Alkaline-earth metals chemical properties (2)
Be and Mg. Be and Mg are stable in dry air as they form natural dense protective coating of oxides and nitrides. These metals flash-ignite with the formation of oxides MO only at high temperature (Be at 900C and Mg at 650C). Heats formation of their oxides per 1 mol-equivalent of metal is the same (301 kJ/mol). Mg burns emitting dazzling bright light.
Water does not act on beryllium. Mg is almost unreactive in cold water, but slowly reacts with boiling water. Magnesium is used for dehydration of organic solvents.
The standard values of electrode potentials, Eo, for Be and Mg are very low, therefore they must be easily decomposed in water already at STP. In fact it is not so due to the presence of protective coating. The coating layer on Be is chemically and mechanically similar to the coating layer on Al. The coating layer of magnesium oxide has a greater thickness, it is more fragile and its protection property is weaker.
Eo for Be is considerably higher, than for the other metals of subgroup ІІA. It is related to its high heat of sublimation and potentials of ionization, which are not completely compensated by large energy of hydratation of ion, as it is observed for other metals, and, above all things, for lithium.
The chemical behavior of compact Be to a great deal reminds Al properties, that is an example of the so-called “diagonal similarity”.
Hydrides. Be does not interact directly with hydrogen. Its polymeric hydride (ВеН2)х is obtained in an indirect way only (decomposition of berillium–organic compounds).
Mg interacts with hydrogen, but only under special conditions: t= 400-500, hydrogen pressure of 10-20 MPa, the presence of traces of I2 (catalyst).
At slight heating already these hydrides are decomposed. They easily hydrolyze :
MH2 + 2H2O = M(OH)2 + H2
Non-metals. With Hal2, S, N2, C, Si and others Be and Mg directly react at heating. Mg forms binary compounds with ordinary valence ratio: MgS, MgHal2, Mg2Si, MgC2, Mg3N2, Mg3P2.
Be3N2 is refractory solid that is practicably stable in cold water. Magnesium nitride, Mg3N2, is not stable at heating and is instantly decomposed in water:
Mg3N2 + 6H2O = 2Mg(OH)2 + 2NH3
Berillium with carbon gives refractory carbide Be2C (it is methanide, like Al4C3), which is slowly decomposed in water with the formation of methane:
Ве2С + 4Н2О = 2Ве(ОН)2 + СН4
Mg with carbon forms acetylenide MgC2, which is easily decomposed in water:
MgC2 + 2H2O = Mg(OH)2 + C2H2
but magnesium also forms not typical for other metals propynide, Mg2C3, which produces propyne СН3–ССН with water:
Mg2C3 + 4H2O = 2Mg(OH)2 + CH3–CCH
Mg is a very strong high temperature reductant. It is widely used in obtaining metals and non-metals by metallothermy:
TiCl4 + 2Mg = Ti + 2MgCl2
B2O3 + 3Mg = 2B + 3MgO
Acids. Mg easily reacts with the diluted acids; Be reacts with HCl and H2SO4, and with HNO3 - only at higher temperatures. The rate of dissolution in acids depends on the purity of metal. For very pure Be the rate change can be represented by the series: HF > H2SO4 > HCl > HNO3.
Mg is unreactive with alkalis, but Be is dissolved gradually in their concentrated solutions :
Be + 2NaOH + 2H2O = Na2[Be(OH)4] + H2