Chapter 3. Alkaline Earth Metals 
Answers to Questions

1.

 
1st IE (kJ/mol)
2nd IE (kJ/mol)
Na
 496
4562
Mg
737
1450
1(a) periodic trend based on increased effective nuclear charge from L to R within a period

1(b) Na: 4562/496 = 9.20; Mg: 1450/737 = 1.97

1(c) it would be extremely large -- perhaps 9X greater than 1450 as we saw with Na
 

2. DH = -19 kJ

3. (a) CaO
 Uses:  a. flux in steel industry to remove P, S, and Si
   b. to produce slaked lime, Ca(OH)2(sat’d)
   c. water treatment with alum
   d. glass manufacture
   e. pulp & paper industry
 

4. (a) Ksp(Mg(OH)2) = 8.9 x 10-12 = [Mg2+][OH-]2
 

[OH-]2 = 8.9 x 10-12/0.0531

[OH-] = 1.29 x 10-5

pH = 9.11

 (b) Similarly for Ca+2, the precipitation should occur at pH = 12.1

(c) Mg+2

(d) Yes, keep the pH above 9.11 and below 12.1
 

5. The Pidgeon process is used to obtain metallic Mg from dolomite:
 
CaMg(CO3)2  CaMgO2

2 CaMgO2 + FeSi  2 Mg + CaSiO4 + Fe


6. Mg is used to make an alloy (90% Mg, 9% Al, traces of Zn, etc) for uses in aircraft, light-weight engine blocks (VWs!), rockets, ladders, docks.  Also used in the cathodic protection of steel.  Fireworks.

7.

(a) BaH2
(b) CaSO4.2H2O
(c) MgSO4.7H2O
(d) CaSiO3
(e) Ca3N2
(f) Mg(OH)2
(g) Ca(OH)2
(h) CaF2
(i) CaMg(CO3)2


8.

(a) Ca(s) + H2O(l)   Ca(OH)2(s) + H2(g)
(b) Ba(s) + N2(l)   Ba3N2(s)
(c) CaH2(s) + H2O(l)   Ca(OH)2(s) + 2 H2(g)
(d) Mg(s) + O2(g) 2 MgO(s)
(e) Ca(s) + S(s) CaS(s)
(f) Sr(s) + C(s) SrC2(s)
(g) Ba(s) + H2(g)  BaH2(s)
(h) CaC2(s) + H2O  C2H2(g) + Ca(OH)2(s)
(i) Mg(s) + HCl(aq)  MgCl2(s) + H2(g)


9.

Ca(s)   Ca(g)            DHatom = +713 kJ/mol
Ca(g)   Ca+(g) + e-           DHie#1 = +590 kJ/mol
Ca+(g)   Ca+2(g) + e-          DHie#2 = +1145 kJ/mol
Cl2(g)   2 Cl(g)                DHCl-Cl = +243 kJ/mol
2 Cl(g) + 2e-  2 Cl-(g)          2 x DHea = -699 kJ/mol
Ca+2(g) + 2 Cl-(g)  CaCl2(s)             DHlattice = -2276 kJ/mol
Ca(s) + Cl2(g)  CaCl2(s)  DHatom = -796 kJ/mol


10.

Mg(s)  Mg(g)            DHatom = +146 kJ/mol
Mg(g)  Mg+(g) + e-           DHie#1 = +737 kJ/mol
Mg+(g)   Mg+2(g) + e-          DHie#2 = +1450 kJ/mol
F2(g)   2 F(g)                    DHF-F = +154 kJ/mol
2 F(g) + 2e-  2 F-(g)          2 x DHea = -659 kJ/mol
Mg+2(g) + 2 F-(g)  MgF2(s)          DHlattice = -2931 kJ/mol

Mg(s) + F2(g)  MgF2(s)              DHatom = -1103 kJ/mol