10.32 Which one of the alkaline earth metal carbonates is thermally the most stable ? (a) MgCO3 (b) CaCO3 (c) SrCO3 (d) BaCO3
Thermal stability size of the cation in carbonates
increasing order of cationic size in alkaline earth metal is
Therefore the most thermal stable carbonates are of Barium
10.31 Which one of the following alkali metals gives hydrated salts ? (a) Li (b) Na (c) K (d) Cs
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
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
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.
(i) is small in size so it has high polarising power and 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, ions are large in size. Hence ion can easily polarise ions and making it unstable and because of that lattice energy of is not very high and so it is soluble in water.
(ii) is soluble because cation is large in size as compare to anion. Size compatibility between them is not good. Therefore is unstable. Hence lattice energy during the formation of their lattice is not high So it can be easily overcome by hydration energy. Therefore is soluble in water. In case of , 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, is not soluble in water.
(iii) The ion has high polarising power. It is very small in size as compare to ion. So, it has a high tendency to distort the electron cloud around the negative iodide () ion. As a result of high polarizability, it has high covalent character than. 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 .
(a)The balanced reaction between and water-
(b) the reaction between water and -
(c) reaction between -
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 ?
(a) when sodium bicarbonate is added to water it gives sodium hydroxide (strong base). As a result, the solution becomes alkaline in nature.
(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.(
[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.
(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 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
(b) Lithium unlike other alkali metals direct react with nitrogen and forms lithium nitride (). This is because lithium-ion is the smallest in size and therefore it has very high size compatibility with . Hence the lattice energy released is very high.
(c) Electrode potential () for 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 . 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 ?
(i) When sodium metal dropped into water it reacts rapidly and forms sodium hydroxide and hydrogen gas.
the chemical reaction is -
(ii) sodium metal heated in the free supply of water-
Sodium reacts vigorously with oxygen and forming sodium peroxides.
(iii) when sodium peroxide dissolved in water it hydrolysed and produce sodium hydroxide and hydrogen peroxide.
10.24 Explain the significance of sodium, potassium, magnesium and calcium in biological fluids
Significance of sodium potassium, magnesium and calcium in biological fluids-
Magnesium and calcium-
10.23 Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in acetone ?
Both ion and 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, cation and 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 . 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?
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.
The chemical formula is .
Importance of limstone is-
Cement is an important building material. It is amixture of triclcium silicate() and tricalium aluminate().
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, , 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?
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 and are dissolve readily in water and Mg and are sparingly soluble in water
10.19 Draw the structure of (i) (vapour) (ii) (solid).
structure of 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.
(i) Caustic Soda-
The chemical name is sodium hydroxide (). Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell.
(ii) Sodium Carbonate-
The common name is washing soda ().Sodium carbonate is generally prepared by Solvay Process.
(iii) Quick lime-
The chemical name is calcium oxide and the formula is . It is prepared on a commercial scale by heating limestone (CaCO3) in a rotary kiln at 1070-1270 K ( )