Chapter – Gp II Alkaline Earth Metals
The PHYSICAL PROPERTIES of the group are as under
- Compared to corresponding alkali metals these are denser. Melting and Boiling are higher than Alkali metals but they don’t have a regular trend.
- Having double charge and not much changed size compare to charge, the corresponding compounds of Gp II metals have higher Hydration Enthalpy, that explains for the Fact that the Halides of Gp II like MgCl2 , CaCl2 are found in hydrated form as MgCl2.6H2O and CaCl2.6H2O , while NaCl and KCl exists as such in aqueous solution.
- All other metals Form ionic compounds, except for Be which forms Covalent compounds, Due to its high ionization enthalpy and small size, high polarizing power etc we have a lot of anomalous behaviour just like Li in gp I.
- Oxidation potential increases down the group, here we must remember that the values of oxidation potentials are negative and by increasing we meant by magnitude wise increase.
Flame Test – Ca Sr Ba
Brick red crimson apple green
Due to higher energy gaps, in Mg and Be the photons released have shorter wavelength and don’t fall in the visible range to be detected by colour.
Comparative study of the groups and noticing the point of difference and reason behind them is the best way to remember the stuff.
CHEMICAL PROPERTIES OF ALKALINE EARTH METALS are under,
Halides- BeF2 is highly soluble in water, while solubility decrease till CaF2 it starts to increase after that from Ca to Ba. Reason is related to their crystal structures.
Here there is an amazing difference that BeF2 is highly soluble and LiF is almost insoluble, reason is high lattice enthalpy of LiF in comparison of BeF2.
Due to smallest size, only Be form complexes, same as Li in gp I.
Metal Hydride – There nature is what important, general formula MH2 , we have covalent nature with Be and Mg, while in case of Ca, Sr, Ba the hydride is ionic.
The formation reaction of Hydride is due to the affinity of the Gp II elements towards non- metal
M + H2 à MH2 M = all except Be
The very important name in the hydride is CaH2 hydrolyth.
The reaction with water can be explained as,
MH2 + H2O –> M (OH)2 + H2
The structure of beryllium hydride is analogous to that of what we call banana bonding, having
3c- 2e non planar structure as shown
H H H H
Be Be Be
H H H H ,this is structure in solid state
Where the headed arrows shown the 3c-2e electrons bonds being out of the plane. Detailed study of these kind of bonding is within he boron family under Diborane.
Reaction with water-
Reaction is of formation of hydroxide as expected
M + H2O –> M(OH)2 + H2
- Reactivity increases down the group as metallic character increases, or we can say the ionic character increases down the group.
- More is metallic nature more is the basicity of the hydroxides, so increases down group.
- Solubility increases down the group because more ionic nature and since water have such high dielectric constant in dissolve polar molecule, or like dissolve like theory.
- Be(OH)2 is amphoteric and insoluble in water showing absolutely different character because of the small size.
Reaction with Air-
Reaction will give oxides of different characteristics according to metal as
M + O2 –> MO –> MO2 this is reaction for M= Sr, Ba due to their large size while in case of other metals the reaction stop at the oxide and don’t go till peroxide.
- Except Be all of them for tarnishing layer of oxide on their surface on reacting with air, affinity towards O increases down the group
- BeO is amphoteric while other is basic in nature.
- BeO and MgO are insoluble while other oxide are soluble and solubility increases down the group.
The simple thing to remember about the oxide is that in every property, Be shows a unique behaviour as exception.
Reaction with Acid – The reaction of the acid is simple ionic reaction to release hydrogen gas.
M + H2SO4 –> M SO4 + H2
M + HCl –> MCl + H2
Now since Beryllium is an amphoteric oxide forming compound it gives reaction with alkali metal hydroxides as well for e.g.,
Be + NaOH –> Na2[Be(OH)4] or Na2BeO2 other metals don’t react with bases.
Reaction with Halogen – The reaction with halogen is simple,
M + H2 –> MH2 where M is all alkaline earth metals
The structure of these halides is important and unique
- In vapour phase, below 1200K BeCl2 exists as dimer and above that, it exists as monomer the structure in dimer state is as follows,
Cl Cl Cl Cl
Be Be Be
Cl Cl Cl Cl
Every alternative quarter of bonds are out of plane, i.e. the red one’s here so this is a unique feature with the dimer of halides.
Reaction with Nitrogen- The reaction can be recalled from the one shown by anomalous behaviour of Li but is characteristic of group two metals as
M + Air –> MO + M3N 2
M3N2 + H2O –> M(OH)2 + NH3
This reaction and one shown by Li is similar so must remember the point, with characterising feature of gp II and diagonal relation between Li and gp II, instead taking it as an exception.
Reaction with Carbon-
Only Li reacts with carbon in alkali metals to give Li2C2 Lithium Carbide, Now similar to the case above we here have a point to consider, LI showed this exception as similar to gp II elements i.e.
M + C –> MC2
MC2 + H2O –> M(OH)2 + C2H2
Not all reactions of carbon with metals are so straight forward e.g.
Mg + C –> MgC2 (EXCESS c)–> Mg2C3 H2O–> Mg(OH)2 + C3H4
BeO + C –> Be2C H2O–> Be(OH)2 + CH4 , where Be difficultly react with carbon
The common name given to the MC2 is called acetylides.
Reaction with S and P-
These reaction are same in both groups and have similar properties, and all metals shows same
M + S –> MS reaction no exception with either with Be or With Li.
M + P –> M3P2
the compound are according to the valences of the atom and there for easy to remember.
Heating Effect –
- Bicarbonates – All bicarbonates gives the same reactions on heating to decompose, into their respective carbonates. General reaction,
M(HCO3)2 heat–> MCO3 + H2O + CO2
Now this is reaction shown by all elements of Gp I also but there was only a bit deviation shown By Li that all other carbonates existed in solid (of alkali metals) while LiHCO3, only existed in liquid state so from here, we can remember that Bicarbonates of Gp II exists only in liquid aq. Solution.
- Carbonates – Now the carbonates are product on heating Bicarbonates this states that they
are thermodynamically more stable. Now the exception that we considered with Li was that all other alkali metal carbonates were stable except Li, so as we could now easily guess that Gp II Metal carbonates also decompose as
MCO3 heat–> MO + CO2 where M = Be, Mg, Ca, Sr, Ba and Li
The reason for this is same as for Li high polarizing power, Now for the properties we have
Stability increases down the group where as the solubility decreases down the group for carbonates of gp II.
The stability of BeCO3 is so less that it decomposes at hot days and so kept in the atmosphere of carbon dioxide to maintain its concentration.
- Nitrates – With nitrates as well Li shows a different behaviour, all of Gp I elements
Decompose but Li decompose to oxides-
MNO3 heat–> MNO2 + O2 where M= Na, K, Rb, Cs
LiNO3 heat–> Li2O + NO2 + O2
Now from so many continuous coming analogy we have that the metal nitrates of Gp II will decompose to their oxides as
M(NO3)2 heat–> MO + NO2 + O2 Where M = Be, Mg, Ca, Sr, Ba and Li
The important part is the similarity of Li with Gp II, no exception of Be and the oxide of nitrogen(+4) because there as too many of nitrogen oxide.
Stability increases down the group.
- Sulphates – The sulphates of Gp II elements also decomposes like their Carbonates into their
oxides as follows,
MCO3 heat–> MO + SO2 + O2 where M = Be, Mg, Ca, Sr, Ba
The solubility decreases down the group