s- Block Elements

s- Block Elements

Elements of group IA and IIA of the periodic table are called s-block elements. Group IA elements are called alkali metals and group IIA elements are called alkaline earth metals. Their outer most configuration is ns1 and ns2 respectively.
General Characteristics
1. They are good conductors of heat and electricity.
2. They are malleable and ductile.
3. Exhibit group valency of 1 and 2 for IA and IIA groups respectively.
4. They are prepared by the electrolysis of their fused salts.
5. They are very reactive as their last shell contains 1 or 2 electrons which can be given off easily (low ionization potential).
6. They form colourless compounds except chromates, dichromates etc.
7. Their cations are diamagnetic.
8. They form ionic compounds (except Li and Be).
9. Their solutions in liquid ammonia are good conductor of electricity and are good reductant.
10. Oxides are basic in nature.

Flame Test of Alkali Metals

Alkali metals have large size. When they are heated in the flame of Bunsen burner, the electrons present in the valence shell move from lower energy level to higher energy level by absorption of heat from the flame. When they come back to the ground state, they emit the extra energy in the form of visible light to provide colour to the flame. Elements and their respective colours imparted to the flame are given below-
Na-----Golden Yellow
Rb-----Red Violet

Why does sodium impart yellow colour in the flame ?

The ionization enthalpy of sodium is low . Therefore , when sodium metal or its salt is heated in Bunsen flame , its valence shell electron is excited to higher energy levels of absorption of energy . When the excited electron returns to the ground state , it emits the extra energy in the yellow region of the electromagnetic spectrum . Therefore , sodium imparts yellow colour to the flame.

Hydration Enthalpy of alkali metal ions

The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.
Li+ > Na+ > K+ > Rb+ > Cs+
Li+ has maximum degree of hydration and for this reason lithium salts are mostly hydrated, e.g., LiCl· 2H2O.

What are Alkaline Earth Metals ? Why are they so called ?

Elements of group 2 of the periodic table are called alkaline earth metals. They are Be, Mg, Ca, Sr, Ba and Ra. Their valence shell configuration is ns2.
They are earth like minerals and their aqueous solutions are alkaline in nature. Hence they are called alkaline earth metals.

Thermal stability of alkaline earth metals carbonates (MCO3)

Thermal stability of alkaline earth metals carbonates (MCO3) are increases down the group. As we go down the group, more heat energy is required to thermally decompose the carbonate. So, down the group the carbonates become more thermally stable. This is explained by the polarizing ability of group IIA metal ions. Smaller the cation, greater the ability to polarize the anion. Hence the polarising ability of the Group IIA ions decreases down the group as the size of cation increases down the group. Group IIA carbonates thermally decompose they produce the metal oxide and carbon dioxide.
MCO3 → MO + CO2
So the correct order of stability of carbonates of Group IIA is
BaCO3 > SrCO3 > CaCO 3 > MgCO3 > BeCO3

How does Be reacts with NaOH and H2SO4 ?

Strong NaOH solutions attack slowly whereas fused NaOH attack it readily forming beryllates and liberates hydrogen gas.
Be + 2NaOH -----> Na2BeO2 + H2
Be displaces hydrogen gas from dilute sulphuric acid but with hot concentrated sulphuric acid, it liberates SO2 gas
Be + H2SO4 → BeSO4 + H2
Be + 2H2SO4 → BeSO4 + SO2 + 2H2O

Alkaline Earth Metals are not obtained by the electrolysis of aqueous solutions. Explain

The aqueous solutions of alkaline earth metals upon electrolysis give metal hydroxide instead of pure metal. That's why alkaline earth metals are not obtained by electrolysis of their aqueous solutions.
MX2 → M+2 + 2X-
2X- → X2 + 2e
2H2O + 2e → 2OH- + H2
MX2 + 2H2O → M(OH)2 + X2 + H2

CaO is basic oxide but ZnO is amphoteric oxide. Why?

CaO is soluble in acids only but ZnO is soluble in acids as well as alkalis. That's why CaO is basic oxide but ZnO is amphoteric oxide.
CaO + 2HCl → CaCl2 + H2O
ZnO + 2HCl → ZnCl2 + H2O
ZnO + 2NaOH → Na2ZnO2 + H2O

Be atom has no unpaired electrons, yet it forms BeF2 Why?

4Be: 1s22s2
In ground state, Be does not have any unpaired elecron but in excited state it has two unpaired electrons. One electron from 2s2 goes into the 2p orbital. These two orbitals (i.e. 2s and 2p)get hybridized and forms two sp hybrid orbitals and arranged linearly in 3D space. Hence, with two F atoms it forms BeF2 molecule.

Solubility of Alkaline Earth Metal (IIA) Hydroxides in Water

The solubility of alkaline earth metal(IIA) hydroxides in water increases down the group as the lattice energy decreases down the group due to increase in size of the alkaline earth metals cation whereas the hydration energy of the cation remains almost unchanged. The resultant of two effects i.e.
ΔHSolution = ΔHLattice - ΔHHydration
Becomes more negative as we move from Be(OH)2 to Ba(OH)2 which accounts for increase in solubility.

Solubility of Alkaline Earth Metal (IIA) Sulphates in Water

The solubility of alkaline earth metal(IIA) sulphates decreases down the groups i.e. BeSO4 > MgSO4 > CaSO4 > SrSO4 > BaSO4. Thus BeSO4 and MgSO4 are highly soluble, CaSO4 is sparingly soluble but SrSO4 > BaSO4 are virtually insoluble.
The magnitude of the lattice energy remains almost constant because the sulphate is so big. However the hydration energy decreases from Be+2 to Ba+2 appreciably as the size of the cation increase down the group. Hence, the solubilities of sulphates of alkaline earth metals decrease down the group mainly due to the decreasing hydration energies from Be+2 to Ba+2. The high solubility of BeSO4 and MgSO4 is due to high hydration energies due to smaller size of Be+2 and Mg+2 ions.

Why the conductivity of Li+ in aqueous solution is less than that of Cs+ ion ?

Due to high charge density and smaller size of Li+ ion, it gets much more hydrated compared to the large Cs+ ion. Thus, size of the hydrated lithium ion is much larger than that of the hydrated cesium ion. For this reason, the mobility of Li+ ion is much lower than that of Cs+ ion and consequently, the conductivity of Li+ in aqueous solution less than that of Cs+ ion.

Lattice energy varies as LiF > NaF > KF > RbF > CsF, Explain.

We know that lattice energy is inversely proportional to the size of the ions involved. Smaller the size of the ions, greater the lattice energy.
The size of cations increases in the following order-
Li+ < Na+ < K+ < Rb+ < Cs+ < Fr+
Hence, the lattice energy of the compounds follows the order-
LiF > NaF > KF > RbF > CsF >

Explain the solubility of iodides and fluorides of alkali metals in water.

We know that lattice energy is inversely proportional to the size of the ions involved. Smaller the size of the ions, greater the lattice energy.
In case of LiI, NaI, KI, RbI and CsI
Since, radius of anion(r) is greater than that of cation(r+), the sum of r and r+ will not change too much as r+ increases. So, the lattice energy will not change significantly. Thus, decreases in lattice energy is not as fast as decreases in hydration energy. Thus, more decrase in hydration energy decreases the solubility. Thus, solubility decreases is-
LiI > NaI > KI > RbI > Csl
With a small anion as (F), the lattice energy decreases more rapidly along a series of salts with increasing the size of cation and thus solubility increases as-
LiF < NaF < KF < RbF < CsF

Explain KOH is a stronger base than Ba(OH)2.

Due to the greater ionic radius and less ionisation energy of potassium as compared to barium, results in KOH being a stronger base than Ba(OH)2

Is Ba(OH)2 strong base or a weak base ?

Ba(OH)2 is a strong base, because on dissolving in an aqueous solution, it completely dissociates into its constituent ions(i.e. Ba2+ and OH), the presence of a large number of OH ions in the aqueous solution of Ba(OH)2 makes it basic in nature.

Basic character of alkaline earth metals hydroixe is increases down the group. Explain.

On moving down the group of alkaline earth metals, the basic character of hydroxides increases from Be(OH)2 to Ba(OH)2. As the size of the metal cation increases, the internuclear distance betwen metal cation and the oxygen of the hydroxide group increases. This increases the ease of ionization of the hydroxide ion. Hence, the basic character increases.

MgCl2 is more stable than MgCl. Why ?

Mg+ ion undergo disproportionation reaction in aqueous solution and converted into Mg+2 ion as Mg+2 ion has high hydration energy in aqueous solution.
Mg+ → Mg+2 + Mg
Hence, MgCl2 is more stable than MgCl.

How would you detect Mg+2 ion ?

By Dry Test-
All Mg compounds when ignited on charcoal in presence of Na2CO3 are converted into white MgO which glows when hot. White color of MgO confirm the presence of Mg+2 ion.
MgCO3 → MgO + CO2

How will you explain that alkali metals are strong reducing agents ?

Lower the ionisation enthalpy, greater is the tendency of an element to lose electrons and hence stronger is the reducing character or higher is the reactivity of the element. Since the ionisation enthalpies of alkali metals decrease down the group, therefore, their reducing character or reactivity in the gaseous state increases from Li to Cs i.e.
Li < Na < K < Rb < Cs.
However, in the aqueous solution, it has been obsorbed that the reducing character of alkali metals follows the sequence
Na < K < Rb < Cs < Li.
In other words, lithium is the strongest reducing agent in aqueous solution.

Why alkali metals give ionic hydrides ?

Alkali metals are more electro positive than hydrogen. They have strong tendency to lose electron and form a uni-positive ion. This electron is accepted by hydrogen to form a hydride ion (H). These cations and anions then combine to form ionic bond. That’s why alkali metals give ionic hydrides. E.g. Na+H and K+H