Solved Exercise, Chem-12, Ch-02


Q.01: Fill in the blanks:

(i) Alkali metals are _______ reactive than alkaline-earth metals. (more)

(ii) Alkali metals decompose water vigorously producing ________ and hydrogen. (metal hydroxide)

(iii) When heated in a current of dry hydrogen, alkaline earth metals form white crystalline _________ of the type MH2. (metal hydride)

(iv) The beryllium hydroxide, like the hydroxide of aluminium is amphoteric, while the hydroxides of the other members of the group IIA are ________ . (basic)

(v) The elements of the group IA are termed as alkali metals, because their _______ are alkaline. (oxides & hydroxide)

(vi) Spodumene is an ore of _______ metal. (Lithium)

(vii) Alkali metal nitrates on heating give the corresponding _______ and oxygen. (nitrite)

(viii) Na2CO3.H2O is the chemical formula of a mineral of sodium which is known as _______ . (Natron)

(ix) Metallic bicarbonates are decomposed on heating into their carbonates, along with ________ and ________ . (CO2, H2O)

(x) Metal nitrates other than the alkali metals on heating decompose into the corresponding metal ______ along with the evolution of nitrogen peroxide and oxygen. (oxide)

Q.02: Indicate True or False:

(i) Group IA elements are called alkali metals because their chlorides are alkaline in nature. (FALSE)

CORRECTION: Group IA elements are called alkali metals because their oxides and hydroxides are alkaline in nature.

(ii) Alkali metals are very good conductor of electricity. (TRUE)

(iii) The hydroxides of alkali metals and alkaline earth metals are soluble in water. (FALSE)

CORRECTION: The hydroxides of alkali metals except lithium hydroxide are soluble in water. The hydroxides of alkaline earth metals are insoluble or sparingly soluble in water except that of barium which is soluble in water.

(iv) Plaster of Paris is a hemihydrate. (TRUE)

(v) Alkali metals have low melting and boiling points as compared to those of alkaline earth metals. (TRUE)

(vi) Lithium carbonate is decomposed to its oxide, but the carbonates of the other alkali metals are stable towards heat. (TRUE)

(vii) All alkali metal sulphates are insoluble in water. (FALSE)

CORRECTION: All alkali metal sulphates are soluble in water.

(viii) Lithium combines with nitrogen to form lithium nitride but other alkali metals do not react with nitrogen. (TRUE)

(ix) Trona is a mineral of lithium. (FALSE)

CORRECTION: Trona is a mineral of sodium.

(x) Alkaline earth metals are stronger reducing agents than alkali metals. (FALSE)

Q.04: Multiple choice questions.

(i) Which one of the followings does not belong to alkaline earth metals?      

(a) Be         

(b) Ra        

(c) Ba         

(d) Rn

EXPLANATION: Radon is a noble gas present in group VIIIA.    

(ii) The oxides of beryllium are:                  

(a) Acidic                      

(b) Basic

(c) Amphoteric              

(d) None of these

EXPLANATION: Beryllium is less electropositive as compared to other group members. So, its oxide is not basic but amphoteric in nature. Amphoteric means it shows both basic and acidic characters, that is, it acts as a base against acid, and as an acid against a base.    

(iii) Which ion will have the maximum value of heat of hydration?        

(a) Na+       

(b) Ca+       

(c) Ba2+      

(d) Mg2+

EXPLANATION: Hydration energy is the amount of heat released when an ion is hydrated i.e., surrounded and interacted by water molecules. Hydration energy depends upon the charge density or the charge to size ratio of ion. The greater is the charge density, the greater will be hydration energy of the ion and vice versa. Mg2+ has greater charge and smaller size, therefore its charge density is higher, and the hydration energy value is also higher.   

(iv) Which one of the followings is not an alkali metal?                       

(a) Francium                  

(b) Cesium

(c) Rubidium                 

(d) Radium

EXPLANATION: Radium is an alkaline earth metal belonging to group IIA.    

(v) Which one of the following sulphates is not soluble in water?                        

(a) Sodium sulphate       

(b) Potassium sulphate

(c) Zinc sulphate            

(d) Barium sulphate

EXPLANATION: Solubility of alkaline earth metal sulphates decreases down the group. Barium lies down in the group IIA, in 6th period. Therefore, barium sulphate is insoluble in water.    

(vi) The element Cesium bears resemblance with:                                        

(a) Ca                           

(b) Cr

(c) Both of these metals  

(d) None of these

EXPLANATION: Cesium (Cs) is an alkali metal, while Calcium is an alkaline earth metal and Chromium is a transition metal. So, the properties of Cs don’t resemble any of these elements.    

(vii) Chile saltpetre has the chemical formula:                                     

(a) NaNO3                                               

(b) KNO2

(c) Na2B4O7                   

(d) Na2CO3.H2O

EXPLANATION: Sodium nitrate is called Chile saltpeter because it was historically mined in large quantities in northern where the largest deposits of naturally occurring sodium nitrate are found.    

(viii) The mineral CaSO4.2H2O has the general name:

(a) Gypsum                   

(b) Dolomite

(c) Calcite                     

(d) Epsom salt

EXPLANATION: The mineral CaSO4.2H2O is named gypsum due to its white chalk like appearance. The word Gypsom has its origin in Greek ‘Gypsos’, meaning chalk or plaster.

(ix) Down’s cell is used to prepare:   

(a) Sodium carbonate      

(b) Sodium bicarbonate

(c) Sodium metal           

(d) Sodium hydroxide

EXPLANATION: In Down’s cell, molten sodium chloride (NaCl) is electrolyzed in the presence of some calcium chloride. The sodium cations are reduced at cathode to give molten sodium metal which is then separated out.      

(x) Which element is deposited at the cathode during the electrolysis of brine in diaphragm cell?                                                  

(a) H2         

(b) Na         

(c) Cl2        

(d) O2

EXPLANATION: In Diaphragm or Nelson cell, the electrolysis of aqueous solution of sodium chloride or brine is carried out. Na+ and H+ are attracted towards cathode, where is H+ ions are reduced to give H2 gas. Na+ ions remain in the solution and are not reduced because its reduction potential or the ability to gain electrons is smaller than that of H+ ions. 

Q.04: Give the names, electronic configurations and occurrence of s-block elements.

Ans: Consult Textbook.

Discuss the peculiar behaviour of lithium with respect to the other members of alkali metals.

Ans: Peculiar Behaviour of Lithium:

In many of its properties, lithium is quite different from the other alkali metals. The deviation shown by lithium can be explained on the basis of its small radius and high charge density. The nuclear charge of Li+ ion is screened only by a shell of two electrons. The so-called ‘anomalous’ properties of lithium are due to the fact that lithium is unexpectedly far less electropositive than sodium. Some of the more important differences of lithium from other alkali metals are listed below:

1. Lithium is much harder and lighter than the other alkali metals.

2. The lithium salts of anions with high charge density are generally less soluble in water than those of the other alkali metals, e.g., LiOH, LiF, Li3PO4, Li2CO3 .

3. Lithium forms stable complex compounds, although complex formation generally is not a property of alkali metals. One of the stable complexes formed by lithium is [Li(NH3)4]+.

4. Lithium reacts very slowly with water, while other alkali metals react violently.

5. Lithium salts of large polarizable anions are less stable than those of other alkali metals. Unlike other alkali metals lithium does not form bicarbonate, tri-iodide or hydrogen sulphide at room temperature.

6. When burnt in air lithium forms only normal oxide, whereas the others form peroxides or super oxides.

7. Lithium hydride is more stable than the hydrides of other alkali metals.

8. Lithium compounds are more covalent, that is why its halides are more soluble in organic solvents and the alkyls and aryls of lithium are more stable than those of other alkali metals.

9. Lithium is the least reactive metal of all the alkali metals.

10. When acetylene is passed over strongly heated lithium, it does not produce lithium acetylide, but other alkali metals form the corresponding metallic acetylides.

2Na + C2H2 ⟶ Na+C≡CNa+ + H2

                                 (Sodium acetylide)

11.Lithium has low electropositive character. Thus, its carbonate and nitrate are not so stable and therefore decompose giving lithium oxide. Carbonates of other alkali metals do not decompose. The decomposition of lithium nitrate gives different products than the nitrates of other alkali metals.

Li2CO3 ⟶ Li2O + CO2

4Li2NO3 ⟶ 2Li2O + 4NO2 + O2

2NaNO3 ⟶ 2NaNO2 + O2

12. Lithium hydroxide when strongly heated, forms lithium oxide but the other alkali metal hydroxides do not show this behaviour.

2LiOH ⟶ Li2O + H2O

13. Lithium reacts with nitrogen to form nitride, while the other members of the group do not give this reaction.

6Li + N2 ⟶ 2Li3N

14. Lithium chloride has an exothermic heat of solution, whereas chlorides of sodium and potassium have endothermic heats of solution.

15. Lithium carbide is the only alkali metal carbide formed readily by the direct reaction.

Q.05: Discuss the trends in chemical properties of compounds like oxides, hydroxides, carbonates, nitrates and sulphates of IA and IIA group elements.

Ans: Consult the textbook article 2.2.3 (Page 27—29)

Q.06: Compare the chemical behaviour of lithium with magnesium.

Ans: Lithium has diagonal relationship with magnesium. It resembles more to magnesium than its own family members. Some of the similarities in their chemical behaviour are:

(1) Both form normal oxides.

(2) The carbonates, nitrates and hydroxides of both Li and Mg decompose into their oxides on strong heating.

Li2CO3 ⟶ Li2O + CO2

MgCO3 ⟶ MgO + CO2

(3) Both can form nitrides and carbides on direct combination with nitrogen and carbon.

6Li + N2 ⟶ 2Li3N

3Mg + N2 ⟶ Mg3N2

4Li + C ⟶ Li4C

2Mg + C ⟶ Mg2C

Q.07: (a) Mention the properties of beryllium in which it does not resemble with its own family.

Ans: Peculiar Behaviour of Beryllium:

Beryllium is the lightest member of the series and differs from the other group IIA elements in many ways. This is due to its small atomic size and comparatively high electronegativity value. The main points of difference are:

1. Beryllium metal is almost as hard as iron and hard enough to scratch glass. The other alkaline earth metals are much softer than beryllium but still harder than the alkali metals.

2. The melting and boiling points of beryllium are higher than other alkaline earth metals.

3. As reducing agents, the group IIA metals are all powerful enough to reduce water, at least in principle. However, with water, beryllium forms insoluble oxide coating that protects it from further attack

4. Beryllium in particular is quite resistant towards complete oxidation, even by acids, because of its BeO coating.

5. Beryllium is the only member of its group which reacts with alkalies to give hydrogen. The other members do not react with alkalies.

Be + 2NaOH ⟶ Na2BeO2 + H2

(b) Why the aqueous solution of Na2 CO3 is alkaline in nature?

Ans: The aqueous solution of Na2CO3 is alkaline in nature. The reason is that when Na2CO3 is dissolved in water, Na+ and CO32- ions are produced. The CO32- ions hydrolyze in water to give a weak acid H2CO3 that does not ionize to an appreciable extent. The Na+ ions, on the other hand, are not hydrolyzed in water to form NaOH. The OH ions remain free in the solution, due to which the solution becomes basic.

Na2CO3 + 2H2O ⟶ 2Na+(aq) + 2OH(aq) + H2CO3

Q.08: (a) Describe with diagram the manufacture of sodium by Down’s cell.

Ans: Manufacture of Sodium by Down’s Cell:

Most of sodium metal is produced by the electrolysis of fused sodium chloride. Since, the melting point of sodium chloride is 801°C, some calcium chloride is added to lower its melting point and to permit the furnace to operate at about 600o C. In the electrolytic cell, the large block of graphite at the center is the anode, above which there is a dome for the collection of chlorine. The cathode is a circular bar of copper or iron which surrounds the anode but is separated from it by an iron screen, which terminated in a gauze. The arrangement permits the electric current to pass freely but prevents sodium and chlorine from mixing after they have been set free at the electrodes.

Sodium metal rises in a special compartment from which it is taken out at intervals. The cell produces dry chlorine and 99.9% pure sodium. The process is carried out at 600°C and it has the following advantages:

(a) The metallic fog is not produced.

(b) Liquid sodium can easily be collected at 600°C. (c) Material of the cell is not attacked by the products formed during the electrolysis.

During the process the following reactions take place:

NaCl ⟶ Na+ + Cl

Na+ + e ⟶ Na

Cl ⟶ 1/2Cl2 + 1e

(a) Point out the three advantages of this process.

Ans: Advantages of Down’s Cell: The advantages of operating Down’s cell at 600oC are:

(1) The metallic fog is not produced.

(2) Liquid sodium can easily be collected at 600oC.

(3) Material of the cell is not attacked by the products formed during electrolysis.

Q.09: (a) Compare the physical and chemical properties of alkali metals with those of alkaline earth metals.

Ans: Consult Textbook at Page 21, 24 — 27.

(b) What happens when:

 (i) Lithium carbonate is heated.

 (ii) Lithium hydroxide is heated to red hot.

 (iii) Beryllium is treated with sodium hydroxide.    

 (iv) Lithium hydride is treated with water.

Ans: (i) When Li2CO3 is heated at high temperature, it decomposes into its oxide and CO2 is released.

Li2CO3 + heat ⟶ Li2O + CO2

(ii) When Lithium hydroxide is heated, it is converted into lithium oxide and water.

2LiOH + heat ⟶ Li2O + H2O

(iii) When Be is treated with NaOH, it produces sodium beryllate and hydrogen gas.

Be + 2NaOH ⟶ Na2BeO2 + H2

(iv) When LiHis treated with water, LiOH is formed and H2 gas is released.

LiH + H2O ⟶ LiOH+ H2

Q.10: Give formulas of the following minerals: (a) Dolomite (b) Asbestos (c) Halite (d) Natron (e) Beryl (f) Sylvite (g) Phosphorite (h) Chile Saltpeter


(a) Dolomite — MgCO3.CaCO3

(b) Asbestos — CaMg3(SiO3)4

(c) Halite — NaCl

(d) Natron — Na2CO3.H2O

(e) Beryl — Be3Al2O4

(f) Sylvite — KCl

(g) Phosphorite — Ca3(PO4)2

(h) Chile Saltpeter — NaNO3

Q.11: Answer the following questions briefly:

(a) Why alkali and alkaline earth metals are among the reactive elements of the periodic table?

Ans: Alkali and alkaline earth metals are among the most reactive elements of the periodic table because they are highly electropositive atoms. They have very low ionization energy values. They lose their valence electrons very easily to form M+ and M2+ ions, respectively. So, they react very quickly with other electronegative elements to form ionic bonds.

(b) Why line water turns milky with CO2 but becomes clear with excess CO2?

Ans: When CO2 is passed through lime water, it turns milky due to the formation of insoluble white crystalline CaCO3.

Ca(OH)2 + CO2 ⟶ CaCO3 + H2O

When excess CO2 is passed through the above solution, it becomes clear again due to the formation of soluble Ca(HCO3)2.

CaCO3 + H2O + CO2 ⟶ Ca(HCO3)2(aq)

(c) How gypsum is converted into plaster of Paris?

Ans: When gypsum (CaSO4.2H2O) is heated above 100oC, it loses three quarters of its water of crystallization, giving a white powder called ‘Plaster of Paris’. Chemically, it is calcium sulphate hemihydrate or CaSO4.½H2O.

       2CaSO4.2H2O  ⟶  (CaSO4)2.H2O

(d) Why 2% gypsum is added in the cement?

Ans: During the grinding of cement clinkers, 2% gypsum is also added. This increases the setting time of cement and prevents it from hardening too rapidly.

(e) Why lime is added to an acidic soil?

Ans: Lime (CaO) is used to neutralize the acidic soils. It also increases the amount of readily soluble phosphorous in such soils.

(f) How lime and sand are used to make glass?

Ans: When lime and sand are heated at high temperature, calcium silicate is formed. This is the basic process in glass manufacture.

CaO + SiO2  ⟶ CaSiO3

(g) How lime mortar is prepared?

Ans: “A mixture of slaked lime, sand and water is called ordinary mortar or lime mortar.” It is prepared by mixing freshly prepared slaked lime (one volume) with sand (three or four volumes) and water to form a thick paste. This material when placed between the stones and bricks hardens or sets, thus binding the blocks firmly together. The equations for chemical reactions when lime mortar hardens are:

CaO + H2O ⟶ Ca(OH)2

Ca(OH)2 + CO2 ⟶ CaCO3 + H2O

Ca(OH)2 + SiO2 ⟶ CaSiO3 + H2O

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