L2 S4
Chemical Engineering
Oil and Gas
Spring semester 2019
Exercice 2
Alkaline and alkaline earth metals
Groupe IIIB, Al&B
1. How do you explain that combustion of Li(s) in air leads to Li2O and to Na2O2
for Na(s) ?
2. In what form is oxygen present in Na2O? What is the reaction of Na2O with
water?
3. Electrolysis of molten LiH led to the formation of dihydrogen at the anode.
Why?
4. Beryllium chloride is not electrolyzable, why? Are beryllium and lithium
chlorides soluble in organic media??
5. Looking at redox potentials, what is the most oxidizing, most reducing, least
oxidizing, less reducing alkali derivative? Is it easier to electrolyze lithium
chloride to lithium metal or sodium chloride to sodium metal?
6. Calculate the Formation Enthalpy for MgBr2 and draw the resulting Born
Haber cycle:
Reticular energy MgBr2
Sublimation of Mg
Ionization Mg into Mg2+
Enthalpy (kJ.mole-1)
-2394
150
2186
Vaporization of Br2(g)
Dissociation of Br2
Electronic Affinity of Br (=-DH° electronic fixation)
31
50
342
7. Is sodium chloride hygroscopic ? Is it more or less hygroscopic than lithium
chloride ?
8. Complete the reactions :
Li +N2 ®
Na + O2 ®
Cs + O2 ®
K + C2H5OH ®
KOH + CO2 ®
9.
Aluminium halides are dimers. Represent their structures, what is the
nature of the bonds in these compounds ? Explain why a monomeric compound is
observed in the case of fluorine.
10.
Complete the reactions :
B2H6 + NaH ®
Al2Cl6 + LiH ®
Al4C3 + 12 H2O ®
H3BO3 + NaOH ®
BF3 + Et2O ®
NH4Cl + LiBH4 ®
Al(OH)3 + H2SO4 ®
Al(OH)3 + NaOH ®
11. BH3 molecule does not exist as such but in the dimeric form B2H6. On the
other hand, trimethylborane exists. Mixed hydride-alkyl compounds such as
B2H5(CH3), B2H4(CH3)2 , B2H3(CH3)3, B2H2(CH3)4 are also observed. Give a
probable representation of these compounds, the nature and order of the bonds
in these compounds.
12.
For the compounds of Type M(OH)3 in Column 13, the following representations
are often found: H3BO3 for M=bore but Al(OH)3 for M = Al. Comment.