NCERT Solutions for Class 11 Chemistry Chemistry Part-2 Chapter 10

The s-Block Elements Class 11

Chapter 10 The s-Block Elements Exercise Solutions

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Exercise : Solutions of Questions on Page Number : 305

Q1 :  

What are the common physical and chemical features of alkali metals?


Answer :

Physical properties of alkali metalsare as follows.

(1) They are quite soft and can be cut easily. Sodium metal can be easily cut using a knife.

(2) They are light coloured and are mostly silvery white in appearance.

(3) They have low density because of the large atomic sizes. The density increases down the group from Li to Cs. The only exceptionto this isK, which has lower density than Na.

(4) The metallic bonding present in alkali metals is quite weak. Therefore, they have low melting and boiling points.

(5) 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 outermost orbital to a high energy level. When this excited electron reverts back to the ground state, it emits excess energy as radiation that falls in the visible region.

(6) They also display photoelectric effect. When metals such as Cs and K are irradiated with light, they lose electrons.

Chemical properties of alkali metals

Alkali metals are highly reactive due to their low ionization enthalpy. As we move down the group, the reactivity increases.

(1) They react with water to form respective oxides or hydroxides. As we move down the group, the reaction becomes more and more spontaneous.

(2) They react with water to form their respective hydroxides and dihydrogens. The general reaction for the same is given as

(3) They react with dihydrogen to form metal hydrides. These hydrides are ionic solids and have high melting points.

(4) Almost all alkali metals, except Li, react directly with halogens to form ionic halides.

Since Li+ion is very small in size, it can easily distort the electron cloud around the negative halide ion. Therefore, lithium halides are covalent in nature.

(5) 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.

(6) They dissolve in liquid ammonia to form deep blue coloured solutions. These solutions are conducting in nature.

The ammoniated electrons cause the blue colour of the solution. These solutions are paramagnetic and if allowed to stand for some time, then they liberate hydrogen. This results in the formation of amides.

In a highly concentrated solution, the blue colour changes to bronze and the solution becomes diamagnetic.

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Q2 :  

Discuss the general characteristics and gradation in properties of alkaline earth metals.


Answer :

General characteristics of alkaline earth metals are as follows.

(i) The general electronic configuration of alkaline earth metals is [noble gas] ns2.

(ii) These metals lose two electrons to acquire the nearest noble gas configuration. Therefore, their oxidation state is +2.

(iii)These metals have atomic and ionic radii smaller than that of alkali metals. Also, when moved down the group, the effective nuclear charge decreases and this causes an increase in their atomic radii and ionic radii.

(iv)Since the alkaline earth metals have large size, their ionization enthalpies are found to be fairly low. However, their first ionization enthalpies are higher than the corresponding group 1 metals.

(v) These metals are lustrous and silvery white in appearance. They are relatively less soft as compared to alkali metals.

(vi)Atoms of alkaline earth metals are smaller than that of alkali metals. Also, they have two valence electrons forming stronger metallic bonds. These two factors cause alkaline earth metals to have high melting and boiling points as compared to alkali metals.

(vii) They are highly electropositive in nature. This is due to their low ionization enthalpies. Also, the electropositive character increases on moving down the group from Be to Ba.

(viii) Ca, Sr, and Ba impart characteristic colours to flames.

Ca – Brick red

Sr – Crimson red

Ba – Apple green

In Be and Mg, the electrons are too strongly bound to be excited. Hence, these do not impart any colour to the flame.

The alkaline earth metals are less reactive than alkali metals and their reactivity increases on moving down the group. Chemical properties of alkaline earth metals are as follows.

(i) Reaction with air and water: Be and Mg are almost inert to air and water because of the formation of oxide layer on their surface.

(a) Powdered Be burns in air to form BeO and Be3N2.

(b) Mg, being more electropositive, burns in air with a dazzling sparkle to form MgO and Mg3N2.

(c) Ca, Sr, and Ba react readily with air to form respective oxides and nitrides.

(d) Ca, Ba, and Sr react vigorously even with cold water.

(ii) Alkaline earth metals react with halogens at high temperatures to form halides.

(iii) All the alkaline earth metals, except Be, react with hydrogen to form hydrides.

(iv) They react readily with acids to form salts and liberate hydrogen gas.

(v) They are strong reducing agents. However, their reducing power is less than that of alkali metals. As we move down the group, the reducing power increases.

(vi) Similar to alkali metals, the alkaline earth metals also dissolve in liquid ammonia to give deep blue coloured solutions.

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Q3 :  

Why are alkali metals not found in nature?


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Q4 :  

Find the oxidation state of sodium in Na2O2.


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Q5 :  

Explain why is sodium less reactive than potassium?


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Q6 :  

Compare the alkali metals and alkaline earth metals with respect to (i) ionization enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.


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Q7 :  

In what ways lithium shows similarities to magnesium in its chemical behaviour?


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Q8 :  

Explain why alkali and alkaline earth metals cannot be obtained by chemical reduction methods?


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Q9 :  

Why are potassium and cesium, rather than lithium used in photoelectric cells?


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Q10 :  

When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change.


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Q11 :  

Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?


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Q12 :  

Discuss the various reactions that occur in the Solvay process.


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Q13 :  

Potassium carbonate cannot be prepared by Solvay process. Why?


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Q14 :  

Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at higher temperature?


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Q15 :  

Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates (c) Sulphates.


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Q16 :  

Starting with sodium chloride how would you proceed to prepare (i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?


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Q17 :  

What happens when (i) magnesium is burnt in air (ii) quick lime is heated with silica (iii) chlorine reacts with slaked lime (iv) calcium nitrate is heated ?


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Q18 :  

Describe two important uses of each of the following: (i) caustic soda (ii) sodium carbonate (iii) quicklime.


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Q19 :  

Draw the structure of (i) BeCl2 (vapour) (ii) BeCl2 (solid).


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Q20 :  

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.


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Q21 :  

Describe the importance of the following: (i) limestone (ii) cement (iii) plaster of paris.


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Q22 :  

Why are lithium salts commonly hydrated and those of the other alkali metal ions usually anhydrous?


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Q23 :  

Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in acetone?


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Q24 :  

Explain the significance of sodium, potassium, magnesium and calcium inbiological fluids.


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Q25 :  

What happens when

(i) sodium metal is dropped in water ?

(ii) sodium metal is heated in free supply of air ?

(iii) sodium peroxide dissolves in water ?


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Q26 :  

Comment on each of the following observations:

(a) The mobilities of the alkali metal ions in aqueous solution are Li+< Na+< K+ < Rb+ < Cs+

(b) Lithium is the only alkali metal to form a nitride directly.

(c) (where M = Ca, Sr or Ba) is nearly constant.


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Q27 :  

State as to 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 ?


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Q28 :  

Write balanced equations for reactions between

(a) Na2O2and water

(b) KO2 and water

(c) Na2O and CO2


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Q29 :  

How would you explain the following observations?

(i) BeO is almost insoluble but BeSO4 in soluble in water,

(ii) BaO is soluble but BaSO4 is insoluble in water,

(iii) LiI is more soluble than KI in ethanol.


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Q30 :  

Which of the alkali metal is having least melting point?

(a) Na (b) K (c) Rb (d) Cs


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Q31 :  

Which one of the following alkali metals gives hydrated salts?

(a) Li (b) Na (c) K (d) Cs


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Q32 :  

Which one of the alkaline earth metal carbonates is thermally the most stable?

(a) MgCO3 (b) CaCO3 (c) SrCO3 (d) BaCO3


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<< Previous Chapter 9 : Hydrogen Next Chapter 11 : The p-Block Elements >>

Chemistry Part-2 - Chemistry : CBSE NCERT Exercise Solutions for Class 11th for The s-Block Elements will be available online in PDF book form soon. The solutions are absolutely Free. Soon you will be able to download the solutions.

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