NCERT Solutions for Class 11 Chemistry Chapter 10
The s-Block Elements Class 11
Chapter 10 The s-Block Elements Exercise Solutions
Exercise : Solutions of Questions on Page Number : 305
Explain why is sodium less reactive than potassium?
Compare the alkali metals and alkaline earth metals with respect to (i) ionization enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.
|
Alkali metals |
|
Alkaline earth metals |
(i) |
Ionization enthalpy: These have lowest ionization enthalpies in respective periods. This is because of their large atomic sizes. Also, they lose their only valence electron easily as they attain stable noble gas configuration after losing it. |
(i) |
Ionization enthalpy: Alkaline earth metals have smaller atomic size and higher effective nuclear charge as compared to alkali metals. This causes their first ionization enthalpies to be higher than that of alkali metals. However, their second ionization enthalpy is less than the corresponding alkali metals. This is because alkali metals, after losing one electron, acquires noble gas configuration, which is very stable. |
(ii) |
Basicity of oxides: The oxides of alkali metals are very basic in nature. This happens due to the highly electropositive nature of alkali metals, which makes these oxides highly ionic. Hence, they readily dissociate in water to give hydroxide ions. |
(ii) |
Basicity of oxides: The oxides of alkaline earth metals are quite basic but not as basic as those of alkali metals. This is because alkaline earth metals are less electropositive than alkali metals. |
(iii) |
Solubility of hydroxides: |
(iii) |
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. This is due to the high lattice energies of alkaline earth metals. Their higher charge densities (as compared to alkali metals) account for higher lattice energies. |
|
|
|
|||
Q3 :
Answer :
Similarities between lithium and magnesium are as follows.
(i) Both Li and Mg react slowly with cold water.
(ii)
(iii) Both Li and Mg react with N2to form nitrides.
(iv) Neither Li nor Mg form peroxides or superoxides.
(v)
(vi) Li and Mg do not form solid bicarbonates.
(vii) Both LiCl and MgCl2are soluble in ethanol owing to their covalent nature.
(viii)
Explain why alkali and alkaline earth metals cannot be obtained by chemical reduction methods?
Why are potassium and cesium, rather than lithium used in photoelectric cells?
All the three, lithium, potassium, and cesium, are alkali metals. Still, K and Cs are used in the photoelectric cell and not Li.
When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change.
The ammoniated electrons absorb energy corresponding to red region of visible light. Therefore, the transmitted light is blue in colour.
Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?
Discuss the various reactions that occur in the Solvay process.
Solvay process is used to prepare sodium carbonate.
When carbon dioxide gas is bubbled through a brine solution saturated with ammonia, sodium hydrogen carbonate is formed. This sodium hydrogen carbonate is then converted to sodium carbonate.
This ammoniated brine is filtered to remove any impurity.
Step 2: Carbon dioxide is reacted with this ammoniated brine to result in the formation of insoluble sodium hydrogen carbonate.
Step 3: The solution containing crystals of NaHCO3 is filtered to obtain NaHCO3.
Step 5: To recover ammonia, the filtrate (after removing NaHCO3) is mixed with Ca(OH)2 and heated.
Potassium carbonate cannot be prepared by Solvay process. Why?
Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at higher temperature?
As we move down the alkali metal group, the electropositive character increases. This causes an increase in the stability of alkali carbonates. However, lithium carbonate is not so stable to heat. This is because lithium carbonate is
Therefore, lithium carbonate decomposes at a low temperature while a stable sodium carbonate decomposes at a high temperature.
What are the common physical and chemical features of alkali metals?
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.
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.
Discuss the general characteristics and gradation in properties of alkaline earth metals.
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)
Why are alkali metals not found in nature?
Find the oxidation state of sodium in Na2O2.
Let the oxidation state of Na be x. The oxidation state of oxygen, in case of peroxides, is –1. Therefore,
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.
(i) Nitrates
LiNO3, on decomposition, gives oxide.
As we move down group 1 and group 2, the thermal stability of nitrate increases.
Nitrates of both group 1 and group 2 metals are soluble in water.
(ii) Carbonates
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.
(iii) Sulphates
Sulphates of both group 1 and group 2 metals are stable towards heat.
Sulphates of alkali metals are soluble in water. However, sulphates of alkaline earth metals show varied trends. BeSO4 Fairly soluble
MgSO4 Soluble
CaSO4 Sparingly soluble SrSO4 Insoluble
BaSO4 Insoluble
Starting with sodium chloride how would you proceed to prepare (i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?
(a) Sodium can be extracted from sodium chloride by Downs process.
Steel is the cathode and a block of graphite acts as the anode. Metallic Na and Ca are formed at cathode. Molten sodium is taken out of the cell and collected over kerosene.
(ii) 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.
(iii) Sodium peroxide
First, NaCl is electrolysed to result in the formation of Na metal (Downs process).
(iv) Sodium carbonate is prepared by Solvay process. Sodium hydrogen carbonate is precipitated in a reaction of sodium chloride and ammonium hydrogen carbonate.
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 ?
(i)
(ii) Quick lime (CaO) combines with silica (SiO2) to form slag.
(iii) When chloride is added to slaked lime, it gives bleaching powder.
(iv)
Describe two important uses of each of the following: (i) caustic soda (ii) sodium carbonate (iii) quicklime.
(i) Uses of caustic soda
(a) It is used in soap industry.
(b) It is used as a reagent in laboratory.
(ii) Uses of sodium carbonate
(a) It is generally used in glass and soap industry.
(b) It is used as a water softener.
(iii) Uses of quick lime
(a) It is used as a starting material for obtaining slaked lime.
(b)
Draw the structure of (i) BeCl2 (vapour) (ii) BeCl2 (solid).
(a) Structure of BeCl2(solid)
In the vapour state, BeCl2exists as a monomer with a linear structure.
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.
Describe the importance of the following: (i) limestone (ii) cement (iii) plaster of paris.
(i) Chemically, limestone is CaCO3.
(a) It is used in the preparation of lime and cement.
(b) It is used as a flux during the smelting of iron ores.
(ii) Chemically, cement is a mixture of calcium silicate and calcium aluminate.
(a) It is used in plastering and in construction of bridges.
(b) It is used in concrete.
(iii) Chemically, plaster of Paris is 2CaSO4.H2O.
(a) It is used in surgical bandages.
(b)
Why are lithium salts commonly hydrated and those of the other alkali metal ions usually anhydrous?
Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in acetone?
Explain the significance of sodium, potassium, magnesium and calcium inbiological fluids.
Importance of sodium, potassium, magnesium, and calcium in biological fluids:
Sodium ions are found primarily in the blood plasma. They are also found in the interstitial fluids surrounding the cells.
(a) Sodium ions help in the transmission of nerve signals.
(b) They help in regulating the flow of water across the cell membranes.
(c) They also help in transporting sugars and amino acids into the cells.
Potassium ions are found in the highest quantity within the cell fluids.
(a) K ions help in activating many enzymes.
(b) They also participate in oxidising glucose to produce ATP.
(c) They also help in transmitting nerve signals.
Magnesium and calcium are referred to as macro-minerals. This term indicates their higher abundance in the human body system.
(a) Mghelps in relaxing nerves and muscles.
(b) Mg helps in building and strengthening bones.
(c) Mg maintains normal blood circulation in the human body system.
(d) Ca helps in the coagulation of blood
(e)
What happens when
(ii) sodium metal is heated in free supply of air ?
Answer :
(i)
(ii) On being heated in air, sodium reacts vigorously with oxygen to form sodium peroxide. The chemical equation involved in the reaction is:
(iii)
Comment on each of the following observations:
(b) Lithium is the only alkali metal to form a nitride directly.
Answer :
(a) On moving down the alkali group, the ionic and atomic sizes of the metals increase. The given alkali metal ions can be arranged in the increasing order of their ionic sizes as:
Li+ < Na+ < K+ < Rb+ < Cs+
Smaller the size of an ion, the more highly is it hydrated. Since Li+ is the smallest, it gets heavily hydrated in an aqueous solution. On the other hand, Cs+ is the largest and so it is the least hydrated. The given alkali metal ions can be arranged in the decreasing order of their hydrations as:
Li+ > Na+ > K+ > Rb+ > Cs+
Greater the mass of a hydrated ion, the lower is its ionic mobility. Therefore, hydrated Li+ is the least mobile and hydrated Cs+ is the most mobile. Thus, the given alkali metal ions can be arranged in the increasing order of their mobilities as:
Li+ < Na+ < K+ < Rb+ < Cs+
(b) Unlike the other elements of group 1, Li reacts directly with nitrogen to form lithium nitride. This is because Li+ is very small in size and so its size is the most compatible with the N3- ion. Hence, the lattice energy released is very high. This energy also overcomes the high amount of energy required for the formation of the N3- ion.
(c) Electrode potential (E°) of any M2+/M electrode depends upon three factors:
(i) Ionisation enthalpy
(ii) Enthalpy of hydration
(iii) Enthalpy of vaporisation
State as to why
(b) alkali metals are prepared by electrolysis of their fused chlorides ?
Answer :
(a)
(b) 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). This is because these elements are highly electropositive. Neither can electrolysis of aqueous solutions be used to extract these elements. This is because the liberated metals react with water.
Hence, to overcome these difficulties, alkali metals are usually prepared by the electrolysis of their fused chlorides.
(c)
Write balanced equations for reactions between
(b) KO2 and water
Answer :
(a)
(b) The balanced chemical equation for the reaction between KO2 and water is:
(c)
How would you explain the following observations?
(ii) BaO is soluble but BaSO4 is insoluble in water,
Answer :
(i) BeO is almost insoluble in water and BeSO4is soluble in water. Be2+is a small cation with a high polarising power and O2–is a small anion. The size compatibility of Be2+and O2–is high. Therefore, the lattice energy released during their formation is also very high. When BeO is dissolved in water, the hydration energy of its ions is not sufficient to
overcome the high lattice energy. Therefore, BeO is insoluble in water. On the other hand, ion is a large anion.
Hence, Be2+can easily polarise ions, making BeSO4unstable. Thus, the lattice energy of BeSO4is not very high and so it is soluble in water.
(ii) 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. It can easily be overcome by the hydration energy of the ions. Therefore, BaO is soluble in water. In BaSO4, Ba2+and are both large-sized. The lattice energy released is high. Hence, it is not soluble in water.
(iii)
Which of the alkali metal is having least melting point?
Answer :
Which one of the following alkali metals gives hydrated salts?
Answer :
Smaller the size of an ion, the more highly is it hydrated. Among the given alkali metals, Li is the smallest in size. Also, it has the highest charge density and highest polarising power. Hence, it attracts water molecules more strongly than the other alkali metals. As a result, it forms hydrated salts such as
Which one of the alkaline earth metal carbonates is thermally the most stable?
Answer :
Thermal stability increases with the increase in the size of the cation present in the carbonate. The increasing order of the cationic size of the given alkaline earth metals is
Mg < Ca < Sr < Ba
Hence, the increasing order of the thermal stability of the given alkaline earth metal carbonates is
MgCO3 < CaCO3 < SrCO3 < BaCO3