Class 11 - Chemistry - The S Block Elements
What are the common physical and chemical features of alkali metals?
Physical properties of alkali metals:
Chemical properties of alkali metals:
Discuss the general characteristics and gradation in properties of alkaline earth metals.
Properties of Alkaline Earth Metals:
Why are the alkali metals not found in nature?
Alkali metals are highly reactive nature. This is why, they always exist in combined state in nature.
Find out the oxidation state of sodium in Na2O2.
Let x is the oxidation state of Na in Na2O2.
2x + 2 (-1) = 0
2x – 2 = 0
2x = +2
x = +1.
Explain why is sodium less reactive than potassium.
Sodium is less reactive than potassium because ionization enthalpy of K is 419 kJ mol-1.
Ionization enthalpy of sodium = 496 kJ mol-1.
Hence, ionization enthalpy of potassium is less than that of sodium, potassium is more reactive than sodium.
Compare the alkali metals and alkaline earth metals with respect to
(i) ionization enthalpy (ii) basicity of oxides (iii) solubility of hydroxides.
In what ways lithium shows similarities to magnesium in its chemical behavior?
Similarities between lithium and magnesium:
(i) lithium and magnesium reacts slow with cold water.
(ii) oxides of lithium and magnesium are less soluble in H2O. Also the hydroxides of both decompose at high temperature.
Mg(OH)2 →MgO + H2O
(iii) Nitrides are formed from both the lithium and magnesium when they react with N2.
6Li+N2 →2Li3 N
3Mg+N2 →Mg3 N2
(iv) Neither Li nor Mg form superoxides nor peroxides.
(v) Both the carbonates of lithium and magnesium are naturally covalent. They decompose on heating.
MgCO3 →MgO + CO2
(vi) They do not form bicarbonates which are solid.
(vii) Both MgCl2 and LiCl are soluble in ethanol because they are naturally covalent.
(viii) Both MgCl2 and LiCl are naturally deliquescent. They crystallize as hydrates from aqueous solutions.
LiCl.2H2O and MgCl2.8H2O
Explain why can alkali metal dissolves in liquid ammonia the solution can acquire different colours.
Explain the reasons for this type of color change.
By using a stronger reducing agent, the oxides of metals gets reduced by the process called chemical reduction.
Alkali and alkaline earth metals are better reducing agents. There are no better reducing agents than the alkali metals.
This is the reason why, these metals are not obtained by chemical reduction methods.
Why are potassium and caesium, rather with lithium used in photoelectric cells?
Potassium (K) and caesium (Cs) have lower ionization enthalpy than that of lithium.
These metals easily emit electrons on the exposure of light. Due to which, K and Cs are used in photoelectric cells rather than lithium.
When an alkali metal dissolves in liquid ammonia the solution can acquire different colours.
Explain the reasons for this type of colour change.
Alkali metals dissolve in liquid ammonia and give deep blue solutions which are conducting in nature.
This is because ammoniated electrons absorb energy in the visible region of light and impart blue color.
M + (x+y)NH3 --> [M(NH3)x]+ + e-3(NH3)y
Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?
Ionization enthalpy of Be and Mg are much higher than those of the other alkaline earth metals due to its small size.
A large amount of energy is needed to excite their valence electrons. Hence, they do not impart color to the flame.
Discuss the various reactions that occur in the Solvay process.
Reactions involved in Solvay process:
Potassium carbonate cannot be prepared by Solvay process. Why?
Potassium carbonate being more soluble than sodium bicarbonate does not get precipitated when CO2
is passed through a concentrated solution of KCl saturated with ammonia.
Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at higher temperature?
Li2CO3 is a covalent compound whereas Na2CO3 is an ionic compound.
Lattice energy of Na2CO3 is higher than that of Li2CO3.
It is decomposed at a lower temperature.
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
Group 2 carbonates are insoluble in water because their lattice energy are higher than hydration energy.
Thermal stability of group 2 carbonates increase down the group because lattice energy keeps on increasing
due to increase in ionic character.
Solubility of group 2 sulphates decreases down the group because lattice energy dominates over hydration energy.
They are thermally stable which increases down the group due to increase in lattice energy.
Starting with sodium chloride how would you proceed to prepare
(i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?
Na + + e- --> Na(l)
2Cl-(melt) ---> Cl2 (g) + 2e-
Cl- - e- -->Cl2
Cl + Cl --> Cl2
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?
Describe two important uses of each of the following: (i) caustic soda (ii) sodium carbonate (iii) quicklime
Draw the structure of (i) BeCl2 (vapour) (ii) BeCl2 (solid).
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.
Group 1 hydroxides and carbonates are large in size.
Due to which they have higher hydration energy than the lattice energy.
So they are easily soluble in water.
In magnesium and calcium, size in small and lattice energy dominates over hydration energy.
Hence, they are sparingly soluble in water.
Describe the importance of the following: (i) limestone (ii) cement (iii) plaster of paris.
Why are lithium salts commonly hydrated and those of the other alkali ions usually anhydrous?
Due to a very small size, Li+ can polarize water molecules easily than the other alkali metal ions.
Why is LiF almost insoluble in water whereas LiCl soluble not only in water but also in acetone?
LiF is almost insoluble in water due to the high lattice energy.
LiCl is soluble in water because its hydration energy is higher than its lattice energy.
Explain the significance of sodium, potassium, magnesium and calcium in biological fluids.
Sodium - Sodium ions participate in the transmission of nerve signals and in the transport of sugars and amino acids into the cell.
Potassium – These activate many enzymes and participate in the oxidation of glucose to produce ATP.
Magnesium – All enzymes that utilize ATP in phosphate transfer require magnesium as a cofactor.
It is the main pigment for the absorption of light in plants.
Calcium – These are present in bones and plays important role in neuromuscular functions.
What happens when
Comment on each of the following observations:
State as to why
Write balanced equations for reaction between
How would you explain the following observations?
Which of the alkali metal is having least melting point?
Correct option is (d)
Size of Cs is the biggest. Thus, its melting point is the lowest.
Which one of the following alkali metals gives hydrated salts?
Correct option is (a)
Li is the smallest so it has highest charge density and hence attracts water molecules more strongly.
Which one of the alkaline earth metal carbonates is thermally the most stable?
Correct option is (d)