S-block (Group 2) Flashcards
The first element …………. differs from the rest of the elements in the group and shows diagonal relationship with ………
Beryllium
Aluminum
Ionization energy order
Be > Mg > Ca > Sr > Ra > Ba
Due to poor shielding of 4f electrons
Hydration Energies G1 G2 comparison
Decreases down the group
HE: G1 < G2
Melting point order
Be > Ca > Sr > Ba > Mg
Boiling point order
Be > Ba > Ca > Sr > Mg
Density order
Ba > Sr > Be > Mg > Cr
Melting and boiling point comparision in G1 and G2
G1 < G2
Due to better metallic bonding in G2
Alkali earth metal hardness
These are comparatively softer than d-block elements because in d-block in addition to ns electrons (n-1)d electrons also take part in metallic bonding
But these are harder than alkali metals
Flame test
Calcium - brick red
Strontium - crimson
Barium - apple green
Due to high ionization energy of Be and Mg electrons do not get excited
G1 G2 reactivity comparison
G1 > G2
Reaction of Be and Mg towards oxygen and water
Be and Mg are inert to oxygen and water due to formation of oxide film on their surface
Powdered form of Be burns brilliantly in air to give
BeO and Be3N2
Magnesium burns in air
with dazzling brilliance to give MgO and Mg3N2
Ca, Sr, and Ba are readily attacked by air to form
their oxides and nitrides
(no peroxides and super oxides are formed)
Reaction with Water (rates)
Be neither reacts with hot water nor with steam of cold water
Mg decomposed hot water
Mg + 2H2O —> Mg(OH)2 + H2 [hot water]
Mg + H2O —> MgO + H2 [Cold water}
Ca, Sr, and Ba react with increasing vigor even with cold water to form hydroxide.
Reaction With Halogens
@ high temperature
M + X2 —> MX2
Reactivity towards hydrogen
All elements except Be react with hydrogen to form their hydrides (MH2)
BeH2 can however be prepared by the reaction of BeCl2 and LiAlH4.
2BeCl2 + LiAlH4 —> 2 BeH2 + LiCl + AlCl3
BeH2 and MgH2 Structure
Polymeric
Reactivity towards acid
M + 2HCl —> MCl2 + H2
Generally dil. HCl and dil. H2SO4 are taken.
Reactivity nature
Standard oxidizing and reduction potential order
- EM/M2+
BE 1.97
Mg 2.36
Ca 2.84
Sr 2.89
Ba 2.92
Thus Ba is the best reducing agent and Be is the worst reducing agent in group 2
Why is Be the worst reducing agent?
The reaction
BE(s) —> Be2+(aq) + 2e-
takes place in two steps.
Step 1:
Sublimation of BE solid to Be gas. This process requires energy to proceed
Step 2:
Ionization. This process requires a lot of energy due to the small size .
Step 3:
Be2+ (g) –H2O–> Be2+ (aq)
In this process energy is released which is much less than the energy required
hence it is worst reducing agent
Solution in liquid ammonia
Like alkali metals the alkaline earth metals dissolve in liquid ammonia to give deep blue black solution ammoniated ions.
M+ (x+y) NH3 —> [M(NH3)x 2+ + 2e(NH3)y]
From these solutions the ammoniates [M(NH3)6]2+ can be recovered by evaporation of ammonia from the solution.
M(NH3)6 —> M(NH3)2 + 4NH3 + H2
Concentrated solution of metals in ammonia are …………. colored
Bronze colored due to the formation of metal ion clusters.
Structure of alkaline earth metal oxides
Rock salt structures
Alkaline earth metal oxides react with water to form
Metal hydroxides
except BeO
MO + H2O —> M(OH)2
Thermal stability and basic character of hydroxides
increase from Mg(OH)2 to Ba(OH)2
The G2 hydroxides are however less basic and less stable than the G1 hydroxides
Be is ….. in nature and reacts with
Be is amphoteric in nature and reacts with both acid and alkali.
Be(OH) + 2OH- —> [Be(OH)4]2- Beryllate ion
Be(OH)2 + 2HCl + 2H2O —> [Be(OH2)4]Cl
Carbonates in water
Carbonates of G2 are insoluble in water.
They can be precipitated by addition of sodium or ammonium carbonate to the solution of soluble salts of these metals.
Solubility of carbonates in water decreases as atomic number increases.
Thermal stability of Carbonates
increases from BeCO3 to BaCO3.
MCO3(s) —> MO+(s) + CO2
BeCO3 is very unstable and can kept in CO2 atmosphere
Solubility of Sulphates
BeSO4 and MgSO4 are readily soluble in water and the solubility decreases from CaSO4 to BaSO4
Solubility of nitrates
Nitrates of all elements are soluble in water.
2M(NO3)2 –heat–> 2MO + 2NO2 + O2
Nature of halides
Except for Be halides all other alkaline earth metal halides are ionic in nature
BeX2 is covalent and soluble in organic solvents
BeCl2 is polymeric
Heating hydrated halides
CaX2 + yH2O —> CaX2 + yH2O
SrX2 + zH2O —> SrX2 + zH2O
BaX2 + zH2O —> BaX2 + zH2O
In case of Be and Mg halides on heating hydrolysis occurs.
MgCl2.6H2O –heat–> Mg(OH)2 + 2HCl + 4H2O
Mg(OH)2 –heat–> MgO + H2O
Preparation of anhydrous BeX2
BeO + C + Cl <—600-900K—> BeCl2 + CO
(NH4)2 BeF4 –heat–> 2NH3 + 2HF + BeF2
Hydrolysis of Nitrides
Be3N2 + 6H2O —> 2NH3 + 3Be(OH)2
Mg3N2 + 6H2O —> 2NH3 + 3Mg(OH)2
Hydrolysis of Carbides
BeC2 + 2H2O —> C2H2 + Be(OH)2 [Acetylides]
CaC2 + 2H2O —> C2H2 + Ca(OH)2 [Acetylides]
Mg2C3 + 4 H2O —> C3H4 + 2Mg(OH)2 [Sesquioxide]
Be2C + 4H2O —> CH4 + 2Be(OH)2 [Methanoids]
