Solutions to Worksheet on Noble gases and Hydrogen

1. Suggest why Pauling considered that the heavier noble gases were more likely to form compounds than the lighter ones?

As with any group, the ionisation energy decreases down the group making it easier to remove electrons. Analogy with the adjacent halogens where iodine is much easier to raise to high oxidation states than e.g. chlorine, also suggests that the heavier elements are more likely to form compounds.

2. Use the "VSEPR" method to predict the structures (3D shapes) of the following molecules:

XeF₂, XeF₄, XeO₃, XeO₄, XeOF₄, XeO₃(OH)^-, XeO₆^4-.

Four stereochemically active electron pairs arranged
         tetrahedrally (i.e. sigma pairs and lone pairs):
      
                     XeO3E                XeO4
                   pyramidal           tetrahedral
      
      Five stereochemically active electron pairs arranged in a
         trigonal bipyramid:
      
                    XeF2E3              XeO3OHE-
                   linear              irregular
      
      Six stereochemically active electron pairs arranged
         octahedrally:
      
              XeF4E2              XeOF4E            XeO64-
          square planar      square pyramidal      octahedral
      

4.* Tritium is radioactive with a half life of 12.4 yr. Why is the proportion of tritium in natural hydrogen more or less constant? What happens to the tritium when it decays?

Tritium is being produced continually in the upper atmoaphere form the reaction between cosmic neutrons and nitrogen atoms:

¹₀n + ¹⁴₇N ---> ¹²₆C + ³₁T

The rate of production is more or less constant, so the amount will stabilise when the decay rate equals the production rate - a dynamic equilibrium. The decay is:

³₁T ---> _-1⁰e + ³₂He

7. Write balanced chemical equations and essential conditions for the following syntheses:

a. XeO₃ from Xe(g)

Xe(g) + 2F_2(g) ---> XeF₄ (heat or light)

6XeF₄ + 12H₂O ---> 2XeO₃ + 4Xe(g) + 3O₂ + 24HF

b. LiH from H₂

2Li + H₂ ---> 2LiH (heat)

c. HI from I₂

H₂ + I₂ ---> 2HI (slowly at room temp.)

d. SnH₄ from Sn

Sn + 2Cl₂ ---> SnCl₄

SnCl₄ + LiAlH₄ ---> SnH₄ + LiCl + AlCl₃

8. Write balanced equations for:

a. the hydrolysis of XeF₄,

See 8.a.

b. the reaction of XeO₃ with dilute base,

XeO₃ + OH^- ---> XeO₃OH^-

c . the reaction of XeO₃ with concentrated base,

4XeO₃ + 12OH^- ---> 3XeO₆^-4 + Xe(g) +6H₂O

d. the reaction of perxenate ion with acid.

XeO₆^-4 + 4H⁺ ---> XeO₄ + 2H₂O

(C) H.J Clase 1999 first version 1999.01.25