agents. The reducing power of alkali metals increases on moving down the group. However, lithium is an exception. It is the strongest reducing agent among the alkali metals. It is because of its high hydration energy.
(iii) Alkali metals hydroxides are highly basic in nature.
(iv) Alkali metals dissolve in liquid ammonia to form blue and conducting solution.
2. Discuss the general characteristics and gradation in properties of alkaline earth metals.
Ans: The general characteristics and gradation in properties of alkaline earth metals are:
(i) Atomic Size: Atomic size goes on increasing down the group.
(ii) Electronic Configuration: The general electronic configuration of alkaline earth metals is [noble gas] ns2.
(iii) Ionization Energy: Ionisation energy goes on decreasing down the group. making it easier to remove the outermost electrons.
(iv) Oxidation State: Alkaline earth metals have an oxidation state of + 2.
(v) Electrical Conductivity: Alkaline earth metals are good conductors of electricity.
(vi) Physical Appearance: These metals are lustrous and silvery white in appearance. They are relatively less soft as compared to alkali metals.
3. Why are alkali metals not found in nature?
Ans: Alkali metals are highly reactive in nature. That’s why they always exist in a combined state in nature. Hence, in nature, they are not found in elemental state. In combined state, they are present in the form of halides, oxides, silicates, borates and nitrates.
4. Find out the oxidation state of sodium in Na2O2.
Ans: Let x be the oxidation state of Na in Na2O2
Therefore,
2(x) + 2(-1) = 0
2x – 2 = 0
2x = 2
x = + 1
Therefore, the oxidation state of sodium in Na2O2 is + 1.
5. Explain why is sodium less reactive than potassium.
Ans: In alkali metals, the atomic size increases down the group while the effective nuclear charge decreases. Because of these factors, the outermost electron in potassium can be lost easily as compared to sodium. Hence, potassium is more reactive than sodium.
6. Compare the alkali metals and alkaline earth metals with respect to
(i) ionisation enthalpy,
(ii) basicity of oxides,
(iii) solubility of hydroxides.
Ans:
| Alkali metals | Alkaline earth metals | |
| Ionisation enthalpy: | Ionisation enthalpy: Alkaline earths have a higher ionisation enthalpy value than alkali metals. This can be because the atomic size of alkaline earths | Alkaline earth metals have higher ionisation enthalpy compared to alkali metals because they have two valence electrons. |
| Basicity of oxides: | Basicity of oxides On dissolution in water, the oxides of alkali metals and alkaline-earth metal metals form basic hydroxides. | The basicity of metallic element oxides is more than that of metallic element metal oxides. And as know it’s all because of the lower ionization enthalpy of alkali metals than that of corresponding alkaline earths. |
| Solubility of hydroxides: | The hydroxides of alkali metals are more soluble than those of alkaline earth metals. | The hydroxides of alkaline earth metals are less soluble than those of alkali metals due to their higher lattice energies. This increased lattice energy results from their higher charge densities compared to alkali metals. |
7. In what ways lithium shows similarities to magnesium in its chemical behaviour?
Ans: (i) Both react with cold water.
(ii) Both react with 02 to form monoxides.
(iii) Both have oxides that are less soluble in water and hydroxides that decompose at high temperatures.
(iv) Both Li and Mg react with N2 to form nitrides.
(v) Both LiCl and MgCl2 are soluble in ethanol owing to their covalent nature.
(vi) Li and Mg do not form solid bicarbonates.
8. Explain why alkali and alkaline earth metals can not be obtained by chemical reduction methods.
Ans: In the process of chemical reduction, oxides of metals are reduced using a stronger reducing agent. Alkali and alkaline earth metals cannot be obtained by chemical reduction methods because of their highly electropositive nature. Hence, they are very strong reducing agents. Hence, other reducing agents cannot reduce oxides.
9. Why are potassium and caesium, rather than lithium used in photoelectric cells?
Ans: Potassium and caesium have much lower ionisation enthalpy than that of lithium. As a result, these metals easily emit electrons on exposure to light. Potassium and cesium are used in photoelectric cells instead of lithium because they have lower ionisation energy and larger atomic sizes, which allows them to lose electrons more easily when exposed to light.
10. When alkali metal dissolves in liquid ammonia, the solution can acquire different colours. Explain the reason for this type of colour change.
Ans: Alkali metals dissolve in liquid ammonia and give deep blue solutions because of the ammoniated electrons. The blue colour is due to the ammoniated electron absorbing energy in the visible region of light.
11. Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?
Ans: Beryllium (Be) and magnesium (Mg) have small atomic sizes, the ionisation enthalpies of Be and Mg are much higher than those of other alkaline earth metals. The energy of flame is not sufficient to excite the electrons of Be and Mg to higher energy levels. Therefore Beryllium (Be) and magnesium (Mg) do not produce a colour in a flame.
12. Discuss the various reactions that occur in the Solvay process.
Ans: The Solvay process, also known as the ammonia-soda process, The Solvay process is an industrial method used to produce sodium carbonate (Na₂CO₃) from sodium chloride (NaCl) and calcium carbonate (CaCO₃).
(i) Formation of ammonium bicarbonate:
2NH3 + H2O + CO2 ⟶ (NH4)2 CO3
This ammoniated brine is filtered to remove any impurities.
(ii) Formation of sodium bicarbonate:
Sodium bicarbonate (NaHCO₃) is heated strongly to convert it into sodium carbonate (Na₂CO₃), water, and carbon dioxide.
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
(iii) Conversion of sodium bicarbonate to sodium carbonate:
The filtrate is mixed with Ca(OH)2 and heated to recover ammonia:
Ca(OH)2 + 2NH4Cl → LNH3 + 2H2O + CaCl2
(iv) Sodium bicarbonate formation:
The solution containing NaHCO3 crystals is filtered to obtain NaHCO3.
13. Potassium carbonate cannot be prepared by Solvay process. Why?
Ans: The Solvay process cannot be used to prepare potassium carbonate because potassium bicarbonate (KHCO3) is too soluble in water and does not precipitate out Potassium carbonate being more soluble than sodium bicarbonate does not get precipitated when CO2 is passed through a concentrated solution of KCl saturated with ammonia. The Solvay process, also known as the ammonia-soda process.
14. Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at higher temperature?
Ans: Lithium carbonate: Lithium carbonate is less stable to heat and decomposes at a lower temperature because lithium ions are small and carbonate ions are large. As lithium carbonate is not stable to heat, it decomposes at lower temperature. Small sized Li polarises large carbonate ions which leads to formation of Li2O and CO2.
Sodium carbonate: Sodium carbonate is very stable and decomposes at a higher temperature because sodium is a very reactive metal and forms a very strong bond with the carbonate ion.
15. Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals.
(a) Nitrates.
Ans: Solubility: Alkali metal nitrates are highly soluble in water and stable at high temperatures. In contrast, alkaline earth metal nitrates are also soluble but decompose at lower temperatures, producing metal oxides, nitrogen dioxide, and oxygen when heated.
Thermal Stability: Alkali metal nitrates are relatively stable upon heating and decompose at high temperatures to form nitrites and oxygen.
(b) Carbonates.
Ans: Solubility: Carbonates of alkali metals are soluble in water with the exception of Li2CO3. Also, the solubility increases as we move down the group. Carbonates of alkaline earth metals are insoluble in water.
Thermal Stability: The carbonates of alkali metals are stable towards heat. However, carbonate of lithium, when heated, decomposes to form lithium oxide. The carbonates of alkaline earth metals also decompose on heating to form oxide and carbon dioxide.
(c) Sulphates.
Ans: Solubility: Sulphates of group 1 are soluble in water except Li2SO4. They are thermally stable. Solubility of sulphates of group 2 decreases down the group because Lattice energy dominates over hydration energy.
Thermal stability: Sulphates of both group 1 and group 2 metals are stable towards heat.
16. Starting with sodium chloride how would you proceed to prepare.
(i) Sodium metal.
Ans: Sodium metal is manufactured by electrolysis of a fused mass of NaCl 40% and CaCl2 60% in Down’s cell at 873 K, using iron as cathode and graphite as anode. Na is liberated at the cathode.
At cathode:
Na+ + e– → Na (l)
At anode:
2Cl– (melt) → Cl2 (g) + 2e–.
(ii) Sodium hydroxide.
Ans: Sodium hydroxide can be prepared by the electrolysis of sodium chloride. This is called Castner – Kellner process. In this process, the brine solution is electrolysed using a carbon anode and a mercury cathode.
The sodium metal, which is discharged at cathode, combines with mercury to form an amalgam.
At cathode:
Na+ + e– → Na
2Na + Hg → Na – Hg + 2H20
2Na- Hg + 2H20 → 2NaOH + H2 + Hg
At anode:
Cl– – e– → Cl
Cl + Cl → Cl2
(iii) Sodium peroxide.
Ans: First, NaCl is electrolysed to result in the formation of Na metal (Downs process).This sodium metal is then heated on aluminium trays in air (free of CO2) to form its peroxide.
2Na + O2(air) ⟶ Na2O2
(iv) Sodium carbonate.
Ans: Sodium carbonate is prepared by Solvay process. Sodium hydrogen carbonate is precipitated in a reaction of sodium chloride and ammonium hydrogen carbonate.
At cathode:
Na+ + e– → Na
2Na + Hg ——-> Na – Hg + 2H 0
2Na- Hg + 2H20 ——> 2NaOH + H2 + Hg
At anode:
Cl– – e– ——-> Cl
Cl + Cl ——–> Cl2
17. What happens when:
(i) magnesium is burnt in air.
Ans: Magnesium burns in air with a dazzling light to form MgO and Mg3N2.
(ii) Quick lime is heated with silica.
Ans: Quick lime (CaO) combines with silica (SiO2) to form slag.
(iii) Chlorine reacts with slaked lime.
Ans: When chloride is added to slaked lime, it gives bleaching powder.
(iv) Calcium nitrate is heated?
Ans: Calcium nitrate, on heating, decomposes to give calcium oxide.
18. Describe two important uses of each of the following:
(i) caustic soda.
Ans: (a) Caustic soda can be used in the manufacturing of soap.
(b) It is also used for the Production of artificial silk, paper, and other chemicals.
(ii) sodium carbonate.
Ans: (a) Used in the softening of water, for laundry and cleaning purposes.
(b) It is used in glass manufacturing.
(iii) quick lime.
Ans: (a) Quicklime is used in the production of iron, steel, paper and pulp.
(b) Used in the purification of sugar and in the manufacturing of cement.
19. Draw the structure of (i) BeCl2 (vapour), (ii) BeCl2 (solid).
Ans: In the vapour phase, BeCl2 exists as a dimer. This means two BeCl2 molecules combine to form a larger molecule with two coordinate bonds between the beryllium atoms.
In the solid state, BeCl2 forms a polymeric structure. This is a chain-like structure where each beryllium atom is bonded to two chlorine atoms and each chlorine atom is bonded to two beryllium atoms.
In the solid state, BeCl2 forms a polymeric structure.
Structure of BeCl2 in vapour phase:
Structure of BeCl2 in Solid-state:
20. The hydroxides and carbonates of sodium and potassium are easily soluble in water while the corresponding salts of magnesium and calcium are sparingly soluble in water. Explain.
Ans: The atomic size of sodium and potassium is larger than that of magnesium and calcium. Thus, the lattice energies of carbonates and hydroxides formed by calcium and magnesium are much more than those of sodium and potassium. Carbonates and hydroxides of sodium and potassium dissolve easily in water, while those of calcium and magnesium exhibit limited solubility due to differences in lattice energy and hydration enthalpy.
21. Describe the importance of the following:
(i) Limestone.
Ans: Limestone. Specially precipitated CaCO3 is extensively used in the manufacture of high quality paper. It is also used as an antacid, mild abrasive in toothpaste, a constituent of chewing gum and as a filler in cosmetics.
(ii) Cement
Ans: A chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. It’s used to make concrete and mortar. It is used in plastering and in construction of bridges. It is also used in concrete. Chemically, plaster of Paris is 2CaSO4.H2O.
(iii) Plaster of Paris.
Ans: Plaster of Paris is used in surgical bandages. Used to make plaster casts to immobilise broken bones while they heal. It is also used for making casts and moulds.
22. Why are lithium salts commonly hydrated and those of the other alkali metal ions usually anhydrous?
Ans: Lithium salts are commonly hydrated because lithium is the smallest alkali metal, Hence, Li+ ion can polarise water molecules more easily than other alkali metals. As a result, water molecules get attached to lithium salts as water of crystallisation. Hence, lithium salts such as trihydrated lithium chloride (LiCl.3H2O) are commonly hydrated. As the size of the ions increases, their polarising power decreases. Therefore, most alkali metal ions tend to form anhydrous salts.
23. Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in acetone?
Ans: LiF is insoluble in water. On the contrary, LiCl is soluble not only in water, but also in acetone. This is mainly because of the greater ionic character of LiF as compared to LiCl. The solubility of a compound in water depends on the balance between lattice energy and hydration energy. Due to the significantly smaller size of the fluoride ion compared to the chloride ion, the lattice energy of LiF is higher than that of LiCl. Additionally, the difference in hydration energies between the fluoride ion and the chloride ion is minimal. Thus, the net energy change during the dissolution of LiCl in water is more exothermic than that during the dissolution of LiF in water. Hence, low lattice energy and greater covalent character are the factors making LiCl soluble not only in water, but also in acetone.
24. Explain the significance of sodium, potassium, magnesium and calcium in biological fluids.
Ans: Sodium, potassium, magnesium, and calcium play crucial roles in biological fluids.
Sodium: Found in plasma and extracellular fluid, sodium helps regulate water levels in the body and cell walls. It also helps transmit nerve signals and transport amino acids and sugars into cells.
Potassium: Potassium ions are found in the highest quantity within the cell fluids.K ions help in activating many enzymes. They also participate in oxidising glucose to produce ATP. They also help in transmitting nerve signals.
Magnesium: Magnesium helps relax muscles and nerves, build and strengthen bones, and maintain normal blood circulation. It’s also involved in ATP metabolism, neurological functioning, and neurotransmitter release. Magnesium contributes to bone structure and is involved in calcium metabolism.
Calcium: Calcium is a primary component of bone and teeth, providing strength and structure. Ca2+ ions are present in bones. plays important roles in neuromuscular function. It is essential for the coagulation cascade, helping in the formation of blood clots to prevent excessive bleeding.
25. What happens when:
(i) Sodium metal is dropped in water?
Ans: 2Na + 2H2O ——–> 2NaOH + H2
(ii) Sodium metal is heated in free supply of air?
Ans: 2Na + 2H2O ——–> 2NaOH + H2
(iii) Sodium peroxide dissolves in water?
Ans: Na2O2 + 2H20 ———> 2NaOH + H2O2
26. Comment on each of the following observations:
(a) The mobilities of the alkali metal ions in aqueous solution are Li+ < Na+ <K+ < Rb+ < Cs+
Ans: The observed trend in mobilities is actually due to the hydration energy of these ions. Smaller ions (like Li+) have a higher hydration energy, meaning they form stronger bonds with water molecules. This makes it more difficult for them to move through the solution, resulting in a lower mobility. As the ion size increases (Na+, K+, Rb+, Cs+), the hydration energy decreases, leading to higher mobilities.
(b) Lithium is the only alkali metal to form a nitride directly.
Ans: It is not possible to prepare alkali metals by the chemical reduction of their oxides as they themselves are very strong reducing agents. They cannot be prepared by displacement reactions either (wherein one element is displaced by another). Alkali and alkaline earth metals cannot be obtained by chemical reduction methods because they are highly electropositive. Due to their strong tendency to lose electrons, they cannot be reduced by standard chemical methods using reducing agents.
(c) Ee for M2+ (aq) + 2e– → M(s) (where M = Ca, Sr, or Ba) is nearly constant.
Ans: It is because reduction potential depends upon sublimation energy, ionisation energy and hydration energy. Their resultant is almost constant for these ions.
27. State as to why:
(a) a solution of Na2CO3 is alkaline?
Ans: When sodium carbonate is added to water, it hydrolyses to give sodium bicarbonate and sodium hydroxide (a strong base). As a result, the solution becomes alkaline.
(b) alkali metals are prepared by electrolysis of their fused chlorides?
Ans: Because the discharge potential of alkali metals is much higher than that of hydrogen, therefore when the aqueous solution of any alkali metal chloride is subjected to electrolysis, H2, instead of the alkali metal, is produced at the cathode. Therefore alkali metals are prepared by electrolysis of their fused chlorides.
(c) Sodium is found to be move useful than potassium?
Ans: Blood plasma and the interstitial fluids surrounding the cells are the regions where sodium ions are primarily found. Potassium ions are present in cell fluids, while sodium ions are essential for nerve signal transmission, water regulation across membranes, and transporting sugars and amino acids, making sodium more crucial.
28.Write balanced equations for reactions between.
(a) Na2O2 and water.
Ans: Na2O2 + 2H2O ——-> 2Na0H + H2O2
(b) KO2 and water.
Ans: 2KO2 + 2H2O ———-> 2K0H + O2 + H2O2
(c) Na2O and CO2
Ans: Na2O + CO2 ———–> Na2CO3
29. How would you explain the following observations?
(i) BeO is almost insoluble but BeSO4 is soluble in water.
Ans: BeO is almost insoluble in water due to high lattice energy. BeSO4 is soluble in water due to high hydration energy of Be2+ ion. The high hydration energy of Be2 + in BeSO4 overcomes the lattice energy factors and therefore BeSO4 is soluble in water.
(ii) BaO is soluble but BaSO4 is insoluble in water.
Ans: BaO is soluble in water, but BaSO4 is not. Ba2+ is a large cation and O2– is a small anion. The size compatibility of Ba2+ and O2– is not high. As a result, BaO is unstable. The lattice energy released during its formation is also not very large. The solubility of BaO in water can easily be attributed to the hydration energy of the ions. Conversely, in BaSO₄, both Ba²⁺ and SO₄²⁻ ions are large in size, resulting in a high lattice energy that prevents its solubility in water.
(iii) Lil is more soluble than KI in ethanol.
Ans: The lithium ion has a smaller size and as a result of that it has a higher polarising capability. This enables it to polarise the electron cloud around an iodide ion thus resulting in a greater covalent character in LiI than KI. Thus LiI is easily soluble in ethanol.
30. Which of the alkali metal is having least melting point?
(a) Na.
(b) K.
(c) Rb.
(d) Cs.
Ans: Cesium (Cs) has the lowest melting point of the alkali metals, (d) is correct.
31. Which one of the following alkali metals give hydrated salts?
(a) Li.
(b) Na.
(c) K.
(d) Cs.
Ans: Lithium (Li) is the alkali metal that gives hydrated salts.
32. Which one of the following alkaline earth metal carbonates is thermally most stable?
(a) MgCO3
(b) CaCO3
(c) SrCO3
(d) BaCO3
Ans: (d) BaCO3
