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10.32 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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R Rakesh
Answered May 05, 2019

Thermal stability $\propto$ size of the cation in carbonates

increasing order of cationic size in alkaline earth metal is  $Mg

Therefore the most thermal stable carbonates are of Barium

Ans. (d) $BaCO_{3}$

10.31 Which one of the following alkali metals gives hydrated salts ? (a) Li (b) Na (c) K (d) Cs

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R Rakesh
Answered May 05, 2019

Answer-

The smaller the size of ion higher is the charge density and high polarising power. And among the alkali metals, lithium-ion is the smallest in size and has the power to attract the water molecules.

Ans is (a) Li

10.30 Which of the alkali metal is having least melting point ? (a) Na (b) K (c) Rb (d) Cs

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R Rakesh
Answered May 05, 2019

The strength of metallic bond decreases down the group in the periodic table because as the size of cation increases the binding energies of their atoms in the crystal lattice decreases.

order of melting point  $Na>k>Rb>Cs$

So, ans is (a) Na

10.29 How would you explain the following observations? (i) BeO is almost insoluble but BeSO4 is 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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R Rakesh
Answered May 05, 2019

(i) $Be^{2+}$ is small in size so it has high polarising power and $O^{2-}$ is also small in size. Compatibility of both the cation and anion are very high. So their lattice energy is also very high. When BeO is dissolved in water it's hydration energy is not sufficient to overcome its lattice energy. So, therefore, it is insoluble in water.

On the other hand, $SO_{4}^{2-}$ ions are large in size. Hence $Be^{+}$ ion can easily polarise $SO_{4}^{2-}$ ions and making it unstable and  because of that lattice energy of $BeSO_{4}$ is not very high and so it is soluble in water.

(ii) $BaO$ is soluble because $Ba^{2+}$ cation is large in size as compare to $O^{2-}$ anion. Size compatibility between them is not good. Therefore $BaO$ is unstable. Hence lattice energy during the formation of their lattice is not high So it can be easily overcome by hydration energy. Therefore $BaO$ is soluble in water. In case of $BaSO_{4}$, we know that down the group hydration enthalpy decreases and both the anion and cation have very good size compatibility. So, lattice energy cannot be overcome by hydration energy. As a result, $BaSO_{4}$is not soluble in water.

(iii)  The $Li^{+}$ ion has high polarising power. It is very small in size as compare to $K^{+}$ ion. So, it has a high tendency to distort the electron cloud around the negative iodide ($I^{-}$) ion. As a result of high polarizability, it has high covalent character than.$KI$ Hence it is more soluble in methanol.

10.28 Write balanced equations for reactions between (a) Na2O2 and water (b) KO2 and water (c) Na2O and CO2 .

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R Rakesh
Answered May 05, 2019

(a)The balanced reaction between $Na_{2}O_{2}$ and water-

$Na_{2}O_{2} +2H_{2}O\rightarrow 2NaOH +H_{2}O_{2}$

(b) the reaction between water and $KO_{2}$-

$KO_{2}+H_{2}O\rightarrow KOH+H_{2}O_{2}+O_{2}$

(c) reaction between $Na_{2}O\ and\ CO_{2}$ -

$Na_{2}O+CO_{2}\rightarrow Na_{2}CO_{3}$

10.27 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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R Rakesh
Answered May 05, 2019

(a)  when sodium bicarbonate is added to water it gives sodium hydroxide (strong base). As a result, the solution becomes alkaline in nature.

$Na_{2}CO_{3}+H_{2}O\rightarrow NaOH+ H_{2}CO_{3}$

(b) Because their oxides are themselves are a very strong reducing agent in nature. So, by chemical reduction, we cannot obtain alkali metals. Also, we cannot use electrolysis of the aqueous solution method because after liberating the metals they again react with water.(

$(2M+H_{2}O\rightarrow 2M^{+}+2OH^{-}+H_{2})$ [M = alkali metals]

10.26 Comment on each of the following observations: (a) The mobilities of the alkali metal ions in aqueous solution are Li+ < Na+ < K+ < Rb+ < Cs+ (b) Lithium is the only alkali metal to form a nitride directly. (c) E 0 for M2+ (aq) + 2e– → M(s) (where M = Ca, Sr or Ba) is nearly constant.

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R Rakesh
Answered May 05, 2019

(a) As we know that down the group ionic size increases. Lithium-ion is the smallest in size and we know smaller the size of ion, the more hydrated it is. So lithium ion is most hydrated and $Cs^{+}$ion is bigger in size so it is least hydrated.

Greater the size of hydrated ion less is mobility. So that the order of mobility of ions $Li^{+}

(b) Lithium unlike other alkali metals direct react with nitrogen and forms lithium nitride ($Li_{3}N$). This is because lithium-ion is the smallest in size and therefore it has very high size compatibility with $N^{3-}$. Hence the lattice energy released is very high.

(c)   Electrode potential ($E^{0}$) for $M^{2+}$ is depend on mainly three-factor- (i)ionisation enthalpy (ii)hydration enthalpy (iii)enthalpy of vaporisation.  But we consider the overall effect of these three factors that should be the same for all $Ca,Sr,Ba$. Hence their electrode potential is constant.

10.25 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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R Rakesh
Answered May 05, 2019

(i) When sodium metal dropped into water it reacts rapidly and forms sodium hydroxide and hydrogen gas.

the chemical reaction is - $2Na+2H_{2}O\rightarrow 2NaOH +H_{2}$

(ii) sodium metal heated in the free supply of water-

Sodium reacts vigorously with oxygen and forming sodium peroxides.

Reaction- $2Na+O_{2}\rightarrow Na_{2}O_{2}$

(iii) when sodium peroxide dissolved in water it hydrolysed and produce sodium hydroxide and hydrogen peroxide.

Reaction-  $Na_{2}O_{2}+2H_{2}O\rightarrow 2NaOH+H_{2}O_{2}$

10.24 Explain the significance of sodium, potassium, magnesium and calcium in biological fluids

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M manish
Answered May 05, 2019

Significance of sodium potassium, magnesium and calcium in biological fluids-

Sodium-

• It is mainly found in blood plasma and also in the interstitial fluid which surrounds the cell.
• $Na^{+}$ ions help in the transmission of nerve signals also for the regulating water in the plasma membrane.
• Also for the transport of sugars and amino acids into the cells.

Potassium-

•  These ions are highly present within cell fluids.
• Helps in activating many enzymes
• To produce ATP it oxidises the glucose molecule.
• Also helps in the transmission of nerve signals.

Magnesium and calcium-

•  Plays an important role in neuromuscular function (by magnesium), interneuronal transmission, cell membrane integrity and blood coagulation(by calcium)
• Mg helps in maintains normal blood circulation in our body.

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

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R Rakesh
Answered May 05, 2019

Both $Li^{+}$ ion and $F^{-}$ ion are smaller in size. So their size compatibility is very high. Hence lattice energy during formation is very high and it is not overcome by the hydration energy. Therefore LiF is almost insoluble in water. On the other hand, $Li^{+}$cation and $Cl^{-}$ anion have comparable size differences, therefore they have low lattice energy and because of small in size, lithium ion has high polarising power as a result is distort the electron cloud of chloride anion. As a result, there is some covalent character in $LiCl$. So, it soluble in water but also in acetone.

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

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R Rakesh
Answered May 05, 2019

Lithium-ion is the smallest among the other alkali metal ions. Hence it has higher polarising power than others and so it can polarise water molecule more easily than other alkali metals. Hence the water molecules are more attracted towards Li salts as the water of crystallisation. Li+ has a maximum degree of hydration and for this reason, lithium salts are mostly hydrated, e.g., LiCl· 2H2O

As the down the group size of ions increase, their polarising power decreases. Hence other alkali metal ions usually form anhydrous salts.

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

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R Rakesh
Answered May 05, 2019

(i) Limestone-

The chemical formula is $CaCO_{3}$

Importance of limstone is-

• It is used as a building material in the form of marble and in the manufacture of quick lime
• Calcium carbonate along with magnesium carbonate is used as a flux in the extraction of metals such as iron.
• It is also used as an antacid, mild abrasive in tooth paste, a constituent of chewing gum, and a filler in cosmetics.

(ii) Cement-

Cement is an important building material. It is amixture of triclcium silicate($Ca_{3}SiO_{5}$) and tricalium aluminate($Ca_{3}Al_{2}O_{6}$).

Importance of cement-

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 a hemihydrate of calcium sulphate. It is obtained when gypsum, $CaSO_{4}.2H_{2}O$, is heated to 393 K.

Importance of POP-

It is used in medicine as surgical bandages and also it is used for making casts and moulds.

10.20 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?

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R Rakesh
Answered May 05, 2019

The atomic size of sodium and potassium is lager than the magnesium and calcium. So, the lattic energy of carbonates and oxides formed by sodium and potassium are less than that of calcium and magnesium. Therefore the carbonates and oxides of $Na$ and $K$ are dissolve readily in water and Mg and $Ca$ are sparingly soluble in water

10.19 Draw the structure of (i) $BeCl_{2}$ (vapour) (ii) $BeCl_{2}$ (solid).

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R Rakesh
Answered May 05, 2019

Answer-

structure of $BeCl_{2}$ in solid phase-

In Gaseous phase-

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

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R Rakesh
Answered May 05, 2019

(i) Caustic Soda-

The chemical name is sodium hydroxide ($NaOH$). Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell.

Uses-

•  the manufacture of soap, paper, artificial silk and a number of chemicals
•  in the purification of bauxite,
•  in the textile industries for mercerising cotton fabrics
•  for the preparation of pure fats and oils, and
•  as a laboratory reagent.

(ii) Sodium Carbonate-

The common name is washing soda ($Na_{2}CO_{3}$).Sodium carbonate is generally prepared by Solvay Process.

Uses-

•  It is used in water softening, laundering and cleaning.
•  It is used in the manufacture of glass, soap, borax and caustic soda.
•  It is used in paper, paints and textile industries.
•  It is an important laboratory reagent both in qualitative and quantitative analysis

(iii) Quick lime-

The chemical name is calcium oxide and the formula is $CaO$. It is prepared on a commercial scale by heating limestone (CaCO3) in a rotary kiln at 1070-1270 K ( $CaCO_{3}\rightleftharpoons CaO+CO_{2}$)

Uses-

•  It is an important primary material for manufacturing cement and is the cheapest form of alkali.
•  It is used in the manufacture of sodium carbonate from caustic soda.
• It is employed in the purification of sugar and in the manufacture of dye stuffs.

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