All the alkaline earth metals are highly reactive elements since they have a strong tendency to lose the two valence s-electrons to form the corresponding dipositive ions having inert gas configuration.
Reactivity arises due to their low ionization enthalpies and high negative values of their standard electrode potential.
The chemical reactivity of alkaline earth metals increases on moving down the group from Be to Ba because the ionization enthalpies decreases and electrode potential become more and more negative with increasing atomic number from Be to Ra.
Beryllium is the least reactive while Ba or Ra is the most reactive element.
Since the ionization enthalpies of alkaline earth metals are higher and their electrode potential are less negative than the corresponding alkali metals ,therefore, alkaline earth metals are less reactive than corresponding alkali metals.
1) Reducing character
The alkaline earth metals are weaker reducing agents than alkali metals. Like alkali metals, their reducing character also increases down the group.
Reason: The alkaline earth metals have a fairly strong tendency to lose two electrons to form dipositive ions because of their low ionization enthalpies and high negative values of standard electrode potentials. Therefore they act as reducing agents.
The reducing character of alkaline earth metal increases as we move down the group from Be to Ba because the ionization enthalpies decrease and electrode potentials become more and more negative with increasing atomic number from Be to Ba.
Since the ionization enthalpies of alkaline earth metals are higher and their electrode potentials are less negative than the corresponding alkali metals, therefore, alkaline earth metals are weaker reducing agents than alkali metals.
2) Reactivity towards water
The electrode potential of beryllium ( Be2+ / Be = -1.97 V) is least negative amongst all the alkaline earth metal. This means that Be is much less electropositive than other alkaline earth metals and hence does not react with water or steam even at red heat.
The electrode potential of Mg (Mg2+ / Mg = -2.36 V) although more negative than that of Be, yet is still less negative than those of alkali metals and hence it does not react with cold water but reacts with boiling water or steam.
Mg + H2O ——> MgO + H2
Mg + 2H2O ———-> Mg(OH)2 + 2 H2
Magnesium forms a protective layer of oxides on its surface ,therefore ,despite its favourable electrode potential , it does not react readily with water unless the oxide layer is removed by amalgamating it with mercury. In the formation of oxide film ,magnesium resembles aluminium.
Calcium, Barium and Strontium have more negative electrode potential and hence react with increasing vigour even with cold water, liberating H2 and forming the corresponding metal hydroxide.
Ca + 2 H2O ——-> Ca(OH)2 + H2
The reactivity of alkaline earth metals increases as we move down the group.The reaction of alkaline earth metals is less vigorous as compared to alkali metals.
3) Reactivity towards air
a) Formation of oxides and nitrites
The alkaline earth metals being less electropositive than alkali metals react with air or oxygen slowly upon heating to form oxides, MO.
Be and Mg are kinetically inert to oxygen because of the formation of a thin film of oxide on their surface. Powdered beryllium is more reactive and burns brilliantly on ignition to give a mixture of beryllium oxide and beryllium nitride.
2 Be + O2 —-> 2 BeO
3 Be + N2 ——-> Be3N2
Magnesium is more electropositive than beryllium and hence burns with dazzling brilliance in air to form a mixture of magnesium oxide and magnesium nitride.
2 Mg + O2 —-> 2 MgO
3Mg + N2 —–> Mg3N2
Ca, Ba, Sr being even more electropositive react with air readily to form a mixture of their respective oxides and nitrites.
Reactivity towards oxygen increases as we move down the group. Thus Ca, Ba and Sr are stored in paraffin but Be and Mg are not because they form a protective layer of oxide on their surface.
b) Formation of nitrides
All the alkaline earth metals burn in dinitrogen to form ionic nitrides of the formula M3N2.
3 M + N2 ——–> M3N2
Be3N2 being covalent is volatile while the nitrides of all other elements are crystalline solids.
All these nitrides decompose on heating and react with water liberating NH3.
Be3N2 ——-> 3 Be + N2
Ca3N2 + 6 H2O ——> 3 Ca(OH)2 + 2 NH3
4) Formation of peroxides
Since larger cations stabilize larger anions, therefore tendency to form peroxide increases as the size of the metal ion becomes larger.Barium peroxide is formed by passing air over heated BaO at 773 K.
2 BaO + O2 ——> 2 BaO2
2 SrO + O2 ——> 2 SrO2
SrO2 is prepared in a similar way but under high pressure and temperature.CaO2 can be prepared as hydrate by treating Ca(OH)2 with H2O2 and then dehydrating the product.
Ca(OH)2 + H2O2 ——> CaO2 .2H2O
Crude MgO2 has been made using H2O2 but peroxide of beryllium is not known.
Treatment of peroxide with acids liberate hydrogen peroxide.
BaO2 + 2 HCl ——-> BaCl2 + H2O2
5) Reactivity towards halogens
All the alkaline earth metals combine with halogens at elevated temperature forming their halides.
M + X2 ——-> MX2
X= F, Cl, Br, I
Thermal decomposition of (NH4)2 BeF4 is the best method to prepare BeF2.
BeO + 2 NH3 + 4 HF —-> (NH4)2 BeF4———> BeF2 + 2 NH4F
BeCl2 is prepared by heating BeO with Cl2 in presence of Carbon.
Beryllium chloride can also be prepared by heating beryllium oxide with carbon tetrachloride.
2 BeO + CCl4 ——–> 2 BeCl2 + CO2
6) Reactivity towards hydrogen
All the alkaline earth metal except beryllium combine with hydrogen directly on heating to form metal hydrides of the formula MH2.
M + H2 —-> MH2
The hydrides of beryllium can be obtained by reduction of BeCl2 with LiAlH4.
2 BeCl2+ LiAlH4 ———-> 2 BeH2 + LiCl+ AlCl3
Beryllium hydride and magnesium hydride are covalent compounds.
Since BeH2 or MgH2 has only 4 electrons in the valence shell, therefore they are electron deficient molecules.To make up their electron deficiency ,each Be or Mg atom forms four three centre two-electron bond or banana bonds. Thus, it is due to electron deficiency that BeH2 and MgH2 have polymeric structure.
The hydrides of other elements of this group i.e. CaH2, SrH2 and BaH2 are ionic and contain the H‾ ions. The stability of alkaline earth metal hydrides decreases down the group from Be to Ba since their lattice enthalpy decrease progressively as the size of the metal cation increases.
All the hydrides of alkaline earth metals react with water liberating H2 gas and thus act as reducing agent.
MH2 + 2 H2O —> M(OH)2 + 2 H2
CaH2 is called hydrolith and is used for large scale production of H2 by action of water on it.
7) Reactivity towards carbon
When beryllium oxide is heated with carbon at 2175-2275 K, a brick red coloured carbide of the formula, Be2C is formed.
2 BeO + 3 C ——> Be2C + 2 CO
It is an ionic compound and reacts with water forming methane
Be2C + 4 H2O ——–> 2 Be(OH)2 + CH4
The rest of the alkaline earth metal form carbides of the general formula, MC2 either when the metal is heated with carbon in an electric furnace or when their oxides are heated with carbon.
Ca + 2 C —-> CaC2
CaO + 3 C ———> CaC2 + CO
All these carbide reacts with water forming acetylene gas.
CaC2 + 2 H2O ——> HC≡CH + Ca(OH)2
MgC2 on heating gives Mg2C3 .This carbide contains C34- unit and hence react with water to form propyne.
Mg2C3 + 4 H2O —-> CH3C≡CH + 2 Mg (OH)2
when CaC2 is heated in an electric furnace with atmospheric dinitrogen at 1375 K , it forms calcium cyanamide, CaNCN or CaCN2.
CaC2 + N2 —-> CaNCN + C
The mixture of calcium cyanamide and carbon is called nitrolim and is used as a slow acting nitrogenous fertilizer as it hydrolyses slowly over a period of months evolving ammonia gas.
8) Reactivity towards acid
All alkaline earth metals react with acids liberating H2.
M + 2 HCl —–> MCl2 + H2
Be being amphoteric also dissolves in alkalis liberating H2.
Be + 2 NaOH + 2 H2O ——> Na2[Be(OH)2] + H2
9) Solution in liquid ammonia
All alkaline earth metals dissolve in liquid ammonia. The dilute solutions are bright blue in colour due to solvated electrons but concentrated solution are bronze coloured due to formation of metal clusters.
These solutions decompose very slowly forming amides and evolving H2 ,but the reaction is accelerated by many transition metals and their compounds.
M + (x+2y) NH3 ——> M2+ (NH3)x +2 e‾ (NH3)y
e‾ (NH3)y ———-> NH2‾ + ½ H2 + (y-1)NH3
Evaporation of ammonia from solution of alkali metals gives the metal, evaporation of ammonia from solution of alkaline earth metals gives hexammoniates the general formula, [M(NH3)6]2+ which slowly decompose to give the corresponding metal amides.
M(NH3)6 —–> M(NH2)2 + 4 NH3 + H2
10)Tendency to form complexes
Among alkaline earth elements Be and Mg have maximum tendency to form complexes. This is due to their small size and higher charge density. BeF2 combines with F‾ ion to form the tetrahedral complex, [BeF4]2‾
BeF2 + F2 —-> [BeF4]2‾